Source memory is defined as memory for not only the core aspect of some event, but additional contextual detail about that core aspect, or item. Source memory tasks are marked by their engagement of prefrontal cortex in addition to the brain circuits required by other episodic memory tasks. The dissertation examines the relationships among source memory accuracy, concurrent brain activity, and general cognitive principles derived from the study of episodic memory more generally. Electrical measures of brain activity (event-related potentials, ERPs) were recorded while manipulating factors hypothesized to improve or worsen source memory accuracy.The first experiment manipulated the task assigned during the encoding phase and its match to the retrieval demands of remembering objects (depicted in drawings) and their colors. As predicted by the principle of transfer-appropriate processing, source accuracy was higher when the encoding task fostered integration of the item (object) and source (color) attributes. Prefrontal activity during the retrieval phase was greatly reduced when retrieval could benefit from transfer-appropriate processing.In associative memory tasks, poor memory performance is observed when the to-be-retained stimuli share elements with other studied stimuli, as in a variety of interference paradigms. The second experiment thus examined the impact of feature overlap on source recognition by varying the quantitative mapping between the shape and color of an object depicted in a drawing. The results showed two frontal processes supporting source retrieval: an early differentiation between stimuli identical to those encoded and those that switch colors from study to test, and a later effect reflecting prolonged memory search that was truncated by reinstating unique object-color pairings at test.The final experiment compared conjunctions of "intra-item" versus "extra-item" features, by placing the features within a single visual object or distributing them across two visual objects. Source accuracy was worse when shape and color were spatially separated, but prefrontal activity did not vary. The insensitivity of prefrontal ERPs to this perceptual manipulation of difficulty stands in contrast to their sensitivity to encoding task. Individual variability in parietal ERPs was strongly correlated with source accuracy, and likely reflects a contribution of visual working memory to long-term memory.

Investigating the Neural Correlates of Crossmodal Facilitation as a Result of Attentional Cueing: An Event-Related fMRI Study. Degree of Masters of Science, 2008 Zainab Fatima Institute of Medical Science, University of Toronto ABSTRACT Attentional cueing modulated neural processes differently depending on input modality. I used event-related fMRI to investigate how auditory and visual cues affected reaction times to auditory and visual targets. Behavioural results showed that responses were faster when: cues appeared first compared to targets and cues were auditory versus visual. The first result was supported by an increase in BOLD percent signal change in sensory cortices upon cue but not target presentation. Task-related activation patterns showed that the auditory cue activated auditory and visual cortices while the visual cue activated the visual cortices and the fronto-polar cortex. Next, I computed brain-behaviour correlations for both cue types which revealed that the auditory cue recruited medial visual areas and a fronto-parietal attentional network to mediate behaviour while the visual cue engaged a posterior network composed of lateral visual areas and subcortical structures. The results suggest that crossmodal facilitation occurs via independent neural pathways depending on cue modality.

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Investigating the Neural Correlates of Crossmodal Facilitation as a Result of Attentional Cueing: An Event-Related fMRI Study. Degree of Masters of Science, 2008 Zainab Fatima Institute of Medical Science, University of Toronto ABSTRACT Attentional cueing modulated neural processes differently depending on input modality. I used event-related fMRI to investigate how auditory and visual cues affected reaction times to auditory and visual targets. Behavioural results showed that responses were faster when: cues appeared first compared to targets and cues were auditory versus visual. The first result was supported by an increase in BOLD percent signal change in sensory cortices upon cue but not target presentation. Task-related activation patterns showed that the auditory cue activated auditory and visual cortices while the visual cue activated the visual cortices and the fronto-polar cortex. Next, I computed brain-behaviour correlations for both cue types which revealed that the auditory cue recruited medial visual areas and a fronto-parietal attentional network to mediate behaviour while the visual cue engaged a posterior network composed of lateral visual areas and subcortical structures. The results suggest that crossmodal facilitation occurs via independent neural pathways depending on cue modality.

Since its creation in 1998, the Implicit Association Test (IAT) has become a commonly used measure in social psychology and related fields of research. Studies of the cognitive processes involved in the IAT are necessary to establish the validity of this measure and to suggest further refinements to its use and interpretation. The current thesis used ERPs to study cognitive processes associated with the IAT. The first experiment found significant differences in P300 amplitude in the Congruent and Incongruent conditions, which were interpreted as a reflection of greater equivocation in the Incongruent condition. The second experiment tested the task-set switching account of the IAT in much greater detail by analyzing each trial type separately. In the Congruent condition, all trial types elicited the same amplitude P300. Local probability, and the consequent checking and updating of working memory, was thought to be responsible for differences between trials of the Incongruent condition that required or did not require a task switch. The final experiment examined the role of working memory in the IAT by introducing obtrusive and irrelevant auditory stimuli. The results of Experiment 3 indicated that the introduction of an obtrusive and irrelevant auditory increment deviant has little overall effect on the IAT, and a similar effect on switch and no-switch trials within the Incongruent condition. This could have been because both the Congruent and Incongruent conditions of the IAT make such extensive demands on central processing resources that few are available to allow for the switching of attention, or it is possible that the IAT does not require significant updating of working memory. The usefulness of ERPs in the study of the IAT effect is demonstrated by the current research. In particular, the finding that behavioural results were not always consistent with the ERP results demonstrates that electrophysiological measures can complement traditional behavioural measures.

Despite the vast number of studies that have examined the relationship between human memory and learning, few have examined learning and memory in more realistically valid environments. The current study examines learning memory in a classroom environment, specifically with students enrolled in a medical anatomy course. In addition to behavioral data, this study also uses electroencephalography (EEG) to examine the neural correlates of successful learning in medical students. A total of 37 students over 2 years was recruited from the Boston University School of Medicine to participate in this study. In the study, medical students were tested on a set of anatomical terms that they learned in the anatomy course. Testing occurred in three sessions: prior to the start of the course, immediately after the completion of the course, and 5 months after the completion of the course. In the experiment itself, students were presented with 176 anatomical terms (132 terms learned in the course and 44 terms deemed outdated) and then given three response choices: whether they "Can Define", are "Familiar" with, or "Don't Know" the term. While testing, the subject's scalp EEG was recorded to measure the brain's neural activity in response to anatomical terms displayed on the computer screen. Resulting EEG waveforms were separated and then averaged based on the response type in order to analyze the difference in amplitude for three neural correlates across distinct scalp sites when the students could define, were familiar with, or did not know the term. Results showed a higher amplitude in ERP readings for "Can Define" and "Familiar" responses for the early frontal effect, which is correlated with memorial familiarity. A higher "Don't Know" ERP wave was observed for the late parietal effect, which reflects memorial recollection. Lastly, a larger ERP amplitude was detected for "Familiar" and "Don't Know" responses for the late frontal effect, which is associated with post memory retrieval processing. Both Pearson correlation and multiple linear regression analyses were then run to investigate if any significant relationship between ERP amplitude and grades existed, and if so, the degree to which these electrophysiological responses can predict the course grades received. Final results found that the early frontal effect for the Can Define responses over the Right Posterior Superior scalp region is the best predictor variable, among the ones tested in the study, for student performance in the medical anatomy course. This finding has the potential to determine whether the information learned in a classroom environment has in fact been incorporated into long-term or even semantic memory. Through the findings of this study, we hope to determine if this method of measuring learning through EEG can be used as a useful indicator of long-lasting learning in classroom environments.

When stimuli processing time varies in an oddball paradigm, the latency of the P300 will vary across trials. In an oddball task requiring difficult response selections, as the variation of stimuli processing time increases, so does the variation of the P300 latency, causing latency jitters in the measurement. Averaging the P300 across different trials without adjusting this latency jitter will lead to diminished P300 amplitude, resulting in inaccurate conclusions from the data. Verleger et al. (2014) reported a diminished P300 amplitude in a difficult oddball task that required subjects to make response selections among stimuli that are difficult to distinguish, but his work did not correct for any latency jitter observed within his sample. The current study replicated the easy and hard oddball tasks conducted in Verleger et al.. Raw ERPs obtained from 16 subjects indicated a successful replication of the study. An examination of the behavioral data showed that there was substantial variation in the P300 during the hard oddball tasks, and a latency jitter correction was applied in the analysis. Results indicated that there was a significant increase in the amplitude of P300 after latency jitter correction, and that this P300 amplitude did not differ significantly between easy and hard oddball tasks. These results suggest that difficult decision requirement does not reduce the amplitude of the P300, and that latency jitter should be accounted for when analyzing data from tasks involving a difficult decision requirement.

The current study used event-related potentials (ERPs) to investigate how motivational salience in the form of expectation violation influences spatial attention. The medial frontal negativity (MFN) ERP indexes expected value, being negative to unexpected punishments and positive to unexpected rewards. The P1 and N1 ERPs index spatial attention, being larger to stimuli in attended locations. This design attached motivational value to locations by making one visual hemifield economically rewarding (greater probability of a rewarding outcome) and the other punishing (greater probability of a punishing outcome). Keypresses to a dot probe following a reward-signifying stimulus were awarded money if correct, and penalized following a punishment-signifying stimulus if incorrect. We predicted that salience would be attached to visual hemifield, thus the MFN would be most negative to punishing outcomes in the rewarding hemifield and most positive to rewarding outcomes in the punishing hemifield. We also predicted that attention would be allocated to a location where expectation was violated, thus the P1 and N1 ERPs would be larger and RTs (reaction times) faster to dot probes appearing in the same side as an outcome that violated expected value. In a sample of 36 participants, there were no significant effects on the MFN, although the means were in the predicted direction, suggesting a lack of power. Contrary to our hypothesis, keypresses were slower, P1 smaller, and N1 larger to probes opposite the location where an expectation violation occurred. This suggested that expectation violation did not direct attention to a particular location, but produced general interference.

Traumatic brain injuries (TBI) are one of the leading causes of disability worldwide (Zitnay, 2008), yet one of the least understood neurological conditions (Duncan, 2005). Research has examined short-term deficits; however, less focus has been on the consequences of multiple concussions. Previous electroencephalography (EEG) concussion research has examined the N200 and P300 human event-related potential (ERP) components, yielding inconclusive results (Duncan, Kosmidis & Mirsky, 2005). An ERP component not as frequently examined is the reward positivity, generated by the anterior cingulate cortex (ACC), a region which experiences increased anatomical stress following injury. In this study, 51 students from the University of Victoria took a ‘Concussion Survey’ to determine participant history and groups; no history of concussion, a single injury or multiple injuries (2+). Participants performed an oddball and decision-making task while EEG data was collected. No significant differences were found between groups for the N200, P300 or reward positivity peak latencies or amplitudes. Both concussion groups yielded attenuated peak amplitudes, but no differences existed between the group with a single concussion versus multiple. Unexpectedly, N200 and reward positivity peak latencies were greater in the group with single injuries, compared to those with a history of multiple concussions. This study adds to a continuous line of inconclusive research on the N200 and P300, suggesting minimal cognitive deficits result from concussive injuries. Furthermore, no noticeable differences were observed between groups with a single versus multiple injuries. While the ACC is located in a region of increased stress following TBI, functional deficits impacting the reward positivity may not be as significant as previously hypothesized. Results may be impacted by confounding variables, including not reliably being able to account for time since injury, injury severity and differences in gender dispersion of participants. With concussions on the rise, continued research, particularly longitudinally and within-subjects is critical for the advancement of both TBI prevention and management. / Graduate

Visual snow syndrome is a persistent visual disturbance characterized by rapid flickering dots in the entire visual field. Hypothesized to arise from reduced inhibition of sensory cortex, visual snow has recently been linked to potentiation (enhancement) of the P100, an event-related potential (ERP) component associated with early visual processing. Here, we investigate whether this potentiation in visual snow is specific to visual responses, by comparing ERPs linked to early, bottom-up perceptual versus late, top-down cognitive processes. Specifically, we examined two components, the N170 and P300, associated respectively with rapid face categorization and attentional orienting towards targets. We predicted that if visual snow predominantly reflects diminished inhibition of perceptual areas, there should be stronger potentiation for the earlier perceptual N170 component. ERPs associated with the N170 (Face > House) and P300 (Target > Nontarget) were recorded in a 22 year-old male with a 2-year history of visual snow symptoms and a set of age- and gender-matched controls. Although N170 and P300 responses in all participants showed appropriate face- and target-selectivity, respectively, the visual snow patient demonstrated consistent potentiation relative to controls, particularly for the early N170 response. Bootstrapped estimates of mean amplitude computed within participants similarly revealed larger and more variable ERP amplitudes in the visual snow patient, especially for the N170 component. These results support an early perceptual locus of ERP potentiation in visual snow, further supporting the idea that this condition arises from diminished inhibition of sensory cortices.