Carl y saab seeing, hearing, and smelling the world (brain works) (2007)

Seeing, Hearing, and Smelling the World

The Brain and Love A Day in the Life of the Brain How the Brain Grows Inside Your Brain Seeing, Hearing, and Smelling the World

Seeing, Hearing, and Smelling the World

Carl Y. Saab SERIES EDITOR Eric H. Chudler, Ph.D.

This book is dedicated to the animals sacrificed for laboratory research.

The author is indebted to Rafa for her editorial contribution and to Samuel Owolabi, M.D., for his review.

Seeing, Hearing, and Smelling the World Copyright © 2007 by Infobase Publishing All rights reserved. No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage or retrieval systems, without permission in writing from the publisher. For information contact: Chelsea House An imprint of Infobase Publishing 132 West 31st Street New York NY 10001 ISBN-10: 0-7910-8945-2 ISBN-13: 978-0-7910-8945-3 Library of Congress Cataloging-in-Publication Data Saab, Carl Y. Seeing, hearing, and smelling the world / Carl Y. Saab. p. cm. — (Brain works) Includes bibliographical references and index.

ISBN 0-7910-8945-2 (hardcover) 1. Senses and sensation—Juvenile literature. I. Title. II. Series QP434.S22 2006 612.8—dc22 2006024117

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Table of Contents 1 Neurons and Nerves

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2 Hearing

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3 The Ear

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4 Vision

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5 The Eye

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6 Visual Abnormalities

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7 Smell and Taste

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8 Synesthesia

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Glossary

88

Bibliography

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Further Reading

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Index

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1 Neurons and Nerves Sensation is a long journey that begins when different stimuli (light for colors and gases for odors) come in contact with their proper receptor organs (eyes for light and nose for gases). This journey of light or gas ends when the stimulus is transformed into messages that are created by connections between cells in the nervous system. The messages are finally transmitted to the brain for perception (light as color and gas as odor). This book takes you on this journey from the outside world of lights, sounds, and odors into your own brain and the deepest memories of your mind.

LIVING AND NONLIVING Any discussion of the senses should begin with the basics of biology. All living creatures possess one fundamental feature: the cell.

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Figure 1.1 A common example of a unicellular organism is the amoeba, photographed above. An amoeba is a type of protozoa, a single-celled organism that has a nucleus and characteristics similar to those of animals, such as mobility. Amoebae are most commonly found in freshwater.

The most basic forms of life are made up of only one cell and are referred to as unicellular organisms. An example of a unicellular organism is a bacterium (plural, bacteria). Unicellular organisms are usually so small that they can be seen only by using a microscope (Figure 1.1). More complicated, multicellular forms of life require multiple cell types and use sophisticated systems of communication between cells to sustain the life of the organism. Any animal big enough to be seen with the naked eye, including a small insect such as an ant, requires

Neurons and Nerves

food for nutrients and air for oxygen to survive. Nutrients and oxygen need to be distributed efficiently to all cells within the animal’s body, generally through the blood that flows through multiple organs and to various cellular destinations. For living organisms to obtain food, water, and air, they first need to be able to move around and sense the environment, to forage for nutrients, to locate prey, or even to chew the food and breathe (exceptions to this rule include plants, which do not move around or chew food as almost all animals do). Indeed, to be able to use any body part, to feel, to speak, or even to think requires coordination by one system: the nervous system. Like any other system in the body, the nervous system is made up of different types of cells that share similar shape and function. Cells that form blood include red blood cells and immune cells. The functional cells that form the nervous system are mainly neurons (Figure 1.2). The nervous system also contains another type of cell, glia, that help support and maintain homeostasis of neurons.

NEURONS AND GLIA Neurons are responsible for giving multicellular organisms (again, except plants) the essential requirements of life itself in the form of an action, a thought, or a feeling. How often do we identify a creature as “alive� by showing that it can move or an animal as dead if it no longer responds to a shout or a poke? Movement is caused by a class of neurons called motor neurons, whereas sensory experiences are possible due to another class of neurons referred to as sensory neurons.

Both motor and sensory neurons, as well as other classes of neurons, have similar basic characteristics. Keep in mind, however, that neurons function differently depending on

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Figure 1.2 The shape and length of a neuron determines the role it will serve in the nervous system. Pictured above are three different types of neurons.

their location within the body (in the periphery, such as in the hand or centrally, such as in the brain), their shape (small neurons, such as those in the brain or neurons that are more than 1 or 2 feet long, such as those in the legs), and their cellular content (neurons are made out of internal parts, known as organelles). Another general characteristic of neurons is that, with few exceptions, each neuron is either connected to another neuron or to a muscle (exceptions include those that connect to a gland or other visceral organs).

When first discovered—and until about 10 years ago—glia were thought to play a supporting role by “gluing” neurons together. In fact, scientists are just starting to recognize other important roles that glia play in maintaining neuronal

Neurons and Nerves

homeostasis. Glia protect the nervous system against invading microbes and repair the system after damage (such as after a severe car accident or a neurological disease).

If one had to describe the most basic function of the nervous system in one word, it would be communication. Think about it: If a person wishes to move an arm, the brain has to command the arm. If a person places his or her hand over a stove accidentally, pain is produced by the activation of specific brain areas. In both cases, a message has to be transferred from the brain to the arm (for movement) or from the hand to the brain (for pain). The message also has to be sent quickly in order to produce an action without too much delay (Figure 1.3). These messages are relayed from one neuron to another, either between two neurons or among thousands! In cases in which only two neurons are involved, the neurons can be remarkably long compared to the dimensions of the human body. For pain messages relayed from the arm or the leg, the first sensory neuron to signal the pain message could be as long as 2 or 3 feet (.6–.9 meters). Certain neurons are in fact the largest cells in the body.

Neurons communicate through synapses, the tiny gaps where two neurons meet. Two neurons typically share one synapse at their meeting point; however, it is not uncommon to find two neurons with multiple synapses. As a result, the message transmitted from one neuron to another can vary, depending on the synapses used to transmit that message.

Even for a single synapse between two neurons, the message transmitted across the synapse can be subject to change (more accurately referred to as modulation) with time or depending on changes in the neuronal environment.

There is a complicated process behind how we react to a stimulus, and how our perception of that same stimulus

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Figure 1.3 Electrical signals travel along the axon of a neuron, also known as a nerve fiber. The speed of the signals can vary, depending on the type of nerve fiber. Speeds of different nerve fibers are compared in the graph above.

changes over time according to varying circumstances. But nevertheless, this process is reflected in the ability of the nervous system to change or adapt. One adaptive characteristic of the nervous system is memory—in other words, neurons can learn. Neurons grow and synapses are formed or broken down constantly in our brains and elsewhere in the nervous system from birth and until death. Living organisms are not robots. They constantly evolve due to the ability of

Neurons and Nerves

the nervous system and synapses to adapt, learn, change, and bounce back.

NEURON = CELL BODY + DENDRITE + AXON Unlike other cells in the body, some neurons have long extensions, which help them communicate over long distances.

The arrangement of neurons is somewhat like the network of telephone wires that connects the homes and businesses in a city. In a telephone network, short wires carry a signal a short distance, while long wires can carry a signal much further. The same type of relationship is at work in the body’s nervous system.

Neurons are equipped with two types of extensions at the head or the tail end (Figure 1.4). The cell body of the neuron contains the nucleus and the rest of the cellular machinery necessary to make proteins, generate energy, and sustain the life of the neuron. Out of this cell body emerges dendrites (head) and an axon (tail end). Neurons generally receive messages through their dendritic synapses and send messages down their axons to synapses on one or many neurons.

A message is relayed from one neuron to another, and the flow of communication is secured. These simple rules and those highlighted in the previous paragraph are essential to understanding more complicated neuroscience facts.

Sometimes the function of a neuron can be predicted based on the structure of the dendrites or the axon. Neurons with long axons transmit messages that need to travel to faraway destinations, such as sensory neurons in the hand relaying information to the brain about objects touching the skin. It is important that these messages are transported faster than other messages in the body (such as hormones

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Figure 1.4 A neuron consists of a cell body, axon, and dendrites. The cell body contains the nucleus, which is the control center of the neuron. Axons carry nerve impulses away from the cell body. They are often wrapped in myelin, which helps increase the speed of transmission of the impulse.

Dendrites receive nerve impulses from adjacent neurons.

transported by the blood). Sensory neurons communicate information that is vital to protect the skin and other body parts, such as warning about very hot surfaces. Information

Neurons and Nerves

about very high temperatures needs to get to the nervous system centers responsible for withdrawal of the hand as quickly as possible in order to prevent or minimize injury.

Other types of neurons with long axons convey messages to the muscles for movement. Imagine how fast the brain needs to communicate with leg muscles to yield a smooth pattern of movement.

Some neurons may have multiple dendrites, often referred to as a dendritic tree. Such neurons receive multiple inputs from many neurons (and thus from many axons) and could be recruited to coordinate or integrate multiple messages.

THE NERVOUS SYSTEM Two structures, the skull and the vertebral column, separate the nervous system into two main compartments (Figure 1.5). The brain rests inside the skull, and the spinal cord is found inside the vertebral column. The brain and spinal cord make up the central nervous system (CNS), and all neurons located outside of this central compartment are contained in the peripheral nervous system (PNS). The thick bones of the skull and the vertebral column shield the CNS against physical injuries. Other tools also help ensure the best protection of the CNS for a good reason: The majority of neurons in the CNS, if damaged, cannot regenerate. Paralysis after spinal cord injury is largely a result of the body’s inability to repair damaged CNS neurons. One example of CNS damage caused by a disease is multiple sclerosis, a condition in which different areas of the CNS degenerate, causing irreversible paralysis and other problems. Research into neuronal regeneration and stem cells may result in new therapies to cure paralysis and CNS degenerative diseases.

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Figure 1.5 The brain and spinal cord make up the central nervous system. The nerves that extend from the spinal cord to the distant parts of the body make up the peripheral nervous system.

The PNS, in contrast, is able to bounce back from injury a bit better. A typical example of PNS tissue is a peripheral nerve. A nerve is a collection of axons generally longer than those found in the brain; examples include sensory or motor neurons. A peripheral nerve, however, may contain only sensory or motor neurons or a collection of both. The

Neurons and Nerves

most prominent nerve in humans is the sciatic nerve, which transmits sensory messages (such as gentle touch or painful pinprick) and conveys motor commands to muscles in the entire leg. The sciatic nerve, like many other nerves, branches out into different smaller nerves as it travels away from the spinal cord.

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2 Hearing Sound, such as music and speech, is physical energy perceived by an organ in the body designed especially for this task. There is no best way to define a sound that has not been heard before. For example, it is impossible to accurately describe such a sound to someone who is deaf, especially if that person has been deaf since birth. To better illustrate this example, imagine describing a color to someone who is blind or a smell to someone who has anosmia (inability to perceive odors). In fact, sharing all feelings that result from sensory perception—touch, sound, or smell—is never exact; in the end, all of our experiences remain deeply personal. Even close friends or family members differ in their interpretation of the same event or phenomenon.

Music produced by a string instrument such as a violin is one pleasant example of sound (if well performed!). The violin is a 18

Hearing

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Figure 2.1 A man plays a violin and appeals to the auditory senses.

The violin, which is a string instrument, is used in many different types of music.

delicately built instrument with a common basic feature: strings attached at both ends. When a bow is brushed against these strings, the friction that results from this mechanical interaction causes the string to vibrate—that is, to move quickly with a speed referred to as frequency. This highspeed vibration causes a similar vibration in the air near the part of the string where the bow strikes (Figure 2.1).

Air is formed of many molecules (mostly nitrogen and oxygen). As a result, when air molecules are “pushed� to vibrate by the moving string, the resulting energy is transmitted to neighboring air molecules (similar to how billiard or pool balls bounce off of each other). How does all this vibration

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and energy reach us as music? Before we answer this question, it is necessary to understand how sound is created.

SOUND IS . . .

It is difficult to imagine how the motion of air molecules, so small as to be invisible to the naked eye, can result in sound. In order to visualize how sound is created, think of the smooth and calm surface of a lake and the disturbance caused when a pebble is thrown in the water (Figure 2.2). The reaction typically takes the form of many rapidly expanding circles, with the point where the pebble hit the surface at the center. Although the pebble may be the size of a fingernail, the waves that spread across the lake are infinitely larger.

Like ever-expanding circles of waves at the surface of the water, air molecules travel by forming waves of compressed (packed together) and decompressed (spread apart) gas molecules. This movement mimics the vibration of a string.

Mechanical friction of the bow causes vibration of the string in a violin and ultimately the sound that spreads throughout an infinitely larger space.

Sound is detected when vibrating air molecules reach the human ear. Many conditions need to be met before sound is heard: â—† Intensity: Faint and loud sounds reflect the strength (loudness) of vibration. Very weak vibrations may reach the ear but may be too weak to be perceived. â—† Attention: Although many sounds reach our ears, we do not perceive all of them. This is especially true of weaker sounds. For example, although thousands of people may be screaming at a concert, we can only

Hearing

Figure 2.2 This bird’s-eye view of a pebble tossed into a pond depicts the way in which sound waves travel through air. The pebble causes ripples moving outward from the point of impact, which mimics vibrating air molecules.

perceive a conversation within the crowd if our attention is shifted to these specific sounds. Another example is sleeping through an alarm bell, even though it is loud enough to wake another person equally distant from the bell. â—† Normal hearing biology: The ear is a complex biological organ with elegant morphology (shape) and design connected to the brain by neurons. In the end, the brain is the organ capable of decoding and unlocking the secrets of sensory messages that bombard us constantly in a busy environment. These sounds that we experience would go unnoticed in this universe without the brain.

Small defects in ear biology, connections to the brain, or the brain itself may lead to hearing abnormalities ranging from

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loss of hearing, to hearing abnormal sounds (even imaginary sounds), to complete deafness.

CONDITIONS FOR NORMAL HEARING Many conditions are necessary for a person to perceive sound.

First, the sound has to travel through an environment, called a medium, that allows the transmission of vibration. Air is the medium in which humans live. It is made of molecules that bounce against each other and is capable of shrinking and expanding, and thus can create a wave-like effect. In contrast, imagine talking to someone underwater. What is heard underwater is mostly mumbled sounds that are much softer than those produced in air. This is largely due to the fact that water molecules are less free to vibrate than air molecules (forces of cohesion between water molecules are stronger than those between air molecules). Sound travels much faster underwater than in air, however, because water molecules are closer together than air molecules are. In fact, the speed of sound in sea water is approximately 1,530 meters per second (3,423 miles per hour), or roughly more than four times faster than the speed of sound in air (343 meters per second; 767 miles per hour).

Another necessary condition is related to the physical property of the sound itself. When air molecules vibrate, they travel in waves (Figure 2.3). Imagine surfing at the beach and waiting for waves. The time spent trying to catch a wave is directly related to the length or distance that separates one wave from another. As a result, longer delays are related to waves being farther apart (longer wavelength). Therefore, even if sound travels through a medium such as air, we may not be able to hear it if it falls outside of the certain wave-

Hearing

Figure 2.3 Waves can be described by their three properties: wavelength, frequency, and amplitude. The wavelength is the distance from the top of one wave, also known as the crest, to the next. Frequency is the number of waves per second. The amplitude measures the height of the wave.

lengths that the human ear can detect. One example is the sound of a special whistle used to call dogs, which the human ear cannot detect.

WHEN VIBRATING AIR MOLECULES REACH THE EAR Vibrating air molecules spread in all directions, just as the smell of dinner cooking on the stove can reach upstairs to a bedroom, out the front door, and into the basement at nearly the same time. This vibration (or waves of molecules

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compressing and decompressing) then undergoes two major transformations to make sound: 1. Transformation by specialized organs in the ear from mechanical energy (vibration) to electrical and chemical energy: mechanical energy ∆ specific receptor ∆ electrical energy 2. Transmission of nerve signals to the brain, ultimately transforming mechanical air vibration into “sound” perception such as music, speech, or even random noise: electrical energy ∆ specific pathway within the nervous system ∆ brain ∆ sensory perception These two major pathways are discussed in detail in the following chapters, which will also highlight similarities and differences among hearing, vision, smell, and other sensory perceptions.

3 The Ear The human ear is not just the part that “sticks out” from the head (outer ear). Another major part of the ear is hidden inside the head and connects to the brain (inner ear). Although the outer ear (pinna) looks complex, it is a simpler biological structure than the inner ear. The main function of the outer ear is to maximize the amount of sound that reaches the ear, almost like a funnel for sound. After being guided through the pinna, vibrating air molecules hit the eardrum (tympanic membrane). The eardrum can be compared to the surface of a real drum that turns tapping or striking into louder sounds. When the surface of a drum is struck, it vibrates and causes air molecules inside the drum to vibrate and “escape” out of the other end of the drum as loud drumbeats. In

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the case of the ear, vibrating air molecules gently “tap” on the eardrum, which then vibrates as well. Any matter lodged in the outer ear (wax or water left over from a shower or swimming) may obstruct airflow to the eardrum or may mechanically prevent the eardrum from freely moving and vibrating with air molecules. As a result, the affected ear will be less sensitive to sounds.

THE MIDDLE EAR The outer ear (eardrum, ear canal) is connected to the middle ear by three small bones (the ossicles). These bones—called the malleus, the incus, and the stapes—are connected to each other and stretch from the eardrum to the inner ear (Figure 3.1).

The main function of the ossicles is to relay the mechanical vibration toward the nervous system. The mechanical properties of these bones are unique in terms of amplifying eardrum vibrations and transmitting them to the inner ear with extreme accuracy. The point of touch between the bones of the middle ear and the inner ear is a thin oval sheet called the oval window. Physical damage to the bones of the middle ear may result in bumping them out of place or even breaking them, which will cause severe hearing loss.

Medical intervention can successfully restore hearing loss that results from damage to the external or middle parts of the ear. Hearing loss caused by nerve damage within the internal ear is more difficult to restore and often is permanent. This is true for other sensory perceptions as well: Damage to the optic (“visual”) nerve results in permanent visual deficits including blindness.

The Ear

Figure 3.1 The external part of the ear receives sound vibrations, which travel down the auditory canal toward the middle ear. In the middle ear, the auditory ossicles (malleus, incus, and stapes) connect to form a chain of bones that is responsible for transmitting sound vibrations from the eardrum (tympanic membrane) to the inner ear. The sound vibrations are converted to an electrical impulse that travels along the auditory nerve to the brain.

THE INNER EAR The inner ear’s oval window is connected to the bone in the middle ear on one side. On the other side (closer to the brain) it is connected to a thin, spiral-shaped covering within a bony

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Figure 3.2 Inner (bottom row) and outer (three upper rows) hair cells within the inner ear are shown in this colored scanning electron micrograph. When sound enters the ear, waves form in the surrounding cell fluid called endolymph. The waves cause the hair cells to move, which generates a nerve impulse that is passed to the brain.

structure called the cochlea, which resembles a snail. The cochlea forms a closed compartment filled with fluid. Small hair cells are immersed inside it. Although referred to as hair cells, they are not biologically similar to typical hairs found on the skin or on the head (Figure 3.2). Instead, these hair cells are tiny extensions with roots attached to a membrane (known as the basilar membrane). The extensions float freely within the fluid space of the cochlea.

The Ear

Air molecules “tap” on the eardrum, which then vibrates the bones of the middle ear. The middle ear in turn vibrates the oval window, which causes the fluid inside the cochlea to vibrate. The slightest fluid motion is sensed by the floating hair cells, which begin to “swing” similar to the way algae or corals sway in shallow ocean waters when moved by gentle waves.

When the fluid inside the cochlea vibrates, the roots of the hair cells are gently pulled and stretched as they sway in the fluid medium. The roots of the hair cells are directly attached to neuronal terminals. Therefore, as hair cells sway, their roots wake up the neurons. This interaction between hair cells and neurons is directly related: the “stronger” the initial vibration that is transmitted to the cochlea, the “stronger” the hair cells sway, the “stronger” the excitation of the neurons, and thus the “louder” the sound is perceived to be.

THE DIRECTION OF SOUND When a person hears a sound, he or she turns to look for the source of the sound. This immediate attempt to locate the sound source is not random but rather is specific and well executed. How do we correctly guess the direction of sound?

This question is especially intriguing when sound happens without any visual cues—that is, when sound or noise is not clearly associated with a visual event. The answer to this question lies in how our system of hearing is set up.

Consider, for example, a sound coming from the left side of the body. This sound will reach the left ear slightly before it reaches the right ear simply because the left ear is closer to the sound source than the right ear is. This is called a time

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Figure 3.3 The cerebral cortex consists of four different areas known as lobes. Regions of each lobe are responsible for different functions, such as hearing, smell, and vision.

delay, and it alerts the brain to the source of the sound and may prompt an immediate rotation of the head in the direction of the sound. Another hint that the brain uses to correctly guess the source of the sound is the difference in the sound intensity that reaches the ears. In the previous example, not only will the left ear receive the sound first, it will also receive a sound that is just a little bit louder than what the right ear receives. What happens if the sound comes from neither left nor right, but from straight ahead? In this case,

The Ear

sound will reach both ears at the same time (and with the same intensity). If the visual event that goes with the sound is not obvious at first (for example, a mosquito too small to be easily observed), the brain will command the eyes to look straight ahead until the source of the sound is located.

FROM THE EAR TO THE BRAIN Neurons in the inner ear gather to form the auditory nerve.

The auditory nerve transmits signals to many brain areas, including deep brain structures and the cerebral cortex (Figure 3.3). The journey of air molecules vibrating because of drumbeats, clapping, or singing ends in the brain, where sound is ultimately perceived. Mysterious as the human sensory experience is, the way neuronal signals in the brain cause a sensory phenomenon is still being studied. This limitation in understanding brain function and its relation to human consciousness is not limited to hearing. Scientists are still figuring out the exact processes involved in touch, pain, vision, smell, taste, and higher brain functions.

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4 Vision Sight is what we perceive when our eyes are open and there is enough light in the environment. Humans cannot see in complete darkness, and therefore any helpful discussion of vision must include an explanation of the physical properties of light and the reflection of light on objects to produce colors. The sight of lit objects, including still images (photographs, trees) or moving ones (a flying bird, a falling star), is our perception of light that is bright enough to stimulate visual neurons when our eyes are open (the term visual neurons here refers to neurons in our eyes that are sensitive to light). Sight is similar to hearing in that they are both the perception of a physical event in our environment that a specialized organ (the ear for hearing and the eyes for sight) changes into an electrical signal that is then sent to the 32

Vision

brain for processing. The whole process of perceiving light is called vision.

To better understand the process of vision, consider the questions below before getting into the details. Keep the following sequence in mind: electrical energy (light) ∆ specific pathway within the nervous system (eye and connections to the brain) ∆ sensory perception (vision) 1. What are the conditions necessary to see an object?

Can you see in total darkness or with your eyelids closed? ∆ Hint: You see “light” reflected off of objects.

2. When you see, do you always see clearly? Do you wear glasses or contact lenses? ∆ Hint: You focus for clear vision.

3. What happens if your eyes do not focus together on the same object? ∆ Hint: Your ability to focus is limited.

4. Why does the sound of a flying airplane usually seem to come after the sight of it? ∆ Hint: The speed of light is faster than the speed of sound.

5. Why do you perceive objects directly in front of you better than those slightly to the sides but within your vision? ∆ Hint: Your field of vision has limits.

These questions form the basis of how vision works. They may be obvious to some people, but nevertheless, further analysis is necessary for normal and abnormal vision to be understood.

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Figure 4.1 The composition of an object affects how it reflects light. In this computer illustration, the sphere at right is opaque and reflects very little light. The sphere in the middle has a mirrored surface that reflects all light that strikes it. The sphere at left is translucent. Light that strikes a translucent surface is both reflected and refracted (bent).

SIGHT IS . . .

With our eyes open, we see objects of different colors lit by an illuminating source—a lamp, a candle, a car’s headlight, or natural sunlight. For humans, the ultimate source of light is the Sun. Without sunlight, we can see only by using artificial light sources. Why is light necessary for vision? Imagine standing in a closed room with only one light source. Objects in that room can be perceived only when the light is on. When

Vision

the light is off, objects disappear from our sight. Switching a light from on to off does not cause objects to mysteriously vanish. It is more logical to assume that the objects remain where they are but that the light “gives them appearance,” or “brings them to life.” Light is physical energy (similar to sound) that travels in space. When light encounters an object, it will hit the object and reflect off of it, just like a ball bounces off of a wall. In this case, the object is referred to as opaque (Figure 4.1). If the object is too “thin,” however, light may penetrate the object, and the object is said to be transparent.

LIGHT IS . . .

Light is similar to sound in a way. Just like sound, it can be described by its wavelength. Sound is a physical event (movement or vibration of air molecules) that obeys physical laws (travels at a specific speed, in all directions). Light is also a physical event (movement or vibration of photons) that obeys the same physical laws. The following concept is hard to grasp at first: Light is not infinitely fast. When a light bulb is switched on, it may appear as though light is generated instantaneously, but this is not the case. In fact, light travels at a defined and measurable speed, just like sound— although light travels much faster than sound. Because of its extreme speed, light reaches us in almost no time for relatively close illuminated objects. For example, light from a source one mile away reaches us in approximately five millionths of a second. The speed of light is approximately 300 million meters per second (approximately 671 million miles per hour). Compare this speed to the typical speed of a car on the highway (27 meters per second; 60 miles per hour),

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Measuring the Speed of Light In the early seventeenth century, many scientists believed that there was no such thing as the “speed of light.” They thought that light could travel any distance in no time at all. Galileo disagreed, and he came up with an experiment to measure light’s speed. He and his assistant each took a shuttered lantern, and they stood on hilltops one mile apart. Galileo flashed his lantern, and the assistant was supposed to open the shutter to his own lantern as soon as he saw Galileo’s light. Galileo timed how long it took before he saw the light from the other hilltop.

He did not find significant delay because it takes light less than 10 millionths of a second to travel a mile, which was too fast to be measured at the time. The speed of light was more or less accurately measured half a century later by two French scientists (Armand Fizeau and Leon Foucault). Each used a slightly different technique but Fizeau relied on a lantern, a mirror, and a fast-rotating toothed wheel. The wheel was placed between the lantern and the mirror so that, as the wheel rotated with a known speed, the light flickered through the gaps in the wheel and hit the mirror. If the wheel rotated at a certain speed, the light would not return to its source because it hit the teeth instead of the gaps in the wheel. Taking into consideration the speed of the wheel, the distance between the mirror and the wheel, and the distance between two teeth of the wheel, the speed of light was measured with an acceptable accuracy. The speed of light was further refined at the turn of the twentieth century by Albert Abraham Michelson to be 186,355 miles per second. In 1983, the value for the speed of light was defined as 299,792,458 m/s (186,282 miles/s).

Vision

or that of a bullet fired from a gun (1,000 meters per second; 2,237 miles per hour).

When illuminated objects are far away from us, although light eventually reaches us, it does so with considerable delay. It is thought that some stars we observe shining at night may not exist at the time we see them. This is because some stars are so far away that light from these shining stars takes months or even years to reach us. Imagine someone running toward you from a starting point a few feet away and another person starting a mile away. Who would reach you first if they both run at the same speed? Similarly, light “escapes” from a star and sets out on a long journey through the vast emptiness of the universe to get to you; that star could have exploded and disappeared before its light reached you. The result of this is that the glittering stars we enjoy on a summer night may not be the real picture of what is in the universe at the time we are gazing up at the night sky. What we see is relative to how far the object is from us.

LIGHT PARTICLES TRAVEL IN WAVES THAT EVOKE COLORS What is the “physical” object that we call light that is capable of traveling from distant places? Light is made up of particles called photons. Photons are so small that we cannot see them with the naked eye or any type of microscope.

Light is created whenever an event frees enough energy to move, or animate, photons. Animated photons vibrate at a certain speed and with certain, specific characteristics that determine the intensity of the light (brightness) and color (reddish for weaker light and bluish for more intense light).

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Figure 4.2 Light that passes through a prism is split into the full spectrum of light. Each color of light has a specific wavelength.

In fact, ordinary artificial light (electric lamp) or natural light (sunlight) is perceived as white to yellowish in color, which is actually a combination of all colors. White light is what you get when all colors of light combine (including blue and green). White color can in turn be separated into individual colors as it travels through space and encounters certain objects. For example, when light hits water vapor in the air during a light rainfall (or just after a heavier rain), a rainbow may appear. That is because the sunlight is bouncing against the tiny drops of rainwater still hanging in the air. This splits sunlight into all the colors of the rainbow—or the spectrum of light (Figure 4.2).

Another way to demonstrate the nature of light is by looking at a natural phenomenon that happens every day: sunset.

At noon on a clear day, the Sun is very bright and white. This is the time of day when the Sun is directly vertical to the

Vision

Figure 4.3 A picture of a sunset reflected on a lake at Superstition Mountain Country Club in Arizona. Sunsets occur at different times each day and are noted for the soft shades of red, orange, and yellow light that cover the sky.

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surface of Earth. The closer the Sun is to Earth (at noon), the more intense the light will be and the brighter the sunlight.

At sunset, however, Earth’s surface (at the point at which we stand to observe the sunset) rotates away from the Sun. As we move away from the sun, the distance between the Sun and us increases. This makes it more difficult for light (made of photons) to reach us because the light has to travel a larger distance at the same speed it travels any other time. When the light finally reaches us, it is less bright than at noon. In addition to the less intense light, sunsets are characterized by the smooth and rather pleasant transition from bright yellowish light to a “softer” orange, and finally a reddish color as the sun sinks behind the horizon (Figure 4.3). Partly cloudy skies at sunset may appear completely red minutes before dark. When the Sun quietly disappears, it does not actually dim the way a light switch in your home might dim. In other words, the light intensity of the Sun is still the same, but it loses more energy as it reaches us, which results in a change in color.

AT FIRST SIGHT With an opaque object, light is reflected off of it and travels in many directions, including toward our eyes. If asked to identify what we see, we usually start by describing the object’s shape and color. The shape is determined by the different reflections of the light off of the different parts of the objects. If the object is a simple box, we describe it as such because light that hits the different corners of the box is reflected in such a way that the front edge of the box emits reflected light a bit more “strongly” than does the back edge (giving the impression of perspective). This results in light

Vision

that reaches our eyes at different intensities. Stated differently, light reflected off of objects can be compared to a mold or cast that, when hit with your fist, will retain the shape of the fist. In a similar way, light that hits an object will retain the exact form of that object and reflect it in many directions.

Once reflected light reaches our eyes, we perceive the shape and color of the object.

LIFE IN COLORS Shape, color, and dimension are what we see in any object.

But though you might say, “My house is brown,” or “My shirt is red,” objects do not “have” colors. They simply reflect light. So what exactly is color, then?

First of all, objects must be opaque (not see-through) in order for them to “have color.” Once you know that, though, you might wonder why certain apples look red and not blue or green, when all they do is reflect light. Where does the quality of redness come from? Remember that all light is white, but that it can be split into the colors of the rainbow.

When light hits a red delicious apple, we see the apple as red.

This is because the apple absorbs all the colors in the light spectrum except red, which is reflected back and absorbed by our eyes.

Transparent objects, however, do not react this way to light. For example, glass looks clear because light passes right through and does not break up. Why, then, even though air is also transparent, does the sky on a clear day appear blue? And why, if the ocean is made of water, does it look blue, green, or gray?

Let’s start with the first question: The sky looks blue because of a layer of air in the sky that reflects the light as

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blue. Molecules of air in that layer split up the white light from the sun, which you see as the color blue. That bright blue sky is part of what causes ocean water to look blue, too.

Ocean life (plant and animals) and sand are opaque objects that mix with the water to give it an opaque surface. The surface of the ocean reflects the blue sky almost like a mirror.

A clean, healthy ocean looks blue on a sunny day because it reflects the color of the sky. On a cloudy day, that same ocean will appear pale or whiter than usual because white light from the clouds is reflected in the water.

5 The Eye It may sound bizarre to compare an eye to an ear, but these two sensory organs have many features in common. A discussion of the similarities may help clarify how the nervous system transforms light into vision and sound into hearing. This will also answer the following questions and explain the links between other types of sensory perception and corresponding organs: 1. Does the eye contain specialized neurons that sense light?

2. Are the light-sensing neurons in the eye connected to a nerve that transmits information about light to the brain?

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3. Can the human eye detect all visual stimuli detected by other animals such as cats or bats?

In contrast to the ear, the eye is connected to a set of muscles that allows it to achieve a wide range of motion without a person having to rotate his or her head to locate a visual cue.

This range, however, is not complete, and rotating the head is often necessary to follow a moving visual target.

EYE MOVEMENTS Unlike the human ear, the human eye can move, and it is protected by an eyelid that closes regularly (blinks). The eye is controlled by muscles that contract to move the eye in all directions except backward. These are the extraocular muscles. A set of two muscles connected to either side of the eye permits left or right gaze. However, the eye actually rotates away from the side of the contracting muscle and toward the muscle that is simultaneously relaxing. Eye movements are mostly under conscious control and therefore obey brain commands. Accordingly, muscles connected to the eye are themselves connected to the brain by nerves and respond to brain commands for eye movements.

Most often eyes move at the same time, and in the same speed and direction. Not only are both eyes under conscious brain control, but they also receive similar commands from neuronal pathways. This descending brain control first comes from both sides of the brain (the left side of the brain controls the right eye and vice versa) and then meets on a specific nucleus in the brain stem before separating again into left and right muscle command pathways. This meeting in the brain stem ensures that the eyes move together.

The Eye

Figure 5.1 The eye converts light into electrical signals that are passed on to the brain by the optic nerve.

ANATOMY OF THE EYE But what is the eye made up of? The answer to this question lies in the anatomy of the visual pathway, which starts from the cornea and the optic nerve in the eye and goes to the back of the brain (Figure 5.1).

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The Cornea The front part of the eye is covered by a transparent sheet, or membrane, that can easily be seen in the mirror. This membrane is the cornea, a clear surface that covers the iris and pupil (discussed in the next sections). The eye is a fragile and important organ that is exposed to the outside environment.

The cornea provides protection against physical damage and foreign objects (such as insects and germs) because it is as strong and durable as plastic. It is also as transparent and clear as glass to allow as much light as possible to enter the eye. In addition, the cornea resembles a special glass that functions like the eye’s outermost lens. This focuses the light onto the retina. Another role for the cornea is protection against damaging ultraviolet (UV) radiation from natural sunlight, which can be harmful to neurons in the retina. Unlike most tissues in the body, the cornea does not receive a blood supply (perhaps to remain as transparent as possible—blood vessels may interfere with light) and therefore relies on tears for nourishment. The cornea is also filled with many neurons that are sensitive to painful events such as rubbing or scratching the surface of the eye.

The Iris The iris is the colored part of the eye. It is in fact a muscle that cannot be consciously controlled. In contrast to the transparent pupil that it surrounds, the iris is opaque. Pigment in the iris gives the external color, such as blue, green, or, more commonly, brown (or a combination of these colors) to the human eye. Pigmentation may change slightly during the first year or two after birth, but eye color almost always remains permanent afterward. Surrounding the iris is another

The Eye

opaque surface of white color called the sclera. Although normally white, the sclera has many small-diameter blood vessels that cross just underneath to supply oxygen and nutrients to neurons inside the eye. These blood vessels may dilate, becoming more visible, when a person hasn’t slept or is exposed to prolonged high winds (driving with the windows down) or chlorinated water (swimming in a pool). Such conditions give the sclera a reddish color (red eye).

The Pupil In the outermost part of the eye, light first penetrates the cornea and then enters through a transparent “hole” or “window” called the pupil, which is surrounded by the iris.

The pupil of the human eye is dark. Dark objects absorb all natural light, while white objects reflect all of the wavelengths of light. Think of it this way: On a hot summer day, if you stand outdoors exposed to the sunlight, you feel warmer wearing dark clothes than you do in paler colors. Dark fabrics absorb more light, “trapping” more energy in the form of heat. (That is a helpful tip for your next trip to the beach: remember not to wear a black T-shirt!) Likewise, having a dark pupil allows the human eye to absorb as much light as possible.

Whereas the diameter of the iris does not change, the diameter of the pupil does change. In fact, the pupil is a “hole” that varies in size depending on the intensity of the light. Small muscles in the iris adjust the diameter of the pupil. When these muscles contract, the pupil’s diameter enlarges, thus widening the pupil’s size to allow more light into the eye; this process is referred to as pupillary dilation (Figure 5.2). In contrast, pupillary contraction occurs when the iris muscles

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Figure 5.2 This photograph, taken in dim light, shows the human eye with a dilated pupil. Pupillary dilation occurs when an iris needs more light, if a person is aroused, or it can be induced by drugs. Extreme dilation is also known as mydriasis.

relax, thus decreasing pupillary diameter. Therefore, in bright light, iris muscles contract and decrease pupil diameter. The size of the pupil changes rapidly in response to light; this is easily observed when bright light is directed into someone’s eye. In fact, pupils in both eyes will diminish in size even if only one eye is subjected to direct light.

Humans cannot consciously adjust the size of their pupils; therefore, it is referred to as a reflex. Similar to quickly and unconsciously withdrawing an arm in response to a hot stove,

The Eye

Figure 5.3 A reflex is an action that is performed without conscious effort. The knee-jerk reflex is controlled by neurons within the spinal cord. When the kneecap is tapped with a mallet, sensory neurons transmit a signal to the spinal cord. The signal is then relayed to the quadriceps muscle, which contracts and causes the leg to kick up. The papillary reflex (increasing or decreasing the size of the pupils) works in a similar way. Unlike the knee-jerk reflex, however, the papillary reflex is controlled by neurons in the brain stem.

the amount of light allowed to enter the pupil is an automatic action that occurs unconsciously. As is the case of an arm jerking away from a hot stove, specialized parts of the spinal cord control the reflex (Figure 5.3). Pupillary reflex, however, is controlled by neuronal structures in the base of the brain

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(the brain stem). The pupils in both eyes contract or relax to the same extent. When body movements are well coordinated, almost in an automatic or robotic way, such as in eye movements, walking or jumping, it is generally a result of the synchronized control of neurons in the spinal cord (for walking and jumping) or the brain (for eye movements). Neurons in the brain may also contribute to the control of walking and jumping in a different way: The brain sends a general order to a neuronal command center in the spinal cord, which then takes care of the second-by-second control of the relevant group of muscles in the legs to work together. The brain also sends another command to speed up or stop.

So, if bright light is directed at only one eye, the pupillary reflex will be evoked in both eyes (even if the other eye is in the dark). Medical doctors and emergency health professionals use this valuable information to test for brain damage after events such as a car accident or a fall. They do this by directing a light into the eye of a traumatized, unconscious person. An absence of the pupillary reflex indicates severe brain damage that involves the brain stem.

The Lens After passing through the pupil, light reaches the lens of the eye. Without a lens, light would spread throughout the internal surface of the eye. Light rays travel straight if uninterrupted, but they may either reflect back on their original source if they encounter an opaque object (for example, a mirror) or continue to travel through a transparent object but change from a straight path (Figure 5.4). As a result, transparent objects bend (refract) light differently, so that each transparent object has a different refractive index.

The Eye

Figure 5.4 Light is refracted when it passes through glass or the lens of the eye. The angle at which the light strikes the surface is known as the angle of incidence (Θ1). The angle of the light that is refracted is known as the angle of refraction (Θ2). The amount of refraction is equal to the difference between the angle of incidence (Θ1) and the angle of refraction (Θ2). The greater the difference, the more the light has been refracted.

For example, a diamond has a very high refractive index compared to glass or water; therefore, light is highly refracted by diamond, resulting in a diamond’s shiny glitter.

As photons (tiny particles that make up light) cross from one medium to another (such as from air to glass or water), their speed is slowed by the atoms within the medium. This loss of speed is evident by the transfer of energy from photons to

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the atoms that make up the transparent object, mainly in the form of heat (objects penetrated by light heat up).

The lens not only refracts light that enters the human eye, but also focuses it on a particularly sensitive area in the back of the eye for the detection of photons. Light rays enter the eye at different degrees, however, depending on the location of the light source. The lens then adapts to different light angles to refract different light rays properly and “bends” them to strike the sensitive area in the back of the eye. The refractive index or power of the lens to adapt is a process called accommodation.

The eye’s lens is connected to muscles located behind the iris within a structure called the ciliary body. In people who have normal vision, the ciliary body flattens the lens in order to bring objects into focus at a distance of 20 feet (6 meters) or more. To see closer objects, this muscle contracts to thicken the lens (Figure 5.5). Young children can see objects at very close range, whereas many older people have to hold objects farther and farther away to see them clearly. This is because the lens becomes less elastic as people age. Just like a camera lens, the eye’s lens focuses light to form sharp, clear images. It is important to note that distant objects tend to emit light in a nearly parallel trajectory, thus requiring minimal refraction by the eye for proper accommodation.

Light emitted by closer objects reaches the eye along a more diverging path, thus requiring “stronger” accommodation to “converge” them back into the photosensitive area in the back of the internal eye. In this case, the ciliary body contracts and thickens the lens, which refracts light more.

Because we are constantly bombarded by light sources at close range, such as computers and televisions, it is recommended that we relax our ciliary bodies by taking a break

The Eye

Figure 5.5 Ciliary muscles relax and the lens flattens to focus on distant objects (top). To focus on close objects, the ciliary muscles contract and the lens becomes more round (bottom).

and looking at faraway objects, such as a landscape in natural sunlight, for at least a few minutes daily.

The Retina Light that enters the eye eventually hits the photosensitive area in the back of the inner eye called the retina. The retina contains a layer of cells sensitive to light known as photoreceptors. All of the structures in the eye serve three main

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purposes: (1) to protect the eye from foreign objects such as insects and microbes, (2) to capture light most efficiently, and (3) to focus light on photoreceptors in the retina. The photoreceptors are connected to neurons that transfer light-related information to the brain so we can see objects. Without the retina, normal vision is impossible.

When a stimulus in the environment comes in contact with our skin, we perceive the touch sensation as a gentle stroke, tingle, pressure, or pinch depending on the properties of the stimulus. In addition, every object has a temperature that is detected by specialized receptors on our skin. Neurons then transform these physical phenomena (touch and temperature) into sensory experiences that are often memorable if either pleasant (such as a kiss) or unpleasant (such as a burn). This principle of transforming physical energy from the external environment into codes that the brain can decode as sensation also applies to vision. Light focused on the retina excites photoreceptors that create electrical activity that in turn excites neurons connected to these receptors.

There are two types of photoreceptors in the human eye, which are named according to their shape (Figure 5.6). These photoreceptors resemble a cone or a rod, and thus they are referred to as cones or rods. They differ not only in size and shape, but also in how they work. Rods are more numerous (roughly 120 million per eye) and are more sensitive to brightness or light intensity than cones. Cones are more sensitive to color, however, so the 6 to 7 million cones provide the eye with color sensitivity. Cones are also “tuned� to certain colors of the light spectrum and are better adapted for vision during the day and in bright light. In contrast, rods are better adapted for dark vision or vision in dim light. Cones also detect details in a visual stimulus such as small-type on a page or the fine

The Eye

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Figure 5.6 Above, a cross-section of the human retina, a thin membrane that lines the back of the eyeball, containing photoreceptor cells known as rods and cones. The rod cells (white) are responsible for distinguishing between light and dark, while the cone cells (yellow) are responsible for color vision and acuity.

texture of an object. In contrast, rods tend to be less sensitive to details and rely mostly on the general features of an object such as its outline or rough dimensions such as height.

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The Macula, the Fovea, and the Foveola Photoreceptors and neurons within the retina are not spread out on the retinal surface equally. These cells are densely packed within one area of the retina called the macula. In addition, at the center of the macula, there is a smaller area where only cones, and no rods, are found. This area in the macula is called the fovea centralis (or simply, the fovea).

Within the fovea, an even smaller area called the foveola is more densely packed with cones.

Cones allow humans to have sharp vision. Loss of cones or damage to the retina at the macula causes legal blindness, which is blindness as defined by law. A legally blind person can detect some light, rough shadows, and shapes, but not letters or signs. The eye moves constantly to keep the source of light reflected from the object of interest falling on the fovea, where cones are found in the highest density.

Cones also provide the eye’s color sensitivity. Each group of cones responds in different ways to different colors. In fact, each group of cones may be sensitive to different wavelengths of light. It is estimated that millions of cones—more than half of the cone population—can be classified as “red” cones. “Green” and “blue” cones make up the rest of the population. The “green” and “red” cones are concentrated in the fovea centralis. The “blue” cones have the highest sensitivity and are mostly found outside the fovea, leading to some distinctions in the eye’s perception of the color blue.

Natural light is the combination of all colors. Daylight, therefore, would be expected to stimulate all types of cones in the fovea, whereas light reflected from a red apple will stimulate “red” cones much more than the “green” or “blue” cones. The fovea is the point of sharpest vision because of the high density of cones.

The Eye

The World Upside Down Light waves from an object, such as a tree, enter the eye first through the cornea, which is the clear dome at the front of the eye. The light then progresses through the pupil, the circular opening in the center of the colored iris. Next, the light passes through the crystalline lens, which is located immediately behind the iris and the pupil.

Initially, the light waves are bent by the cornea and then further by the crystalline lens, to a nodal point (N) located immediately behind the back surface of the lens. At that point, the image becomes reversed (turned backward) and inverted (turned upside down).

Images formed on the retina are reversed and upside down. When the image is processed by the brain, it is restored to its correct orientation.

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Because the fovea is located roughly in the center of the macula (itself located in the center of the retina in the back of the eye), the ability to see details and colors is dependent on light hitting the fovea. In other words, a person must look straight at the object in question by coordinating eye movements and head rotation to bring that object into his or her central field of vision.

Rod Photoreceptors In spite of the contribution of cones to color vision and sharpness of vision, cones are in fact less sensitive to light than rods are. Rods are incredibly efficient photoreceptors, about a thousand times more sensitive to light and much more numerous than cones. Being less sensitive to light, cones respond better to strong light, whereas rods, being more sensitive, respond to both weak and strong light. It turns out that rods require more time (a few seconds to as much as 10 to 20 minutes) than cones to adapt when suddenly exposed to light.

Daylight vision (cone vision) adapts much more rapidly to changing bright light levels. Cones can adjust to rapid color and intensity changes in less than a few seconds.

Differences in daylight vision and night vision can be demonstrated easily. For example, a person needs a few minutes to adjust fully when stepping from bright daylight into a dark room, or when driving in the open on a clear day and suddenly entering a tunnel. This type of vision is mediated by cones. Night vision is not affected by colored lights because rods are not sensitive to color. This is partly why cars are equipped with red taillights, which do not disturb night vision as much as “white light,� such as that produced by a car’s headlights.

The Eye

HOW DO CONES AND RODS WORK? From its source, light (natural sunlight or artificial electrical light) bounces off objects and toward the eye, where it enters through the cornea and the pupil. The light travels all the way to the retina, where photoreceptors (cones and rods) await the light after its long journey in space and within the eye. The cones and rods contain proteins that are deformed by light photons and initiate a chemical reaction that results in an electrical current. In fact, cones and rods are neurons. Like all other neurons, they generate electrical signals in response to proper stimuli or relay the message from other neurons.

Cones and rods contain different light-sensitive proteins.

In rods, the protein is called rhodopsin. Rhodopsin breaks down into two different molecules called opsin and retinal when it is exposed to even one photon. Interestingly, retinal is a derivative of vitamin A. Because carrots provide a natural nutrient source for vitamin A, it is commonly believed that eating carrots aids vision. Although there is some truth to this belief, it is misleading to think that carrots can treat serious visual problems such as astigmatism or cataracts.

Light causes electrical activity in rods and cones that are connected to other neurons that are, in turn, connected to other neurons. In this visual pathway, the message is carried from one neuron (starting in the rods and cones in the retina of the eye) to the next in the pathway until it reaches specific brain areas. Like all neurons, cones produce an electrical impulse that travels along the nerve fiber and then must reset to fire again. The light adaptation is thought to occur through adjustment of this reset time, which simply takes longer in cones. Brain areas that contain neurons that finally receive and process this neuronal electrical signal are located mainly

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Figure 5.7 Visual signals crossover to the opposite side of the brain for processing. The left side of the brain is responsible for processing the right visual field (red area in front of eyes), while the right side of the brain processes the left visual field (blue area in front of the eyes).

in the back of the brain. All the light information that has been converted in the retina to an electrical signal is sent outside of the retina through the optic nerve in each eye.

Tracing the visual pathway from neurons in each eye to the visual centers in the back of the brain is complicated, because

The Eye

some neurons send their axons along a twisted path to many brain centers, including in the brain stem. There are two portions of the optic nerve in each eye that meet in the middle of the brain, forming the optic chiasm (Figure 5.7).

Beyond the optic chiasm, the nerves carrying visual information toward the back of the brain are referred to as optic tracts (instead of optic nerves). This complex migration of nerves from the eye ensures that the left side of the brain is responsible for vision of objects viewed on the right side of the body (or in the right visual field) and vice versa. This is not surprising if one compares the visual system to other sensory pathways, such as that for touch, whereby the right side of the body is “felt� by the left side of the brain and vice versa. This setup is not limited to sensory systems, but is also a property of the motor system: Willful orders to move the right side of the body are initiated on the left side of the brain and vice versa.

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6 Visual Abnormalities The cones and rods are nourished by many blood vessels that lie just beneath the surface of the retina, forming a layer known as the choroid. Cones and rods in the retina are highly active (almost all the time your eyelids are open), requiring maintenance and nutrients from the choroid layer. The photoreceptors also generate waste chemicals as by-products of their high activity level. The outermost surface of the retina creates a critical passageway for nutrients from the choroid to the retina and helps remove waste products from the retina to the choroids. This layer is called the retinal pigment epithelium (RPE).

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Visual Abnormalities

COMMON VISUAL DEFECTS The RPE normally deteriorates with age and can lose its pigment and become thin. As a result, the waste-removing and nutritional functions between the retina and the choroid can gradually deteriorate. Lacking nutrients, the light-sensitive cells of the macula become damaged. The damaged cells can no longer send normal signals through the optic nerve to the brain, and vision may become blurry. This is often the first symptom of the condition known as macular degeneration.

Other visual abnormalities include astigmatism, nearsightedness, farsightedness, strabismus, cataracts, and color blindness. The first three of these conditions relate to one common mechanism in the eye called accommodation. Recall how light enters the eye through the cornea first and then through the pupil and the lens and travels all the way to the back of the eye, where the retina contains photoreceptors. The main function of the lens is to focus the entering beam of light onto the retina.

If the muscles that control the accommodation power of the lens are weak, light from far-away objects will be focused behind the retina and therefore the image of the object will be out of focus. Similarly, nearby objects could be out of focus with the image focused in front of the retina. Both of these conditions can be corrected by wearing eyeglasses or contact lenses. These lenses help bring images within the retina in focus. â—† Astigmatism is a visual defect that causes blurred vision as a result of an abnormal curve of the cornea.

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Figure 6.1 Nearsightedness and farsightedness are corrected by using a lens to move the focal point to the correct location on the retina. The path of light without correction is marked by black lines in the illustration above.

Wearing eyeglasses or contact lenses can generally improve vision. â—† Nearsightedness (myopia or shortsightedness) occurs when the lens of the eye focuses light in front of the retina instead of directly on it (Figure 6.1). This causes the image of an object to form slightly in front of the retina, making it blurry. People with myopia do not see well far away but can see close objects clearly. This condition tends to become gradually worse with age.

Laser eye surgery is a treatment option that changes the shape of the cornea. â—† Farsightedness (hyperopia) is a condition in which the incoming image is focused behind the retina, resulting in a blurred image of close objects (distant objects

Visual Abnormalities

Figure 6.2 The grey, opaque mass obscuring the pupil of the eye is a mature cataract. Aging, steroid use, and diseases such as diabetes can all lead to the onset of cataracts. Although cataracts never cause complete blindness, a person’s sight becomes limited and vision progressively worsens if not corrected by surgery.

are still seen clearly). Corrective lenses can help this defect, but the condition may get worse with age. ◆ Cataracts are a cloudiness of the eye’s lens that prevents light from reaching the retina (Figure 6.2).

Having cataracts has been described as looking through a dirty window. Corrective lenses are not a good option for people with cataracts. Rather, surgery to remove the cataracts is common. It is advisable to treat cataracts at a young age to prevent permanent visual defects. ◆ Color blindness is the inability to detect or perceive one color, some colors, or all colors. It is generally

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caused by the absence of certain cones in the retina that are responsible for detecting colors. People with color blindness usually do not have other visual defects. The most frequent type of color blindness is the inability to distinguish red and green pigments.

This condition is mostly genetic. Unfortunately, it cannot be corrected. ◆ Complete blindness is complete insensitivity to light.

This severe condition may be temporary or permanent.

Damage to any of the structures of the eye, by disease (such as diabetes), accident, or old age may lead to blindness. Permanent blindness, for example, can be caused by an object penetrating the eye and severely damaging the retina (where photosensitive neurons transform light into electrical signals). It could also be caused by a tumor growing outside the eye and pressing against the optic nerve, interrupting the flow of information from the eye to the brain. ◆ Conjunctivitis is inflammation of the “external” eye. It is associated with redness around the pupil and sometimes pain. One type of conjunctivitis is called pinkeye.

Some forms of conjunctivitis result from allergies or a scratch on the surface of the eye and can be easily treated with medicine in the form of eyedrops. ◆ Strabismus is a condition in which the eyes look crossed. Strabismus results from a muscle coordination defect that may later lead to a visual defect because images formed on the retina may not match in both eyes. Corrective lenses cannot solve the problem of strabismus. Instead, surgery, especially early in childhood, can be used to prevent permanent visual defects.

Visual Abnormalities

Untreated visual defects may also cause severe headaches.

Proper eye care may prevent damage that causes visual defects. Such care includes periodic eye exams by an eye specialist.

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7 Smell and Taste Smell and taste are chemical senses that provide us with valuable information to explore our environment. Thanks to smell, we are constantly testing the quality of the air as we breathe.

Aside from being used to smell perfumes and food, the sense of smell can save lives. For example, people often detect the smell of smoke from a hazardous fire before they hear an alarm.

Even newborn babies make faces that indicate their dislike of fishy or rotten odors; however, the sense of smell declines with age. Older people gradually lose their sense of smell to the point of being anosmic (unable to smell a certain odor or several odors, probably because of the loss of neurons). With the loss of the sense of smell comes the loss of the sense of taste. As is most obvious in conditions of nose blocks caused by a common cold or 68

Smell and Taste

a runny nose, the sense of smell contributes to the sense of taste (this is why food tastes different when you are sick and stuffed up). Smell and taste are both essential for animals to explore their environment. Some scientists estimate that humans can distinguish among as many as 10,000 different smells.

Gases mixing with objects or evaporating from them carry certain molecules characteristic of these objects. These molecules may reach the nose and dissolve in the mucus, helping to generate particular odors. These molecules are referred to as odorants. To be able to reach us, odorants need to be volatile (dispersed in air).

Our sensitivity to smell depends on more than just the strength of our senses. For instance, we are less sensitive to odorants in the cold for two reasons. One reason for this is that gases evaporate less in cold weather than in warm weather; therefore, spring and summer are the best times to smell. Another reason is that warm weather makes odorants more soluble and so they mix better with our mucus. This enhances our sense of smell.

Odors are distinctive—so distinctive that certain animals, including common pets, use them to identify other animals or humans. Odorants can have a powerful influence over mating behavior, whereby secreted molecules may prepare an animal for pairing. In addition, strong odorants secreted in a dog’s urine are used to mark territory. In these cases, odorants are more accurately called pheromones (Figure 7.1).

Although the sense of smell is highly developed in humans, pheromones are thought to influence human behavior less than animal behavior.

The sensitivity of a dog’s sense of smell allows it to determine the direction or trail of a human by odor. Apparently,

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Figure 7.1 Pheromones are chemicals produced to send messages to other members of the same species. In the above photograph, a honeybee fans pheromones from its Nasanoff gland, a common form of communication with other bees.

each one of us has a unique smell or body odor. In fact, even certain types of twins have distinct body odors. Identical twins, however, who share similar genes, also share similar odors; therefore, dogs cannot distinguish between identical twins based on odor.

THE NOSE Odorants, when inhaled, enter the nose through the nostrils.

In the roof of each nostril is a region called the nasal mucosa.

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Figure 7.2 Smell receptors are located in the nasal cavity. Once a receptor is stimulated, an electrical signal is created and passed on to the olfactory bulb, which then relays the information to the brain.

This region contains the mucus-covered olfactory epithelium that in turn contains the sensory receptors or neurons.

Humans have approximately 10 million olfactory receptors; other animals such as rats and cats have more. The sensory sheath at the roof of the nostril also contains glands that produce mucus that bathes the surface of the receptors. This is where odorant gas molecules dissolve.

Inside the nose, air travels toward odorant-sensitive cells (neurons) that lie close to the bony structure at the top of the nasal cavity (Figure 7.2). These cells have extensively branched dendrites with receptors for different gaseous odorants.

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When activated by an odorant, these neurons generate an electrical current that is relayed to another neuron above the nasal cavity. These two types of neurons are separated by a bony structure with small holes that allow communication of electrical signals. Above the nasal cavity, a collection of axons forms a swelling called a bulb (one per nostril). The axons then carry odorant information to the brain through distinct pathways. The brain processes this information as a perception of smell. Some odorants are strongly linked to powerful memories and therefore are processed by multiple brain areas.

ANOSMIA Severe head injury may damage communication among neurons in the smell pathway. This may cause a medical condition known as anosmia. Anosmia can be either the complete absence of smell or loss of the ability to smell particular odorants. Anosmia may also be temporary, caused by less serious conditions such as a common cold with a running or stuffy nose. In this case, the nasal cavity and membrane may be inflamed and neurons do not process odorants as well if the nasal cavity is very wet. This common type of anosmia is obviously reversible, whereas anosmia caused by brain injury is usually permanent. Other conditions may trigger anosmia, including allergy to certain odorants or thick smoke.

The sense of smell also undergoes adaptation. Notice how we become accustomed to an odorant after we are exposed to it for a long time: We lose our awareness of the smell. A typical example of adaptation to smell is reduced sensitivity

Smell and Taste

to one’s own perfume or natural smell after a short period of time.

TASTE The last sensory experience to be discussed in this book is taste. This is not because taste is the least important of the senses, but it is the least understood. Perhaps if most humans were asked to choose between losing sight or hearing versus losing taste, many would prefer to keep their visual and auditory capacity rather than the ability to taste food.

However, savoring food is not the only function of taste.

Before meats and vegetables were available in stores, people had to hunt for meat or grow plants for food. Many chemical substances available in nature, such as certain mushrooms, are in fact toxic to humans. While gathering foods in nature, humans had to rely on taste to distinguish toxic substances from healthy ones. It is interesting to note that, generally, toxic materials tend to have a sour or bitter taste, and therefore, it is natural to develop food aversion to bitter-tasting food. (Many sour foods are healthy though—for example, citrus fruits.)

THE TONGUE AND TASTE BUDS INSIDE THE MOUTH CAVITY A multitude of colors and a variety of sounds can be distinguished using the senses of vision and hearing. With taste however, only four tastes, or a combination of these four, can be distinguished: sweet, sour, bitter, and salty. Chemicals within the mouth trigger each of these tastes. Just like stimuli

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Figure 7.3 The above image is a colored scanning electron micrograph of the surface of the human tongue. The tongue is covered with protrusions called papillae. Taste buds line the surface of fungiform papillae (round, red). Filiform papillae (pink) are the most numerous and give the tongue its rough surface.

for vision and hearing, stimuli for taste are transformed into electrical signals in neurons that relay taste-related information from the mouth to specific brain centers for conscious experience. Inside the mouth, the tongue is the major organ that contains taste-sensitive neurons. Although it is common to think about taste being intimately associated with the tongue, the sense of smell also plays a major role in human taste experience. A simple way to demonstrate the association between taste and smell is by considering how food tastes when your nose is stuffy.

Smell and Taste

In the tongue, taste-sensitive receptors are found in grooves referred to as taste buds (Figure 7.3). The number of taste buds is estimated to be approximately 10,000. Each bud contains taste receptors that can detect different chemicals in food (or other objects placed on the tongue). These receptors are in turn connected to neurons that generate messages that are ultimately sent to the brain.

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8 Synesthesia We have learned about sound, sight, smell, and taste, and how people experience these audio, visual, odorant, and taste phenomena. This chapter looks at abnormalities, mostly abnormal visual experiences. It then asks, “How can someone see an object that is not really there?” “How can someone hear a color or see a musical tune?” “Is this possible?” “Is there any plausible scientific explanation?” These are sensory abnormalities together known as synesthesia, a unique and rare sensory experience (Figure 8.1). People who have synesthesia are called “synesthetes.” A synesthete is not psychologically disturbed. He or she simply judges sensory stimuli differently than others. Let’s imagine, for example, that our five senses get mixed together, so we hear colors, smell taste, or see noise. This “mix-up” is thought 76

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Figure 8.1 Synesthesia can be described as the mixing of the senses. For example, a person with synesthesia might “hear” the color red or “see” music.

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to be caused by “cross-wiring” that occurs in brain areas that mediate these sensations. Let’s “see” further how this might happen . . .

WHAT IS SYNESTHESIA? The word synesthesia comes from the Greek syn, meaning “together” and aesthesis, or “perception.” Certain synesthetes can hear colors, whereas others can see sounds or feel tastes.

Let us consider John, Mary, and Eddie to be synesthetic.

When John listens to music, he sees the color red. When Mary is playing with her blocks, she feels a sweet taste like chocolate in her mouth. When Eddie sees a red light, he hears a soft melody instead. Synesthesia is a strange biological condition, yet people who experience it are ordinary human beings in other aspects of their lives; the only difference is that some of their five senses are mixed up instead of being separate. We will try to better explain synesthesia by answering the following questions: ◆ ◆ ◆ ◆ ◆

When was it discovered?

How did scientists discover it?

Is it real or fake?

What do people with synesthesia really experience?

What does it teach us about the mysteries of the brain?

WHEN WAS SYNESTHESIA DISCOVERED? Scientists have been aware of synesthesia for many years, but it was not understood very well. Because synesthesia

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Figure 8.2 For many years, scientists have doubted the reality of synesthesia, however, several experiments have affirmed that it is indeed a real condition.

is not a “disease,� little attention was paid to the condition and only a few studies were conducted. Scientists have only recently investigated the mechanisms that underlie synesthetic feelings and explored them within the nervous system, particularly in the brain. Several experiments conducted on volunteer subjects have reaffirmed to many skeptical scientists that those who can see smells or hear colors are not guessing or pretending. What they are sensing is in fact real (Figure 8.2).

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A QUICK LOOK AT SYNESTHETIC EXPERIMENTS Here are the details of a modified experiment: Consider a square. Inside this square, the numbers 2 (in red) and 5 (in black) are grouped together as follows: 2552225 5252552 5522225 2255225 In this example, it is easy for a normal volunteer observer to tell the difference between the number 2 and the number 5 with a quick glimpse because 5 is in black and 2 stands out in red.

Now consider the same square containing the numbers 2 and 5 in which these numbers are both in black color: 2552225 5252552 5522225 2255225 In contrast to the previous example, it is more difficult to distinguish between the two numbers because they seem to be fused together. The only way to tell the difference would be to take the time to read each number.

Now consider a volunteer synesthetic person, someone who happens to associate the number 2 with the color red and 5 with the color blue, for example. For this person, making a clearer distinction between the black 2s and the black 5s does

Synesthesia

not require much extra effort. For this particular synesthetic person, the square shown in the second example (both numbers in black) will appear as: 2552225 5252552 5522225 2255225 Scientists also conducted this pop-out test on volunteers to determine whether an individual who is synesthetic lives a real sensory experience evoked by a physical stimulus (in this example, the visual stimulus of the square and colored numbers) or if synesthesia was merely a product of the imagination of nonexistent stimuli or physical objects. The same numbers were now put in a square, but this time the 2s were grouped together to form the shape of a triangle, as follows: 5525555 5222555 2222255 2222225

5525555 5222555 2222255 2222225

Figure A

Figure B

Results of this experiment were as follows: A. For a normal volunteer, nothing unusual was reported.

As expected, with a quick glance at the numbers, the subject saw black 2s and black 5s but no distinct pyramid (Figure A).

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B. For a synesthetic volunteer, however, the red pyramid was easily identified. In this case, the synesthete associated the number 2 with the color red and the pyramid popped out.

Many other experiments have been performed, and scientists now have significant evidence that synesthesia is real and that it deserves to be explored more thoroughly. In the course of their investigation, scientists have discovered fascinating facts related to sensory experience in the human brain.

WHAT CAN SYNESTHESIA TELL US ABOUT THE MYSTERIES OF THE BRAIN? We need to understand how a normal human brain functions in order to get a better grasp of how a synesthete’s brain works. This book has discussed the mechanisms that underlie normal vision, smell, sound, and taste, and how specific neuronal pathways transform various stimuli into electrical signals that are carried to specific brain areas.

Take the case of a normal subject “seeing” a number, for example, the number 8. When light refracting from the number 8 reaches the retina of the eye, photoreceptors in the retina (rods and cones) generate electrical signals that are relayed through the optic nerve, optic chiasm, and optic tract to the back of the brain. Let us call that visual brain area X. In a similar way, an odorant elicits a smell after it has been transformed into electrical activity that the brain smell area will decode as a sensory smell experience; let us call that area Y. In a simplistic way, a synesthetic person might have a pathway for vision that leads to Y (that of smell) and a pathway that leads to X (vision). Or, that person might have

Synesthesia

a vision pathway that leads to Y (smell), and a smell pathway that leads to X (vision). Other synesthetics may have direct connections between X and Y centers with powerful communication pathways, so that any activation of X may also lead

Synesthesia Experiment Here is a simple synesthesia experiment that you can try at home with a group of friends.

1

2

3 4

5 6

Read a list of random numbers between 0 and 9 at a rate of about one every 3 seconds. For example: 7, 9, 4, 0, 3, 8, 2, 5, 1, 6.

After each number is read, ask people to write down the number and what COLOR that they associate with each number.

Collect the answers. These will be called “Answers #1.”

Two to three weeks later, repeat the experiment, but change the order of the numbers. For example: 3, 6, 5, 9, 4, 1, 7, 0, 5, 2, 8.

Collect the answers. These will be called “Answers #2.”

Compare Answers #1 with Answers #2. A person with synesthesia will have all or most of the same number-color pairs for both Answers #1 and Answers #2.

This experiment can also be done using letters instead of numbers.

Source: Chudler, Eric H. Neuroscience for Kids. http://faculty.washington.edu/ chudler/syne.html.

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to activation of Y. Therefore, any visual experience would also cause a smell associated with an image.

FOLLOW YOUR HEART From the above examples, one can conclude that the brain can trick us. It may elicit thoughts and feelings that others may not judge to be valid. If so, how can we ever trust our brains? To take this point even further, imagine you visit a

Seeing and Perception To the right is an illustration called “Duck-Rabbit.” This is a copy of an original sketch that was drawn in June 1945 by Ludwig Wittgenstein, one of the most influential philosophical minds of the twentieth century. He drew it to emphasize that “seeing” is not enough and that careful investigation and attention are often necessary to “look” and perceive a shape correctly.

In this sketch, two heads can be perceived depending on the concentration or state of mind we are in. If we focus on the ears, we might think that it is a rabbit, but if we focus on the mouth, we are more inclined to say that it is a duck instead. The result is that we can change our perception of the drawing (flip back and forth between duck and rabbit) while the drawing itself does not change.

This is just a fantastic capability of the mind: to interpret what we see differently depending on our concentration, mood, and previous experiences. It is also an extremely valid point to make: The mind is made out of a bunch of connections between cells in the nervous system, but our brains are not like machines and humans are not

Synesthesia

foreign country where the majority of people are synesthetic; they can all “see” perfumes. Wouldn’t it be strange to be the only person who could not see odors? After all, the color red is “red” because the majority of us see it as “red”—but who is to say what is really “red” and what is really “green”?

As you reflect on this dilemma, remember that all of our sensory experiences cannot be analyzed or judged as unmistakable facts, realities, or universal truths; they are just the product of our brains and what our brains decide to interpret

like robots. What to one person is red may be orange or purple to someone else, and what to you smells like an exquisite perfume may be intolerable to your friend.

Figure 8.2 Illustration of a Duck-Rabbit.

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depending on mood, experience, and memories. How many times have we “seen” something that others claim was not there? How many of us hold strong beliefs without scientific evidence? Sometimes we just go with our hearts and feelings instead of our rational minds, and often this is the best course of action.

DO WE TRUST THE SENSES? BE CAREFUL WITH WORDS Can you describe a taste without saying a word? How would you describe “sweet” to a person who cannot hear you? How would a deaf person react to the cry of a baby? These questions are intended to emphasize the importance of language in communicating our sensations and feelings to others. In fact, language is central to human communication, and our sensory experience would go largely unnoticed if we did not talk about it with others.

With this in mind, what is the best way to pick the words that perfectly match our sensations? When we say, “This apple tastes sweet,” how do we know the other person really appreciates sweetness? Does “sweetness” give pleasure to everyone? Does everyone enjoy chocolate? Have you ever met someone who likes sour or salty or spicy food more than an apple pie?

NOT EVERYONE SHARES THE SAME TASTE Not everyone likes classical music. Not everyone even agrees what classical music really is or what it should sound like.

Isn’t that confusing? By the same logic, not everyone totally agrees that what you might be wearing is a burgundy, a

Synesthesia

reddish-purple, or a dark red shirt. Taken further, a person who is colorblind might disagree with the rest of us completely. An anosmic person might walk next to a skunk and smile. In the midst of this confusion about how to bring all people into agreement with regards to words we use so frequently, there must be a rule, some source that we can refer to for the final word in describing sensory perceptions. Right?

Well, not exactly . . .

Take your time to reflect on this rather confusing problem, which has occupied the minds of many famous philosophers.

Talk to others about it, keep your eyes open for disagreements regarding colors and odors. Travel overseas, if possible, and get to know how people from other cultures are moved by different melodies, different images, and the wide variety of tastes. When you do, you will realize that the way that we describe sensations is not written in stone. Language is agreed upon by a society. The majority (but not necessarily everyone) accepts what is “good” music, “bad” food, or a “wonderful” experience.

Next time you use similar words, do not be offended if others do not share your views, or look surprised if someone else does not see at all what you see in front of your own eyes.

After all, what is “green” to you may be “blue” to others. In the end, trust your senses, not the words.

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Glossary Accommodation Ability of the lens to change shape in order to bring

an object into focus.

Adapt To accommodate; to respond favorably to changes in the environment.

Anatomy The physical structure of a body or its parts.

Anosmia Complete or partial inability to perceive or smell odors.

Auditory nerve The collection of neurons that carry information transmitted from the ear to the brain for sound perception.

Axon An extension of a neuron that transmits information to another neuron; may be as long as 3 feet (0.9 m).

Bacterium A unicellular organism.

Brain stem The region at the base of the brain that controls many unconscious functions, including respiration and heart rate.

Central nervous system (CNS) The part of the nervous system contained within the skull (brain) and the vertebral column (spinal cord).

Cerebral cortex The outermost layer of the brain, which contains cell bodies of neurons. The human cerebral cortex is the largest part of the brain in mammals.

Decode To translate a code into a meaningful signal.

Degenerate To degrade or shrink.

Dendrite An extension of the cell body of the neuron that receives information.

Dendritic tree An extension of a neuron that receives information from another neuron; may be one dendrite or many, forming a tree shape.

Descending brain control Commands from the brain that travel down through specific pathways.

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Glossary

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Eardrum A thin membrane (also called the tympanic membrane) that

separates the outer ear from the middle ear. Its function is to transmit sound from the air inside the middle ear.

Field of vision The part of space within sight.

Food aversion Repulsion from food because of a bad experience, odor, or taste.

Forces of cohesion Forces that bring molecules or objects together; molecules are subject to forces that are either repulsive or cohesive.

Frequency The scientific description of the number of events that happen for a given time; the universal unit is the Hertz, abbreviated as Hz or one event per second, for example, a musical beat of three Hz is a beat that occurs three times per second.

Gland Body tissue formed by many cells that release small molecules into the bloodstream. These molecules usually target organs of the body that could be far away from the gland.

Glia Supportive cells of the nervous system that provide nutrients and maintain homeostasis of neurons.

Homeostasis Maintaining balance to achieve a beneficial stable environment.

Incus Small bone (ossicle) shaped like an anvil that together with the malleus and stapes relays vibration from the eardrum to the auditory nerve for perception.

Inner ear The part of the ear inside the eardrum and the skull, directly connected to the brain.

Iris The contractile membrane of the eye perforated by the pupil and located between the cornea and the lens.

Malleus Small bone (ossicle) shaped like a hammer that together with the incus and stapes relays vibration from the eardrum to the auditory nerve for perception.

Molecule The smallest unit into which a substance can be divided without changing its chemical properties. A molecule is made out of two or more atoms.

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Glossary

Morphology The shape of a biological organ, tissue, or cell.

Multiple sclerosis A disease of the central nervous system associated

with degeneration of the myelin around the axons in the brain or the spinal cord, leading to muscular weakness or psychological deficits.

Nervous system A collection of neurons that forms a network and includes the peripheral nervous system and the central nervous system.

Neurons Cells that are building blocks of the nervous system and contain a nucleus inside a cell body, axon(s), and dendrite(s).

Neuroscience A field of science related to the nervous system.

Nucleus A collection of neuronal cell bodies clustered together (or the part of a cell that contains genetic material).

Odorant A substance or object that causes odor.

Olfactory epithelium Cells lining the nose that contain smell receptors.

Opaque Opposite of transparent; an adjective that describes objects that deflect light (or reflect it completely) and therefore prevent light behind them from reaching our eyes (hide objects behind them).

Optic chiasm The intersection of the left and right optic nerves.

Outer ear The part of the ear outside of the skull and the eardrum.

Perception Conscious interpretation of a sensory stimulus.

Peripheral nervous system (PNS) The part of the nervous system contained outside the skull (brain) or the vertebral column (spinal cord); any neuron other than one in the brain or the spinal cord.

Pheromones Chemicals produced by living organisms that transmit messages to other members of the same species.

Photon A particle of light.

Photosensitive Sensitive to light.

Receptor An organ that receives or senses a stimulus.

Reflex A reaction that is fast and does not require brain commands (unconscious).

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Refractive index The ability of objects to deflect light to certain

degrees, for example, water has a higher refractive index than air and therefore “bends” or deflects light more than air.

Regenerate To regrow.

Retina Structure at the back of the eye that receives light from the lens and converts it into a nerve impulse.

Spectrum of light The “rainbow of colors,” or light of all possible wavelengths.

Stapes Small bone (ossicle) shaped like a stirrup that together with the malleus and incus relays vibration from the eardrum to the auditory nerve for perception.

Stem cells Cells that have the capacity to differentiate into different organs, for example, a stem cell can become a heart fiber, a liver cell, or a neuron.

Synapse The site of contact between neurons for the relay of messages.

Synesthesia Abnormal sensory experience.

Transparent Opposite of opaque; an adjective that describes objects that do not deflect light and therefore appear “clear,” allowing light behind them to reach our eyes (show objects behind them).

Unicellular Formed by only one cell, for example, organisms such as bacteria (singular, bacterium).

Visceral Pertaining to internal organs such as the stomach (in contrast to somatic or external organs such as skin).

Visual cues Sensory information received by the eyes.

Wavelength If sound can be represented by waves, wavelength is the scientific description of the distance between repeating peaks of the wave pattern. Longer wavelength is a “slower” wave.

Bibliography Chudler, Eric H. Neuroscience for Kids. http://faculty.washington.edu/ chudler/neurok.html.

Cleveland Clinic. “The Cleveland Clinic Health Information Center.” http://www.clevelandclinic.org/health/.

Farabee, M.J. “The Nervous System.” http://www.emc.maricopa.edu/ faculty/farabee/BIOBK/BioBookNERV.html.

Fowler, Michael. “The Speed of Light.” http://galileo.phys.virginia.edu/classes/109N/lectures/spedlite.html Hegedüs, Katalin. “Neuroanatomy Structures and Their English and Latin Names.” http://www.neuropat.dote.hu/anastru/anastru.htm.

Kandel, Eric R., James Schwartz, and Thomas Jessell. Principles of Neural Science. New York: McGraw Hill, 2000.

Khowaja, Fahad, Chen Jun Lee, Sarabjit Singh, et al. “Speed of Light.” Thinkquest. http://library.thinkquest.org/C006027/html-ver/nat-vel.html.

Leiner, Henrietta C., and Alan L. Leiner. “The Treasure at the Bottom of the Brain.” New Horizons for Learning. http://www.newhorizons.org/neuro/leiner.htm.

Lundbeck Institute. “Brain Atlas.” http://www.brainexplorer.org/brain_atlas/Brainatlas_index.shtml.

Nolte, John, and Jay B. Angevine. The Human Brain. St. Louis: Mosby, 1995.

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Further Reading Cassan, Adolfo. The Senses. Philadelphia: Chelsea House, 2005.

Cleveland, Donald. How Do We Know How the Brain Works. New York: Rosen, 2005.

Cobb, Vicki. Feeling Your Way: Discover Your Sense of Touch.

Brookfield, Conn.: Millbrook Press, 2001. _____. Open Your Eyes: Discover Your Sense of Sight. Brookfield, Conn.: Millbrook Press, 2002.

Evans-Martin, Fay. The Nervous System. Philadelphia: Chelsea House, 2005.

Hayhurst, Chris. The Brain and Spinal Cord: Learning How We Move.

New York: Rosen, 2002.

Light, Douglas. The Senses. Philadelphia: Chelsea House, 2004.

Morgan, Jennifer R., and Ona Bloom. Cells of the Nervous System.

Philadelphia: Chelsea House, 2006.

Newquist, H.P. The Great Brain Book: An Inside Look at the Inside of Your Head. New York: Scholastic, 2004.

Oleksy, Walter. The Nervous System. New York: Rosen, 2001.

Ripoll, Jaime. How Our Senses Work. Philadelphia: Chelsea House, 1994.

Roca, Nuria, and Marta Serrano. The Nervous System and the Brain.

Philadelphia: Chelsea House, 1996.

Silverstein, Alvin, Virginia Silverstein, and Laura Silverstein Nunn.

Smelling and Tasting. Brookfield, Conn.: Twenty-First Century Books, 2002. _____. Touching and Feeling. Brookfield, Conn.: Twenty-First Century Books, 2002.

Vera-Portocarrero, Louis. Brain Facts. New York: Chelsea House, 2007.

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WEB SITES Body Basics: The Brain and Nervous System http://www.kidshealth.org/teen/your_body/body_basics/brain_nervous_ system.html Brain Backgrounders http://apu.sfn.org/index.cfm?pagename=brainBackgrounders_main Brain Museum http://brainmuseum.org/ Brain Web http://www.dana.org/brainweb/ How the Brain Controls the Heart http://www.childrenheartinstitute.org/educate/syncope/brainhrt.htm Medical College of Wisconsin Health Link http://healthlink.mcw.edu/article/924451309.html Nemours Foundation Kids Health http://kidshealth.org/kid/talk/qa/taste_buds.html Nervous System Fact File http://www.imcpl.org/kids/guides/health/nervoussystem.html Neuroscience for Kids http://faculty.washington.edu/chudler/neurok.html Sheep Brain Dissection: The Anatomy of Memory http://www.exploratorium.edu/memory/braindissection/index.html The Brain Is the Boss http://www.kidshealth.org/kid/body/brain_noSW.html

Picture Credits page: 49: 51: 53: 55:

8: © Wim van Egmond / Visuals Unlimited 10: Infobase Publishing 12: Infobase Publishing 14: Infobase Publishing 16: Infobase Publishing 19: © Shutterstock / Dennis Sabo 21: Infobase Publishing 23: Infobase Publishing 27: Infobase Publishing 28: Steve Gschmeissner / Photo Researchers, Inc. 30: Infobase Publishing 34: Laguna Design / Photo Researchers, Inc. 38: Infobase Publishing 39: Frank Zullo / Photo Researchers, Inc. 45: Infobase Publishing 48: © Dr. Frank T. Awbrey / Visuals Unlimited

60: 64: 65: 70: 71: 74: 77: 79:

Infobase Publishing Infobase Publishing Infobase Publishing Eye of Science / Photo Researchers, Inc. Infobase Publishing Infobase Publishing Sue Ford / Photo Researchers, Inc. Scott Camazine / Photo Researchers, Inc. Infobase Publishing Omikron / Photo Researchers, Inc. Dominique Duval / Photo Researchers, Inc. Voisin / Photo Researchers, Inc.

Cover: AbleStock.com; ©Magdalena Bujak / Shutterstock.com; ©Andrea Leone / Shutterstock. com

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Index A accommodation, 52, 63 anosmia, 18 causes, 68–69, 72–73, 87 astigmatism, 59, 63 treatment, 64 auditory nerve, 31 axons functions of, 13, 15–16, 61, 72

cochlea fluid space of, 28–29 color descriptions, 18, 34, 41–42, 56, 58, 66 sensitivity, 56, 58, 73 and shapes, 40–41 and synesthesia, 76, 78, 87 color blindness, 63, 87 defined, 65–66 communication systems of, 8, 11, 13–15, 72 cones and color, 54, 58 conjunctivitis, 66 function, 54, 56, 59–62, 82 loss of, 56, 66 cornea, 45 damage to, 63–64 function of, 46–47, 57, 59

B bacterium, 8 blindness causes, 18, 26, 56, 66 brain activities, 7, 12 functions, 11, 13, 15, 21, 24, 27, 30–31, 33, 43–45, 49–50, 63, 66, 72, 75, 82, 84 protection, 11, 15 structures, 10, 16, 31, 59–61 and synesthesia, 78–79, 82–85 brain stem functions, 44, 49–50, 61

D dendrites functions, 13, 15 diabetes, 66 diseases and defects protection against, 11, 15 repair of, 11

C cataracts, 59, 63 treatment, 65 cell body functions, 13 central nervous system (CNS) protection of, 15 structures of, 15 cerebral cortex functions, 31 CNS. See central nervous system

E ear, 25–31 compared to the eye, 43–44 defects of, 21–22 function, 23–24, 30, 32 morphology of, 21 organs in, 24–29

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Index emotions and moods control of, 84, 86 eye, 41–63 activities, 32 anatomy of, 43, 45–61, 66, 82 care and exams, 67 compared to the ear, 43–44 defects of, 62–67 and light, 7, 34, 41 movements, 31, 44, 50, 56, 58 protection of, 44, 46, 54 F farsightedness. See hyperopia Fizeau, Armand, 36 food aversions, 73 Foucault, Leon, 36 fovea, 56, 58 foveola, 56, 58 G Galileo, 36 gases and odors, 7, 69, 71–72 glia cells functions, 9–11 H hearing abnormalities, 21–22, 26, 73 control of, 18–24, 29, 31–32, 43 and synesthesia, 76, 78–79, 82 homeostasis of neurons, 9–11 hyperopia (Farsightedness), 63 treatment, 64–65 I injuries, brain effects on smell, 72–73 protection from, 11, 15 signs of, 50

97

inner ear functions, 31 structures of, 25–26, 27–29 iris function, 46–47, 57 muscles, 47–48 L language, 86 lens damage to, 64–65 functions of, 50–53, 57, 63 light absorption, 47–50, 53–54, 57–60, 63 and colors, 7, 34 defined, 35, 37 intensity, 37, 40, 54, 58 perception of, 32–33, 37–38, 43, 46, 56 photons, 37, 40, 51–52, 59 properties of, 32, 35 reflections, 40–42, 50–52, 56, 82 research, 36 sources of, 34–35, 59 spectrum of, 38, 41, 54 speed of, 33, 35–37, 40, 51 travel, 35, 37–38, 40, 50, 59 waves, 56–57 M macula, 56, 58 damage to, 63 macular degeneration, 63 memory, 86 control of, 12 and smell, 72 storage, 7 Michelson, Albert Abraham, 36 middle ear bones of, 26–27, 29 damage to, 26 incus, 26

98

Index

malleus, 26 stapes, 26 motor neurons function, 9, 16–17 movement and reflexes control of, 9, 11, 15 multicellular organisms neurons of, 8–9 multiple sclerosis, 15 myopia (nearsightedness), 63 treatment, 64 N nearsightedness. See myopia nerves damage to, 26 signals, 24 types, 16–17, 45 nervous system adaptive characteristics, 12 functions of, 11, 13, 15, 24, 26, 33, 79 protection of, 11 structures of, 7, 9, 11, 15, 84 neurons characteristics, 9 classes of, 9, 11, 14–17, 71 damage of, 15 functions of, 9–14, 21, 29, 31, 43–44, 46, 50, 54, 56, 59, 61, 66, 68, 74–75, 82 homeostasis of, 9–11 structures of, 10, 11–15, 49, 61, 72 night vision, 58 nose blockage, 68–69, 72, 75 and gases, 7, 69–72 mucus in, 69–72 O odorants allergy to, 72 perception of, 18, 69–72, 76, 82, 85, 87

olfactory epithelium, 71 optic chiasm, 61, 82 optic nerve damage to, 26, 63, 66 function, 45, 60–61, 82 outer ear function of, 25–26 obstruction of, 26 P pain control of, 31 receptors, 11 paralysis causes of, 15 and research, 15 perception and the brain, 7, 78 and stimulus, 11, 18, 24, 32–33, 43, 87 peripheral nervous system (PNS) functions, 15 injury to, 16 pheromones, 69–70 photoreceptors, 53 types of, 54–56, 58–59, 62–63, 82 photosensitive, 53, 66 PNS. See peripheral nervous system pupil, 46 contraction, 47–50 damage, 66 function of, 47–50, 57, 59, 63 pupillary reflex absence of, 50 function of, 47–50 R refractive index, 50–52 research on neuronal regeneration, 15 on sensations, 31 on the speed of light, 36

Index stem cell, 15 and synesthesia, 79–80, 82 retina damage to, 56, 63–64, 66 functions of, 46, 53–56, 58, 60 neurons in, 46, 54, 56 photoreceptors of, 53–55, 62–63, 66, 82 Retinal Pigment Epithelium (RPE), 62–63 rods function, 54–55, 59–62 photoreceptors, 58, 82 protein, 59 RPE. See Retinal Pigment Epithelium S sciatic nerve, 17 sensation research, 31, 82 and stimuli, 7, 61 senses hearing, 18–33, 35, 43–44, 73, 76, 78–79, 82–84 sight, 7, 24, 26, 29–67, 73, 76, 78–79, 82–85 smell, 7, 18, 23–24, 31, 68–76, 79, 82, 85 taste, 31, 68–76, 78, 82, 86–87 touch, 18, 31 sensory neurons function, 9, 11, 14–17, 71 skull function, 15 smell activities, 7 animal, 69–70 control, 18, 23–24, 31 function of, 68–71 loss of, 68–69, 72–73 sensitivity, 69, 72–73 and synesthesia, 76, 79, 82–85 and taste, 69, 75

soma. See cell body sound attention to, 20–21 creation, 20 defined, 18, 20–22 detection, 20–21, 24–25, 43, 73 direction of, 29–31 frequency, 19 intensity of, 20, 30 and synesthesia, 78, 82 travel of, 22, 24, 33, 35 and vibration, 19–20, 22–26, 29, 31, 35 and visual cues, 29–31, 44 wavelengths, 22–23, 35 spinal cord functions, 50 nerves, 16–17 protections, 15 stem cell research, 15 strabismus, 63 treatment, 66 synapses formation, 12 function of, 11, 13 synesthesia and brain function, 78–79, 82–85 defined, 76, 78–79 discovery, 78–79 experiments, 80–83 research on, 79–80, 82 T taste, 86–87 control, 31 function, 68, 73 loss of, 68 receptors, 73–75 and smell, 69, 75 and synesthesia, 76, 78, 82 types, 74–75

99

100

Index

tongue and taste buds, 73–75 touch control, 18 and pain, 31 tympanic membrane function of, 25–26, 29 U unicellular organisms, 8 V vertebral column function, 15 vision abnormalities, 26, 33, 59, 62–67, 73, 76

activities, 7 control of, 24, 32–33, 43, 61 correction, 63–65 and light, 35–38, 40 perceptions, 73, 84–85 photoreceptors, 54–55 shape and color, 40–41 sight defined, 34–35, 41–42 and sound, 29–31, 44 and synesthesia, 76, 79, 82–85 W Wittgenstein, Ludwig, 84

About the Author Carl Y. Saab is an active neuroscience researcher and assistant profes-

sor at Brown University in Providence, Rhode Island. Dr. Saab studies pain sensation and underlying mechanisms of abnormal pain. He is the author of several scientific manuscripts and neuroscience books for college students. He is also an avid cyclist and an occasional disc jockey.

ABOUT THE EDITOR Eric H. Chudler, Ph.D., is a research neuroscientist who has investi-

gated the brain mechanisms of pain and nociception since 1978. Dr. Chudler received his Ph.D. from the Department of Psychology at the University of Washington in Seattle. He has worked at the National Institutes of Health and directed a laboratory in the neurosurgery department at Massachusetts General Hospital. Between 1991 and 2006, Dr. Chudler was a faculty member in the Department of Anesthesiology at the University of Washington. He is currently a research associate professor in the University of Washington Department of Bioengineering and director of education and outreach at University of Washington Engineered Biomaterials. Dr. Chudler’s research interests focus on how areas of the central nervous system (cerebral cortex and basal ganglia) process information related to pain. He has also worked with other neuroscientists and teachers to develop educational materials to help students learn about the brain. Find out more about Dr. Chudler and the fascinating world of neuroscience by visiting his Web site, Neuroscience for Kids, at http://faculty.washington.edu/chudler/neurok.html.

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