Effect of encoding variability on mnemonic discrimination: An ERP study

Alexis Khuu, Katherine Goida, and Natasha Woods

Faculty Sponsor: Dr. Leslie Rollins

Mnemonic discrimination is the ability to distinguish between similar stimuli. Huffman and Stark (2017) assessed this ability by developing the forced-choice Mnemonic Similarity Task (MST). The task involves three conditions: A-X (target paired with novel object), A-A′ (target paired with similar lure), and A-B′ (target paired with different lure). Performance is best for A-X condition, followed by A-A′ condition, then A-B′ condition (Huffman & Stark, 2017; Rollins et al., 2019). A previous eye-tracking study revealed that errors on A-B′ trials were associated with more fixations to the B stimulus than the A stimulus at encoding (Rollins et al., 2019). The current project seeks to examine whether encoding variability, as indexed by electrophysiological brain activity, explains performance on the forced-choice MST. Friedman and Johnson (2000) found that items subsequently recognized elicit a larger amplitude ERP response during memory encoding compared to those later forgotten at retrieval. Therefore, we hypothesize that accuracy will interact with the stimulus such that incorrect A-B′ trials will be associated with a higher mean amplitude to the stimulus B than the A stimulus during encoding. However, correct A-B′ trials will either be associated with a higher mean amplitude to the A stimulus than the B stimulus during encoding or there may be no difference in the amplitude of the waveforms. Eighteen young adults (15 females, 3 males) completed a modified version of the forced-choice MST and provided a sufficient number of ERP trials for the following conditions: A-A′ correct A-A′ incorrect, A-B′ correct, and A-B′ incorrect. Behaviorally, individuals performed better in the A-A′ condition than the A-B′ condition. Baseline-corrected mean amplitudes 500-700 ms poststimulus onset were exported and analyzed using a 2 Stimulus (A, B) x 2 Accuracy (Incorrect, Correct) x 3 Coronal (F, C, P) x 3 Sagittal (Left, Middle, Right) repeated-measures ANOVA. Consistent with our hypothesis, the mean amplitude elicited to the B stimulus was larger than the A stimulus when individuals subsequently responded incorrectly and larger to the A stimulus than the B stimulus when individuals subsequently responded correctly. These findings suggest that encoding variability accounts for performance on the forced-choice MST.

Alexis Khuu is member of the class of 2020 at Christopher Newport University with a Bachelor of Science in Neuroscience and Psychology. Her achievements earned her membership in Alpha Chi Honor Society and Nu Rho Psi Honor Society. While at the University, she conducted research in the Developmental Cognitive Neuroscience Laboratory with Dr. Leslie Rollins, participated in the Research Apprentice Program, presented at the Paideia Undergraduate Research Conference, and co-authored a manuscript that was published in July 2019. Alexis expanded her research involvements beyond the University as a volunteer research trainee at the Center for Autism Spectrum Disorders at Children’s National Hospital. To further her contributions to the scientific community, she aspires to pursue a doctorate in clinical psychology.

Katherine Goida is a member of the class of 2021 at Christopher Newport University. She is majoring in Neuroscience and pursuing a minor in Leadership Studies. Katherine is part of the Honors Program, President’s Leadership Program, Alpha Chi

Honors Society, Omicron Delta Kappa Leadership Honors Society and has completed the Research Apprenticeship Program during the 2019-2020 academic year. After graduation, she hopes to attend graduate school to pursue a master’s program focused on applied behavioral analysis (ABA) and wants to become an ABA therapist for children with disabilities.

Natasha Woods is a member of the class of 2020 at Christopher Newport University. She majored in Neuroscience and pursued a minor in Biology. She currently works at McDonalds as a Department Manager. In addition, is pursing many job opportunities around the Hampton Roads area. She is hoping to pursue a Master’s in Public Health in Fall 2020. She enjoys spending time with her family and watching scary movies.

Dr. Rollins is an Assistant Professor of Psychology and Neuroscience. Her research focuses on the development and neural bases of memory.

Effect of encoding variability on mnemonic discrimination: An ERP study

Alexis Khuu, Katherine Goida, and Natasha Woods Faculty Sponsor: Dr. Leslie Rollins

Christopher Newport University

• Mnemonic discrimination refers to the ability to distinguish between previously encountered and novel stimuli

• Forced-choice assessment of mnemonic discrimination

• Huffman and Stark (2017) developed the forced-choice Mnemonic Similarity Task (MST), which included three test formats: (A-X), (A-A’), and (A-B’). A was the target, X was an unrelated lure, A’ was a similar lure, and B’ was a non-corresponding lure Mathematical models suggested that encoding variability may account for errors, especially on the A-B’ test format. Specifically, errors (i.e., selecting B’ stimulus, which was a lure, rather than the A stimulus, which was a target) may be due to the enhanced encoding of the B stimulus than the A stimulus at encoding. A previous eye-tracking study in the lab provided support for this hypothesis (Rollins et al , 2019)

• The goal of the present study was to examine whether encoding variability, as indexed by electrophysiological brain activity, explains performance on the forced-choice MST.

• Hypotheses:

• We hypothesized that participants would perform best in the A-A’ condition compared to the A-B’ condition (Huffman & Stark, 2017; Rollins et al , 2019)

• Given the subsequent memory effect (Friedman & Johnson, 2000) is typically larger for subsequently recognized than forgotten stimuli, we hypothesized that

• On incorrect A-B’ trials, mean amplitudes would be higher for the B stimulus than the A stimulus

• On correct A-B’ trials, mean amplitudes would either be higher for the A stimulus than the B stimulus or display no difference between the two stimuli

Methods Participants

•

• At retrieval, participants were presented with 192 trials. For each trial participants viewed two images and were asked which of the two images they viewed at encoding.

• A-A’ Condition: The target was paired with a corresponding lure

• A-B’ Condition: The target was paired with a non-corresponding lure

Figure 1

Encoding Retrieval

288 pictures, 2000 ms

192 trials (96/condition)

Results

Event-Related Potentials (ERPs)

• EEG was recorded with a sampling rate of 512 Hz (BioSemi Active 2) from 64 active AgAgCl scalp electrodes and two vertical and two horizontal electrooculogram (EOG) channels re-referenced offline to a linked mastoid configuration using Brain Electrical Source Analysis (BESA) software (MEGIS Software GmbH, Gräfelfing, Germany). Ocular artifacts were corrected applying the Ille, Berg, & Scherg (2002) algorithm Trials were hand-edited to remove movement related artifact Data were high and low pass filtered at 0.1 Hz and 30 Hz, respectively. A minimum of 10 trials were required per condition. Trials were epoched with a 100 ms baseline and continued during stimulus presentation for 1500 ms The following analyses were conducted on baseline-corrected mean amplitudes 500-700 ms poststimulus onset

Behavioral Performance (Figure 2)

• Consistent with previous research, participants performed better on the A-A’ condition than the A-B’ condition, t(17) = 3.525, p = .003. ERP Data

• A 2 Stimulus (A, B) x 2 Accuracy (Incorrect, Correct) x 3 Coronal (F, C, P) x 3 Sagittal (Left, Middle, Right) mixedmodel ANOVA on mean amplitudes 500-700 ms poststimulus onset revealed a marginal stimulus x accuracy interaction, F(1, 17) = 3.302, p = .087.

• On incorrect trials, the mean amplitude elicited to the B stimulus was larger than the A stimulus (Figure 3)

• On correct trials, the mean amplitude elicited to the A stimulus was larger than the B stimulus (Figure 4)

Discussion and Future Directions

• Errors on the A-B’ condition may may occur if encoding of the subsequent lure stimulus (B) superseded the encoding of the target stimulus (A). This explanation was supported by Rollins et al. (2019), which showed that individuals exhibited more fixations to the B stimulus than the A stimulus for subsequently incorrect trials. Similarly, the current study suggests enhanced neural processing of the B stimulus than the A stimulus when they later answered incorrectly. Taken together with our previous research (Rollins et al., 2019), these studies suggest that encoding variability partially accounts for errors observed on the A-B’ test format of the forced-choice MST.

• In contrast, in the A-A’ condition participants are presented with both versions of the stimulus. We had expected either similar neural activity to the A and B stimuli at encoding or enhanced encoding of the A stimulus, which was observed in the present study.

Friedman, D., & Johnson, R. Jr. (2000). Event-related potential (ERP) studies of memory encoding and retrieval: A selective review. Microscopy Research and Technique, 51(1), 6-28. Huffman, D. J., & Stark, C. E. L. (2017). Age-related impairment on a forced-choice version of the mnemonic similarity task. Behavioral Neuroscience, 131(1), 55-67. doi:10.1037/bne0000180

Rollins, L., & Khuu, A, & Lodi, N. (2019). Encoding variability accounts for false recognition of noncorresponding lures on the forced-choice Mnemonic Similarity Task. Learning & Memory, 26(8), 280-283. doi:10.1101/lm.047142.117

Stark, S. M., Kirwan, C. B., & Stark, C. E. L. (2019). Mnemonic Similarity Task: A tool for assessing hippocampal integrity. Trends in Cognitive Sciences, 23(11), 2442-2449. doi:10.1016/j.tics.2019.08.003

Yassa, M. A., & Stark, C. E. L. (2011). Pattern separation in the hippocampus. Trends in Neuroscience, 34(10), 515-525. doi:10.1016/j.tins.2011.06.006