DUJS 08S

Note from the Editorial Board Dear Readers, In the spring of 1999, a group of pioneering Dartmouth undergraduates sought to establish a forum for undergraduate scientific thought. Now, quarterly issues of the DUJS greet readers worldwide in print and on our newly redesigned website. Online, the DUJS now has weekly updates on science at Dartmouth and around the globe.

On May 22, the DUJS celebrates its 10th Anniversary in conjunction with the Wetterhahn Science Research Symposium, co-sponsored by the Women in Science Program. In the coming terms, the DUJS will continue to expand its online presence and bring leaders in the scientific community to the Dartmouth campus.

The commemorative section of this issue features a collaboration between the first DUJS president Amar Dhand ’01 and the outgoing president Frank Glaser ’08, who offer their perspectives on 10 years of the DUJS. Will Schpero ’10 and Colby Chiang ’10 look back on 10 years of scientific progress at Dartmouth, while Peter Zhao ’10 speculates on what the next 10 years may hold for the world of science. Dillon Lee ’08 delves into the history of physics at Dartmouth, highlighting some of Dartmouth’s lesser-known milestones. Finally, Laura Sternick ’08 offers a tribute to the late Professor Karen Wetterhahn, as this year also marks the 10th anniversary of the dedication of the Wetterhahn Science Symposium.

We also continue to bring you the best of Dartmouth’s undergraduate science writing. Patrick Karas ’08 analyzes the chaotic behavior of an oscillating chemical reaction. Qinggong Wu ‘09 explains the creation of supernovae. Tim Shen ’08 describes the efficacy of iron chelation treatment for mucormycosis, and discusses the future directions for the treatment of depression, while Colby Chiang ’10 considers the results of the genetic battle of the sexes. Theresa Yang ’08 and Allison Baker ’09 describe the mechanisms behind epilepsy and autism, respectively.

On the research front, Laura Calvo ’11 reveals how explosives may continue to persist in soil long after they are expected to, raising serious environmental questions.

Sam Haynor ’08, Chad Gorbatkin ’08, Sarah Isbey ’08, and Zachary Mayer ’08 present the results of their study on army ant behavior in a paper written for the Biology Foreign Study Program.

And, to conclude this anniversary issue, we are pleased to present a special feature: a scene from Senior Fellow Latif Nasser’s ’08 play on Albert Einstein.

The DUJS has come a long way since its inception. Its sustainability, evolution, and expansion are direct consequences of the hard work and dedication of countless writers, editors, and staff members. In this final issue of DUJS Volume X, we would like to thank them as well as our sponsors and faculty advisors, both past and present, for their continuing support and good wishes. Last but not least, we take this opportunity to thank you, dear readers, for having supported the DUJS in its endeavors to promote science at Dartmouth and throughout the world.

The DUJS Editorial Board

Cover Image Cover manipulations

by Tim Shen ‘08.

The Dartmouth Undergraduate Journal of Science aims to increase scientific awareness within the Dartmouth community by providing an interdisciplinary forum for sharing undergraduate research and enriching scientific knowledge.

EDITORIAL BOARD

President: William Schpero ’10 Emeritus: Frank Glaser ’08 Editor in Chief: Shreoshi Majumdar ’10 Emeritus: Laura Sternick ’08 Managing Editor: Hannah Payne ’11 Managing Editor, Images: Peter Zhao ’10 Emeritus: Tim Shen ’08 Asst. Managing Editor: Sean Currey ’11 Online Content Editor: Laura Calvo ’11 Publicity: Edward Chien’ 09 Secretary: Dillon Lee ’08

DESIGN STAFF

Kristin Bonello ’11 Alison Flanagan ’10 Edward Yu ’11

ISSUE STAFF WRITERS Colby Chiang ’10 Chad Gorbatkin ’08

Faculty Advisors

Alex Barnett Mathematics Ursula Gibson Engineering Marcelo Gleiser Physics/Astronomy Gordon Gribble Chemistry Carey Heckman Philosophy Richard Kremer History Leslie Sonder Earth Sciences Megan S. Steven Psychology Roger Sloboda Biology Special Thanks

Dean of Faculty Associate Dean of Sciences Thayer School of Engineering Provost’s Office Whitman Publications Private Donations The Hewlett Presidential Venture Fund Women in Science Project

DUJS@Dartmouth.EDU Dartmouth College Hinman Box 6225 Hanover, NH 03755 (603) 646-9894 www.dartmouth.com/~dujs Copyright © 2008 The Trustees of Dartmouth College

Spring 2008

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DU The Dartmouth Undergraduate Journal of Science aims to increase scientific awareness within the Dartmouth community by providing an interdisciplinary forum for sharing undergraduate research and enriching scientific knowledge.

In this issue: X: The DUJS’s 10th Anniversary

From Idea To Reality: Ten Years of the DUJS 4

Amar Dhand ‘01 and Frank B. Glaser ‘08

A Decade of DUJS, A Decade of Science at Dartmouth 7

Will Schpero ‘10 and Colby Chiang ‘10

What Else Has Happened? A Celebration of the Legacy of Physics at Dartmouth 12

Dillon Lee ‘08

The Future of Science at Dartmouth and Elsewhere: The Developments of Today and Realities of Tomorrow

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Remembering Karen Wetterhahn

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Peter Zhao ‘10

Laura Sternick ‘08

Features

Future Directions for Treating Depression: Deep Brain Stimulation and the Brain’s Reward System

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Autism Spectrum Disorder: Theory of Mind Impairment in Autism as a Result of Failed Implementation of Integrated Social Percepts

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Tim Shen ‘08

Allison Baker ‘09

Dartmouth Undergraduate Journal of Science

UJS Spring 2008

Volume X, No. 3

DUJS

Battle of the Sexes: How X and Y Chromosomes Are Engaged In Perpetual Warfare

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The Neurobiology of Epilepsy: Mechanisms of the Disease and Network and Structural Changes Involved

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Iron Chelation As A Novel Therapy: Treatment of Mucormycosis with Deferasirox in the Laboratory and in a Clinical Case

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Type 1a Supernovae: Properties, Models, and Theories of Their Progenitor Systems

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Colby Chiang ‘10

Theresa Yang ‘08

Tim Shen ‘08

Qinggong Wu ‘09

Research

Army Ant Emigration: Diel Emigration and Foraging Behaviors of the Army Ant Eciton hamatum (Subfamily Ecitoninae)

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Persistent Explosives Present A Problem: Analyzing the Biodegradation of Nitroglycerin and 2,6-dinitrotoluene in Camp Edwards Soil

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Chaos In Oscillating Chemical Reactions: The Peroxidase-Oxidase Reaction Patrick Karas ‘08

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Philistines! (Scene Two): Or the Electrodynamics of a Moving Body

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Sam J. Haynor ‘08, Chad Gorbatkin ‘08, Sarah C. Isbey ‘08, Zachary A. Mayer ‘08

Laura Calvo ‘11

Latif Nasser ‘08

Image Credits from left to right: Joseph Mehling ’69, Professor Paul Whalen, CDC and Dr. Libero Ajello, © David A. Hardy/www.astroart.org, J.T. Longino of The Evergreen State College, J.T. Longino of The Evergreen State College, and Patrick Karas ’08.

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dujs x

From Idea To Reality: Ten Years of the DUJS

Amar dhand ’01 and frank b. glaser ‘08

Amar Dhand ‘01, one of the founders of the DUJS. Image courtesy of Amar Dhand ‘01.

We propose a medium of scientific expression for the students, by the students.

The community needs a medium that will focus on recognizing and unifying Dartmouth undergraduate research while also, motivating more students to get involved in the advancement of scientific thought. We need some entity to become the source of scientific dialogue and expression within the community. Therefore, we propose the establishment of The Dartmouth Undergraduate Journal of Science (1).

In Project Proposal written by the Founders February 1, 1999

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n late 1998, the DUJS was conceived in order to satisfy a need. Too often, students would write culminating research reports that made it only as far as supervisors’ desks. Occasionally, students would have the opportunity to participate in writing a formal article with a research mentor. Such experiences, however, did not always afford students the freedom to creatively ‘play’ with ideas, or consider alternative explanations.

Moreover, students’ work was largely unknown to each other, resulting in a lack of potential peer learning and coconstruction of ideas. Therefore, if research ever made it to the Collis coffee table, it was usually a superficial recounting of moments in the laboratory. In essence, the spirit of creative thinking and scientific dialogue relating to undergraduate research was missing.

The aim of the DUJS was to change this experience. As the founders, Tim Lesle ‘01, Soon Hyouk Lee ‘01, Arvindh Kanagasundram ’01 and Amar Dhand ’01, stated in the quotation above, the DUJS sought to

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recognize, unify, and motivate undergraduate research.

College administrators and alumni swiftly supported this vision, perhaps because it served to bolster Dartmouth’s research identity, an asset in the perpetual jostling among premier universities. The DUJS was awarded grants from the Hewlett Presidential Venture Fund, the Dean of Sciences, and the Dean of Faculty.

Subsequently, it received substantial donations from trustees and prominent alumni including Peter Fahey ‘68, Barry MacLean ‘60, Dr. TJ Rogers ’70, and Dwight L. Allison ’51.

The first issue, published in Spring 1999, was an experiment in defining the journal’s identity as a ‘science’ periodical. Fittingly, the cover was a collage of images designed to represent the diversity of ideas embodied by science. The issue included papers ranging from the organic by-products in the disinfection of natural waters to a reflective piece about a cubic mathematical equation in a student’s journey to understand himself. The first issue also established the tradition of highlighting Dartmouth science with articles about scanning probe microscopy work at the College and the Nobel Laureate alumnus Owen Chamberlain ’41 (2).

Students and alumni were impressed with the journal and the quality of its articles. As Anura Abeyesinghe ’01 stated, “The journal is online and that made it easy for me to simply mention the website in my application so that the grad school people could read my article and get an idea of the kind of work I have done” (3). Alumni were frequent readers as Dr. Colin B. Holman ‘39 attested: I surely do wish we had such a journal as you now have to display our efforts. Anyway, I am very happy to have your journal and hope to see more of it. I have published over a hundred papers and book chapters during my time on the staff of the Mayo Clinic where I worked after serving in the Medical Corps of the army during WW 2 and am sorry my bibliography doesn’t contain a contribution in your fine journal (4).

Central to the early development of the journal was its faculty-student collaboration, most evident in the editorial process. The initial group of faculty advisors Dartmouth Undergraduate Journal of Science

included Drs. Paul Corballis, Ursula Gibson, George Langford, Delo Mook, Leslie Sonder, and Samuel Velez.

The first student editors were Bryan Coffing ’00, Brian C DeSchuytner ’00, Karen Glocer ’00, Gwendolyn M. McKee ’02, Jacob Waldbauer ’01, and the four founders.

This faculty-student group was critical in soliciting articles from different departments, creating parameters of “high quality” research, and providing specialized expertise in organizing and presenting ideas and data in a persuasive manner. Using faculty guidance, the first editorial board conceived an intensive review and editing process. At least two editors and one faculty advisor read each submitted article, and then debated its inclusion during a full meeting. Subsequently, each editor would work one-on-one with an author to sharpen arguments, implement the board’s suggestions, and format the article appropriately. A novel scientific dialogue began to flourish on multiple levels.

The journal would not have survived without transitioning the leadership to new generations of editors. Therefore, fostering editorial and leadership skills in the younger generation was a goal of the original editorial board in the journal’s third year of existence. Experienced editors often paired up with inexperienced editors to demonstrate how to work with authors and faculty advisors to perfect articles. Specific hands-on knowledge was also transmitted about layout, fund-raising, and working with the administration.

These activities constituted an embedded curriculum developed by the board to ensure the longevity of the journal. When the original editorial board began graduating in 2000 and 2001, this foresight paid off as the journal was recognized as one of nation’s pioneering student journals in the May 2001 issue of the journal Nature (3).

It was in the spring of 2003 that the journal experienced one of the most dramatic single improvements in its history. Hereafter, the body of the journal including graphs, tables, and images would be produced in color. This change, undertaken by journal president Peter Chalmers ’05 and editor-in-chief Laura Berzak ‘04, significantly improved the quality and attractiveness of the final product. Just one year later, nearly every graphic was printed with high-quality color inks, which was likely responsible for the subsequent increase in readership. According to Chalmers, “color printing layers an entirely new dimension of information onto figures.” The transition, he says, “provided innumerable benefits to the journal’s ability to advance scientific knowledge at Dartmouth” (5).

In addition to printing student research in the journal, the journal’s staff has sought innovative opportunities for exposure of student work. In the summer of 2004 members of the DUJS staff helped organize Spring 2008

Frank Glaser ‘08, current President Emeritus of the DUJS. Image by Tim Shen ‘08.

a series of science broadcasts on WDCR Dartmouth College Radio. In 2005, a new partnership was forged with UGResearch.org, an organization founded by Dartmouth graduates to expand the availability of undergraduate research beyond schools at which it was conducted. Many articles published in previous editions of the DUJS were manually uploaded into a database that features student research from around the country.

Projects such as these very much complement the DUJS’ mission of striving to promote undergraduate interest in science and writing.

Another aspect of the journal’s mission is to encourage interaction between members of the Dartmouth community and the professional world of science journalism. To this end, in 2004, the journal’s staff organized a seminar series on science writing featuring career journalists. As part of this program, writers from the New England Journal of Medicine and deputy editor Craig Whitney of the New York Times visited the DUJS in April and May of 2004. That year, the journal also began organizing regular outings to the offices of Dartmouth Medicine. The goal was to introduce the staff to the editors and writers of a professional science magazine and to promote collaboration between the two publications. In fact, members of the DUJS staff have served as interns at Dartmouth Medicine, and Dartmouth Medicine has reprinted original DUJS articles.

Additionally, the DUJS staff has helped coordinate events for its members to meet with visiting scientists from the Montgomery Fellows Program.

Rita Colwell, a former director of the National Science Foundation, Oliver Sacks, a British neurologist and author of case histories collection Awakenings (which was adapted to a film starring Robin Williams and Robert De Niro), and both Sidney Altman and Thomas Cech, co-recipients of the 1989 Nobel Prize in Chemistry for 5

work on the catalytic properties of RNA, are among the distinguished guests with whom the DUJS staff has had an opportunity to meet through the program. Certainly, the journal’s staff has benefited from these incredible opportunities to engage scientists in a dialogue on science writing and their professional careers.

One of the most exciting milestones of Dartmouth science history was celebrated in 2006. That year marked the semicentennial of the establishment of artificial intelligence as a research discipline by Dartmouth professor John McCarthy. To help celebrate this occasion, the college hosted the AI@50 conference, which brought 175 leading researchers from around the world to campus (including five of the surviving founders of the discipline – one of whom was McCarthy himself).

For its part, the DUJS dedicated an issue to artificial intelligence and many staff members had an opportunity to meet with these distinguished visitors. Also, with the initiative of journal president Jacob Goldberg ’07, the journal conducted its first ever essay contest for original articles written about artificial intelligence. Winning submissions were featured in the AI issue, which was released concurrently with the conference.

Currently, a decade after the journal’s inception and acceptance into the Dartmouth science community, the organization is stronger than ever before. The future seems promising for the journal: its incoming leadership is capable and ambitious, its staff – writers, editors, and technical personnel – is hard-working, and the intellectual community to which it belongs is presently producing some of the most impressive science achievements in recent history.

This provides a logical and appropriate opportunity for celebration of the journal’s own

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accomplishment. Thanks to this thriving academic community, the DUJS has itself been able to thrive over the past ten years. A decade’s worth of Dartmouth students have been significantly influenced by the journal’s strong presence in their midst. Scientists here and elsewhere have been genuinely interested in the developments propagated by the journal, as evidenced by worldwide subscriptions. Over the years, the DUJS has also aided and advised in the establishment of undergraduate science journals across the country.

Some, such as that of Harvard University, have been quite successful. This prompted editor-in-chief James Klaas ’04 to see the DUJS as a trailblazing publication, “a model for all undergraduate journals of science” (6).

In 1998, the DUJS was conceived to satisfy a need on campus. Reflecting on ten years of the journal’s existence, it is fair to say that now the reality is larger than the idea proposed. The journal has undergone an evolution spurred by the changing needs of the Dartmouth community, innovative undergraduate research, and successive generations of creative editorial boards. As a result, the journal has endured and developed into a dynamic enterprise fit to be a “medium of scientific expression for the students, by the students.”

References 1. A. Dhand, S.H. Lee, T.K. Lesle, A. Kanagasundrum, unpublished document entitled The Darmouth Undergraduate Journal of Science: Project Proposal (1999).

2. See Dartmouth Undergraduate Journal of Science 1, 1 (1999) for all of the above-mentioned articles.

3. J. Chen, Nature 411, 6833 (2001).

4. A. Dhand, personal communication.

5. F. Glaser, personal communication.

6. S. Okazaki, VOX Of Dartmouth 22 (2004). Available at http:// www.dartmouth.edu/~vox/0304/0209/journals.html.

Dartmouth Undergraduate Journal of Science

dujs x

A Decade of DUJS, A Decade of Science at Dartmouth William Schpero ‘10 and Colby Chiang ‘10

During the ten years of the DUJS’s existence, science at Dartmouth has seen breakthrough after breakthrough. Once the site of the first clinical x-ray in North America, Dartmouth College over the last decade has had a hand in the discovery of microRNA and cancer chemoprevention, and its researchers have worked to produce ethanol from biomass and to understand “dark matter.” Here we review just a few of the recent scientific accomplishments at the College.

Biology

miRNA In 1993, Victor Ambros, a geneticist at Dartmouth Medical School, disproved the traditional paradigm that only proteins can regulate the expression of DNA. While examining defects in C. elegans larvae, he discovered a short 22 nucleotide piece of RNA, called lin-4, that was never transcribed into protein but was still able to negatively regulate expression of another gene (1, 2, 3).

Even 10 years after Ambros discovered the lin-4 RNA, the significance of his finding was still not fully realized by the scientific community. By 2001, only two of these regulatory “microRNA” (miRNA) genes had been observed, and they were both found in the genome of Caenorhabditis elegans, a worm whose name is rarely heard outside of laboratories and biology textbooks.

In fact, at the time, no one had even named this type of RNA. It was not dubbed “microRNA” until 2001, when Ambros published a paper in Science that described 15 other examples of miRNA gene regulation in the C. elegans. Furthermore, many of these miRNAs were in ancient regions of the genome that are highly conserved, and appeared to have homologs in insects, mammals, and perhaps other vertebrates. The finding demonstrated that miRNA was not simply a biological anomaly, but a prominent mechanism of gene regulation in nature (4).

In his 2001 paper, Ambros used cDNA and computational approaches to compare the two known miRNAs, lin-4 and let-7, to DNA sequences in the genomes of C. elegans and other organisms. The 15 genes that he discovered varied in expression at different times in development and appeared to have diverse functions in C. elegans. Furthermore, three of the genes had homologs in higher vertebrates. One of these homologs Spring 2008

is expressed in the human heart and in mouse embryos, demonstrating the diversity and prevalence of miRNA (4).

Two other papers were published simultaneously in the same journal, describing other instances of miRNA regulation in worms, flies, and humans, and bringing the total miRNA count up to about 100 (4, 5, 6).

Since then, miRNA has continued to play a major role in genetics. Today, over 5,000 miRNAs have been identified in many organisms including plants, flies, worms, and vertebrates (7). A recent discovery at Dartmouth demonstrated that miRNAs may have played a major role in vertebrate evolution (8). Other labs report that miRNA is an important element in the pathology of cancer (7, 9). Since Ambros’ discovery the impact of miRNA has resounded through the world of science with a booming voice, but its humble beginnings were right here at Dartmouth.

Chemistry

Chemoprevention It began with a simple idea: trees have long lives, and all trees produce triterpenoids. Might triterpenoids have some application in medicine (10)?

Thus began a quest by chemistry professors Gordon Gribble and Tadashi Honda, together with Dartmouth Medical School professor Michael Sporn, to revolutionize the field of chemoprevention.

Beginning in 1995, this pharmacological team set out to examine the various types of triterpenoids available commercially and naturally to see what might be applicable as a cancer treatment in vivo. Triterpenoids have long been a fixture in Asian medicine.

It was not until 1998, however, that Honda and Gribble isolated a triterpenoid that was powerful enough to function in an anti-cancer capacity. This substance, labeled triterpenoid 151 or CDDO, was able to reduce tumor growth and protect non-tumor cells. In addition, it also had the ability to reduce chronic inflammation (11).

The research group published this landmark discovery in the January 1999 edition of Cancer Research.

The anti-inflammatory function makes CDDO a “triple threat” in chemoprevention. Inflammation causes rapid cell proliferation, which increases the 7

The X-ray structure of CDDOMe, the compound currently in Phase II clinical trials for the treatment of pancreatic cancer and melanoma.

Image courtesy of Professor Gordon Gribble, Dartmouth Chemistry Department.

chance of mutagenesis and tumor development.

Any reduction in inflammation, therefore, can serve to reduce the chance of mutagenesis.

Specifically, the researchers found that CDDO is capable of suppressing COX-2 and nitric oxide, two agents that commonly promote inflammation and have recently been shown to characterize the tumor microenvironment and immunological tumor promotion (11, 12). This is the logic behind use of arthritis medications in cancer treatment.

Successive publications stemming from the chemists’ work since the 1999 Cancer Research paper have indicated that CDDO can prevent lung cancer in mice and can induce apoptosis, or programmed cell death, in mouse myeloma and lung cancer cells (13).

A CDDO derivative is currently in Phase II trials with the F.D.A., with the hope that the drug could be on the marketplace by 2009 (10).

Anesthesia As Gribble and Honda’s work shows, many of the world’s biggest discoveries lie at the intersections of the scientific disciplines. For them, it was chemistry and biology. For chemistry professor Robert Cantor, discovery lay in the overlap among chemistry, biology, and physics.

Cantor’s work has focused on the dynamics of the cell’s lipid bilayer in the context of anesthesia.

Scientists have theorized that anesthetics function by binding to protein channels within the bilayer. This binding can block the ionic flow between the cell and its outer environment, thereby interrupting communication between cells (14).

Cantor sat down with DUJS in 2005 to discuss his analysis. He explained that past models of anesthesia did not take into account several important factors, including (i) the variety of viable anesthetic substances; (ii) the viability of similar anesthetics across species; and (iii) the high concentration of anesthetics needed for an in vivo effect. Cantor said these factors indicate 8

that anesthetics could not function through a direct binding mechanism because such a specific process is at odds with the apparently non-specific nature of anesthetics (15).

Cantor, in the 2001 edition of Biophysical Journal, had hypothesized that a different mechanism must be at play. In short, he proposed that anesthetic substances enter the bilayer and change its “lateral pressure profile.”

The profile describes the amount of pressure along the plane of the bilayer. As anesthetics enter the bilayer, they increase the membrane pressure, which results in conformational changes to the membrane’s proteins.

The proteins are thus inactivated and the ionic flow, as with the previous model, is interrupted (14).

These conclusions have led Cantor to pursue the evolutionary implications of anesthesia. “I’ve become more interested in the why of anesthetics,” he said in the 2005 DUJS article. “A more fundamental question is why would we be anesthetized in the first place? They affect all humans, and there is nearly zero genetic diversity.

If a sensitivity to a drug is narrow in a population, then any mutation that changes your sensitivity, it must affect survivability” (15).

Earth Sciences

Glacial Periodicity The traditional theory of Milankovitch cycles asserts that the 100,000 year periodicity of ice ages is the result of gravitational attractions, orbits, and tilts of celestial bodies (16). In 2002, Mukul Sharma, an assistant professor in Dartmouth’s Earth Science department, offered a new alternative to this longaccepted theory. He proposed that changes in magnetic Dartmouth Undergraduate Journal of Science

activity on the surface of the Sun may play a large role in glacial periodicity.

Solar activity was known to vary over shorter time periods, but cyclical fluctuations on a scale of 100,000 years had yet to be explored. Sharma devised a method of measuring these long-term fluctuations by exploiting the Sun’s effect on Beryllium-10 production in the Earth’s atmosphere. Different levels of atmospheric 10Be production on Earth reflect variances in the geomagnetic field strength and the solar modulation factor of the Sun. Sharma analyzed 10Be production data that had been derived from deep marine sediments. Using this data, he estimated solar surface magnetic activity. Because 10Be has a long half-life of 1.5 million years, Sharma was able to gather data for the last 200,000 years (17).

Sharma’s results showed not only that the magnetic activity of the Sun was variable over this period, but also that it appeared to be cyclical. In addition, the solar modulation was in phase with the 100,000 year glacial cycles. Solar magnetic intensities are believed to affect radiation and cloud formation on Earth. Thus, the variations that Sharma observed may be responsible, in part or in whole, for the Earth’s glacial cycles. While Sharma acknowledges the need for further investigation of this novel theory, it is an exciting new explanation for an ancient phenomenon (17).

however, have been that the synthesis of the fuel is often expensive, time consuming, and results in the creation of secondary products that are unnecessary and reduce the efficiency of energy provision.

Thayer School professor Lee Lynd and colleagues, however, have made significant strides toward finding a solution. The research, based on a partnership between Dartmouth and the Mascoma Corporation (a company founded in part by Lynd), is focused on producing ethanol using thermophilic organisms and what is called “consolidated bioprocessing,” where “biological conversion is solidated into a single step without added cellulase enzymes.” Cellulase enzymes are the primary substances required for the breakdown of cellulose-containing biomasses (21). In short, consolidated bioprocessing makes cellulase production, cellulose hydrolysis, hexose fermentation, and pentose fermentation a one-step process (22).

Lynd’s research has focused on the use of several organisms, including an engineered version of Thermoanaerobacterium saccharolyticum, to supply ethanol given certain biomasses. In engineering these organisms, Lynd has centered his work on reducing their production of lactic and acetic acid, substances that correlate with reduced ethanol production (22).

Engineering

Antarctic Cooling In January 2002, environmental studies professor Ross Virginia, along with a team of researchers from across the country, found that average temperatures in Antarctica declined over the last decade despite a general global warming trend. They also discovered that this cooling corresponded with a 10 percent decrease in the number of soil invertebrates. The discovery, published in Nature, received widespread national attention as some pundits interpreted the results as proof that global warming does not exist.

Using weather data and lake level measurements, Virginia and his co-authors found that the air temperature at Lake Hoare in Antarctica decreased by 0.7OC from 1986-1999. Wind speed during this period also decreased, while the amount of solar radiation increased.

The amount of discharge from major streams decreased while lake levels rose.

By collecting samples of nematodes, tardigrades, and androtifers from the soil, the researchers were also able to determine whether the environmental trends correlated with changes in the Antarctic biosphere. The researchers concluded that the population of nematodes and tardigrades was declining significantly.

“Given the low diversity and long generation times of these invertebrates, these declines in population represent important shifts in the diversity, life cycles,

Protein Glycosylation Many of the therapeutic proteins used in the medical world today are artificially produced and engineered using mammalian cells lines. These proteins, or glycoproteins, must often have sugar structures attached in vivo through a process called glycosylation before they can be fully functional (18).

In 2003, professor Tillman Gerngross and others at the Thayer School of Engineering genetically engineered the yeast strain Pichia pastoris to conduct human-like glycosylation. This process allows the synthesized proteins to be more uniform in structure, and also allows for more control over important protein properties, including solubility. The engineering was accomplished by deleting the non-human glycosylating gene from the yeast and incorporating its human counterpart (19).

In January 2006, Gerngross and his colleagues were able to synthesize human monoclonal antibodies in yeast, having formed GlycoFi Inc., to market their discovery by that time. GlycoFi was acquired by Merck & Co. in 2006 (20).

Cell-Mediated Production of Ethanol from Biomass Researchers have long focused on how to develop usable fuel from cellulose biomass. Major roadblocks, Spring 2008

Environmental Science

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The first ten frequency modes of an irregularly shaped drum.

Image courtesy of Professor Alex Barnett, Dartmouth Mathematics Department.

trophic relationships, and functioning of dry valley soils,” the Nature paper states (23).

Research since the 2002 paper has indicated that the hole in the ozone layer may be somewhat responsible for the Antarctic cooling. The hole causes west winds to blow toward the South Pole. This decreases the air pressure, which in turn causes a reduction in temperature (24).

Mathematics

The Quantum Drum How is the surface of a drum like a billiards table? Alex Barnett, an assistant professor of mathematics at Dartmouth, is investigating the notion that these two systems are one and the same. The key to their similarities lies in the overlap between quantum and classical mechanics.

The quantum interpretation is analogous to a drum skin stretched over an irregularly shaped frame.

When struck, the edges of the drum skin remain fixed, but the interior can oscillate at different frequency modes. In the lowest mode, the entire drum oscillates in phase as one, while at higher modes, areas of the drum oscillate differently. Theoretically, there are an infinite number of modes, with higher nodes dividing the drum surface into smaller and smaller oscillating areas (25).

In the classical system, imagine a frictionless billiard table in the same shape as the drum frame in the quantum problem. A ball follows a path over the 10

billiard table by bouncing off of the edges. In this “chaotic” system, the path of the ball is specific to small deviations in initial conditions. In other words, two balls that begin their journeys with nearly identical initial conditions quickly diverge into wildly different paths (25).

The nature of the quantum drum is controversial at very high frequency modes. For example, it is unknown whether the oscillating patterns arrange themselves uniformly over the surface, or whether they cluster together in “scar” patterns. If the quantum modes do in fact exhibit scars, these scars may form along closed, periodic paths of the chaotic billiard ball.

Barnett’s research focuses on methods of calculating the patterns for these higher nodes, and examining the connection between the classical and quantum systems (26). The consequences of these questions have practical applications in determining the behavior of heat dissipation in microchip semiconductors, and theoretical significance in the distribution of prime numbers (27). Barnett’s research was published in the January 2006 issue of Communication on Pure & Applied Mathematics, and on the cover of the January 2008 issue of Notices of the American Mathematical Society (26, 28).

Physics & Astronomy

Dark Matter and Quintessence In 2005, Dartmouth physics professor Robert Caldwell, along with Eric Linder of Berkeley Laboratory, shed light on our understanding of dark energy. Scientists had purported “dark energy” to be the force behind the surprising 1998 discovery that the expansion of the universe is accelerating, not decelerating. The discovery shook the foundations of cosmology, since under the traditional paradigm, gravity pulls galaxies together, decreasing the rate of expansion. In order to explain the mysterious observation of accelerating expansion, scientists proposed dark energy, which would counter gravity to pull the universe apart (29).

Dartmouth Undergraduate Journal of Science

The concept of dark energy is still poorly understood. One proposal for the nature of dark energy is Albert Einstein’s cosmological constant, which Einstein suggested in 1917, but would later call his “greatest blunder” (30). However, the cosmological constant theory received renewed interest following the accelerating expansion discovery, since it would provide the force needed to balance out gravity. The other theory was that dark energy was a dynamic force, called “quintessence.” Caldwell and Linder’s research outlined two possibilities for the fate of the universe under the quintessence model: thawing and freezing. In the thawing scenario, the acceleration of the universe will slow down and eventually stop, possibly leading to recollapse. The freezing scenario describes a state where the acceleration continues to increase, pulling galaxies further and further apart (31).

Their findings were attractive, because the models for neither of these scenarios required Einstein’s cosmological constant, which had been called into question. Particularly, the cosmological constant seemed to overcompensate for gravity, providing much more expanding force than was necessary (29).

Caldwell and Linder’s ideas also provided possibilities for observational tests on the nature of dark energy (30).

Their work provided new clues into the mystery behind the fate of the universe.

Acknowledgements

We would like to thank professors James Aronson, Alex Barnett, Andrew Friedland, David Glueck, and Thayer Dean Joseph Helble for their assistance in helping us review the past 10 years of science at Dartmouth.

Spring 2008

References 1. R. C. Lee, R. L. Feinbaum, V. Ambros, Cell 75, 843 (1993).

2. V. Ambros, Nature 431, 350 (2004).

3. E. Chien, Dartmouth Undergraduate Journal of Science 9(1), 4-7 (2006).

4. R. C. Lee, V. Ambros, Science 294, 862 (2001).

5. M. Lagos-Quintana, R. Rauhut, W. Lendeckel, T. Tuschl, Science 294, 853 (2001).

6. N. C. Lau, L. P. Lim, E. G. Weinstein, D. P. Bartel, Science 294, 858 (2001).

7. E. Barbarotto et al., International Journal of Cancer 122, 969 (2008).

8. A. M. Heimberg et al., Proceeding of the National Academy of Sciences 105, 2946-2950 (2008).

9. M. Kato and F. Slack, Biology of the Cell 100, 71 (2008).

10. J. Durgin, Dartmouth Medicine (2002).

11. N. Suh et al., Cancer Research 59(2), 336-41 (1999).

12. S.J. Roberts et al., Proceedings of the National Academy of Sciences 104(16), 6770-5 (2007).

13. K. Liby et al., Clin. Cancer Res. 12(14), 4288-93 (2006).

14. R.S. Cantor, Biophys. J. 80(5), 2284-97 (2001).

15. B. Huang, Dartmouth Undergrad. Journal of Science 7, 49-53 (2005).

16. K. D. Bennett, Paleobiology 16, 11 (1990).

17. M. Sharma, Earth and Planetary Science Letters 199, 459 (2002).

18. GlycoFi Technology, GlycoFi Available at: http://www.glycofi. com/engineered_glycosylation.htm.

19. B. Choi et al., Proceedings of the National Academy of Sciences 100(9), 5022-7 (2003).

20. GlycoFi Technology, GlycoFi Available at http://www.glycofi. com/glycoproteins.htm.

21. Market Leadership, Mascoma Corp Available at http://www. mascoma.com/welcome/market_leadership.html.

22. Professor Lee Lynd, Thayer School Available at http:// engineering.dartmouth.edu/biomass/.

23. P. Doran et al., Nature 415, 517 (2002).

24. Current Understanding of Antarctic Climate Change, Pew Center on Global Climate Change Available at http://www. pewclimate.org/global-warming-basics/antarctic_facts.

25. A. H. Barnett, Alex Barnett: Nontechnical Introduction to My Research (Dec. 2004). Available at http://www.math.dartmouth. edu/~ahb/nonres.html (27 Mar. 2008).

26. A. H. Barnett, Communications on Pure and Applied Mathematics 59, 1457-1488 (2006).

27. A. H. Barnett, Personal correspondence, 27 Mar. 2008.

28. Z. Rudnick, Notices of the AMS 55, 32-34 (2008).

29. G. Ellis, Nature 452, 158-161 (2008).

30. L. Yarris and E. Linder, Finding a Way to Test for Dark Energy (29 Aug. 2005). Available at http://www.lbl.gov/Science-Articles/ Archive/Phys-SNAP-dark-energy.html (28 Mar. 2008).

31. R. R. Caldwell and E. V. Linder, Phys. Rev. Lett. 95:141301 (2005).

11

dujs x

What Else Has Happened?

A Celebration of the Legacy of Physics at Dartmouth DILLON LEE ‘08

I

n 1769, Reverend Eleazar Wheelock founded Dartmouth, the ninth college in the nation.

In 1998, four daring, inspired undergraduates established the DUJS.

What else has happened?

I. Half a Century Ago…

Dartmouth

provided adequate protection against X-rays, the emitted neutrons easily penetrated it. Hence, substances with high concentrations of hydrogen like oil, paraffin, or water were used to absorb the neutron emissions (2).

In contrast with the contemporary models, like the two-mile-long accelerator at Stanford, Dartmouth’s accelerator was one of only two in the country specifically designed for undergraduate courses. Even the first-year students had access to radioactive materials produced by this accelerator. At the same time, it certainly served as a magnificent research tool for graduate students and professors. Physicists at Dartmouth enjoyed having the impressive device for their use, and it remained as a symbol of Dartmouth’s commitment to undergraduate learning until the late 1960’s (3).

The Wilder Laboratory witnessed the birth of New Hampshire’s first nuclear accelerator. This device, also known as the “atom smasher,” is an electric device used to study the nucleus of atoms. Scientists use it to accelerate particles such as electrons and protons II. A Century Ago… that are smashed into the nucleus to study the result (Dartmouth’s accelerator smashed deuterium ions into James Clark Maxwell (1831-79), who enlightened

deuterium targets) (1). the world with his equations of electromagnetism, Professors Leonard M. Rieser, Jr. and William T. concluded that “in a medium in which waves are Doyle, shown in the accompanying picture from Rauner propagated there is a pressure normal to the waves and Special Collections Library numerically equal to the spent nearly one year energy in unit volume” planning and constructing (4). The significance of the accelerator for physics this conclusion was such education at Dartmouth. that Italian physicist, The accelerator Adolfo Bartoli, declared could produce the total in 1876 that the validity spectrum of atomic of the second law of radiation, and thus, the thermodynamics at least experimenters needed to partially depended on be careful. When using the presence of such the device, precautions pressure. A few centuries included X-ray dosimeters before Bartoli’s statement, that measured the X-ray Johannes Kepler had output and a radioactivity also conjectured that the indicator that used a pressure of solar light Rieser, center, and Professor Doyle, right, assembling New blinking light and a Professor accounted for the tail of Hampshire’s first nuclear accelerator. ticking sound to display comets blown away from the radioactivity output. the sun.

A neutron counter took the central role of maintaining Many had attempted to experimentally prove the a safe environment by alerting scientists of neutron Maxwell’s equations without success. As Ralph Gibson output. explains it, such pressure only amounts to as much as To inhibit the radioactivity exposure, lead that exerted by one sixty-thousandth of one sheet of shielding surrounded the device, with an extra layer Xerox paper (5). However, in 1901, E. F. Nichols and behind the control panel. Although the lead shielding G. F. Hulls produced a convincing result in the Wilder Image courtesy of the Rauner Special Collections Library (Photo Record 6-55-252).

12

Dartmouth Undergraduate Journal of Science

Laboratory, which Nichols had helped build in 1900.

This celebrated experiment took place in what is now Wilder 115 (the room that currently hosts classes such as Physics 15 and 16: Introductory Physics I and II, Honors Section). In 1901, they published their result in The Physical Review, one of the most distinguished journals in the field of physics, and their article was extensively read in Europe and in the United States (6).

In this section, the details of the experiment are only briefly treated; the full-length article may be found in The Physical Review as noted in the references. The process of measurement will be described using Figures 1 and 2, which come directly from their published article.

Nichols and Hull set up a lamp to provide the light coming from the left of the Figure 1, and the series of diaphragms and lenses (d­­1, d2, d3, L1 and L2) both intensified and focused the light into a beam. The slanted mirror, d5, was used to redirect a tiny bit of the beam to a small device above. This allowed them to keep track of the brightness of the light and consider possible irregularities in their final calculation. Also, a shutter (S2) was placed in front of the slanted piece of mirror, and it controlled the period of time the light was allowed to pass, providing the necessary Δt in the equation of impulse, ΔP=FΔt, to obtain the magnitude of force (F).

Following this path, the light would enter the Nichols radiometer, which was probably the best of its kind at the time. As shown in Figure 2, a fine quartz fiber was suspended in vacuum with a weight (m3) at its end. The light would shine on either silvered disc (G or S), causing the torsion wire to rotate slightly due to the pressure of light on the disc. The light was also allowed to shine on the other disc to take into account any asymmetries, while the experimenters guided magnets (M) around the upper body of the chamber to align the discs to be perfectly perpendicular to the projection of light.

How did they measure such an infinitesimal twist produced by the light? A tiny mirror, m1, was Spring 2008

Figure 1 (Top), showing the apparatus used by Nichols and Hull in the light pressure experiment. Figure 2 (Bottom), showing the Nichols radiometer.

Images courtesy of the Rauner Special Collections Library.

the key. In Figure 1, a telescope (T1) was placed in the distance on the side of the radiometer, and the telescope was positioned with a ruling engine, acting like a meter stick. When one looked through the telescope, the mirror would reflect the reading of the ruling engine, and Nichols and Hull could compare how much the torsion wire twisted by comparing initial and final values of the reading, thus obtaining the measurement of the pressure on the disc. And finally, the second telescope (T2) on the right determined the exact aim of the beam to ensure that it followed the designated path (4).

Although Nichols passed away a few years later during an American Physical Society meeting, their 13

achievement left a lasting impact on the world of physics. Some even suspected that Einstein received his inspiration for his famous equation, E=mc2, from this very experiment (6).

III. A Century and a Half Ago… The oldest scientific building on campus, Shattuck Observatory, was built in 1854. The story begins with a comment made by the Cambridge astronomer Royal George Airy in 1832: “I am not aware that there is any public observatory in America, though there are some able observers.” Before long, an “observatory movement” swept the country, resulting in over twenty new observatories, including the one we see on the hilltop east of the Green (3).

Shattuck Observatory represents Dartmouth’s first major scientific investment. Ira Young, professor of mathematics and natural philosophy, promoted the idea and made extensive travels and negotiations towards accomplishing this vision. The College Catalogue for 1849-50 proudly reads: “The lectures in astronomy are accompanied by celestial observations and instructions in the use of instruments. The splendid telescope obtained during the past year, ranking as the third in the United States, in magnitude and power, supplies important facilities for these purposes.” After the acquisition of the telescope, in 1852, he contacted a reputed Boston physician, George C. Shattuck, Dartmouth 1804, and his generous donations of $8,900 with the additional input by the trustees finally realized the dream of observatory.

Young was sent to Europe to purchase the apparatus and books, and Young’s brother, Ammi Young, was commissioned to design the building. Ammi Young was an architect of U.S. Treasury Department and had designed several other Dartmouth buildings and Boston’s Common House. Overall, the expenditure for the building cost $4,800 with total outlay of $10,000. In comparison, Young calculated that since the beginning, the College had only spent $2,300 for philosophical apparatuses.

At the time, the observatory included a twostory, domed rotunda with the equatorial telescope on the upper level and a library on the lower level, a meridian transit room to the east, a prime vertical transit room to the north, and a bedroom and additional observer’s room with a slit roof to the south. Revolving on six cannon balls, the 2800-pound dome allegedly could be turned with a force of only six pounds.

Charles Young, Ira’s son and a world-renowned scientist, along with John M. Poor and Edwin B. Frost actively used the observatory for research from about 1870 to 1890. Since then, it has served for teaching and public viewing. From the 1860s through the 1920s, students even lived in the building, working as 14

custodians or weather observers. In 1865, one of the earliest boarders wrote to a friend: I am now rooming at the Observatory. There is only one room in this building occupied by students & this is given out free to the best student in the Senior Class. The Sen. wanted to be absent three months & asked me to take his place which I accordingly did. I have the keys to every room in the building and have complete control of all the instruments…If you will make me a call I will let you look thro’ the Spy Glass all night if you want too” (3).

Today, one will notice a weather station adjacent to Shattuck. It is about a century old under continuous operation by the National Oceanic and Atmospheric Administration (NOAA). The departments of physics and environmental studies are currently pursuing plans to develop a system that will acquire, analyze and display weather data directly for Dartmouth’s use. Back when most of New Hampshire was open farmland, the observatory overlooked the town of Hanover and stood as a tangible symbol of science and learning. Although its heyday has now passed, and the vegetation returning to the hill limits its view, it still stands visible as a symbol of undergraduate learning to all who enter Dartmouth.

Acknowledgement

In this article, I have identified three underrepresented events that have momentously contributed to the scientific history of Dartmouth College.

My intention is to inform the readers of these scientific legacies and help them appreciate the opportunity to pursue their studies here. I thank the following individuals for their help in gathering resources for this article: Richard Kremer, professor of history, whose publication, Study, Measure, Experiment: Dartmouth’s Allen King Collection of Scientific Instruments (2005) proved singularly informational on all the subjects, and Ralph Gibson, manager in the physics department, who shared much insight on the Pressure of Light Experiment.

References 1. L. Evans 2002. Glossary o to z. [Online]. (available at http://www.ecotao.com/holism/glosoz.htm). Accessed 2008 March 25.

2. F. Pemberton. New Hampshire Profiles. 22-24 (November 1955).

3. D. Pantalony, R. L. Kremer, F. J. Manasek. Study, Measure, Experiment: Stories of Scientific Instruments at Dartmouth College (Terra Nova Press, Norwich, Vermont, 2005).

4. E. L. Nichols, G. F. Hull. The Physical Review. XIII, 307-320 (1901).

5. Ralph Gibson, Personal Communication (2008).

6. P. Bruskiewich. Canadian Undergraduate Physics Journal.

VI(Issue 2), 36-37 (2008).

Dartmouth Undergraduate Journal of Science

dujs x

The Future of Science at Dartmouth and Elsewhere: The Developments of Today and Realities of Tomorrow PETER ZHAO ‘10

A

s the Dartmouth Undergraduate Journal of Science celebrates its tenth year, scientists from all over the world continue to chip away at the unknown. Slowly but surely, we are learning more about the myriad components that comprise our universe, from the very small to the very large, and from the physical to the biological to the philosophical. Undoubtedly, we are the main beneficiaries of these achievements. A better understanding of our universe enables us to better appreciate our place in it. In addition, technological progress translates into improvements in the quality of ours lives, although it can sometimes be many years before the benefits of a scientific discovery are available to us.

Science is often unpredictable. Significant discoveries are made not only through diligent research, but also sometimes by accident. But, a brief look at some overall trends in research from recent years can give us an idea about the looming questions in each of the scientific disciplines. The following is a general exposition of the uncharted fronts that scientists are currently mapping out through research, both here at Dartmouth and around the world.

Chemistry

The field of chemistry contains a variety of disciplines, all of which study the various properties of matter. According to Dartmouth Professor David Glueck, the field as a whole has many goals to achieve and challenges to overcome in the near future, including “finding solutions to the energy crisis and global warming, better understanding the structures of biomolecules and their role in human disease, and creating advanced materials for new applications.” The Dartmouth Department of Chemistry conducts research aimed at addressing these three challenges.

To help reduce energy consumption, the Department develops better catalysts for chemical reactions, which results in more energy-efficient syntheses. Catalysts are used to speed up the rates of specific chemical reactions but are themselves not used up in the reactions. They are especially important to the chemical industry because even modest improvements in efficiency for large-scale operations have the potential to significantly reduce the amount of waste produced.

Spring 2008

The Department also studies the structure and function of biologically important molecules and synthesizes new molecules for use in the pharmaceutical industry. Both of these research areas have implications for human health and medicine. Understanding the structure and function of specific biomolecules allows for the development of drugs to treat diseases in which those biomolecules are malfunctioning. Synthesizing new molecules can either lead directly to the development of new drugs for the treatment of a disease or introduce new synthetic methods for other researchers to build on.

Lastly, the Department focuses on creating advanced materials. In association with the Center for Nanomaterials Research, which is sponsored by the Thayer School of Engineering, the faculty conducts research on nanoparticles and advanced polymers.

Nanotechnology research is a relatively new and rapidly expanding field, and advocates of nanotechnology anticipate that this research will have many applications in medicine, optics, and electronics. In contrast, polymer research is a more established field of study. Polymers are the basis for everything from plastic bags to styrofoam to Kevlar. Professor Barney Grubbs conducts research focusing on special types of polymers called block copolymers. Block copolymers are polymers in which two or more one-subunit polymers are linked together by covalent bonding. At Dartmouth, research into new, advanced materials with unique properties continues, with the vision that these materials will have important practical applications.

Physics and Astronomy

Physics is touted as the most fundamental of sciences, because its laws and theories form the physical basis for many other fields of study. The greatest scientists now face are in the fields of quantum computing, astronomy, and cosmology.

Spurred by the allure of machines with computational power far exceeding that of today’s computers, the field of quantum computing has been expanding at a frenetic pace since the late 1990s. Today, the most complex encryption technologies utilize quantum communication protocols. While normal computers use conventional bits to store information, 15

Map of the cosmic microwave background radiation in the universe as observed by the Wilkinson Microwave Anisotropy Probe.

Image courtesy of NASA/WMAP Science Team

the various theories of universal expansion are put to the test, scientists will also be able to better predict the ultimate fate of the universe.

Biology

quantum computers store information in quantum bits, or qubits, which are much more information-dense due to a phenomenon known as superposition. Consequently, quantum computers can perform calculations much faster than conventional computers. A Scientific American article from 1998 gives the example of factoring a number with 400 digits. This computation would take the fastest conventional supercomputers billions of years. In contrast, a quantum computer could solve such a problem in a year or less (1). However, as Dartmouth Professor Lorenza Viola explains, there is still a long way to go: “[devising a working quantum computing machine] is incredibly more challenging due to difficulties of reaching fault-tolerant operations…the major issues are well-identified, and creative solutions continue to be proposed.”

Astronomy is the study of objects and phenomena that originate outside of planet Earth, most of which are in our galaxy, the Milky Way. Professor James LaBelle believes that astronomy is a “high profile area of future science in which Dartmouth will play a big role.” Faculty members at Dartmouth conduct research on numerous different topics, including modeling the structure and life of stars, and predicting the weather in outer space.

The field of cosmology examines the structure and composition of the universe at large. Professor Robert Caldwell, an expert in the field of cosmology, believes that “the number one question is why the expansion of the universe is accelerating.” Cosmologists believe that dark energy and dark matter may be contributing to this acceleration, but neither interacts with conventional matter. They hope new studies will shed light on dark energy and dark matter, allowing us to better understand exactly why universal expansion is accelerating. When 16

The biological sciences focus on investigating all the aspects of life. Today, two major fronts of research are cell biology, and ecology and evolutionary biology. The Biology professors at Dartmouth foresee many breakthroughs in the near future.

Cell biology is the study of life at the molecular and cellular level. This field has seen some exciting developments in recent years, including the advent of genomic sequencing and increasing research on stem cells. Moreover, as Professor George Schaller explains, two major trends in the field are paving the way for future research: “[the ability to] visualize smaller and smaller particles within the cell, and [the ability to] track the dynamics of cellular processes in real time.”

Professor Roger Sloboda envisions that in the next ten years there will likely be several significant breakthroughs in cell biology that will have considerable impact. He believes that more light will be shed on the molecular basis of memory: not just how the brain as a whole stores and recalls memories, but also what the molecular medium for the storage of memory is.

Sloboda also believes that gene targeting using designed viruses or other means will be able to treat or even cure diseases such as cystic fibrosis. He predicts that the first medical applications of stem cells will emerge and that people suffering from diseases such as Type I and Type II diabetes will be the first to benefit.

Ecology and evolutionary biology focus on how organisms, species, and populations interact with their environment. Most recently, proof of global warming has prompted further research into how species and populations are affected by climate change and how they adapt to these changes. Professor Mark McPeek believes that one result of climate change is that scientists will pay closer attention to ecology and evolution. In Dartmouth Undergraduate Journal of Science

addition, climate change is providing scientists with valuable information on how species and populations adapt. McPeek also considers the ability to quickly sequence genomes an exciting development because it allows biologists to study natural selection at the genetic level and reconstruct the demographic and evolutionary histories of species.

Engineering

The field of engineering is concerned with applying scientific knowledge to create practical systems and machines. The benefits from advances in engineering can very rapidly become available to the public. The Thayer School of Engineering recently developed the Strategic Research Initiative to identify the major fronts of research in engineering today. According to Professor Brian Pogue, at the Initiative’s symposium the faculty identified three main areas of interest in field of engineering—medicine, energy, and complexity—and problems in these areas that the School of Engineering will concentrate on investigating in upcoming years.

In the field of medicine, engineers are working to increase the efficacy and dependability of machines and materials that help physicians diagnose and treat disease. Biomedical engineers are aiming to better understand why various medical devices fail so they can engineer better devices with lower failure rates. They are also focused on improving medical imaging devices and creating better biomaterials for a variety of applications.

Moreover, the intersection of nanotechnology and medicine has the potential to become a flourishing field in the near future.

The second major field of research involves energy. With the global warming crisis looming, engineers around the world are looking for ways to increase the efficiency of the systems that we use to generate energy. By doing so, they hope to help society move towards more sustainable energy.

The third major front in engineering is a field known as complexity engineering. Some systems are difficult or impossible to characterize definitely because of the number of interacting yet individual components.

For example, imagine two cars that are identical except for their engine systems. One is a hybrid-electric vehicle; the other uses a regular internal combustion engine. The car with the hybrid-electric engine has more components: batteries to power the electric motor, switches that decide whether the engine should use the batteries or gasoline, and energy-recovering brakes. All of these components work together to achieve a desired effect: better fuel economy. However, the increased complexity of the machine also makes it more difficult for engineers to predict where and how the system might Spring 2008

fail. Engineering complexity attempts to model complex systems so engineers can better predict the behavior of these systems.

Philosophy

The field of philosophy is grounded on the principles of asking and attempting to answer a variety of questions using rational arguments and reason.

Professor Walter Sinnott-Armstrong predicts that “the most exciting developments in philosophy, which are bound to progress in the next couple of years, are in naturalistic and experimental philosophy.”

Philosophers are now looking at natural and real experiments, including past studies in behavior and neuroscience. This radical new approach to philosophy brings quantitative evidence into a field that has long been based on qualitative rational arguments and has far-reaching implications for other areas of philosophy.

A striking example of this is in a study which concerned the interpretation of Saul Kripke’s story of Gödel and Schmidt:

Suppose that Gödel was not in fact the author of [Gödel’s] theorem. A man named ‘Schmidt’…actually did the work in question. His friend Gödel somehow got hold of the manuscript and it was thereafter attributed to Gödel. On the view in question, when our ordinary man uses the name ‘Gödel’, he really means to refer to Schmidt, because Schmidt is the unique person satisfying the description ‘the man who discovered the incompleteness of arithmetic’ (2).

An opinion poll conducted in America and in Hong Kong showed that while people in America generally agreed that the statement was incorrect, the majority of people in Hong Kong agreed that the word ‘Gödel’ did refer to Schmidt. In this case, experimental philosophy suggests that people from different cultural and linguistic backgrounds do not necessarily share the same intuitions. Thus, experimental philosophy is bringing old philosophical questions back into the foreground, and it is also raising new questions. The Dartmouth philosophy department is widely recognized as one of the leaders of this movement, and more philosophers will likely use this approach in the next ten years.

Conclusion

Since 1998, there have been many new and exciting developments in the world of science, particularly here at Dartmouth. Moreover, new fields of study are emerging to answer today’s most prominent questions. Just as 1998 was not so long ago, 2018 is not as far off into the future as it might seem. By then, we might have some answers to the great questions of today.

In addition, many new questions will be posed. But no 17

matter what questions and answers arise over the next ten years, the DUJS will be there to illuminate them.

Acknowledgements

The main purpose of this article is to inform the readers of the research occurring in a variety of scientific fields today. I would like to thank the following individuals for helping me assemble this exposition of research: Professors David Glueck and Gordon Gribble of the chemistry department; Professors Robert Caldwell, James LaBelle, and Lorenza Viola of the physics and astronomy department; Professors George Schaller, Roger Sloboda, and Mark McPeek of the biological

sciences department; Professor Brian Pogue of the Thayer School of Engineering; and professor Walter Sinnott-Armstrong of the philosophy department.

References (1) N. Gershenfeld, I.L. Chuang, Quantum Computing With Molecules. Sci. American, June 1998. Available at http:// www.media.mit.edu/physics/publications/papers/98.06. sciam/0698gershenfeld.html.

(2) J. Knobe. What Is Experimental Philosophy? The Philosophers’ Magazine, (in press). Available at http://www.unc.edu/~knobe/ ExperimentalPhilosophy.pdf.

The new DUJS Online! http://www.dartmouth.edu/~dujs The DUJS Online features:

The DUJS archives

The latest Dartmouth Science News

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Current DUJS articles

Dartmouth Undergraduate Journal of Science

DUJS x

Remembering Karen Wetterhahn laura sternick ‘08

O

n June 8, 1997, the Dartmouth community In 1995, Wetterhahn began the Toxic Metals suffered a tragic loss: the death of Karen Research Program at Dartmouth, after receiving Wetterhahn, a beloved professor. A specialist in a $7 million grant from the National Institute of metal toxicology, Wetterhahn had spent over 20 years in Environmental Health Sciences to study how toxic heavy the Chemistry Department elucidating the mechanisms metals affect the human body (1,2). The grant reflected of chromium and nickel metabolism in cells. In August both the strength of her past work and her potential to of 1996, while following standard safety protocol, further expand our understanding of metal toxicology Wetterhahn spilled a few drops of dimethyl mercury and carcinogenesis. The development of the Toxic onto her latex glove. Five months later, she began to Metals Research Program also highlighted Wetterhahn’s notice curious health problems – impaired coordination, promotion of the interdisciplinary study of science. John nausea, weight loss, and slurred speech. As was revealed Winn, a Professor of Chemistry who was the chair of the in subsequent laboratory tests, department at the time of Wetterhahn’s acute mercury poisoning was to accident, recalls that she demonstrated blame for these symptoms. The to her colleagues the importance of latex gloves Wetterhahn had worn chemistry of the life sciences. Winn were thought to be protective, but credits Wetterhahn with the development were in fact highly permeable to of strong research connections linking the toxic compound. The level chemistry to other departments (3). of mercury in her blood was 80 In her roles as Associate Dean of the times the threshold of toxicity (1).

Sciences and later, Acting Dean of the Despite intense chelation therapy, Faculty, she worked to encourage such her condition worsened rapidly. collaboration, arguing that partnership In February, she fell into a coma. enriches scientific research (1).

Four months later, Wetterhahn

In addition to her died at the age of 48, leaving research, Wetterhahn was passionate behind loved ones, colleagues, and about teaching and getting students students, who were all devastated excited about science. She was an by her death. advisor and mentor to scores of Wetterhahn was known postdoctoral researchers, graduate as an inspired researcher and students, and undergraduates in her lab an accomplished scientist. She (2). Winn asserts that “one of her goals of Karen Wetterhahn, which can be found received her undergraduate degrees Portrait from day one was to be able to encourage in the west wing of Baker Library. in chemistry and mathematics from all students who were interested in St. Lawrence University, and completed a doctorate science in general and in her work in particular to have in inorganic chemistry and physical biochemistry at opportunities here at Dartmouth to do that kind of work” Columbia University. Soon after, she began her career (3). Concerned about the higher rate of dropout from at Dartmouth, becoming the first female professor in the the sciences among undergraduate women compared to Chemistry Department (1). Wetterhahn specialized in men, Wetterhahn worked with Carol Muller ’77, who studying how carcinogenesis is induced by chromium was then the Assistant Dean of Engineering, to develop and nickel, and was a tremendous force in her field. the Women in Science Project (WISP) in 1990-1991 (1).

A prolific author, she contributed to over 85 research Since its inception, the program has grown significantly. papers. Her work illuminated the mechanisms by The stated goal of WISP is to “encourage more Dartmouth which chromium damages DNA and promotes cancer women to persist in science, math, and engineering development. According to Brooke Martin, who worked by creating and fostering a supportive academic and in the Wetterhahn lab as a doctoral student, Wetterhahn social climate that will aid women in pursuing science “established one of the major paradigms in chromium as a major and a career” (4). WISP works toward this toxicology” (1). objective by providing first-year women with research experiences that will engage their interest in science, Image courtesy of Joseph Mehling ‘69.

Spring 2008

19

and encourage them to develop strong research skills.

Since the initiation of the program, 1255 first-year women have participated in research internships with over 300 faculty sponsors (5). WISP also seeks to connect upper-class women in science with their firstyear counterparts, in the belief that peer mentoring gives students the opportunity to share advice and information that will help them succeed in their scientific pursuits.

Reflecting on the success of the program with respect to Wetterhahn’s goals, Winn commented, “It has been more than what she hoped for in the sense that so many faculty have embraced it as a way of getting really good students into their labs” (3). The success of WISP and the continuing support that it provides to undergraduate women in science indicate Wetterhahn’s profound impact on Dartmouth.

Wetterhahn’s death provoked an intense investigation by her colleagues to discover how standard safety precautions had failed. Material Safety Data Sheets (MSDS) for dimethyl mercury at the time recommended the use of rubber or neoprene gloves, or “chemically impervious gloves” (1), upon which the MSDS did not further elaborate. The latex gloves in Wetterhahn’s lab were tested for their permeability to dimethyl mercury, and the results suggested that the nonpolar compound could pass through a typical latex glove in under 15 seconds (1). According to Winn, knowing the structure of dimethyl mercury and latex, it is unsurprising that the latex gloves failed to serve as a barrier. However, he argues, “She wasn’t at fault. It wasn’t something she should have done differently, given the information everyone had at the time” (3). After Wetterhahn’s death was reported to the Occupational Safety and Health Administration (OSHA), safety guidelines were changed to reflect the incredible risk associated with the use of dimethyl mercury. The original memorandum produced in response to Wetterhahn’s death may be found on OSHA’s official website (6). Within the notice are the revised recommendations for the safe use and handling of dimethyl mercury. First among these recommendations is a discouragement of the future use of dimethyl mercury, unless strictly necessary. This information came at an enormous cost, and, thankfully, it will help to protect the health and safety of researchers to come.

A decade later, new students and faculty pass through the halls of Burke and Steele. Although many are aware of the accident that resulted in Wetterhahn’s death, there are fewer who are as familiar with the events of her life. Winn remembers her even temper and vibrant personality: “I always heard her laughing – I never heard her yelling. For the most part, I’ll remember her giggle” (3). She was passionate about her work and equally devoted to her family, including her husband 20

and her two children, who were teenagers at the time of her death (1). When asked about what the past ten years would have entailed for Wetterhahn, Winn imagined more of the same: competent stewardship of the Toxic Metals Research Program (now directed by Professor Joshua Hamilton, who worked as a postdoctoral student in Wetterhahn’s lab), further illuminating research in metal toxicology, and the raising of her children. This past June, Winn was struck by the fact that ten years had passed since her death: “I think that Karen’s memory is still quite strong among those of us who knew her, and we pass that on to those who weren’t here when she was.

I think of her often” (3).

As students who are deeply interested in scientific research, we at DUJS recognize how exceptional Karen Wetterhahn was, as a scientist, a teacher, and an individual. We honor her enthusiasm for her work and her dedication to helping students explore science. She is remembered by those who knew her as a brilliant researcher and an insightful mentor, and Dartmouth was changed for the better as a result of her work. This year marks the tenth anniversary of the dedication of the annual undergraduate scientific research symposium to Karen Wetterhahn. It serves as a fitting tribute to her legacy at Dartmouth.

Acknowledgements

The author would like to thank Professor John Winn for his thoughtful reflections, which were invaluable in the writing of this article.

References 1. K. Endicott, Dartmouth Alumni Magazine, April 1998. Available at: http://www.udel.edu/OHS/dartmouth/drtmtharticle.html (16 January 2008).

2. N. Serrell, A Tribute to Karen Wetterhahn, Available at: http:// www.dartmouth.edu/~toxmetal/HMKW.shtml (16 January 2008).

3. J. Winn, personal communication.

4. Women in Science Project at Dartmouth College: Mission Statement (2008). Available at: http://www.dartmouth.edu/~wisp/ menu_test_frames.html (28 March 2008).

5. M. Pavone, personal communication.

6. U.S. Department of Labor, Occupational Safety and Health Administration, Dimethyl Mercury: Hazard Information Bulletin (15 February 1991). Available at: http://www.osha.gov/dts/hib/ hib_data/hib19980309.html (28 March 2008).

Dartmouth Undergraduate Journal of Science

X Volume X It’s the DUJS’s 10th Anniversary! We hope that you have enjoyed the special DUJS retrospective articles, and we thank all of our loyal readers, advisors, and sponsors for their interest and help.

Please join us on May 22 as we celebrate our anniversary in conjunction with the Wetterhahn Science Research Symposium.

Keynote Speaker: Dr. Pamela Hines, Science Magazine

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psychology

Future Directions for Treating Depression:

Deep Brain Stimulation and the Brain’s Reward System Tim shen ‘08

Introduction

U

nipolar depression is one of the fastest growing and most widespread disorders recognized today. Particularly distressing is the fact that depression now strikes young people at an unprecedented rate. Approximately one-sixth of all high school students will be affected before graduation, and one-quarter of all college students may have symptoms of depression (1).

Treatment of unipolar depression is a pressing issue in the field of abnormal psychology, yet our knowledge of depression remains incomplete. The etiology is still uncertain – in fact, most of the available treatments for depression were not the result of directed research, but rather were the result of accidental discoveries over 50 years old (2). It is obvious that research into new and alternative treatments for depression to supplement the current treatments must be a priority. Fortunately, several promising approaches are already emerging.

Deep Brain Stimulation

Current evidence suggests that depression is a disorder that is the result of multiple issues. Genetic predisposition, brain damage, and stress are all contributors to the development of depression, and may involve multiple brain pathways and neurotransmitters.

Neuroimaging has shown that multiple areas of the brain may be involved in depression, including the subgenual cingulated region (Cg25). Cg25 has been shown to be involved in acute sadness, and has shown reduced activation in response to common antidepressant treatments, including specific serotonin reuptake inhibitors and electroconvulsive therapy. Areas linked to Cg25 are also involved in controlling many of the behaviors that are affected by severe depression (3). As a result, Cg25 theoretically presents a prime target area for applying deep brain stimulation (DBS) to unipolar depression. Delivering electrical pulses to Cg25 in order to compensate for reduced activation may alleviate particularly difficult cases of depression.

Deep brain stimulation involves the use of electric current to stimulate a target brain area. Four electrodes are implanted into the targeted brain area under local anesthesia. The electrodes are tested and connected to a pulse generator which is implanted in 22

the body under general anesthesia. The pulse generator allows patients to continue a normal life by continuously delivering electrical current pulses to the target brain area even after the patient leaves the hospital (3).

The use of deep brain stimulation can effectively treat tremors caused by Parkinson’s disease. Use of DBS for Parkinson’s patients has demonstrated that repeated stimulation of overactive brain regions may be beneficial (3, 4). Additionally, DBS has shown the ability to modulate brain activity in obsessive-compulsive disorder (4). As a result, researchers in Canada attempted to use DBS to modulate Cg25 activation in order to treat severe, treatment-refractory depression (TRD) (3).

TRD patients have shown a distinctive lack of response to even aggressive treatment plans. The effective use of DBS to treat TRD patients would be a significant step forward in the treatment of depression.

Although DBS targeting Cg25 was effective as described by the Canadian researchers Mayberg et al., the sample size used was very small (n=6), and DBS was completely ineffective for one patient and effective only in the short-term for another (3).

The patient population was selected to meet standardized DSM IV-TR descriptions for major depressive disorder, with a major depressive episode of one year or longer. Each of the selected patients also met the criteria for TRD, as all failed to improve after treatment with antidepressants, psychotherapy and electroconvulsive therapy (3).

During the course of the initial implantation of DBS electrodes, the electrodes were sequentially tested with varying voltages. The patients were blind to testing, but all reported beneficial effects, including “sudden calmness,” “disappearance of the void,” “heightened awareness,” “sudden brightening of the room,” “sharpening of visual details,” and “intensification of colors” (3). The effects were associated with stimulation through the electrodes, and none were associated with “sham” stimuli. Additional improvements were noted in motor speed and volume (3).

Magnetic Resonance Imaging confirmed DBS electrode placement in Cg25. Placebo effects were controlled by blinding patients to the electrode settings and to whether the electrodes were active.

This testing confirmed that placebo effects were not Dartmouth Undergraduate Journal of Science

Postoperative X-Ray showing location of DBS electrode leads in a patient’s cranium.

Image courtesy of Nature Publishing Group and Schlaepfer TE, Cohen MX, Frick C, Kosel M, Brodesser D, Axmacher N, Joe AJ, Kreft M, Lenartz D, Sturm V (2008): Deep brain stimulation to reward circuitry alleviates anhedonia in refractory major depression. Neuropschopharmacology 33:368-377.

responsible for the improvements. Testing six months later confirmed that, ultimately, the depression of three patients had gone into remission. At peak response two months after implantation, five of the patients showed improvement. However, by the time the study ended at the 6 month time point, two patients had failed to show sustained improvement. Many effects were observed, including normalization of sleep, increased energy, increased psychomotor speed, decreased anhedonia, and improvement in ability to perform tasks, improvement in intellectual function, and improved mood and emotional states (3).

Since the sample size was very small, conclusions are difficult to draw. However, the research performed by Mayberg et al. seems to show promising results for the use of DBS to treat TRD. Four of six patients responded, a conspicuously high response rate for a patient population that is notoriously difficult to treat. Neuroimaging established that in patients with a lasting positive response to the DBS targeting, Cg25 normalized brain activation where many abnormalities were observed before treatment (3).

If further research into DBS for depression shows promise, it will be an important advancement in treatment for this disorder. DBS offers multiple advantages, including reversibility and the ability to tailor the treatment parameters to the individual patient (4). DBS in the mode used by Mayberg et al. also offers compactness and portability because the circuitry and the pulse generated are all implanted within the body (3).

However, there are many special considerations to take into account with TRD patients and this mode of treatment. DBS requires delicate surgery to implant electrodes, and there is always the possibility that even TRD patients may eventually respond to less risky Spring 2008

conventional treatment (4). Care should be taken to ensure that if DBS is adopted as a common treatment for depression, treatment be limited to appropriate patients.

There is the dangerous possibility that patients may see DBS as a panacea for their troubles.

Reward Circuit

Another future target for the treatment of depression involves the reward system of the brain, including the nucleus accumbens (4). The nucleus accumbens (NAc) receives dopamine inputs from an area known as the ventral tegmental area (VTA), and together these areas form the mesolimbic dopamine system – the brain’s reward circuit for food, sex, and narcotics.

The NAc is an interesting target of action for treatment of depression primarily because of the depressive symptoms of anhedonia, lack of motivation, and low energy. Additionally, studies performed with dopamine receptor antagonists indicated the involvement of the reward pathway in controlling depression symptom behaviors. (2).

Depression appears in more females than males, with a mean ratio of 2:1. This ratio is perplexing, but hypotheses range from sex differences in coping with depressive events to genetic theories regarding a “depressive gene” (1). One of these hypotheses notes that males may turn to alcoholism or other narcotics abuse to cope, and that these cases may lower the apparent rate of depression in men (1). If this proves to be true, treatments targeting NAc may have strong beneficial effects. Dopamine and the NAc are involved in the reward effects of many abused drugs. Opiates, for instance, activate dopamine transmission and receptors in the NAc (2). Treatment targeting the NAc may therefore show promise in treating disorders similar to, but separate from depression.

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The NAc may also be able to mediate the abnormal sleep patterns of severely depressed individuals.

Studies of circadian rhythm genes have shown that narcotic reward mechanisms at the reward pathway are influenced heavily by these genes (2). Therefore, it is also possible that depression’s symptoms may be rooted in these same circadian genes, influencing sleep pattern disruption as well as errors at the NAc and the reward pathway (2).

Based on the role of the NAc and the reward circuit in motivation, treatment targeting the NAc may also be helpful for mood disorders other than depression.

In depression, it seems logical that an impairment of NAc and the reward circuit may be responsible for many of the symptoms of the disease. The development of treatments targeting NAc is lagging behind even the development of a procedure like DBS, which is still being researched for depression. DBS at least has concrete evidence of effectiveness in similar situations; treatments targeting NAc are still theoretical. However, these treatments are well worth researching. Treatments targeting NAc could supplement DBS or supplant it in cases where DBS is undesirable for its surgical aspects or for other reasons.

Conclusion

Finding future targets for treatment of depression may be difficult due to the limited current knowledge of the etiology and mechanisms of depression and its symptoms. Still, promising research has emerged in the form of deep brain stimulation targeting the subgenual cingulate in Brodmann’s Area 25. The effectiveness of this approach is still uncertain, given the limited nature of the research (3). Furthermore, studies have shown that there are important mechanistic differences across the different types of depression. These differences are likely relevant to the treatment of the types of depression (3). Deep brain stimulation’s successes

24

in treating TRD cannot show effectiveness across the several types of depression. However, if it is possible to use DBS to treat TRD, it will offer a treatment for a type of depression that currently has few if any other treatments. Furthermore, it is possible that other types of depression may be localized to other brain regions that would be potential targets for DBS. It is also important to look further ahead for alternative targets. Molecular targets are an intriguing choice because, once developed, this type of treatment is much simpler and less risky – many molecular targets are treated with chemical pills like selective serotonin reuptake inhibitors. One promising location in which to hunt for molecular targets includes the nucleus accumbens. Targeting the reward pathway through the NAc seems like a promising and logical method of at least alleviating many symptoms of depression, including anhedonia, low energy, and lack of motivation.

The NAc also offers the possibility of treating other disorders, including narcotic additions, alcoholism, and possibly sleep disorders (2).

Research into these future directions for treatment of depressive disorders is an important priority in the field of abnormal psychology. The apparent increase in rate of depression and the apparent decrease in age of onset are disturbing, but the outlook is hopeful.

With many possible avenues for new research moving forward, and with promising treatments already gaining increased attention, there is hope that depression can be alleviated and that its effects may be reduced.

References 1.M.E.P. Seligman, E.F. Walker, and D.L. Rosenhan, Abnormal Psychology, 4th Edition (W.W. Norton and Company, New York City, 2001).

2.E.J. Nestler and W.A. Carlezon, Jr., Biological Psychiatry 59, 1151 (2006).

3.H.S. Mayberg et. al, Neuron 45, 651 (2005).

4.T.E. Schlaepfer and K. Lieb, The Lancet 366, 1420 (2005).

Dartmouth Undergraduate Journal of Science

psychology

Autism Spectrum Disorder:

Theory of Mind Impairment in Autism as a Result of Failed Implementation of Integrated Social Percepts Allison baker ‘09

Introduction

A

utism Spectrum Disorder (ASD) is a complex neurodevelopmental disorder characterized by an array of perceptual, social, and neurological abnormalities. Diagnoses of ASD are typically made between ages two and four and are based on broad behavioral criteria including impairments in verbal and nonverbal communication, a lack of social or emotional reciprocity, restricted repetitive and stereotyped patterns of behavior, and persistent preoccupations with parts of objects (1). Individuals with autism often may demonstrate heightened abilities in rote memory and attention to detail. These characteristics are common across different severities of the disorder. Asperger’s syndrome is a closely related, yet milder variant that is distinguished from autism by its lack of linguistic or cognitive delay.

Overall, the neuroanatomical abnormalities in autism seem to affect regions of the brain crucial for social interactions. Questions remain, however, as to how these neurological abnormalities correspond to autism’s complex array of perceptual and social deficits. Further research is also necessary to elucidate the developmental relationship between irregularities in lower-level processing areas and those in higher cognitive areas: to what extent do autism’s secondary executive functioning deficits exacerbate the neuroanatomical abnormalities in the primary perceptual areas? Current research initiatives address this question and others: is there a more quantitative means of discerning the severity and characteristics of different autism phenotypes? Can the neurological correlates of the disorder’s cognitive deficits be pinpointed?

Strategic methods employed to answer these questions include developmental studies, sibling and first-degree relative studies, lesion studies, theory of mind studies, and studies of symptomaticallyrelated disorders (e.g., Asperger’s Syndrome, Fragile X Syndrome). Mapping the constellation of deficits and characteristics of autism will require an array of integrative and comparative studies. Certainly, these efforts will lead not only to a better understanding of an enigmatic and pervasive disorder, but also to an illumination of some of the complicated integrative processes that characterize human cognitive complexity (i.e., top-down modulation and theory of mind).

Spring 2008

Cognitive Developmental Aspects of Autism

Autism is characterized by an array of cognitive disorders, many of which comprise the current diagnostic criteria. Some of the hallmarks include abnormal language development such as muteness, language delay, echoing of speech, and idiosyncratic use of language (2).

Overall, autistic individuals display impaired verbal as well as nonverbal communication. Furthermore, autistic individuals exhibit a striking impairment in perceptual processes that require rapid attentional shifting or sensory integration (3, 4, 5). Social deficits are marked by an inability to empathize and a lack of implementation of theory of mind processes (3, 6).

Deficits in Integration of Sensory Perceptions Many of the cognitive deficits in autism result from impairments in sensory integration (4, 5, 7).

Without the ability to synthesize their perceptions, autistic individuals have difficulties interacting with their environments, particularly in social situations. A recent study by Westerfield et al. (2007) confirms previous research that crossmodal sensory integration in autism is neither early nor automatic (5). The study examined the temporal, spatial, and attentional factors that could disrupt integration of auditory and visual information.

Varying the congruence of spatial and temporal proximity of simple auditory and visual stimuli revealed that autism subjects were not affected by mismatch in picture and sound except in the most extreme mismatch, in which the picture and sound were separated in time by more than 150 milliseconds. This finding suggests that auditory-visual integration takes place at a much later stage in autistic individuals than in typically-developing individuals (5). By extension, the slower integration of perceptual stimuli indicates a likely contributing factor to the social deficits characteristic of autism. The lack of rapid and automatic integration of multiple sensory percepts and social observations would impair autistic individuals in their ability to make inferences about the mental states of others and to provide context for their social interactions.

In a related study, Townsend et al. (2007) extended the hypothesis that social deficits in autism may be caused in part by impaired sensory integration. The 25

new postulation specifies that these impairments occur in the automatic implementation of integrated perceptions.

Their study employed a series of behavioral and EEG/ ERP studies to examine implicit learning of a sequence rule to predict the appearance of a certain target. The authors compared the subjects in their ability to use the implicitly-learned rule to improve performance versus their ability to use the rule to improve performance when it was explicitly supplied. Results indicated that autistic individuals were not able to implement implicitlylearned information in order to improve performance, but they were able to improve their performance when the rule was made explicit. The authors suggest that autistic individuals lack an automatic implementation of sensory information, but are able to do so with conscious effort.

Normal and Impaired Theory of Mind Processes Theory of mind refers to the ability to explain and predict others’ behavior by inferring their thoughts, feelings, and motivations (2, 8). It involves rapid and automatic implementation and modulation based on context (2, 8). As far as is known, theory of mind processes are universal and follow a particular stereotyped development that is similar across individuals (2, 8). One of the earliest manifestations of a child’s mentalizing abilities is seen at around 18 months in the toddler’s enjoyment of pretence (2, 8). Playing “make believe” is a favorite game of most young children and requires an understanding of others’ mental states, including intent and belief (2). That these mentalizing abilities follow a stereotyped development suggests control by a dedicated neural mechanism (2). Structures thought to be involved include the paracingluate cortex, the temporal-parietal junction, the amygdala, the temporal poles adjacent to the amygdala, and the frontal lobes (7, 8, 9).

Most of the central deficits in autism occur in theory of mind processes. Many autistic individuals have described their bewilderment at the mysterious “mind reading” capabilities of other, normal individuals (2). Autistic children lack the ability to mentalize from birth; some of the earliest symptoms of the disorder are evident when young children fail to follow another person’s gaze or understand games of “make believe” (2). Autism’s hallmark abnormalities in language development may be a by-product of the theory of mind deficits (2). Without normal abilities to infer another’s intent or to follow another’s eye gaze, children with autism show difficulty in mapping the word to the object to which the speaker is referring.

Carrying out normal social interactions requires a constant incorporation of contextual information and prior social knowledge. This ability is impaired in autism, 26

but it is possible for some individuals with less severe phenotypes of the disorder to carry out relatively normal social behaviors. This normal behavior, however, is accomplished by alternative compensatory processes that circumvent the persistent physiological abnormalities.

High functioning autistic (HFA) individuals thus share similar atypical processing methods to individuals with more severe phenotypes (3). For example, HFA individuals display the same abnormal face gaze pattern as individuals with more severe phenotypes: aversion to direct eye gaze and increased fixation on the mouth. A recent study by Spezio et al. (2006) employed a new method of “Bubbles” to quantify the extent to which HFA subjects avoid eye gaze and instead seek information from the mouth area (10). The “Bubbles” method varies the facial information available on a given trial by revealing only small parts of the face. Measurements of the eye movements made as participants view these stimuli provide novel detail about the abnormal way in which autistic individuals look at faces. The findings

by Spezio et al. showed marked differences in HFA eye gaze patterns compared to normal subjects. A possible extension of this study would be to repeat it with individuals of a more severe autistic phenotype and to compare the different patterns of face gaze. This type of analysis could lead to more specific and quantitative diagnostic criteria based on the varying extents to which individuals follow autistic face gaze patterns.

Neuroanatomical Abnormalities

Neuroanatomical abnormalities in autism are numerous and follow a complex developmental pattern. Studies of ASD are still in the process of elucidating the primary deficits. By the time a diagnosis is made, it is difficult to discern the trajectory of the neurodevelopmental abnormalities. Studies so far seem to indicate two patterns: (1) Early brain overgrowth followed by a premature cessation of growth that ultimately results in a brain size comparable to that of typically-developing individuals (but an enlarged amygdala persists); and (2) Primary deficits in lowerlevel processing areas that lead to aberrant development of higher cognitive areas and abnormal neural connectivity.

Early Brain Overgrowth Followed By Premature Cessation of Growth Neuroimaging studies and head circumference data suggest that one of the earliest deviations from normal brain development involves a transient period of postnatal macrocephaly followed by a premature cessation of brain growth (11). Courchesne et al. (2001) found that maximum brain size was reached in autistic Dartmouth Undergraduate Journal of Science

individuals by about three to five years of age, which is roughly six to 10 years earlier than typicallydeveloping individuals (11).

A recent MRI longitudinal study by Schumann et al. (2007) confirmed and extended previous studies on the pattern of brain growth in autism (12). Their work included a total of 220 clinical evaluations and MRI scans collected at roughly twelve-month intervals from 18-60 months from two groups of subjects: (1) typically-developing toddlers (n=48); and (2) toddlers with autism spectrum disorder (n=43). Each MRI underwent a detailed examination by the automated program Freesurfer, which segmented the frontal lobe into grey and white matter as well Diagram illustrating the roles of many different areas of the brain, all of which are active during social as further parcellated the cortex into processes. Researchers are aiming to understand how these interactions differ in autistic individuals. superior, middle, and inferior frontal gyri, orbitofrontal cortex, frontal previous findings of increased amygdala volume pole, precentral gyrus, and anterior cingulate cortex. in autistic individuals (13). Their work focused on A preliminary regression analysis of a subset of investigating brain structure in very young children with longitudinal data revealed significant enlargement of the autism, a relatively unstudied area of research. Nordahl collective frontal lobe and frontal gray matter volumes et al. attribute the sparseness of MRI studies of very (p<.05) in toddlers with ASD compared to typicallyyoung children with autism to challenges in acquiring developing controls.

These data confirm earlier high quality data. A significant outcome of their study findings that the frontal cortex â&#x20AC;&#x201C; more specifically, the was the development of a new protocol for obtaining cerebrum, frontal, and temporal lobesâ&#x20AC;&#x201D;exhibits the brain images of very young children. Using the fMRI highest degree of enlargement during the period of to observe this age group has been notoriously difficult. transient macrocephaly in autistic brain development Instead of resorting to sedation or anesthesia to still the (11). A possible explanation for this excessive frontal children during the scan, Nordahl et al. scanned their lobe volume is that the developing brain may be subjects during natural, nocturnal sleep. If this new compensating for sensory filtering deficits in the lowerprotocol is consistently successful and replicable, it may level processing areas. Without proper preliminary be a valuable tool for future studies that target earlier information processing, input to the frontal lobe is in stages of brain development in autism. It is hoped that an unusually low signal-to-noise level (i.e., the input is these will lead to elucidations regarding the primary relatively unfiltered and nonspecific and, therefore, is neuroanatomical deficits in autism spectrum disorder. difficult for the higher cognitive areas to process). This An fMRI study by Wallace et al. (2007) confirms condition may lead to abnormal neural connectivity and previous findings that autistic individuals have reduced an inability to develop normal executive functioning grey matter volume and less pronounced frontal lobe (11). asymmetry (14). Significantly, their study also extends Although autistic individuals have larger these characteristics to correlate symptom severity with brains compared to typically-developing individuals the degree of frontal lobe asymmetry. While typicallyduring early developmental stages, the latter catch up developing individuals display marked left frontal after the former experience a premature cessation of lobe asymmetry, the most impaired autistic individuals growth. Notably, although adults with autism possess display comparably little frontal lobe asymmetry. similar brain volumes to typically-developing adults, High functioning autistic individuals (HFA) who may the amygdala (which, as mentioned earlier, is thought often achieve normal behavioral output possess less to be a key region involved in theory of mind processes) frontal lobe asymmetry than typically-developing remains enlarged (11, 13). A recent study by Nordahl individuals but more asymmetry than individuals with and Amaral, et al. (2007) replicated and extended more severe autistic phenotypes. This structural data Image courtesy of Professor Thalia Wheatley

Spring 2008

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thus supports previous physiological data that normal behavioral output by HFA subjects is accomplished by abnormal compensatory neural circuits (3). The precise developmental relationship between lowerlevel processing areas and higher cognitive areas remains uncertain. Questions persist as to the extent to which secondary effects in higher cognitive areas exacerbate the original lower-level abnormalities.

Earlier developmental studies may lead to a better understanding of the severity and characteristics of the primary neuroanatomical deficits.

Lower-Level Deficits Lead to Aberrant Development of Higher Cognitive Areas Ultimately, autistic brains (including those of HFA individuals) show decreased grey matter in anterior regions such as the paracingulate sulcus – a region thought to be involved in theory of mind processes and, specifically, mentalizing of the self (2, 15)—and the inferior frontal gyrus (2). Conversely, increases in grey matter are found in posterior regions such as the periamygdaloid cortex and the middle temporal and inferior temporal gyrus (2). These findings are inconsistent with behavioral neuroimaging studies that show hyperactivation and abnormal patterning of primary perceptual processes coupled with hypoactivation of more complex cognitive processes (3). Both structural and functional data support the hypothesis that hyperarousal occurs as a result of aberrant filtering in lower-level processing regions, which leads to abnormal neural connectivity and, ultimately, impairments in topdown control.

In the typically-developing child, experience and normal neural growth processes act in tandem to develop organized and finely-tuned higher cognitive areas. Autistic individuals, however, are unable to carry out proper primary processing of their experiences.

As a result, their higher cognitive areas develop abnormally in spite of intact neural growth mechanisms (7). Neuropathology studies show abnormally dense packing of neurons occurs in limbic regions such as the hippocampus and the amygdala as well as a reduction in the size of cortical minicolumns. Furthermore, cells within the minicolumns show increased dispersion.

These alterations could increase the total number of minicolumns and, consequently, the degree of connectivity between minicolumns (3, 7).

The over-connectedness within cortical regions and the abnormally densely packed neurons in the limbic regions suggest that higher cognitive areas have developed in a compensatory manner in response to excessive noise in the input from the lower-level processing areas. The inhibition of higher cognitive functions in autistic individuals causes impairments in 28

top-down modulation processes such as the selection of perceptual input1. This lack of top-down control likely contributes to deficits in theory of mind processes that require informed inferences based on an integration of real-time observations and prior experience.

The Amygdala as a Possible Cause for Social Deficits

Studies of the amygdala strongly suggest that it is involved in mediating experiences of fear, anxiety, and surprise (16, 17). More specifically, the amygdala has been shown to play a role in perceiving facial emotion – particularly negative emotions such as anger—and then linking it to its social meaning (6, 15, 17, 18). Because of its important role in recognizing others’ emotions, the amygdala has become an area of interest in theory of mind studies. Furthermore, because of the volumetric enlargement of the amygdala in autistic individuals compared to typically-developing individuals, it has gained attention as a possible source of many of autism’s theory of mind deficits (2, 6, 15, 18). In order to investigate the role of the amygdala in normal and impaired theory of mind processes, studies have focused on comparing its activation in normal subjects with subjects who have amygdala. They also have focused on comparing the related behavioral output in subjects with brain damage to autistic subjects (e.g., in studies of perception of facial expression). Comparing these data may lead to a better understanding of the amygdala’s role in the neural network involved in theory of mind as well as to a more specific sense of the neural correlates of autism’s social deficits.

Some of the strongest evidence for the amygdala’s specific role in recognizing social cues from faces comes from a study how amygdala damage affects the ability to discern emotion from facial expressions (Adolphs et al., 2003) (18). Overall, bilateral amygdala damage did not seem to impair recognition of emotion from non-facial cues such as body language. All four subjects with bilateral damage, however, were significantly impaired in recognizing angry faces shown in isolation, which they frequently mistook for expressing happiness (Adolphs et al. postulate that this may be because these expressions of anger often showed bared teeth). This impairment in discerning emotion from facial expressions could lead to deficits in empathizing and in inferring the mental states of others, both of which are hallmarks of autism spectrum disorder (6).

Current challenges in the field of amygdala research are similar to those in theory of mind research: how can experimental paradigms be extended from static images to more realistic, real-time social interactions?

More complicated paradigms are required to elucidate how the amygdala functions as a part of the complex Dartmouth Undergraduate Journal of Science

An MRI scan of the brain. The amygdala is highlighted in green.

Image coutesy of Professor Paul Whalen, Dartmouth Department of Psychological and Brain Sciences.

neural network responsible for making subtle social inferences from an integration of real-time observations with prior knowledge and self-understanding. A recent study by Spezio et al. (2007) examined eye gaze in an individual with bilateral amygdala damage (participant S.M., a 42-year-old woman) compared to five matched controls during real-life social interactions (10). In a face-to-face session, an actor posed questions to a subject fitted with eye tracking machinery. Previous studies have shown that autistic individuals tend to avoid direct eye gaze and instead attempt to glean social information from the mouth (7, 10, 15). Significantly, Spezio et al. showed that bilateral amygdala damage in S.M. produced similar face gaze patterns to autism: there was almost no direct eye contact and nearly exclusive gaze to the mouth.

Although the study by Spezio et al. presents data from only one subject with bilateral amygdala damage, the findings are striking. These data not only confirm previous findings that autistic individuals and individuals with amygdala damage show similar cognitive deficits, but they also further indicate that these deficits seem to come from identical abnormalities in physiological processes. Together, these results strongly suggest that the amygdala is one of the central structures responsible for autism’s social deficits.

Future initiatives in amygdala and theory of mind research likely will center on elucidating how the amygdala comes into play in the neural network involved in theory of mind. A recent study by Kim et al. (2004) examined the differential responses of the amygdala to surprised faces based on varying contextual information (17). Normal, healthy subjects viewed faces showing surprised expressions, which were preceded Spring 2008

by either a positive sentence or a negative sentence.

Using fMRI neuroimaging, Kim et al. examined the functional relationship between the amygdala and the medial prefrontal cortex with regard to their differential responses based on the context given to the surprised facial expression by the preceding sentence. Results indicated that responses to negative versus positive sentences were greater within the ventrolateral prefrontal cortex, while responses to positive versus negative sentences were greater within the ventromedial prefrontal cortex (17). But what is the machinery responsible for this connectivity between the amygdala and the medial prefrontal cortex? Is there a third brain region involved, or are morphological constructs such as glia responsible for this critical networking?

Uncovering the interconnections of the neural network involved in theory of mind processes will lead to a better understanding of autism’s social deficits and to a clearer sense of the neural mechanisms that govern human social interactions.

Current and Future Directions

In the near future, autism and theory of mind studies can begin to look toward illuminating a more complicated neural network. The priority of most current research initiatives, however, remains to elucidate the structures and behaviors involved in isolation. There is still much to learn about the specific behavioral functions of many neurological structures and the details of autism’s abnormal neurodevelopmental trajectory.

One major area of study compares the cognitive behaviors and physiological characteristics of autistic individuals to those of their first-degree relatives. A recent fMRI study by Belmonte et al. (2007) examined the physiology of visual attention in autism families (19). Their results showed that both autistic individuals and their non-autistic siblings were impaired at a visual divided-attention task and displayed atypical frontal activation in comparison to normal controls. Analysis of correlations across brain regions, however, revealed that autistic individuals exhibited decreased functional correlation compared to normal siblings. Belmonte et al. thus suggest that although atypical frontal activation may reflect processes that are permissive but not determinative of autistic brain development. Future research likely will attempt to illuminate these determinative factors, which may be related to functional interconnectedness across an array of brain regions.

Other current research initiatives are exploring symptomatically-related disorders.

Asperger’s Syndrome is a disorder that often is regarded as a “less severe” form of autism and a more manageable venue in which to study cognitive and social deficits. An fMRI study by Moran et al. (2007) investigated self-referential 29

processing, a process integral to theory of mind functions in individuals with Asperger’s Syndrome (AS) (20).

When asked to attribute adjectives to themselves and to others, normal subjects did so significantly faster and with more robust activation in cortical midline structures such as the medial prefrontal cortex and the posterior cingulate cortex compared to AS subjects.

It is notable to recall that findings by Kim et al. (2004) suggest that the medial prefrontal cortex is connected to the amygdala in a network involved in contextual processing of emotion from facial expressions.

Therefore, certain theory of mind deficits in inferring the emotional states of others – specifically from facial expressions—may come from impairments in this collaborative process by the medial prefrontal cortex and the amygdala. In autism, an enlarged amygdala likely causes excessive activation of fear and anxiety responses. A weakly activated medial prefrontal cortex may lead to an impaired ability to empathize (i.e., to use self-referential knowledge of one’s own emotions), which could inhibit the contextual modulation of the amygdala’s response, thereby further magnifying the amygdala’s fear and anxiety response. Such a combined effect could help to explain how autistic individuals might develop an abnormal aversion to direct eye gaze.

Advances in neuroimaging may help to obtain earlier and higher quality data in developmental studies.

Increased detail in images taken during sensory perception and theory of mind tasks could reveal more specific information about which regions are being activated and how they interconnect. Furthermore, developmental studies may explore morphological abnormalities in children with autism. A recent study

by Morgan et al. (2007) suggests that examining the development of glial infrastructure could lead to important information about the developmental abnormalities in autism (21). The study examined microglial cell populations in a 3.8-year-old with autism and a 1.8-year-old control. Results indicated that robust activation of microglial cells – normally seen after there has been trauma to the brain—can occur in the autistic brain at an early developmental age. Although the subject pool was limited, the data obtained recommends further investigation of microglia development in larger samples of young autistic cases. Given that glia is emerging in the larger field of neuroscience as an increasingly important neurobiological construct, it may be a valuable area of study in autism research.

The widespread interconnectivity of glia in the brain suggests that glial abnormalities may contribute to the atypical functional connectivity found in autism.

Future research using lesion studies and more advanced and realistic theory of mind paradigms will lead to a better understanding of autism’s cognitive 30

deficits. Experimental tasks that more closely resemble real-life social interactions could provide more detailed information about the neural and physiological mechanisms involved in complex social behavior.

Comparing the functioning of these mechanisms in control subjects to autistic subjects could help to reveal the locus (or loci) of autistic impairments in implementing integrated perceptual information and prior knowledge (22). Particular challenges in creating these new paradigms will involve segregating which elements are testing theory of mind and which are testing other aspects of social interactions such as rapid attentional shifting. Isolating regions of interest will also be an obstacle in lesion studies, in which finding subjects with specific patterns of brain damage can be exceedingly difficult (3).

A possible starting point in lesion studies may be to continue investigating eye gaze and recognition of emotion from facial expressions. One study of interest might be to compare amygdala activation in autistic individuals versus controls during direct eye gaze. Given that autistic individuals have an enlarged amygdala and seem to show aversion to direct eye gaze, the expectation would be to see hyperactivity in the autistic amygdala compared to controls. Another study might repeat the experiment developed by Kim et al.

(2004), except using subjects with brain damage to the medial prefrontal cortex. Comparing the performance of brain damaged subjects to that of control subjects might illuminate more about the role of the medial prefrontal cortex in the contextual modification of amygdala responsivity. Because amygdala studies most often involve negatively-valenced emotions, a difficulty in any amygdala study will be to keep anxiety levels low across subjects. This will likely be particularly challenging in autistic subjects, for whom direct face gaze and social interactions are unusually anxietyprovoking. Steps to alleviate this anxiety might involve providing substantial practice training for all subjects as well as frequent rest periods in between trials (3).

A priority in current and future autism studies will be to delineate more specific diagnostic criteria.

Quantitative gradations of the autism phenotype might be developed as more knowledge is gained about the determinative characteristics of the disorder. A significant study by Chiu et al. (2007) has developed a preliminary algorithm for determining the severity of different autism phenotypes (4). This algorithm is called a “self-eigenvalue” and was derived from fMRI studies of cingulate cortex activation during self-referential studies. Primary evidence for the algorithm was provided by a related study by Kishida et al. (2007), which found striking correlations in cingulate activation in the brains of Division I athletes (23). Neuroimaging data indicated Dartmouth Undergraduate Journal of Science

an inverse activation of the cingulate cortex when athletes viewed clips of other athletes playing sports of their expertise compared to when they were subsequently asked to “do it” (i.e., visualize themselves carrying out the action in the clip). Chiu et al. observed a similar pattern of cingulate activation in control subjects during an active social exchange game with a human partner (the activation disappeared in the absence of an interactive social partner). Significantly, high functioning autistic subjects lacked the cingulate response pattern of control subjects. Perhaps most striking is that this activation was not “all-or-none,” but rather the extent of cingulate activation varied parametrically with symptom severity.

An algorithm developed from the parametric relation between cingulate activation and symptom severity would be an important step in outlining quantitative autism phenotypes.

Conclusion

Progress in autism research is contingent upon expanding our understanding of complex cognitive processes such as top-down modulation and theory of mind. Future studies will require new and complex paradigms that come closer to recreating the complex social interactions in which these neural networks are activated. For the present, primary goals in autism research include gaining a better understanding of the complex pattern of deficits in autism and developing a more precise means of diagnosing different autistic phenotypes. Hopefully, these advances will eventually lead to finding a means of helping even severely autistic individuals to reach a higher level of social functioning.

References 1. American Psychiatric Association, Diagnostic and Statistical Manual of Mental Disorders: Fourth Edition. (American Psychiatric Pub., Inc., 2000). Available at http://www.autism-watch.org/general/ dsm.shtml 2. U. Frith, Neuron 32, 969 (2001).

3. M. Belmonte and D. Yurgelun-Todd, Cognitive Brain Research (17)3, 651 (2003).

4. P. Chiu, P. Montague, et al., Active interpersonal exchange evokes quantitative neural phenotype for high functioning autism. Program No. 304.19. 2007 Neuroscience Meeting Planner. San Diego, CA: Society for Neuroscience, 2007. Online.

5. M.A. Westerfield , P.T. Lai, G.L. Smith, A. Lincoln, J. Townsend, Auditory-Visual Integration in Autism. Program No. 175.15.

2007 Neuroscience Meeting Planner. San Diego, CA: Society for Neuroscience, 2007. Online.

6. V.E. Stone, S. Baron-Cohen, A. Calder, et al., Neuropsychologia 41, 209 (2003).

Spring 2008

7. M. K. Belmonte, E.H. Cook, Jr., G.M. Anderson, J.L.R. Rubenstein, W.T. Greenough, A. Beckel-Mitchener, E. Courchesne, E., L.M. Boulanger, et al., Molecular Psychiatry 9, 646 (2004).

8. V.E. Stone, S. Baron-Cohen, R.T. Knight, Journal of Cognitive Neuroscience 10(5), 640 (1998).

9. K.K.W. Kampe, C.D. Frith, U. Frith, The Journal of Neuroscience 23(12), 5258 (2003).

10. M.L. Spezio, R. Adolphs, R.S.E. Hurley, J. Piven, Neuropsychologia 45, 144 (2007).

11. E. Courchesne, Mental Retardation and Developmental Disabilities Research Reviews 10, 106 (2004).

12. C. Schumann, G.M. Wideman, C. Carter Barnes, J.A. Buckwalter, D.J. Hagler, Jr., R.A. Carper, E. Courchesne, MRI longitudinal study of the frontal cortex through early childhood in autism. Program No. 61.5. 2007 Neuroscience Meeting Planner. San Diego, CA: Society for Neuroscience, 2007. Online.

13. C.W. Nordahl, T.J. Simon, and D.G. Amaral, Amygdala Enlargement in Very Young Children with Autism. Program No. 61.7. 2007 Neuroscience Meeting Planner. San Diego, CA: Society for Neuroscience, 2007. Online.

14. G.L. Wallace, J. Silvers, A. Martin, and J. Gledd, Reduced Gray Matter Volume and Frontal Lobe Asymmetry in High Functioning Autism Spectrum Disorders. Program No. 172.9. 2007 Neuroscience Meeting Planner. San Diego, CA: Society for Neuroscience, 2007.

Online.

15. M.L. Spezio, S. Po-Yin Huang, F. Castelli, and R. Adophs, The Journal of Neuroscience 27(15), 3994 (2007).

16. M. Davis, and P.J. Whalen, Molecular Psychiatry 6, 13 (2001).

17. H. Kim, L.H. Somerville, T. Johnstone, S. Polis, A.L. Alexander, L.M. Shin, and P.J. Whalen, Journal of Cognitive Neuroscience 16(10), 1730 (2004).

18. R. Adolphs, D. Tranel, Neuropsychologia 41, 1281 (2003).

19. M.K. Belmonte, M. Gomot, S. Baron-Cohen, Visual Attention in Autism Families: ‘Unaffected’ Sibs Share Atypical Frontal Activation But Not Atypical Functional Connectivity. Program No. 61.14. 2007 Neuroscience Meeting Planner. San Diego, CA: Society for Neuroscience, 2007. Online.

20. J.M. Moran, A. Qureshi, M. Singh, J.D.E. Gabrieli, Neural Underpinnings of Self-Referential Processing in Asperger’s Syndrome. Program No. 304.18. 2007 Neuroscience Meeting Planner. San Diego, CA: Society for Neuroscience, 2007. Online.

21. J.T. Morgan, G. Chana, J. Buckwalter, E. Courchesne, I.P. Everall, Microglial Activation in a 3-Year-Old Autistic Brain. Program No. 172.12. 2007 Neuroscience Meeting Planner. San Diego, CA: Society for Neuroscience, 2007. Online.

22. M. Belmonte and D. Yurgelun-Todd, Cognitive Brain Research (17)3, 651 (2003).

23. K. Kishida, P. Chiu, R. Montague et al., “Self” responses revealed independently by fMRI during visualization. Program No. 304.16.

2007 Neuroscience Meeting Planner. San Diego, CA: Society for Neuroscience, 2007. Online.

31

BIOLOGY

Battle of the Sexes:

How X and Y Chromosomes Are Engaged In Perpetual Warfare Colby Chiang ‘10

I

n the summer of 1925, Russian geneticist Sergey Gershenson collected 19 female fruit flies of the species Drosophila obscura from a forest near Moscow. He brought the captured flies back to his laboratory and mated them with normal males to study their offspring. By the second generation of matings, Gershenson noticed a puzzling trend: some of the male flies were producing almost all female progeny. He repeated the matings to confirm that this was not just an extraordinary coincidence, but the result remained the same. These otherwise healthy males exhibited a shocking propensity to sire daughters (1).

Gershenson resolved that this abnormality was related to the fertilization of the egg. During mating, a male fly fertilizes the egg with a sperm cell that contains either an X chromosome or its homologue, the Y chromosome. The egg always has an X chromosome, and the content of the sperm determines the gender of the offspring. If the sperm has an X chromosome then the fertilized egg will develop into an XX female, and if it has a Y chromosome it will become an XY male.

The mutation in Gershenson’s flies exploited the diploid nature of the fly genome. Like most organisms, fruit flies have two copies of each chromosome, and pass either one to their offspring with approximately equal probability. Gershenson’s flies, however, carried a mutated chromosome that distorted the transmission probability in favor of itself, a mechanism that is known as “meiotic drive” (2, 3, 4, 5). In this case, the affected chromosome was the X chromosome, leading to the sex-ratio bias.

The meiotic driving X chromosome mutation, known as Xd, was attacking and destroying Y-bearing sperm, leaving a vast majority of X-bearing sperm to fertilize the egg. As a result, nearly all of the fertilized eggs received an X chromosome and developed into XdX females. The mutation then lurked in the genome of the daughters, who would mate with healthy males and pass the defective X chromosome to their offspring. As the generations progressed, the number of fathers with the ability to sire sons plummeted, nearly eliminating males from the population entirely (1).

While the destructive and selfish nature of the X-linked meiotic drive mutation is detrimental to the species as a whole, evolution actually favors 32

it, at least in the short term. Since females have two X chromosomes and males only have one, the X chromosome spends two-thirds of its existence in females (4, 6). So if an X chromosome has a gene that poisons and kills Y-bearing sperm, it increases its own chances of being transmitted to progeny. In pursuit of this advantage, the X chromosome has no qualms about wiping out males entirely, even if it means eventual extinction of the species. Evolution has no foresight and no plan. The only goal is propagation, and the Xd mutation was effectively forcing its own propagation through the destruction of its competitor.

The idea that evolution occurs on the level of the genes was most famously articulated by Richard Dawkins in The Selfish Gene: “Genes are competing directly with their alleles for survival, since their alleles in the gene pool are rivals for their slot on the chromosomes of future generations. Any gene which behaves in such a way to increase its own survival chances in the gene pool at the expense of its alleles will, by definition, tautologously, tend to survive. The gene is the basic unit of selfishness” (7). One way that a gene can triumph over its competing allele is to improve itself, but another, more devious method, is to sabotage its competitor.

Meiotic drive requires several factors to accomplish this feat. First, the affected pair of chromosomes must each contain both a driving and a target locus. In the hostile chromosome, the target locus becomes resistant to the driving locus, which prevents the chromosome from destroying itself. In the susceptible homologue, the driving locus is inactive and the target locus has no protection against the attack, leaving it at the mercy of the hostile chromosome. Meiotic drive also requires that the driving and target loci have a low recombination frequency, since recombination may lead to the suicidal pairing of driving and target loci on the same chromosome (3).

Since the sex chromosomes do not undergo recombination, they are hotspots for meiotic driving mutations (3). The long and widespread history of sex chromosome antagonism may have shaped the structure of the genome. In a paper published in the Philosophical Transactions of the Royal Society of London, William Amos and John Harwood hypothesize that evolutionary pressure from meiotic drive genes on the X chromosome is responsible for the reduced content Dartmouth Undergraduate Journal of Science

A comparison by region between the X and Y chromosomes Imge courtesy of the Wellcome Trust Sangar Institute.

of the Y chromosome (4, 8). The human Y chromosome, (~60 million nucleotides), is less than half the size of the X chromosome (~165 million nucleotides) (9, 10).

Homologies in the X and Y DNA sequences suggest that they originally descended from a pair of identical autosomes, but the X chromosome repeatedly targeted loci specific to the Y chromosome. To escape these attacks, the Y chromosome shed all non-essential genetic material that the X chromosome could lock on to. Little by little, the Y chromosome grew shorter, eventually assuming its current abbreviated and specialized form.

In humans, todayâ&#x20AC;&#x2122;s Y chromosome has lost all but 45 of its original 1000 genes, and the majority of surviving genes are those involved in the development of male traits (9).

But the Y chromosome has other methods of recourse than to simply retreat. It can also take the offensive against the X chromosome, and sometimes even beat the X chromosome at its own game. The stalk-eyed fly (Cyrtodiopsis dalmanni) population is 33-35% male as a result of a meiotic drive gene on the X chromosome. This particular Xd mutation exerts its destructive power during spermiogenesis, the last stage of sperm formation. During spermiogenesis in stalkeyed flies, the immature X- and Y-bearing spermatids are arranged into sperm bundles. Males that carry the Xd mutation exhibit varying levels of sperm bundle Spring 2008

degeneration. In some bundles, all spermatids are degenerate, while in others only a portion is affected. It is believed that in partially degenerate bundles only the Y-bearing spermatids are destroyed, while the X-bearing spermatids are protected by a resistant locus. But 66% of the sperm bundles are completely degenerate, and X- and Y-bearing spermatids alike are destroyed.

Hence, the Xd mutation fights a semi-kamikaze mission against the Y chromosome, destroying many of its own in its quest to eliminate Y-bearing sperm.

Though it suffers heavy losses, the X chromosome, along with the Xd mutation, is carried by nearly all sperm that emerge from the battle (3).

As an adaptive response to the Xd mutation, some male stalk-eyed flies carry a Y chromosome resistance mutation (Ym) to protect Y-bearing sperm. The Ym mutation makes the Y-bearing spermatids strongly resistant to the Xd attacks. In fact, it appears that the Ym mutation is better at protecting the Y-bearing sperm than the Xd mutation is at protecting itself. In the face of the Ym defense, the Xd mutation causes more collateral damage than directed damage. This resistance actually reverses the drive to produce fewer X-bearing sperm than Y-bearing sperm, resulting in a 63% male-biased progeny (3).

But the Y chromosome has another secret weapon in its defense. The autosomes enter the war as its allies, and offer reinforcements against the X chromosome. There is good reason for this behavior.

Once the population becomes heavily female biased, males enjoy a huge evolutionary advantage (11). The rare males are surrounded by eligible females clamoring for a chance to mate. This evolutionary pressure affects not only the Y chromosome, but the autosomes as well.

An autosome that finds itself in a male is assured of plentiful matings and transmission to many progeny.

Thus, an autosome that develops resistance to the Xd 33

mutation would be increasing its own propagation in its defense of the Y chromosome. This alliance between the embattled Y chromosome and the autosomes has been observed in the stalk-eyed fly of Malaysia (12, 13, 14).

In Malaysian stalk-eyed flies, the eyes are positioned on the tips of antenna-like eye stalks. These eye stalks extend far from the body, and in some species the width between the eyes exceeds the animal’s entire body length. The Ym suppressor is closely linked to an allele that is responsible for wide eyes. Because of the chromosomal linkage, males with wide eye stalks are likely to also carry the Ym mutation. Females that have a sexual preference for wide-eyed mates are indirectly attracted to the drive suppressing mutation. These mothers pass the autosomal preference gene to their sons who, by way of female mating demand, flood the population with wide eye preference. Once it establishes itself in the population, the female preference enters the feedback loop of runaway sexual selection. The males, reinforced by mating success, develop wider and wider eye stalks, leading to the comically exaggerated width in today’s flies. In closely related species that lack the X-linked meiotic drive however, female eye width preference never evolved, and these flies have normal eyes. Thus, X-linked meiotic drive may be the basis for female sexual preference in some species (14).

But even with this valiant defensive effort, the Y chromosome may be fighting a losing battle. An analysis of the chromosome’s history reveals that it has been losing genetic material at an alarming rate. In humans the Y chromosome may eventually be eliminated entirely. Estimates for Y chromosome extinction vary widely, from 125,000 years to 14 million years to never, but other species have shown that Y chromosome loss is certainly possible. In the Japanese spinous country rat (Tokudaia osimensis), both males and females are XO, meaning that they have a single X chromosome and no

34

Y chromosome (15). In a species of mole vole (Ellobius tancrei), both males and females are XX (9).

From the Gershenson’s Drosophila to stalkeyed flies to humans, many animals may be in danger of Y chromosome loss. Or perhaps this process is cyclical, and the Y chromosome is replaced by another autosome that follows it on the path to complete deterioration.

Whatever the fate, the X and Y chromosomes will continue their blind evolutionary battle for propagation, selfishly teetering the gender balance on the brink of disaster.

Acknowledgments

The author would like to thank Professor Ryan Calsbeek for his consideration and guidance in this manuscript.

References: 1. S. Gershenson, Genetics 13, 488 (1928).

2. L. Sandler and E. Novitski, The American Naturalist 91, 105 (1957).

3. D. Presgraves et al., Genetics 147, 1169 (1997).

4. M. Ridley, Genome: The Autobiography of a Species in 23 Chapters. (HarperCollins, New York, 1999) pp. 107-121.

5. G.D.D. Hurst and J.H. Werren, Nature Reviews Genetics 2, 597 (2001).

6. B. Charlesworth et al., Genetics 134, 1291 (1993).

7. R. Dawkins, The Selfish Gene. (Oxford University Press, Oxford, 1976) pp. 38-39.

8. W. Amos and J. Harwood, Phil. Trans. R. Soc. Lond. B. 353, 177 (1998).

9. J.A.M. Graves, Cell 124, 901 (2006).

10. J.A.M. Graves, Current Opinion in Genetics & Development 16, 219 (2006).

11. W.D. Hamilton, Science 156, 477 (1967).

12. G.S. Wilkinson et al., Nature 391, 276 (1998).

13. K. Reinhold et al., Proc. R. Soc Lond. B. 266, 1391 (1999).

14. R. Lande and G.S. Wilkinson, Genet. Res., Camb. 74, 245 (1999).

15. Y. Arakawa et al., Cytogenet. Genome Res. 99, 303 (2002).

Dartmouth Undergraduate Journal of Science

biology

Neurobiology of Epilepsy:

Mechanisms of Disease and the Network and Structural Changes Involved theresa yang â&#x20AC;&#x2DC;08

T

he neurobiology of epilepsy is an active field of investigation. Numerous mechanisms of inhibitory and excitatory deregulation have been suggested, and the interactions of these pathways are observed almost ubiquitously. The compromised inhibitory compensation of Gamma-Aminobutyric Acid (GABA) during seizures has been hypothesized to be a direct result of aberrant glutamate metabolism. Recent research has also implicated the role of endocannabinoids in modulating epileptic activity in the hippocampus. A stimulating area of epilepsy research is the examination of different models of circuitry participating in seizure activity. Other physiological changes that occur in the epileptic brain include glial composition changes and synaptic modification. Together, these findings provide a better understanding of the neurobiology of epilepsy, and could lead to improved management and cures.

Introduction

Epilepsy is a neurological disorder characterized by unprovoked, spontaneous seizure activity due to abnormal, excessive, or synchronous neuronal activity.

It can be caused by specific cortical deformities such as lesions, tumors, developmental malformations, or other neurological complications such as Alzheimerâ&#x20AC;&#x2122;s or cerebral palsy, or be of idiopathic origin. Epilepsy and seizures affects more than three million people in the United States, with approximately 200,000 new cases being diagnosed each year (1). It is one of the most frequent disorders among children, and causes impairments in psycho-motor and cognitive development ranging from mild to severe (2). Patients with epilepsy suffer from debilitating social problems and stigmas, as well as increased risk for co-morbid psychiatric disorders such as depression, anxiety, and suicidality (3).

The majority of seizure disorders are treated with administration of anticonvulsive or antiepileptic drugs (AEDs), while a smaller percentage of patients receive neurosurgery or surgical implants to control their seizures. The treatment of epilepsy is varied and tailored to each patientâ&#x20AC;&#x2122;s situation. Understanding the underlying neural networks of epilepsy is crucial in helping physicians assess the appropriate treatment for a better outcome. Seizure disorders affect a variety of brain pathways and mechanisms that interact in different ways. A wide range of receptors and neurotransmitters Spring 2008

are modulated in epileptic patients, including GABA, glutamate, and endocannabinoids. Investigating the precise functions of these neural components in seizure activity will help formulate more specific and efficient AED treatments so that side effects can be reduced and seizures better managed. Here, I will review the recent findings of the role of different mechanisms and receptors in seizure activity, followed by network and structural changes that accompany the epileptic brain.

Mechanisms of Epilepsy

Different inhibitory and excitatory pathways undergo modification and contribute to epileptogenic activity. Deregulation can occur at the synaptic level, and may result in a change in receptor expression, neurotransmitter reuptake, or binding affinity.

The Glutamate-Glutamine Cycle Modulates Neuroinhibition GABA and glutamate imbalance is a key cause of the hyper-excitation and hyper-synchronization of neurons, which lead to synaptic changes. An experiment examined the changes in GABA and glutamate neurotransmission in rats with generalized absence seizures, which are seizures that spread to the entire cortex and occur frequently in human populations. In vivo uptake of glutamate was found to be compromised in the cortex of epileptic rats compared to controls, while extracellular levels of GABA and glutamate were unchanged in the cortex and thalamus (4). This finding suggests a dysfunction in glutamate transportation, which may cause spike wave activity indicative of brain dysfunction in the cortex of rats with absence epilepsy.

The regulation of extracellular glutamate levels is important because sustained activation of glutamate receptors results in excitotoxicity and leads to neuron death. Glutamate transporters EAAT-1 and EAAT-2 are found in astrocytes and take up extracellular glutamate against the concentration gradient to prevent CNS injury (6). Near glutamatergic synapses, astrocytes express an enzyme called glutamine synthetase (GS) which converts glutamate to its inactive form, glutamine. Glutamine is then shuttled into the neuron through a system A transport system, protecting the cell from toxicity (7). These two mechanisms serve to regulate deleterious glutamate activation. Inside the neuron, glutamine can be converted 35

Illustration of glutamine synthetase, the enzyme which intercoverts glutamate and glutamine. Pictured here are the binding sites for ATP and glutamate.

Image courtesy of the Protein Data Bank (Illustrated by David S. Goodsell of The Scripps Research Institute).

back to glutamate by mitochondrial glutaminase and stored in vesicular glutamate transporters (VGLUT) until release (6). The glutamate-glutamine cycle therefore functions as a crucial neuroprotective mechanism to prevent glutamate-related excitotoxicity and oxidative stress.

The excess of excitatory activity is mainly a glutamatergic event, and consequently extracellular glutamate is elevated in hippocampi of patients with intractable mesial temporal lobe epilepsy (MTLE).

Extracellular glutamate levels also increase during a seizure and the glutamate is cleared more slowly than usual afterwards. In a recent study by Eid et al., the accumulation of extracellular glutamate in MTLE hippocampi is proposed to be due to a deficiency in astrocytic GS enzyme levels. The researchers performed enzyme activity assays and found that the enzyme catalyzing glutamate synthesis, phosphate activated glutaminase (PAG), was increased in the epileptic hippocampus (9). This finding suggests that there is increased potential for glutamate production resulting in increased excitation in the MTLE hippocampus.

What needs to be further investigated is what controls the changes in GS and PAG. Speculations can be made about the upstream involvement of transcription and translation, and studies examining the expression 36

patterns of mRNA for these two enzymes need to be performed to confirm this.

Glutamate has an additional role in inhibitory transmitter regulation. Glutamate and GABA interterminal levels exist in equilibrium, and the glutamateglutamine cycle also functions as a regulator of synaptic GABA release. In neurons, glutamate can be decarboxylated by glutamic acid decarboxylase (GAD) to generate GABA, which is packaged into vesicles.

Studies have shown that disturbing the glutamateglutamine cycle results in decreased hippocampal GABA reserves, and that GS levels are significantly decreased in epileptic human hippocampi (8). These data indicate that glutamate and GABA participate in the convergent regulation of excitatory and inhibitory networks.

In a recent experiment by Liang and Coulter, the deficiency of the glutamate-glutamine cycle in rats with temporal lobe epilepsy (TLE) was hypothesized to compromise GABAergic inhibitory synaptic efficacy.

They performed whole-cell patch recordings from the pyramidal neurons of CA1, a region of the hippocampus containing several output pathways. They found that inhibitory post-synaptic currents (IPSCs) were much slower and smaller in epileptic rats, which was consistent with a decrease in synaptic GABA concentration as well as vesicular GABA content (5). The decrease in vesicular GABA release was thought to be due to the Dartmouth Undergraduate Journal of Science

compromised function of the glutamate-glutamine cycle. To test this speculation, they examined changes in IPSCs induced by extracellular stimulation using a patch pipette and measuring synaptical GABA release. In the presence of a glutamine uptake inhibitor, a reduction in synaptic strength of IPSCs was observed in control CA1, but not in neurons from rats with TLE. They concluded that extracellular glutamine was not used by the cell to make GABA in inhibitory synapses of epileptic rats.

Interestingly, the administration of exogenous glutamine restored inhibition in CA1 of epileptic animals, but had no effect on controls (5). Combined, these results show that lowered vesicular GABA synthesis and release are caused by a deficiency in the glutamate-glutamine cycle. Therefore, glutamate affects the efficacy of the GABA inhibitory response during seizures, and this compromised GABA release fails to reduce neuronal activity in a timely fashion.

Endocannabinoid Hypotheses The endocannabinoid system is an important neuromodulatory system, and is involved in the regulation of excessive excitatory neuronal activity.

It consists of cannabinoid receptors, which bind endogenous lipid ligands. The type 1 receptors (CB1) are G-protein coupled seven-transmembrane domain proteins mainly expressed in the forebrain, and their activation by endocannabinoids activates a cascade of functions including inhibition of neurotransmitter release, long-term plasticity, and control of excitatory activity (10). In the hippocampus, amygdala, and neocortex, CBI receptors are colocalized in the terminals with GABA receptors, but in the cerebellum and striatum, are found in glutamatergic neurons (11). Therefore, endocannabinoids can either have either have inhibitory or excitatory effects depending on the receptor which they activate and the brain region on which they bind.

In a current study by Bhaskaran and Smith, changes in the synaptic composition of cannabinoid receptors in the dentate gyrus of a pilocarpineinjected mouse model were examined. After inducing status epilepticus in mice, mossy fiber sprouting and spontaneous firing of neurons were found in the hippocampus, and especially in the dentate gyrus.

These fibers are bundles of axons projecting from granule cells through the hilus, and connecting to the pyramidal cells in CA3 (18). The reorganized tissue contained CB1 receptors as well as a class of capsaicinbinding villanoid receptors (TRPV1), which integrate inflammatory stimuli and play an important role in the pathophysiology of sensitization (11). Anandamide is an endocannabinoid which can activate both CB1 and TRPV1 receptors and was used to explore the potential therapeutic advantages of endocannabinoids Spring 2008

in the treatment of TLE (12). Anandamide was found to diminish excitatory post-synaptic currents (EPSCs) in the altered dentate gyrus of seizure-induced mice, but sometimes it enhanced excitatory responses. When CB1 receptors were blocked with AM251 antagonist, administering anandamide caused an increase in EPSCs. However, when TRPV1 receptors were blocked with capsazepine, anandamide had the opposite effect, and suppressed neuronal activity. When CB1 was activated by a receptor-specific agonist, EPSCs decreased in duration and strength, and when TRPV1 receptors were activated by capsaicin, EPSCs were enhanced (12). These results indicate that anandamide can have either an inhibitory or excitatory effect depending on the receptor it stimulates.

Another recent experiment by Ludanyi et al. found that epileptic hippocampi exhibited a downregulation of the neuroprotective CB1 receptor.

Quantitative PCR measurements showed that mRNA levels of the CB1 receptor in epileptic hippocampi were one third of the value in controls. Similarly, the cell surface expression of cannabinoid receptor-interacting protein was decreased. Finally, the researchers performed immunoblotting studies and showed that the density of CB1 was decreased in the hippocampi of epileptic patients, most robustly in the dentate gyrus (13). That this experiment was performed in human samples should be of note. Control hippocampal tissue was obtained from post-mortem samples from subjects with no known neurological disorders, and epileptic tissue was obtained from surgically resected patients with intractable TLE. Although more human studies are needed, results from post-mortem samples may not have significant in vivo implications. The composition of tissue changes slightly after death, and previous studies even found a small decrease in the post-mortem localization of the astroglial transporter protein in TLE (14). Therefore, more experiments exploring endocannabinoid pathways need to be made in both mice models and humans in vivo to provide more convincing support.

Endocannabinoids have been associated in the regulation and modulation of various physiological behaviors, including eating, anxiety, pain, and aversive memory extinction (11). Their potential therapeutic use in neuroprotection, specifically down-regulation of excitatory brain activity in epilepsy deserves further investigation. Future studies should examine the role of plasticity in the endocannabinoid system and determine if any of the harmful synaptic reorganization and neuronal damage due to aberrant excitatory activity can be reversed.

37

Network and Structural Changes

Seizures are the outward expression of abnormal brain activity, which can be seen in irregular EEG patterns. Several physiological changes can be observed in the hippocampus of epileptic patients, including neuronal degeneration, mossy fiber sprouting, basal dendrite formation on granule cells, aberrant synaptogenesis, astriogliosis, and microglial activation (17). Here I will examine the findings on connectivity changes caused by seizures, and observe how the brain as a whole behaves in epilepsy.

Network Topology on Seizure Spread The complex neural networks involved in epileptic activity are not well understood. Standard explanations for balanced neural activity rely on groups of inhibitory neurons that suppress excitatory neuron firing. Kaiser and Hilgetag investigated the effect of network topology on preventing large-scale seizure spread, specifically the propagation of signal through hierarchically clustered neurons. They created a model where neurons were represented as binary nodes which were activated by a certain number of surrounding active neurons, and were deactivated over time. This basic model imitates the cerebral cortex network which spans several levels of organization, and was compared to isolated and random circuits of the same size. They found that higher degrees of cluster connectivity resulted in more frequent and complete spreading activity, which was not true for isolated and random models. In contrast, hierarchical clustering prevents large-scale activation, and provides stable neural activity (15). Inhibition at the hierarchical level would therefore provide efficient regulation of neural activity. These computational models can have significant applicational value. Additional research is needed to determine if epileptic patients do in fact have higher inter-cluster connections, and if this is the cause of widespread and sustained electrical activity during a seizure. In addition, the role of inhibitory neurons needs to be factored into the model to provide a more complete and accurate understanding of the neural circuits that underlie epilepsy.

Another recent study by Schiller and Cymerblit examined the network dynamics leading up to the development of seizures, and possible ways to predict them. They conducted multi-electrode single unit recordings for hippocampi of rats with induced pilocarpine and picrotoxin epilepsy (16). They recorded action potentials of individual neurons and assessed the synchrony of the network and firing patterns during seizures. Changes in EEG patterns were observed 15 minutes before seizure onset, which can be separated into two phases. In the early phase, there is a slowing 38

of activity, and fewer neurons fire together and at the same time. In the later phase, neurons tend to fire in synchronous bursts, causing sharp waves in the EEG recordings, suggesting possible activations of positive feedback loops (16). These findings suggest that in pharmacologically-induced seizures, specific and predictable changes in firing patterns of hippocampal neuron populations occur during the pre-ictal state.

Understanding the network dynamics responsible for seizure initiation helps detect seizures before they start, so that timely treatment can be given.

Astrocytic and Microglia Changes Glial activation is known to occur after brain injury, trauma, or seizures, and play a critical role in neuronal degeneration and survival. Significant changes in glial composition take place in the epileptic brain of pilocarpine-injected rats, which closely mimics the human condition (16). Shapiro et al. conducted a study examining the precise temporal changes in epileptic hippocampi glial composition of rats over a period of eight days. They labeled astrocytes with GFAP and labeled microglial cells with IBA-1. One day after seizures, an increase in labeled astrocytes was evident in specifically the hilus of the hippocampus, followed by a transient decrease over two to five days. Amounts of labeled microglia increased initially in the hilus, CA1, and CA3 regions, which progressively decreased over a period of three days, then increased again at day eight (18). These results indicate that astrocytes and microglia exhibit specific temporal and spatial changes after pilocarpine-induced seizures.

More precise descriptions of these changes, such as specific cell counts, are needed for a better understanding of glial mechanisms involved in epilepsy.

The release of cytokines activates gliosis during neuronal damage, and recruits astrocytes and microglia to the site of injury to induce apoptosis and neuronal repair (17).

The signals for glial activation need to be elucidated so that better pharmacological management can be used to exploit their many reparative roles.

Conclusion

Combined, the recent findings help us formulate a better understanding of the causes and effects of epilepsy. We are still at the early stages of investigation, and recent technology such as fMRI and other imaging techniques has helped advance our knowledge of the disorder in recent years. However, for future research, more standards need to be established so that results can be compared across studies. For example, results could be less variable across experiments if a consistent protocol were developed for the induction of pilocarpine seizures in rat or mouse models. The general procedure Dartmouth Undergraduate Journal of Science

for making rat or mouse models of epilepsy is to inject pilocarpine locally in the rat brain, causing status epilepticus, and then stopping SE by administration of an anticonvulsant. This causes the animal to develop epilepsy, which is characterized by the occurrence of spontaneous and unprovoked seizures (19). Too many variables in procedure exist presently, such as length of drug administration, amount in each injection, length of time between injections, and length of time to wait before stopping SE. Data could be compared across experiments with more confidence and results could be replicated more easily if most of these variables were eliminated.

In addition to aiming for less variable data acquisition, more in vivo studies should be conducted.

While insightful knowledge can be gained from examination of tissue and single-cell recordings, what occurs in a live model can differ significantly from the in vitro study. Most studies focus on the hippocampus as a key structure involved in epileptic activity and possibly epileptogenesis. However, additional experiments that investigate other afferent and efferent circuitries and their role in propagation of seizures are needed. Research on neighboring brain structures involving the limbic and thalamic systems can clarify the methods by which a seizure can spread to the whole brain and compromise consciousness. Genetic studies are worthy of conducting, and may provide invaluable information about the predisposition and prevalence of epilepsy among human populations. The influence of environmental factors on the neurobiology of epilepsy is also an area of potential investigation. The future of patients with seizure disorders depends greatly on further research leading to improved clinical management.

of patients with mesial temporal lobe epilepsy. Program No 259.4/ Q12. 2007 Neuroscience Meeting Planner. San Diego, CA: Society for Neurosience, 2007. CD-ROM 10. K. Monory et al., Neuron, August 2006, p. 455-466.

11. B.E. Alger, Epilepsy Currents, September 2004, p.169-173.

12. Bhaskaran, M.D., Smith, B.N. Cannabinoid and villanoid interactions in the dentate gyrus of a mouse model of temporal lobe epilepsy. Program No 257.20/P7. 2007 Neuroscience Meeting Planner. San Diego, CA: Society for Neurosience, 2007. CD-ROM 13. A. Ludanyi et al., Down-regulation of the neuroprotective CB1 cannabinoid receptor and related molecular elements of the endocannabinoid system in epileptic human hippocampus. Program No 374.8/U22. 2007 Neuroscience Meeting Planner. San Diego, CA: Society for Neurosience, 2007. CD-ROM 14. S. Tessler et al., Neuroscience, February 1999, p. 1083-1091.

15. M. Kaiser, C.C. Hilgetag, Critical topology of seizure spreading: hierarchically clustered neural connectivity prevents large-scale network activation. Program No 333.6. 2007 Neuroscience Meeting Planner. San Diego, CA: Society for Neurosience, 2007. CD-ROM 16. Y. Schiller, A. Cymberblit, Network dynamics during the development of seizures in pilocarpine and picrotoxin treated rats in-vivo. Program No 333.7. 2007 Neuroscience Meeting Planner.

San Diego, CA: Society for Neurosience, 2007. CD-ROM 17. M. Rizzi et al., Neurobiology of Disease, February 2003, p.

494-503.

18. S. Xiu-Yu, S. Ruo-Peng, W. Ji-Wen, Brain and Development, April 2007, p. 157-163.

19. R.S. Fisher, Brain Research Reviews. July-September 1989, p.

245-278

References 1. Epilepsy Foundation Website. Available at http://www. epilepsyfoundation.org/about/statistics.cfm (7 December, 2007) 2. M. Lassonde, H.C. Sauerwein, MĂŠdecine Sciences, November 2007, p. 923-928.

3. M. Pompili, P. Girardi, R. Tatarelli, Epilepsy & Behavior, November 2006, p. 641-648.

4. M. Touret, S. Parrot, L. Denoroy, M.-F. Belin, M. Didier-Bazes, Glutamatergic alterations in the cortex of genetic absence epilepsy rats. BioMed Central, November 2007.

5. S.-L. Liang, D.A. Coulter, Epilepsy-induced reduction in glutamate-glutamine cycle efficacy depletes vesicular release of GABA from hippocampal inhibitory synapses. Program No 164.6/ Z8. 2007 Neuroscience Meeting Planner. San Diego, CA: Society for Neurosience, 2007. CD-ROM 6. G. Seifert, K. Schilling, C. Steinhäuser, Nature, March 2006, p, 194-206.

7. G. Gras, F. Porcheray, B. Samah, C. Leone, Journal of Leukocyte Biology, November 2006, p. 1067-1075.

8. S.-L. Liang, G.C. Carlson, D.A. Coulter, Journal of Neuroscience, August 2006, p. 8537-8548.

9. T. Eid, Y. Wang, J.H. Heador-Woodruff, D.D. Spencer, N.C. De Lancerolle, R.E. McCullumsmith, Perturbation of glutamine synthetase and phosphate activated glutaminase in the hippocampus Spring 2008

39

biology

Iron Chelation as a Novel Therapy:

Treatment of Mucormycosis with Deferasirox in the Laboratory and in a Clinical Case tim shen â&#x20AC;&#x2DC;08

Background

M

ucormycosis is a fungal infection with an extremely high mortality rate (over 50%), with patients often succumbing despite the use of modern state-of-the-art treatments. For particular patient populations, including those with central nervous system involvement, mortality rises to 80%-100% (1). Fungi of the order Mucorales, ubiquitous in the environment, cause this disease. These fungi cause mucormycosis by invading the host organism primarily through inhalation of conidia, ingestion, or infection of open wounds (2). Rhizopus oryzae is the most common cause of mucormycosis (1). Mucormycosis generally infects immunocompromised individuals. Diabetic ketoacidosis and neutropenia are risk factors because these patients become immunocompromised as a result of their conditions (2).

It has previously been noted that many microbial pathogens require iron. In fact, infected mammals often use iron sequestration as a defense mechanism. It follows, therefore, that the use of iron chelating drugs

may be an effective treatment for many pathogens (1).

Mucormycosis has also been shown to require iron to grow in the host (3). However, this possible treatment has long been ignored because of conflicting data from previous trials with the iron chelating drug deferoxamine.

Deferoxamine could successfully sequester the iron available in the host serum, but the risk of developing mucormycosis for animals treated with deferoxamine would paradoxically increase. Eventually, it was found that Mucorales fungi were able to specifically bind deferoxamine and remove the sequestered iron.

Deferoxamine therefore facilitated the iron uptake of Mucorales (1).

The approval of the new oral iron chelating drug deferasirox by the United States Food and Drug Administration again brought forth the possibility of treating microbial pathogens, like those causing mucormycosis, with iron chelation. As a result, a collection of researchers at the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center, the UCLA David Geffen School of Medicine, Charles River

Image of inflamed heart valve tissue infected with Mucor pusillus, one of several fungi that can cause mucormycosis. The hyphae of the fungus are highlighted

by Methenamine silver stain.

Image courtesy of the Centers for Disease Control and Prevention and Dr. Libero Ajello.

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Dartmouth Undergraduate Journal of Science

Laboratories, and the National Center for Agricultural Utilization Research conducted research and produced a paper detailing the effectiveness of deferasirox on mucormycosis in murine animal models. Consideration of this paper, “The iron chelator deferasirox protects mice from mucormycosis through iron starvation” (1) along with a clinical case, “Deferasirox, an ironchelating agent, as salvage therapy for rhinocerebral mucormycosis” (3) reveals the promising possibility of the future use of iron chelation as a novel treatment for mucormycosis.

Results

The initial step in the investigation was to confirm deferasirox’s iron chelation ability. One R. oryzae gene, rFTR1, was known to show upregulated expression in response to iron starvation. The authors therefore decided to use rFTR1 to indicate a state of iron starvation. R. oryzae strain 99-880, a clinical brain isolate, was tested in three different environments: “iron replete”, “iron depleted”, and a control. “Iron replete” was a growth medium with additional 350 μM ferric chloride, whereas “iron depleted” added 350 μM ferric chloride and 2 mM deferasirox to sequester the iron, and the control added 2 mM deferasirox with 6 mM ferric chloride to render the deferasirox ineffective. RT-PCR amplification of the extracted RNA showed rFTR1 expression only in the “iron depleted” scenario. 18S rDNA was used to provide a baseline expression level for comparison and to show that there was no genomic DNA contamination (1).

Two DNA vectors were constructed for the purpose of visualizing rFTR1 expression in vivo. The GFP gene was placed behind either the rFTR1 promoter or the actin gene ACT1 promoter. Both versions included pyrF, required for uracil synthesis, and were placed in the M16 strain, which was mutagenized from 99-880 to be pyrF-null. This allowed the use of a selection system to create a population of M16 cells that would express GFP in response to the upregulation of rFTR1 or ACT1 expression. These cells were grown in the same control, “iron replete” and “iron deficient” media previously used.

The M16 cells with rFTR1 promoter vectors showed GFP expression only in the “iron deficient” scenario, whereas those with the ACT1 promoter vectors showed GFP expression regardless of the scenario, confirming the effectiveness of the vectors. Flow cytometry was then used to confirm and quantify the GFP fluorescence of the M16 cell cultures (1).

A quick test was then performed on 29 Mucorales organisms to determine susceptibility to deferasirox.

Deferasirox was determined to be fungicidal against nearly all the tested organisms (the only exception was R. oryzae type I NRRL13440). Deferasirox was considered Spring 2008

fungicidal if the minimum fungicidal concentration was at most 4 times the minimum inhibitory concentration required to inhibit 90% of the isolates. Even at low deferasirox concentrations, fungicidal effects were noted within 24 hours, suggesting a time dependent effect rather than a concentration dependent effect. A short test was then performed by growing R. oryzae, adding deferasirox, and then adding iron to half of the samples. The iron addition reversed the deferasirox’s cidal effects, and even supported growth, but this data was not included in the paper (1).

Having demonstrated the ability of deferasirox to induce iron starvation to a fungicidal point, the next step was to demonstrate deferasirox’s effects in vivo with a murine model. Diabetic ketoacidosis was induced in BALB/c male mice with 210 mg/kg streptozotocin to destroy the pancreas’s insulin producing β cells. R. oryzae strain 99-892, a clinical pulmonary isolate, was used to simulate disseminated infection by injection of 2.2 x 104 spores into the tail vein. The experimental mouse cohorts were treated with 1, 3, or 10 mg/kg of deferasirox. Control mice were treated with a placebo, deferasirox with excess ferric chloride, or ferric chloride alone, but this data was not shown. Deferasirox was shown to improve survival markedly, with some suggestion of a minor dose dependent response, but this observation was not followed up on. More mice were tested and colony forming units were counted in the kidneys and brain, and deferasirox was shown to also significantly reduce the fungal burden. More mice were infected with 4.2 x 104 spores of 99-892. Histology with hemotoxylin and eosin staining showed that hyphae failed to develop in samples from mice that underwent deferasirox treatment. Improved survival with deferasirox treatment also resulted when testing was performed with 1.3 x 103 spores of the more virulent 99-880 strain (1).

To simulate the usual pulmonary route of infection, diabetic ketoacidotic mice were infected intranasally with 107 spores of 99-880 to deliver a median 6.5 x 104 spores to the lungs. Mice were treated with a placebo, deferasirox, or deferoxamine. Deferoxamine was shown to worsen survival rates compared to the placebo, whereas deferasirox improved survival rates (1).

The authors then surmised from their observations that excess iron could directly suppress the host inflammatory response. Helper T cells Th1 and Th2 levels as well as levels of inflammatory cytokines TNF and IFN-γ were tested in diabetic ketoacidotic mice infected with 3.1 x 104 spores of 99-892. Treatment with deferasirox resulted in a rise in splenic Th1 and Th2 levels over levels observed in mice treated with either placebo or deferasirox and excess iron. The deferasirox regimen also resulted in mice with significantly increased 41

TNF and IFN-γ levels in the spleen, as well as increased IFN-γ levels in the kidney. No effects were noted on Treg cells or on lymphocyte apoptosis frequency (1).

Since one current front-line treatment for mucormycosis is amphotericin B, deferasirox was tested alongside amphotericin B treatment to determine the efficacy of this combination. Again, diabetic ketoacidotic mice were infected with 99-880 through the tail vein.

The mice were treated with either a placebo, liposomal amphotericin B (LAmB), deferasirox, or both LAmB and deferasirox. This series of experiments showed a surprising synergistic effect in the combination of LAmB and deferasirox that drastically improved survival rates even over the improved survival rates of either LAmB or deferasirox alone. The tissue burden in both the brain and the kidneys were also drastically reduced by the combination of LAmB and deferasirox, whereas LAmB was able to significantly reduce kidney tissue burden only (1).

Finally, the effect of deferasirox in a neutropenic murine model was tested. BALB/c mice were “myeloablated with cyclophosphamide” (1) at a dosage of 200 mg/kg, removing the ability to produce immune cells like neutrophils. These mice were infected with 2.7 x 103 spores of 99-892. Oddly, this study showed that treatment every other day with deferasirox was much more effective than daily treatment. Theorizing that a toxicity effect may be the root of this phenomenon, the authors performed toxicity tests, but found no evidence of deferasirox toxicity (1).

The clinical study involved a 40 year old male presenting with diabetic ketoacidosis, pain in the left retrobulbar area, and palsy in the left cranial nerve VI (3). The patient was therefore at extreme risk with diabetic ketoacidosis as well as central nervous system involvement. Developments in the left eye included paralysis of the extraocular muscles responsible for eye movement, and increasing pressure causing the eye to protrude outward (3), known as “proptosis” (4). The patient began receiving amphotericin B. A few weeks later, the patient received surgery to remove the contents of the left orbital area, which revealed an extensive ischemic area with much necrosis in the extraorbital muscles. LAmB treatment quickly followed, along with caspofungin. A postoperative MRI showed heavy clotting in the left cavernous sinus (3), common for rhinocerebral mucormycosis infections (4).

Follow up MRIs revealed continuing mucormycosis progression to involve cranial nerve V and the development of a new brain lesion in the pons and left cerebellum. Renal insufficiency developed, requiring a dilution of LAmB dosage. Meanwhile, subsequent MRIs exposed the continued growth of the mucormycosis infection. Salvage therapy was attempted 42

with a seven day course of deferasirox (3). At this point, the synergistic relationship of LAmB and deferasirox must have come into play. The patient improved rapidly, and a final MRI showed significant improvement to the point where LAmB dosing was discontinued after the deferasirox therapy was completed. Four months later, the patient was asymptomatic, with no worsening neurological effects, and stabilization with no apparent changes noticed in subsequent MRIs (3).

Discussion

The paper and clinical case in conjunction provide evidence that the iron chelating drug deferasirox can successfully sequester iron and starve R. oryzae to treat mucormycosis. The laboratory testing first confirmed deferasirox’s ability to sequester iron and stress R. oryzae through iron starvation. Next, deferasirox was confirmed to have fungicidal activity against various Mucorales fungi, and it was demonstrated that this activity was based on iron starvation. Mice with diabetic ketoacidosis were tested as a model for humans with the same risk factor. Deferasirox was shown to be able to reduce fungal burden, reduce R. oryzae hyphae formation in tissue, and improve survival for infected mice. Improved survival was shown for both disseminated infection and the more common intranasal infection. The paper then revealed conflicting data regarding iron availability and the immune response. Deferasirox iron chelation raised Th1 and Th2 levels as well as TNF and IFN- γ levels in the spleen. Deferasirox also raised IFN-γ in the kidneys. Finally, deferasirox showed a synergistic effect with LAmB in the treatment of diabetic ketoacidotic mice. Deferasirox in combination with LAmB also more successfully reduced fungal burden in the brain and kidneys. Finally, deferasirox was shown effectively increase survival rates of neutropenic mice, although not as effectively as with diabetic ketoacidotic mice, and optimally with treatment every other day. Treatment in combination with LAmB was not tested (1). It appears clear that at least diabetic ketoacidotic patients with mucormycosis infection by R. oryzae should be able to be treated with some success with deferasirox.

It would have been interesting to investigate the dose response of deferasirox treatment on R. oryzae strains 99-892 and 99-880 to determine why a dose response trend in survival was noted when testing diabetic ketoacidotic mice infected with 99-892. The comparison of the two strains in a dose response test may have revealed some interesting responses between the more virulent strain and 99-892. Further testing with similarly infected diabetic ketoacidotic mice could also have helped determine an optimal treatment concentration for 99-892.

Dartmouth Undergraduate Journal of Science

Ball-and-stick model of two molecules of the iron-chelating drug deferasirox binding an atom of iron. Iron chelated in such a manner is unavailable to the fungi that cause mucormycosis.

Image retrieved from http://en.wikipedia.org/ wiki/Image:Deferasirox%E2%80%93iron%28II I%29_complex.png (5 April 2008)

The clinical case study provided anecdotal evidence that deferasirox can successfully act as salvage therapy for mucormycosis. Since the patient embodied many of the characteristics of mucormycosis sufferers, including diabetic ketoacidosis, central nervous system and cranial nerve involvement, and facial sinus infection extending into the brain, this case is a promising example (2, 3). Further study is clearly needed before deferasirox can be widely employed against microbial infections, but this is a positive first step.

Obvious future directions for further research are brought to mind by this research. Due to the sudden increase in the amount of data withheld during the immune response testing, it appears that the authors are already considering further research into effects of the iron chelator on the host immune response. This testing should aim to explain the elevated TNF and IFN-γ levels when deferasirox was added, as well as the increased Th1 and Th2 splenocyte levels and the unexplained Th1 cytokine and Th2 cytokine levels (1). The authors suggest that the effect of iron on host immune response could vary from host to host. This assertion was based on conflicting findings between their own data and other studies, sometimes involving iron chelators other than deferasirox (1). Due to the involvement of other drugs, the investigation of this matter may need to become more complex. However, it is still important for this matter to be pursued further in order to concretely establish iron’s effects on the host immune response.

The ultimate goal of future research should be to better establish the parameters for clinical use of deferasirox against mucormycosis. Perhaps if this future research determines that iron can directly suppress the host’s inflammatory response, then clinical treatment with deferasirox would increase the inflammation in response to mucormycosis while helping to clear the infection simultaneously. This combined effect may be more successful at fighting mucormycosis than either alone. On the other hand, if iron has the opposite effect on the immune response, then deferasirox treatment would Spring 2008

relieve inflammation while killing the fungal infection.

This relief may reduce the body’s ability to deal with the mucormycosis, and therefore more deferasirox may be necessary to fully clear the infection. Either way, detailed knowledge of the effects of deferasirox treatment will be important for clinical use. Additionally, it is important that the optimal treatment regimen be determined, whether that is the daily treatment with deferasirox, treatment every other day with deferasirox, treatment in conjunction with LAmB, or some other combination of factors. Furthermore, as the authors suggested, better toxicity data for deferasirox should be determined, since the effect of toxicity in clinical deferasirox applications at this time is still unclear (1).

Lastly, since iron chelation holds such promise for widespread application across many different microbial infections, the authors suggest future research into the applicability of iron chelators, and deferasirox in particular, as a future therapy against other important pathogens. The availability of an iron supply is important to many microbial pathogens, and if iron chelation and deferasirox treatment can be found to reduce the iron supply and starve microbes, iron chelation as a clinical therapy may find applications across many fields (1).

References 1. A. S. Ibraham, et al., The Journal of Clinical Investigation 117(9), 2649 (2007).

2. N. F. Crum-Cianflone and D. Eisen, Mucormycosis (2006).

Available at http://www.emedicine.com/MED/topic1513.htm (04 March 2008).

3. C. Reed, et al., Antimicrobial Agents and Chemotherapy 50(11), 3968 (2006).

4. D. S. Smith, Mucormycosis (2006). Available at http://www.nlm. nih.gov/medlineplus/ency/article/000649.htm (04 March 2008).

43

physics

Type Ia Supernovae:

Properties, Models, and Theories of Their Progenitor Systems Qinggong wu ‘09

Introduction

S

upernovae are magnificent phenomena in the night sky, and have always been a marvel to human beings. A supernova is a stellar explosion that emits a burst of radiation resulting in an extremely luminous object that may outshine its entire host galaxy before fading from view over several weeks or months.

One class of supernovae, known as Type Ia Supernovae (SNe Ia), is characterized by the absence of hydrogen emission lines in spectra and the presence of a prominent silicon Si II absorption line near maximum light (1).

With uniform light curves and spectral evolution, SNe Ia have been increasingly used as reliable indicators of distance in measuring important cosmological constants (2). This usage has led to a need for a more intensive study of the nature of SNe Ia. A SN Ia explosion is often the result of thermonuclear disruption of a carbon-oxygen white dwarf that accretes mass from its companion in a binary system, and thereby reaches the Chandrasekhar limit of 1.4 M⊙ (solar mass) (3). However, there is no simple means of identifying the immediate progenitor of a SNe Ia, nor of deriving information about its properties from observations (1). One way to determine the progenitors of SNe Ia is to eliminate the unlikely candidates from a pool of possible systems if they show any significant contradiction with the physical principles or observational data. Since there is not yet a best candidate whose properties agree with all the theoretical or observational criteria, identification of progenitor systems of SNe Ia remains difficult.

Properties of SNe Ia Progenitor Systems

The spectroscopic properties of SNe Ia give some indication of the composition of their progenitors.

The absence of hydrogen emission lines indicates that the star contains little (less than 0.1 M⊙) to no hydrogen before explosion; the presence of a silicon Si II absorption line near maximum light suggests that nuclear fusion from pre-explosion matter into intermediate-mass elements like silicon takes place in the explosion (1).

The observed velocity (mean v = 5000 km/s and peak v > 20000 km/s) of SNe Ia explosion ejecta agrees with the calculation result of about 1 M⊙ of C and O fusing into iron-group elements or intermediate-mass elements.

44

This fact implies that the progenitor star is composed of mostly carbon and oxygen (1).

According to observational data, most SNe Ia share very similar peak luminosities, light curves, and spectra. This strongly indicates that a unique class of progenitor systems exists. Upon closer study of these properties, Chandrasekhar-mass (1.4 M⊙) white dwarfs are suggested to be the best-fitting model (1). Since 85% of observed white dwarfs have masses of no more than 0.8 M⊙ and large-mass white dwarfs are extremely rare, the only way for a SN Ia progenitor white dwarf to reach the Chandrasekhar limit is to be in a close binary system where it can accrete mass from the companion star (3).

The results of a radio observation program lasting over two decades at the Very Large Array, a radio observatory located in New Mexico, USA, imply a very low density for any possible circumstellar material established by the progenitor before explosion.

This conclusion would rule out the possibility of white dwarf mass-accretion via stellar wind from a massive binary companion. Hence, the progenitor system could only be a white dwarf that accretes mass from a low mass companion by Roche lobe flow due to gravity, as suggested in single-degenerate models, or the merger of two white dwarfs, as suggested in double-degenerate models (4).

The Origin of Diversity of SNe Ia Luminosity

The above discussions all point to the currently favored model for SNe Ia progenitors: a relatively homogeneous class of C+O white dwarfs accreting mass from their companions in binary systems. However, SNe Ia also have many observed differences, among which the most important is the diversity of luminosity.

Since SNe Ia are used as standard distance indicators in cosmology, this diversity and its origin requires an answer. Below are possible explanations based on various explosion models.

I. C/O ratio of white dwarf progenitors: The brightness of a SN Ia is determined by the mass of 56Ni synthesized during the explosion, which ranges from 0.4 – 0.8 M⊙ for most SNe Ia (5).

It is postulated that as the C/O ratio increases in the Dartmouth Undergraduate Journal of Science

Artistâ&#x20AC;&#x2122;s rendition of a white dwarf accumulating mass from a nearby companion star. This type of progenitor system would be considered singly-degenerate.

Image courtesy of David A. Hardy, Š David A. Hardy/www.astroart.org

progenitors, the mass of 56Ni will increase, and this consequently causes a greater luminosity (2).

systems. They can host luminous SNe Ia as well as dim ones (2).

II. The age of progenitor systems: As suggested by Nomoto et al (2003), in an older binary system, the mass of the companion star of the white dwarf is smaller, and the mass which can be transferred from the companion to the white dwarf is smaller. This implies that the original total mass of carbon and oxygen of the white dwarf is larger as the white dwarf reaches Chandrasekhar mass. By calculation, the explosion of a larger portion of carbon and oxygen will lead to smaller luminosity. Therefore older progenitor systems produce dimmer SNe Ia (2).

Models of Pre-Supernova Evolution

III. Morphology of the Host Galaxy: It is observed that the most luminous SNe Ia occur only in spiral galaxies. Both spiral and elliptical galaxies can have dimmer SNe Ia (6). This may due to the different ages of the companion stars. As suggested above, SNe Ia that occur in older progenitor systems have smaller luminosities. In elliptical galaxies, star formation has long since ceased, and most of the progenitor systems are too old to produce very luminous SNe Ia. However, in spiral galaxies, star formation continues to occur, and so these galaxies can have both old and young progenitor Spring 2008

Two ways by which white dwarfs in binary systems can accrete mass toward Chandrasekhar mass and cause SNe Ia have been proposed: single-degenerate and double-degenerate. Models for both scenarios have some elements that explain the observed data, and some that do not.

Double-degenerate models I. Mechanism: Two C+O white dwarfs in a close binary system are brought together by the emission of gravitational radiation. When the lighter white dwarf with the larger radius fills its Roche lobe, the Roche lobe is dissipated within a few orbital periods and forms a massive and hot disk configuration around the heavier white dwarf.

Then the two merge into one, reaching Chandrasekhar mass and giving rise to SN Ia explosion (7).

II. The Weaknesses of Double-Degenerate Models: (i) When the lighter white dwarf forms a disk configuration around the primary white dwarf, the 45

disk is rotationally supported, and so carbon ignition cannot happen immediately. The most likely result of this scenario is off-center carbon ignition if the massaccretion rate is higher than 2.7×10-6 M⊙/year. This reaction will convert the composition of the white dwarf from C-O to O-Ne-Mg. The consequence, however, is more likely to be an accretion-induced collapse to a neutron star rather than a SN Ia explosion (7). (ii) Galactic chemical evolution results do not agree with the double-degenerate models. In particular, Kobayashi et al. (1998) performed the chemical evolution calculations for both double-degenerate and single-degenerate models and argued that the early heavy element production of double-degenerate models, which is formulated as O/Fe as a function of Fe/H, is inconsistent with the observations (7,8). (iii) The observed SNe Ia have a similar amount of 56Ni as a production of explosion. The merging of two white dwarfs of different mass, composition, and angular momentum with different impact parameters will lead to very different burning conditions with a different amount of 56Ni produced, which disagrees with the observations (1).

III. The Strengths of Double-Degenerate Models: (i) The absence of hydrogen lines in SNe Ia spectra can be naturally explained by double-degenerate models since only C+O white dwarfs with little or no hydrogen are involved in this scenario (1). (ii) Merging white dwarfs can reach Chandrasekhar mass easily, while in single-degenerate models, achieving a sufficient mass-accretion rate is a major difficulty. (iii) Many binary systems with two white dwarfs are identified. Among the eight known systems with orbital periods of less than half a day, there is one system [KPD 0422+5421 (9)] whose mass could exceed Chandrasekhar mass. Population synthesis predicts that there could be more sufficiently massive merger candidates found in short-period white dwarf binary systems (1).

Single-degenerate models I. Mechanism: A C+O low mass white dwarf in a binary system accumulates hydrogen-rich or helium-rich matter from the companion star by mass overflow, reaches a critical mass near the Chandrasekhar mass and explodes due to thermonuclear disruption. Another model, known as the sub-Chandrasekhar model, suggests an alternative road of evolution: before the white dwarf reaches a critical mass limit, a layer of helium forms on top of C+O core and ignites the C+O fuel (1).

46

II. The Weaknesses of Single-Degenerate Models: (i) According to single-degenerate models, since a great portion of the mass that the white dwarf accumulates is hydrogen, hydrogen lines should be seen in the spectra of SNe Ia. However, hydrogen has not yet been found in any SNe Ia. The failure to detect hydrogen in SNe Ia is a factor that may rule out single-degenerates as appropriate candidates for SNe Ia progenitor systems (8). (ii) Theoretically, few mass-accretion rates can lead to thermonuclear explosion. For low accretion rates below 10-8 M⊙/year, repeated nova outbursts will occur before the white dwarf reaches Chandrasekhar mass, and in these eruptions, more mass will be lost than accreted between eruptions. On this track the white dwarf will never reach Chandrasekhar mass (8). For higher rates (10-8 – a few ×10-7 M⊙/yr), the white dwarf will lose mass due to helium shell flashes (1). For even higher rates of accretion above a few ×10-7 M⊙/year, a hydrogen-rich red-giant envelope will form outside the white dwarf and mass will be lost due to winds.

Moreover, no observation has given evidence to the existence of the debris of this red-giant envelope in SNe Ia explosion (1).

III. The Strengths of Single Degenerate Models (i) A class of binary systems, namely the Supersoft X-ray Sources, has been identified. In this system hydrogen-rich matter is being transferred from the companion star at so a high a rate that hydrogen burns steadily outside the C+O core of the white dwarf (10). If the accreted mass can be retained, the mass of the white dwarf can actually increase toward Chandrasekhar mass.

These systems may serve as strong candidates for SNe Ia progenitors in the single-degenerate scenario (1). (ii) There are other good candidates that exist, such as symbiotic systems or recurrent novae (8).

Summary and Conclusions

Based on current knowledge of observational evidence and physical principles, it can be confidently concluded that the progenitors of SNe Ia are a homogeneous class of compact white dwarfs composed of carbon and oxygen that accrete mass from binary companion stars.

The luminosity of a SN Ia can offer some indications about its progenitor. Generally, the progenitor white dwarf of a brighter SN Ia has higher C/O ratio and a younger age, and appears in a spiral galaxy; the progenitor white dwarf of a dimmer SN Ia has lower C/O ratio and an older age, and appears in either a spiral or an elliptical galaxy.

Two kinds of models, double-degenerate and single-degenerate, are proposed to explain pre-supernova Dartmouth Undergraduate Journal of Science

evolution. As discussed, there are observational and theoretical arguments that support and contradict each. However, double-degenerate models have more significant conflicts with theories, and with the discovery of Supersoft X-ray Sources, single-degenerate models are favored today.

As new observation technologies in x-ray, radio, and high resolution optical spectroscopy are being developed, more information concerning the properties of SNe Ia progenitor systems will be obtained. In particular, an unambiguous choice between singledegenerate and double-degenerate models can be made if the absence or presence of hydrogen in SNe Ia is determined conclusively by observations.

References 1. W. Hillebrandt and J. Niemeyer, Annual Review of Astronomy and Astrophysics 38, 191 (2000).

2. K. Nomoto, et al., in From Twilight to Highlight: The Physics of Supernovae, W. Hillebrandt & B. Leibundgut, Eds., ESO/Springer Series “ESO Astrophysics Symposia” (Springer, Berlin, 2003).

3. M. Partharsarathy, D. Branch, D. Jeffery, & E. Baron, New Astronomy Reviews 51, 524 (2007).

4. N. Panagia, et al., in American Institute of Physics Conference Proceedings, Cefalu’, Italy, 11-24 June 2006, (American Institute of Physics, Melville, NY, 2006).

5. P. Mazzali and L. Lucy, Monthly Notice of the Royal Astronomical Society, 295, 428 (1998).

6. M. Hamuy, M. M. Phillips, R. Schommer, and N. B. Suntzeff, The Astronomical Journal, 112, 2391 (1996).

7. C. Kobayashi, T. Tsujimato, K. Nomoto, I. Hachisu, and M. Kato, Astrophysical Journal, 503, L155 (1998).

8. M. Livio, Type Ia Supernovae: Theory and Cosmology, J. Niemeyer, & J. Truran, Ed. (Cambridge Univ. Press, Cambridge, 1999).

9. C. Koen, J. Orosz, and R. Wade, Monthly Notice of the Royal Astronomical Society, 200, 695 (1998).

10. P. Kahabka, E. Van Den Heuvel, Astronomy and Astrophysics, 35, 69 (1997).

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Spring 2008

47

ecology

Army Ant Emigration:

Diel Emigration and Foraging Behaviors of the Army Ant Eciton hamatum (Subfamily Ecitoninae) SAM J. HAYNOR ‘08, CHAD S. GORBATKIN ‘08, SARAH C. ISBEY ‘08, AND ZACHARY A. MAYER ‘08

A

s predators of many social insects and larger facultative associate species, including 50 neotropical arthropods, army ants are a key component of bird species, phorid flies, parasitic beetles, and mites tropical forest ecosystems. Specifically, their (3,4). foraging behaviors and food preferences can affect Recent genetic and fossil evidence suggest forest invertebrate abundance and composition. To better that the 298 ant species with ‘army ant adaptive understand army ant behavior, we observed foraging syndrome’ (i.e. obligate social foraging, nomadism, and and movement patterns dichthadiigyne queens of Eciton hamatum for (blind queens with large a continuous 20 hour gasters and ovaries)) period. We measured evolved from a common movement rates in subterranean ancestor in and out of the bivouac the subfamily Dorylinae and mapped changes (1,5). Only true army in foraging column ants display all three and bivouac locations. characteristics of army We also characterized ant adaptive syndrome, substrate use by foraging although some non-army columns. The number of ant species display two columns and movement of the three. Many army rates varied with foraging ant genera have remained status (e.g. active foraging partially or completely or zero foraging). E. subterranean, but some hamatum moved only in exhibit derived epigaeic dense columns (~4 ants (above-ground) foraging wide), showing no swarm behaviors.

Foraging foraging behavior. E. strategies differ among Front view of the long sickle-shaped mandibles of an Eciton hamatum major hamatum workers foraged lineages, ranging from worker. during the morning hours swarm feeding to the and only on larvae of social insects, including wasps, specialized raiding of termite, ant, or wasp colonies in ants, and termites. From the evening into the night, the columns (6,7,8). Many species also exhibit diel and/or ants spent seven hours (1600-2300) emigrating from seasonal variation in prey selection, raid formations, and their old bivouac to their new bivouac approximately 90 emigration cycles (9). meters away (column distance). Army ants were more We studied the foraging and emigration commonly found traveling on branches than on leaves behavior of Eciton hamatum (Subfamily Ecitoninae), a or bare soil. E. hamatum may have considerable effects species of epigaeically and arboreally foraging army ant on the reproductive success and colony structures of in the tropical moist forests of Corcovado, Costa Rica. their primary prey sources (social insects).

We aimed to better understand the ecological roles of E. hamatum as arthropod predators by measuring the ant’s diel foraging behavior, movement patterns, and Introduction interactions with other species. We hypothesized that In many tropical and subtropical forests, army the colony would have periods of maximum biotic ants are important predators of social insects, large interaction (e.g. intense foraging) and minimum biotic arthropods, and small vertebrates (1,2). They can interactions (e.g. retraction of all ant columns to the influence tropical biodiversity by two mechanisms: bivouac). Based on previous field observations of Eciton (1) migrating army ant colonies can alter arthropod burchelli emigration at Corcovado on 3 February 2007, community composition and abundance, and (2) army we predicted that maximum foraging would occur during ants can create niches for approximately 200 obligate and Image courtesy of J.T. Longino of The Evergreen State College

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Dartmouth Undergraduate Journal of Science

Figure 1: Number of E. hamatum ants entering and exiting the old and new bivouacs per minute (averaged across five 10-second observation periods at each halfhour, except during periods of high activity, during which there was no replication). Immediately before bivouac emigration, the number of ants entering the old bivouac greatly increased. The increase in activity between hours 16 (0345) and 20 (0630) represents foraging from the new bivouac after it was established.

daylight while bivouac emigration and rest would occur at night.

Methods

One army ant colony was observed continuously from 1045 on 4 February to 0630 on 5 February in Corcovado National Park, on the Osa Peninsula of Costa Rica. As soon as the colony was located on 4 February, the rates of movement of ants both in and out of the bivouac were recorded, and all columns leaving the bivouac were mapped. Natural history data (personal observations and notes about the surroundings and ant activity) were collected throughout the day and night.

Every half hour, five categories of ant movement rates (number of ants per minute in and out of the bivouac) were recorded. During periods of highly fluctuating movement (e.g. during bivouac dismantling and relocation), movement rates were recorded continuously. When the bivouac contained more than one column, the rates were recorded for each column.

Ant movement rates were estimated by counting the number of ants crossing a designated point on the substrate in a ten second interval. Specifically, for each 10-second count, the number of ants moving into the bivouac, moving out of the bivouac, carrying larvae, and carrying larval prey were recorded. In all cases but those in which ant activity was changing at an interval of 10 seconds or less (high activity fluctuation), movement rates for each of the four movement types were recorded for five replicate 10-second sampling intervals per halfhour time period. In times of high activity fluctuation, rate samples were not replicated because ant movement changed rapidly between replicates. The only prey items the ants were observed to carry were larvae of other Spring 2008

social insects (i.e. wasps, termites, and ants), which could be differentiated from their own larvae based on size and direction of movement (i.e. ants moved prey into the bivouac, while they only moved their own larvae out of the bivouac).

Bivouac emigration occurred from 1600-2300 on 4 February 2007. The columns of the colony were mapped every 2.5 hours during the study, except during bivouac dormancy from 2215 on 4 February to 0345 on 5 February. The lengths of all ant columns (main foraging columns plus side branches) were measured. For each distinct column segment, four items were recorded: the substrate type (i.e. sticks/ twigs, bare soil, or leaves), the total number of ants carrying food sources, the number of ant body-bridges, and the length of the column segment. For each of the six replicate mapping periods, the proportion of each substrate type used was measured, summing across all columns and column branches sampled. A bridge was noted any time one or more ants served as a connector between two objects on the path; army ants link together to form these bridges, which expedite movement by other individuals in the colony.

To characterize the diel patterns of ant movements into and out of the bivouacs, average movement rates (with and without larvae, in and out of the nest) were plotted over the entire 20-hour period (averages of five replicate 10-second counts per movement type in each 30-minute time interval and single rates from periods of rapid change in movement rates). To calculate the rate of ants moving into the new bivouac, data from the ants moving out of the old bivouac were shifted forward three hours, the lag time between departure from old bivouac and arrival at new bivouac. Here it was assumed that all ant velocities were equal to the velocities of the final departing ants (used for lag time calculations). The 49

Figure 2: Eciton hamatum ant activity (sum of ingoing and outgoing ants) in all columns and total number of columns emanating from the bivouac over a 20-hour period.

Hours 0-12 represent the old bivouac and hours 12-20 represent the new bivouac.

In the morning of 4 February 2007, the number of columns declined as the foraging columns were eliminated, and activity focused on a single bivouac emigration column. After the number of columns dropped to one, bivouac emigration intensified and ant activity spiked. After the bivouac emigrated, the colony was dormant for several hours. At 0345, multiple foraging columns formed and ant activity increased.

mean percent of total column length of different substrate types used by the ants was compared with a one-way ANOVA (n = 6 mappings of the entire colony).

Results

Diel Patterns During the 20-hour observation period, the army ants were observed both foraging and moving their bivouac. When we found the army ant colony at 1045 on 4 February 2007, they had an established bivouac and three foraging columns. At 1300, the ants from the foraging columns began to return to the nest, and at 1600, activity of one column greatly increased (Fig. 1): at this point, the ants began to carry their larvae towards the new bivouac along this column (Fig. 2). A new bivouac was discovered at the end of this column, and, as the movement of larvae towards the new bivouac increased, the number of foraging ants returning with prey items to the old bivouac decreased (Fig. 2). All other columns disappeared by the time activity in the column moving towards the new bivouac increased greatly

(Fig. 3). There was a time lag of approximately three hours between complete desertion of the old bivouac (1937 hours) and arrival of the entire column of ants at the new bivouac (2216 hours; see Fig. 1). After the new bivouac was established, activity ended for approximately 5.5 hours until 0345 on 5 February, when the ants established three new foraging columns (Fig. 1).

Foraging Columns Throughout the six replicate mapping periods, a total of 1159 meters of ant columns on three different substrates were surveyed. The ants did not use substrate types equally (ANOVA, F(2,15) = 17.00, P =0.0001, Fig. 4). Throughout the entire 20-hour sampling period, branches and twigs comprised 65% of total column length, leaf litter comprised 32%, and bare soil comprised 3% (mean values).

Early morning ant foraging activity was much lower than ant activity during bivouac emigration (Fig.

Figure 3: Number of E. hamatum ants moving into the bivouac with prey and out of the bivouac with larvae throughout the observation period. The large spike in larvae movement corresponds with bivouac emigration. Immediately before bivouac emigration, the number of ants entering with prey decreased greatly. Increases at hour 18 (0430) and 19 (0520) represent the reestablishment of foraging.

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Dartmouth Undergraduate Journal of Science

Figure 4: Mean total column length of E. hamatum ant foragers on each of three substrate types (branches, leaf litter and bare soil) in six replicate mappings, evenly spaced at 2.5 hour intervals over the 20-hour study period, excluding the bivouac dormancy period. There was little dirt available for ant travel. Error bars represent + 1 SE.

1). The maximum number of columns (13) was recorded when foraging recommenced at the new bivouac (Fig. 2). These columns extended as far as 200 meters away from the bivouac and typically ended with raids of arboreal insect nests (Fig. 3). Calculations showed that the longest foraging columns would require a threehour round trip from the bivouac to their prey location (0.0025 hour/m x 400 meters of column). Finally, it was observed that E. hamatum would not consume live or dead adult arthropods placed in their foraging columns, and the ants would notice and collect prey larvae only if larvae were placed within seven centimeters or less of the foraging column.

Bivouac movement Bivouac emigration took place over seven hours, followed by a 5.5 hour dormancy period after the entire colony had reached their final destination (Fig. 1).

The ants moved their bivouac to a fallen log, 90 meters along a foraging column from the original bivouac location (71 meters absolute distance). Prior to bivouac emigration, this column had reached a length of 140 meters; it then retracted in the early afternoon before bivouac emigration. After soldier ants near the bivouac were observed to be erasing foraging paths using anallytransmitted pheromones, only a single column remained from the old bivouac to the new bivouac site (Fig. 2).

Flanked by soldiers on the sides of the column, ants transported larvae in a narrow (3-4 cm wide) column Spring 2008

through the forest floor at sunset. During the sevenhour emigration period (including preparation time), no foraging activity was observed. In total, the E. hamatum colony did not prey upon any insects in the trees or in the leaf litter for almost 12 hours, from 1600 on 4 February to 0345 on 5 February.

Discussion

Army ant foraging strategies and their subsequent ecological effects can vary both by species and by season (8). While approximately seven-weeklong cycles of either bivouac dormancy or nomadism have been recorded in many army ant species (3), we found evidence for shorter diel behavioral cycles in E. hamatum. As predicted, peak foraging occurred in the early morning while bivouac movement occurred in the evening and early night. These diel foraging and movement patterns may reduce parasitism rates by diurnal phorid flies, the primary parasites of many army ant species. In addition to their direct lethal effects (i.e. fatal oviposition into head, thorax, or abdomen), some phorid flies elicit alarm pheromones from both sedentary and nomadic ants, which greatly decrease foraging and movement efficiency (10). Because E. hamatum bivouac emigration occurs in a single column, alarm pheromone release could devastate emigration.

Additionally, larvae and the queenâ&#x20AC;&#x201D;the individuals directly linked to colony fitnessâ&#x20AC;&#x201D;would be exposed to parasitism if the bivouac emigration occurred during the day. Experimental manipulations of phorid fly density during different foraging and emigration stages would help to isolate the effects of phorid flies on E. hamatum diel behavior patterns.

Foraging Columns Army ants are obligate social foragers, requiring pheromones to maintain colony-scale efficiency.

Without pheromone trails, ants that were experimentally displaced rarely found their way back to foraging columns, and often did not even return to the bivouac.

Pheromones may also be necessary to recruit enough ants to overcome adult prey arthropods that defend their own colonyâ&#x20AC;&#x2122;s larvae.

Army ants are also selective foragers, in contrast to more generalized swarming army ant species (e.g. Eciton burchellii), which entirely clear leaf litter of adult arthropods and even small lizards (2). The distinct foraging columns observed are likely to affect the terrestrial invertebrate community differently than mass foraging swarms characteristic of some other army ant species, such as the well-studied E. burchellii. Columnar foraging is extremely selective, as foragers ignore nearby food sources to raid large larval caches farther away. Therefore, instead of creating large changes in 51

Side view of an E. hamatum worker. Observe the extended, spider-like legs of these army ants.

Mandibles are used in the transport of colony larvae as well as defensive and predatory devices.

Image courtesy of K.T. Ryder Wilkie of Boston University

invertebrate community composition and abundance, E. hamatum reduces reproductive success of a few social invertebrate species.

Even though the forest floor we studied consisted mostly of leafy material (personal observations), E. hamatum foraging columns primarily moved on woody substrates (Fig. 4). Therefore, they are spatially removed from most of the terrestrial invertebrates that live only in the leaf litter, such as the prey harvested by swarming army ant species. Because E. hamatum forages in columns on woody substrates without flushing out leaf-litter arthropods, they do not create niches for antfollowing birds which feed on flushed-out arthropods (3).

Bivouac Movement Foraging and bivouac emigration periods occurred at different times of day, and the transition between the two was documented. Because foraging occurred in columns, retraction of the dayâ&#x20AC;&#x2122;s raid was rapid, at a rate of up to two meters per minute. Immediately before the colony moved their bivouac, workers formed bridges with their bodies to span gaps in the leaf litter. In foraging columns, bridging behavior was only observed when movement rates were high. The occurrence of bridges in high-traffic columns suggests that bridging is important for efficiently transporting large numbers of ants across broken terrain. The army ant body-bridges observed were composed of as many as 90 bodies 52

per bridge, and bridges sometimes occurred in densities greater than 10 per meter of column. All bivouac emigration was concentrated in a single fourcentimeter-wide column with many ant body bridges.

Therefore, larger colonies may spend a greater proportion of their time transporting their bivouac, which could cut into overall foraging time.

Single column emigration may limit E. hamatum colony size. Further research should investigate emigration patterns of different colony sizes and how this relates to the colony energy budget.

Ecological Impacts Army ants can influence tropical forest biological diversity through two mechanisms: predation leading to changes in arthropod community composition and abundance (3,11) and facilitation of obligate and facultative associates (4). In addition, the selective larval predation exhibited by E. hamatum may impact life history strategies in their arboreal social insect prey. When an E. hamatum worker successfully communicates the location of a potential food source to the rest of the colony, forager recruits proceed to remove larvae from that source until it is depleted. Therefore, column army ant predation may select small, wellhidden colonies as its prey (e.g. wasps), while having little effect on litter-dwelling arthropods. Thus, columnforaging species of army ants may have concentrated, species-specific effects on invertebrate communities.

In contrast, a mass swarming foraging strategy might create changes in invertebrate communities that affect more invertebrate individuals and species, which can subsequently influence invertebrate competition and diversity in tropical ecosystems (3).

Dartmouth Undergraduate Journal of Science

Compared to swarming foragers such as E. burchelli, the intensity of E. hamatum foraging is mitigated by both its propensity for selective foraging as well as the time required for bivouac emigration in columns. However, the effects of E. hamatum on its individual prey species are still likely very strong, removing all or the majority of larvae from a given nest (personal observation).

Social insects, such as those targeted by E. hamatum, are ecologically important species that make up a large portion of forest biomass and perform essential ecosystem functions (12). By decimating colonies of these ecologically important insects, E. hamatum fits the similar characterization of E. burchelli as a keystone species.

References 1. S.G. Brady, Proc. Natl. Acad. Sci. U.S.A 100, 6575 (2003).

2. S. Powell, N.R. Franks, Proc. R. Soc. Lond [Biol] 272, 2173 (2005).

3. G.P. Boswell, N.F. Britton, N.R. Franks Proc. R. Soc. Lond [Biol] 265, 1921 (1998) 4. D.H. Kistner, in Social Insects, H.R. Hermann, Ed. (Academic Press, New York, 1982), vol. 3.

5. S. Berghoff, Curr. Biol. 23, R676 (2003).

6. V. Witte, U. Maschwitz, Insectes Socieoux 47, 76 (2000).

7. I.D. Couzin, N.R. Franks, Proc. R. Soc. Lond [Biol] 270, 139 (2003).

8. M. Kaspari, S. Oâ&#x20AC;&#x2122;Donnell, Evol. Ecol. Res. 5, 933 (2003).

9. A. Forsyth, K. Miyata, Tropical Nature (Scribner, New York, 1984).

10. M.A. Bragnaca, A. Tonhasca, T.D. Lucia, Entomologia Exper. et Appl. 89, 305 (1998).

11. B. Brown, D. Feener, Biotropica 30, 482 (1998).

12. E.O. Wilson, B. Holldobler, Proc. Natl. Acad. Sci. U.S.A 102, 7411 (2005) All authors contributed equally to this manuscript.

Image courtesy of the Centers for Disease Control and Prevention.

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Spring 2008

53

BIOCHEMISTRY

Persistent Explosives Present a Problem:

Analyzing the Biodegradation of Nitroglycerin and 2,6-dinitrotoluene in Camp Edwards Soil LAURA CALVO â&#x20AC;&#x2122;11

Introduction

D

uring winter and spring of 2008 I had the opportunity to participate in the Women in Science Project (WISP) internship program for first-year students. As an intern in the Biochemical Sciences branch at the Cold Regions Research and Engineering Laboratory (CRREL), I conducted research in the laboratory of Dave Ringelberg, sponsored by Jay Clausen. In my research I used state-of-the-art equipment to study contaminate remediation and microorganisms. This analysis stems from an ongoing comprehensive study of the soil mobility of nitroglycerin and 2,6-dinitrotoluene being conducted by a team of researchers at CRREL. Soil samples were collected from Camp Edwards, Massachusetts, where soldiers train by firing live rounds. New small arms ammunition is typically loaded with a double-based propellant consisting of 84% nitrocellulose (NC), 10% nitroglycerin (NG), and 6% filler compounds (1). Since the propellant does not completely combust when the arms are fired, the initial soil samples were analyzed for the presence of various by-products, such as flash suppressor dinitrotoluene (DNT), which are released into the soil when weapons are fired. In these soils, a high level of NG, 42 mg/ kg, was measured (1). The literature suggests that NG typically has a halflife of one to two days. Training at Camp Edwards ceased two years ago.

There is still a large quantity of NG left in the soil despite ample time for natural degradation. Thus, the objective of this study is to investigate whether or not certain biological and chemical factors are causing NG to persist in this soil.

This is an important issue to resolve, since there is potential for NG to reach ground water.

Indigenous Microbial Activity

The first phase of the analysis was to determine whether there was sufficient microbial activity in the soil.

An insufficient indigenous bacterial community could limit biodegradation of NG and 2,6-DNT. A microbial profile of the soil samples was first determined using a respirometer that measures CO2 produced by bacterial metabolism. Five sets of samples of 15 g of air-dried NG- and DNT-contaminated soil samples were analyzed in triplicate, yielding 15 total samples. The five different preparations would compare soil moisture at two different levels (natural moist soil and wet saturated soil), a comparison of a soil with bacteria (live) and without bacteria (kill), and the effect of adding a nutrient source (acetate). The five sample sets were wet-live, wet-kill, wet-acetate, moist-live, and moist-kill. For the wet-live preparation, 30 mL rainwater collected at the same site in March 2007 was added. Thirty mL of a biocide solution of 1% glutareldehyde and 90 mM mercuric chloride was added to the wet-kill set. The wet-acetate set received 30 mL of 50 mM acetate. The added acetate provides excess carbon for bacterial respiration to stimulate growth of

Figure 1. Microbial respiration as cumulative CO2 over a 5-day period. Addition of acetate stimulated respiration, however, the indigenous microbes also showed significant activity relative to the killed controls. Note: The wet/kill set is not visible because the data corresponds almost identically with the moist/kill data.

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Dartmouth Undergraduate Journal of Science

Figure 2. The total concentration of nitroglycerin and 2,6-DNT recovered from 5 g of site soil following seven days of incubation as a slurry with water. Results show there was no significant biodegradation of either explosive.

through high-pressure liquid chromatography (HPLC) and then measured for NG and 2,6-DNT. The second experimental study (Figure 2) yielded interesting results indicating that the presence of bacterial communities in the live samples were not involved in the biodegradation of NG and DNT. There was no significant difference in degradation rates between the kill and live samples even though the microbial profile study suggested the presence of a healthy microbial community in the live soils, and the absence of microbes in the killed soils.

Surprisingly, there was still a large quantity of NG present in the soil samples after a week. There was virtually no NG or 2,6-DNT in the aqueous state.

Future Investigation

carbon-limited bacteria. The moist-live set were given 1.5 mL rainwater, while 1.5 mL of a biocide solution of 1% glutareldehyde and 90 mM mercuric chloride was added to the moist-kill set.

An analysis of microbial respiration indicated that bacterial growth was occurring in the soil. The soil displayed typical exponential growth and approached stationary phase after four days (96 hours). The growth of bacteria was suggested by a notable increase in CO2 and a decrease in O2 levels, indicating that cell respiration was occurring. The acetate treatment enhanced bacterial growth, and bacterial communities flourished much more in wet conditions rather than moist conditions.

The results of the bacterial profile display that the soil harbors active bacterial communities under natural conditions. This data ruled out the possibility that a deficient bacterial population was the restricting factor for NG and 2,6-DNT biodegradation in Camp Edwards soil.

Future studies of these soil samples will reveal other mechanisms for NG conservation in field soil.

Bacteria can only biodegrade NG in aqueous solution, so something may be preventing the NG from dissolving; there may be an issue relating to a lack of bioavailability because NG is not dissolving into the aqueous phase.

One possibility is encapsulation by nitrocellulose. If this is the case, grinding or chemically processing the soil may release NG from within the nitrocellulose for accessibility by the bacteria. Moreover, certain microbes, such as the fungi Sclerotium rolfsii and bacterium Fusarium solani, are known to successfully break down nitrocellulose, and thus could be used as a treatment in these soils (2). Introduction of such bacteria could allow for NG biodegradation returning to normal rates and resulting in elimination from the Camp Edwards site.

Exploring the Biodegradation Potential

I would like to thank my sponsor Jay Clausen as well as David Ringelberg and Karen Foley for their assistance at CRREL. Thank you to the Women in Science Project for providing this internship.

In the next phase of the study, a set of samples was examined over a period of one week for NG degradation. Thirty soil slurry flasks were set up with 5 g soil and 20 ml solution. Three live and three kill samples were analyzed at five time intervals: days 0, 1, 2, 5, and 7. Three samples from each set (live and kill) were centrifuged at each time interval to separate the solid and aqueous phases. The solid phase was processed on a sonicator to release materials from the soil surface. All samples were extracted with acetonitrile for processing Spring 2008

Acknowledgements

References 1. D. Margolis, I. Osgerby, J. Macpherson, and J. Clausen, Site Specific Sorption/Desorption Measurements for Nitroglycerin and Dinitrotoluene. 1, 7 (2007).

2. A. Sharma, S.T. Sundaram, Y. Zhang, and B.W. Brodman, Journal of Industrial Microbiology and Biotechnology. 1 (1984).

55

mathematics

Chaos In Oscillating Chemical Reactions: The Peroxidase-Oxidase Reaction Patrick Karas â&#x20AC;&#x2DC;08

T

he peroxidase-oxidase (PO) reaction is an important example of how oscillating reactions arise in living organisms. The reaction is important mathematically because it exhibits many characteristics of chaos. Of these characteristics, sensitive dependence on initial conditions illustrates how predicting the future state of the PO reaction is nearly impossible.

For this reason, chaotic behavior in living organisms presents many obstacles to chemists and biologists in their attempts to predict how systems will react to perturbations. This paper explores the chaotic behaviors of the PO reaction, using a system of four differential equations as a model. The topics analyzed are timeseries data, chaotic attractors, bifurcations, tests for sensitive dependence, Lyapunov exponents, and one-dimensional time delay embedding.

What is Chaos?

Many everyday occurrences act in unpredictable ways. Weather, the stock market, billiards, and planetary orbits are all examples of systems that have a degree of uncertainty due to either a myriad of factors influencing them or some other complexity. Chaos and the study of dynamical systems provide tools to model and analyze

these complex and often seemingly random systems.

Sensitive dependence on initial conditions is a classical characteristic of chaos. A system has sensitive dependence if a small perturbation in the initial conditions of the system results in rapidly increasing deviations in the state of the system. A common example is the butterfly effect on weather: the small perturbations in the wind from a butterfly flapping its wings in Asia could build up to cause a hurricane in the southeastern United States.

A good way to test for sensitive dependence is using Lyapunov exponents. Lyapunov exponents measure the change in the natural log of the difference between the perturbed and the unperturbed state of a system at a given time. Thus a Lyapunov exponent greater than zero is characteristic of chaos.

Another characteristic often observed in chaotic systems is a period doubling route to chaos. For timeseries data, as in this paper, period doubling refers to a doubling of the number of maxima (as one of the parameters is varied monotonically) in the timeseries data. The number of maxima continues to double until the chaotic region is reached, in which there are infinite maxima.

Period doubling is most easily illustrated in bifurcation diagrams. The bifurcation diagrams show the maxima of the timeseries data on the vertical axis and the value of the parameter being varied on the horizontal axis. The location that the lines split on these diagrams indicate where period doubling occurs. For a good introductory text on chaos, including more formal definitions of the terms above, see (1).

Figure 1: Timeseries plot of [O2] and [NADH] to illustrate the correlation of the concentrations between the two species. Here mechanism I and mechanism II are constantly battling to be the dominant reaction mechanism, resulting in oscillating concentrations of [O2] and [NADH].

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Dartmouth Undergraduate Journal of Science

Figure 2: Projections of the four dimensional phase space ([O2], [NADH], [NAD.], [Co III]) of a chaotic attractor for the PO reaction with k1 = 0.35, and k3 = 0.035.

Oscillating Reactions

Less than a century ago, oscillating chemical reactions were thought to be nothing more than erroneous results caused by chemical impurities. Few believed that a reaction, in proceeding to equilibrium, could have intermediates with oscillating magnitudes of concentration. The Lotka-Volterra Oscillator and the Belousov-Zhabotinsky (BZ) reactions were two of the first oscillating reactions to be studied extensively and

universally accepted to have oscillating concentrations of reactants (1). Finding chaos in oscillating reactions is a much more recent undertaking, gaining popularity with the rise of computers. In, fact there is still debate as to whether oscillating chemical reactions truly exhibit chaos, or if they are merely oscillatory with very large period (2). However since the discovery and eventual acceptance of oscillatory reactions, many more such reactions have been designed and discovered. One of the main areas of discovery of oscillating reactions is in living systems. The peroxidase-oxidase (PO) reaction is a prime example of such a reaction. It was one of the first reactions outside of the BZ reactions to be classified as oscillating and chaotic, and is an extensively studied example of an in vivo oscillating reaction (the reaction can be carried out both in vivo and in vitro). Molecular oscillating reactions are an important area of study, as they are essential to understanding the more complex oscillating systems of organism (e.g. a heartbeat) (1).

The PeroxidaseOxidase Reaction

The peroxidaseoxidase reaction is an enzyme-catalyzed redox reaction. Nicotinamide adenine dinucleotide (NADH) is oxidized and

Figure 3: Bifurcation diagram of [O2]max on k1 (here k3 = 0.035). Decreasing k1 leads to perioddoubling and chaos. Decreasing k1 past the chaotic range once again yields periodic oscillations in [O2]. Bifurcations on k1 all have a similar structure no matter what value of k3 is used. However, the values of k1 that show chaotic behavior change depending on k3.

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molecular oxygen acts as an electron receiver. The net reaction is

2NADH + O2 + 2H+ g 2NAD+ + 2H2O

O2 and NADH are continuously replenished and products are continuously removed from the experimental system via the use of a continuous-flow, stirred tank reactor (CSTR). Experimentation has shown that the PO reaction exhibits oscillatory behavior and a period-doubling route to chaos (3). These characteristics have been effectively modeled using a simplified eightstep mechanism. The Olsen model (4) is

k k k B + X g 2X 2X g 2Y A + B + Y g 3X

k k k X g P Y g Q X0 g X

k k

A0 g A B0 g B 1

2

4

5

7

3

6

8

where A and B are reactants (O2 and NADH respectively), P and Q are the products, and X and Y are reaction intermediates. Experimentally, [X] corresponds with [NAD] and [Y] corresponds with the concentration of oxyferrous peroxidase (compound III), however it is still unknown how well these intermediate variables correlate with the true intermediates. It is also important to note that k1 = [enzyme], the concentration of peroxidase enzyme, and there is also a strong correlation between k3 and the concentration of 2,4-dichlorophenol, [DCP] (3). Thus k1 and k3 are variable parameters. Oscillatory behavior arises because of competing mechanisms in the PO reaction, appearing in the Olsen model as follows: call mechanism I the net reaction of the autocatalytic production of X from B and X, and the production of 2Y from 2X.

Mechanism I B + X g 2X 2X g 2Y B + X g 2Y

Mechanism I dominates the PO reaction when the concentration of X is high, but uses up X to create Y. The rate of mechanism I can be varied by changing the concentration of peroxidase enzyme, k1. Once the concentration of X falls below some critical value, [X] , a second mechanism, mechanism II, takes over. crit Mechanism II is the termolecular reaction of A, B, and Y to form X.

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Figure 4: Bifurcation diagram of [O2]max on k3 (here k1 = 0.35). The points on the bifurcation diagram correspond to the local maxima of the timeseries data. In (A), where k3 = 0.3, there are two alternating peaks, corresponding to the two points above k3 = 0.3 on the bifurcation diagram. In (B), there is no easily discernable pattern to the peaks in the timeseries data, corresponding to the chaotic region in the bifurcation diagram at k3 = 0.035.

In (C) there are three alternating peaks. Note the doubling route to chaos as k3 increases.

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Mathematically Approximating the Olsen Model

Methods All numerical analyses were carried out using MATLAB version 7.2.0.232 (R2006a). Systems of differential equations were solved using ode45 (a RungeKutta numerical approximation method).

Timeseries data were numerically compared by fitting a cubic spline (using the MATLAB ‘fit’ toolbox) to the data from ode45 and then interpolating values from this fit.

Linear regressions were also carried out using the ’fit’ toolbox. The Olsen model can be approximated by the following system of four first-order differential equations: Figure 5: A plot of time versus the natural log of the absolute value of the difference in [O2] concentration when initial conditions differ by 1e-10 in [NADH]. The slope of regression line (in red), the Lyapunov exponent, is 0.022, indicating chaotic behavior of [O2]. k1 = 0.35 and k3 = 0.035

Mechanism II A + B + Y g 3X

A = k7(A0-A) - k3ABY B = k8 - k1BX - k3ABY X = k1BX - 2k2X2 + 3k3ABY - k4X + k6 Y = 2k2X2 - k5Y - k3ABY

The following parameters were used in all of the calculations in this paper: k2 = 250, k4 = 20, k5 = 5.35, k6 = 10-5, k7 = 0.1, k8 = 0.825, and A0 = 8. Values of k1 and k3 are indicated in the diagrams. Initial conditions, except where indicated, are [O2] = A = 6, [NADH] = B = 58,

Mechanism II dominates when the concentration of Y is high, using up Y and turning it to X. Once the concentration of Y falls below some [Y]crit, mechanism I takes over once again. Thus the concentrations of the intermediates X and Y oscillate. Since the rate at which the reactants are converted Figure 6: k1 and k3 phase plane illustrating combinations of k1 and k3 that exhibit chaotic into intermediates depends on the concentration behavior. The color bar indicates colors corresponding to the Lyapunov exponent. Areas of the intermediates, the concentrations of where the Lyapunov exponent is greater than 0.005 are thought to be chaotic. Note that the Lyapunov exponents in this plot are conservative estimates due to the sampling the reactants also oscillate and are strongly method, i.e. for an exponent greater than 0.005, true value of the Lyapunov exponent correlated. See figure 1 for an example of the may be larger than stated. The area in black represents where the concentration of oxygen is in a steady state. correlation of [O2] and [NADH].

Also note that in mechanism II, the reaction rate is governed by k3, the concentration of DCP. Since k1 and k3 are simple to control experimentally, there is an easy way to change the rate constants that govern mechanisms I and II. In fact k1 and k3 determine the characteristics of how the concentrations of A, B, X, and Y vary with time, and if chaos arises. In fact, either decreasing k1 or increasing k3 results in a period-doubling route to chaos (3).

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Figure 7: One-dimensional time delay embedding for [O2]. This figure graphs the maxima versus the successive maxima in the [O2] timeseries data.

Zooming in on the figure reveals a fractal structure. The structure supports the claim that [O2] exhibits chaotic behavior. k1 = 0.35 and k3 = 0.035.

[NAD] = X = 0, and [Co III] = Y = 0. Phase space diagrams were computed to compare [O2], [NADH], [NAD.], and [Co III]. No matter what values of k1 and k3 were used to compute the phase space, they all resulted in attractors.

Chaotic attractors (as in figure 2) resulted from values of k1 and k3 corresponding with values of k1 and k3 that were found to yield chaos in figure 6. Similarly, periodic attractors resulted from values of k1 and k3 that were found to yield periodicity in figure 6.

Bifurcations of the concentration of reactants over k1 (see figure 3) or k3(see figure 4) both illustrate a period-doubling route to chaos. This property appears from either decreasing k1 or increasing k3. The perioddoubling behavior on k1 appears independently of the choice of k3, as does the behavior illustrated in the bifurcation on k3 for any choice of k1. Furthermore, experimentation has verified the predicted behavior of the bifurcations (4, 5). Although the bifurcation diagrams display areas indicative of chaos, further testing is required to determine whether the disordered regions do indeed give rise to chaos. Sensitive dependence on initial conditions was found from calculating the timeseries data from the Olsen model differential quations. Timeseries data was calculated two times for initial conditions differing in [NADH] concentration by 1x10-10. Then the natural log of the magnitude of the difference of [O2] concentration was calculated, and a line was fit to the section with positive overall slope, if it existed (see figure 5). The slope of the regression line is therefore the Lyapunov exponent corresponding to the orbit of the oxygen concentration in the system.

Tests for sensitive dependence were carried out by monitoring [O2], [NADH], [NAD.], and [Co III], while varying the difference in initial conditions among all four variables. Also, different values of k1 and k3 were used.

The Lyapunov exponents resulting from the sensitive dependence tests verify the data in the bifurcation diagrams: no sensitive dependence (slope < 0) was 60

found where the bifurcation diagram shows periodic behavior of [O2], while sensitive dependence was found (slope > 0) where the bifurcation diagrams predict chaos. The cutoff on positive Lyapunov exponents was taken to be 1e-4. Testing for sensitive dependence in the above manner proved to be very tedious, and is impractical for understanding how k1 and k3interact to either give rise to a steady state, periodic oscillations, or chaos. In order to characterize these interactions, a plot over k1 and k3 was constructed, displaying Lyapunov exponents for the different k values (figure 6). The black area in figure 6 represents values of k1 and k3 for which the concentration of oxygen is in a steady state. Note that when k3is increased just past the steady state area, the behavior of oxygen concentration becomes chaotic (light grey/white). Then as k1 decreases and k3increases, there is a large area of periodic behavior (grey), eventually getting to a region of chaos denoted by white and light shades of grey in the figure. As k1 continues to decrease and k3 continues to increase, there is once again a large area of well behaved, periodic oscillations in [O2]. Figure 6 supports the general trend seen in the bifurcation diagrams (figures 3 and 4): decreasing k1 leads from periodic to chaotic to periodic behavior, as does increasing k3. Furthermore figure 6 illustrates that this trend is relatively universal across values of k1 and k3. Lyapunov exponents were calculated as the slope of a linear regression of the natural log of the difference in [O2] (as described above for the sensitive dependence test). The Lyapunov exponents are a conservative approximation: the linear regression was taken from time 50 to time 250 in all cases, so in a case of extreme Dartmouth Undergraduate Journal of Science

sensitivity to initial conditions, the Lyapunov exponent in figure 6 is too small (but still larger than 0.005 and therefore indicative of chaos). Finally, in an attempt to verify the chaotic nature of the imeseries data, a onedimensional map was constructed (figure 7). The map constructed by plotting the preceding amplitude of the [O2] timeseries data against the following amplitude. The map appears to be a fractal since zooming in on an area of the map yields an ordered structure. Furthermore, in preliminary calculations (due to time constraints) with mediocre resolution, the box counting dimension of the fractal appeared to be around 1.1 or 1.2. This indicates that indeed the map is a fractal. The fact that the timedelay embedding of the successive peaks in the [O2] time series is a fractal supports the claim that the PO reaction indeed exhibits true chaos. If the orbit of [O2] was merely periodic with a very long time scale, the fractal dimension of figure 7 would be zero (just a finite collection of points). Showing that the fractal dimension of the map is greater than zero would provide strong evidence for [O2] exhibiting chaos.

Conclusions

There is significant and conclusive evidence that the PO reaction displays true chaotic behavior.

This statement holds true only for certain choices of k1 and k3; however the existence of chaos in molecular reactions occurring inside an organism is a significant result. We have shown that the attractors are bounded for the Olsen model. Furthermore, for a given chaotic orbit we have seen that there exists a corresponding positive Lyapunov exponent. These two facts by themselves justify calling the orbit chaotic (this is the definition of chaos). Additionally we have shown that there indeed exists sensitive dependence on initial conditions, and that a one-dimensional map illustrating the successive change in maxima of the timeseries data for a reactant is a fractal. Finally, there is a period-doubling route

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to chaos. Thus it is clear that the PO reaction can exhibit chaos. One issue requiring further attention is exactly how accurate the Olsen model is at describing the chemical concentrations in the PO reaction.

Experimentation has proven that the model describes periodic oscillations correctly: the number of different peaks in the experimental timeseries data corresponds with the predictions from the bifurcation diagrams.

Similarly, experimental results mimic the chaotic behavior where predicted on the bifurcation diagrams.

However since the Olsen model displays sensitive dependence for certain choices of k1 and k3 (presumably the PO reaction does also), it is impractical to expect the model to properly predict the chaotic timeseries data for an experimental procedure. Indeed that is the nature of chaos: it is extremely difficult to predict. The most important tests of how good the Olsen model is are those that test how well the model predicts periodic behavior, and how well the model predicts when chaos will arise.

Acknowledgements

I would like to thank Professor Alexander Barnett for all of his mathematical guidance. Also I thank Professor Robert Ditchfield and Charlie Ciambra for their help with the chemistry.

References 1. K. T. Alligood, T. D. Sauer, J. A. Yorke. Chaos: An Introduction to Dynamical Systems. (Springer-Verlag, New York, 1996).

2. I. R. Epstein, K. Showalter, J. Phys. Chem. 100, 13134 (1996).

3. I. R. Epstein, Physica D. 7, 47 (1983).

4. C. G. Steinmetz, T. Geest, R. Larter, J. Phys. Chem. 97, 5649 (1993).

5. L.F. Olsen, Physics Letters A. 94, 454 (1983).

6. I. R. Epstein, J. A. Pojman, An Introduction to Nonlinear Chemical Dynamics: Oscillations, Waves, Patterns, and Chaos.

(Oxford University Press, New York, 1998).

7. L. Gyorgyi, R. J. Field, J. Phys. Chem. 95, 6594 (1991).

8. R. J. Field, L. Gyorgyi, eds. Chaos in Chemistry and Biochemistry. (World Scientific Publishing Company, 1993).

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dujs presents...

Philistines! (Scene Two)

Or the Electrodynamics of a Moving Body Latif nasser ‘08

Nasser is a Senior Fellow at the College. Each Senior Fellow completes an independent, interdisciplinary project in lieu of taking classes or finishing a major.

His project ‘Playing with Science’ is advised by faculty from the Theater, History, and Physics departments, and examines the inherently human character of science. PHILISTINES! dramatizes the hardships faced by a young, brash, and unemployed Albert Einstein, soon after his graduation from the Swiss Polytechnic.

ACT ONE, SCENE TWO

The turn of the twentieth century. A cramped dark office at the Swiss Polytechnic. HEINRICH FRIEDRICH WEBER sits behind his desk uncomfortably. ALBERT sits across from him comfortably; he thinks he is about to be offered a job.

WEBER I’m sorry.

ALBERT I don’t understand.

ALBERT It was either the battery or theWEBER Herr Einstein, the electromagnetic induction demonstration has been a part of my lectures since 1871. It is a simple demonstration.

Electrons flow from the power supplyALBERT I know howWEBER -from the battery through the conductive wire. The movement creates voltage. The voltage creates magnetism. Never before has the power supply been a problem.

WEBER Based on your trial lecture, I cannot offer you a teaching assistantship for next year.

ALBERT The apparatus was quite old.

ALBERT Because of -?

(Holds up his bandaged right hand)

WEBER Well if you had tested it before you started, as you had been instructed –

WEBER There are a number of factors-

ALBERT I did.

ALBERT Herr Professor, you saw it. It was the apparatus. The power supply must have-

WEBER you would not have sabotaged your own trial lecture.

WEBER The decision was not simply based on ALBERT But Herr Professor, you do understand that. It was the battery.

ALBERT I tested it with the battery in the laboratory, not with the battery in the classroom.

WEBER Herr Einstein, your actions were negligent and dangerous. Not to mention the damage you did to the classroom, the apparatus-

WEBER It was not the battery.

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Dartmouth Undergraduate Journal of Science

ALBERT Herr Weber, it will never happen again.

WEBER No, it won’t. Because you won’t lecture here again.

ALBERT Herr Weber, it was one accident.

WEBER Yes. It was one accident – that capped off a – I am sorry to say - lazy and feeble lecture on electromagnetism. (pause.)

WEBER I gave you my notes as a guide. Now I did not expect you to deliver my lectures word for word, in fact, if you remember, I discouraged it. On the other hand, there are a number of critical principles that you simply omitted.

In addition, you lacked any semblance of a structure.

ALBERT - I wondered why everything seemed to stop at 1850.

You didn’t even mention Maxwell’s equations. Not once.

WEBER I’m starting to resent your tone, Herr Einstein.

ALBERT I apologize if I’m coming off as ungrateful, but I learnt about Maxwell’s equations from my own reading. After I graduated.

WEBER One cannot understand Maxwell without first understanding – ALBERT What about the ether drift experiments? Your lectures talk about the luminiferous ether as if Newton had the last word. You didn’t even mention-

ALBERT It’s not that … I … When I was a student here, when I took your class, you taught it as a history of electromagnetism: “1820: Oersted finds moving electrons carry magnetism. 1831: Faraday finds moving magnets carry electricity.” It’s not -

WEBER In ten years no one will remember those experiments.

WEBER Yes?

WEBER That may or may not-

ALBERT I just thought it would be more interesting to teach electromagnetism through contemporary research.

ALBERT They say he’s going to finish physics. Finish the entire field. In the next two years. And you still want to teach Newton?

WEBER Poincaré?

ALBERT Exactly. Someone alive everybody has heard of.

ALBERT And Poincaré? The work of Poincaré, you must admit, is changing how we understand electrodynamics-

WEBER I’m sorry my physics course is not fashionable enough for you, Herr Einstein. (pause.)

WEBER What you seem not to understand, Herr Einstein, however dry you might find the material, is that this is an introductory course.

ALBERT Right. And when I was taking the classWEBER Three years ago.

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ALBERT I … I can redo the lecture.

WEBER That won’t be necessary.

ALBERT I can have it on your desk tomorrow. Read it and then decide.

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WEBER I’m afraid the decision has already been made.

ALBERT You did. I’m the only one.

ALBERT Please.

WEBER Yes. Fine. You’re the only one.

WEBER All of the teaching assistantships for next year have already been filled.

ALBERT So who’s the fourth?

ALBERT What?

WEBER Now if you’d like I could forward a letter of reference on your behalf toALBERT I’m sorry?

WEBER -to several colleagues of mine.

ALBERT I thought you had four slots.

WEBER And they’ve all been filled.

ALBERT Kollros, Ehrat, Grossman and Einstein. You had four graduates and four slots.

ALBERT It does to me.

WEBER It shouldn’t.

ALBERT Is it Reinhold?

WEBER Herr Einstein.

ALBERT It’s Reinhold, isn’t it?

WEBER Herr Einstein, you’re being quite petty, and I’d advise you to stop. (short pause.)

WEBER It seems not to have worked out that way.

ALBERT I apologize.

ALBERT Did you offer Grossman a job?

WEBER Yes.

WEBER Herr Einstein.

(short pause)

WEBER That’s not-

ALBERT I appreciate your … constructive criticism about the lecture. Again, I apologize for the damage to the classroom and if it isn’t too much trouble, I would appreciate it if you … would send out the letters you mentioned.

ALBERT Did you?

WEBER No trouble at all.

ALBERT Did you?

WEBER That’s not your business.

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WEBER It doesn’t matter.

A free public reading of PHILISTINES! will take place in the Hopkins Center on May 29, 2008.

Dartmouth Undergraduate Journal of Science