Abstract

Effective memory representations must be specific to prevent interference between episodes that may overlap in terms of place, time, or items present. Pattern separation, a computational process performed by the hippocampus overcomes this interference by establishing non-overlapping memory representations. This project explores pattern separation and how it is impacted by age and sleep. Experiment 1. Structures of the medial temporal lobe (MTL) are known to be involved in declarative memory processes. However, little is known about how age-related changes in MTL structures, white matter integrity, and functional connectivity affect pattern separation processes in the MTL. In the present study, we used magnetic resonance imaging (MRI) to measure the volumes of MTL regions of interest, including hippocampal subfields (dentate gyrus, CA3, CA1, and subiculum) in healthy older and younger adults. Additionally, we used diffusion tensor imaging to measure white matter integrity for both groups. Finally, we used functional MRI to acquire resting functional connectivity measures for both groups. We show that, along with age, the volume of left CA3/dentate gyrus predicts memory performance. Differences in fractional anisotropy and the strength of resting functional connections between the hippocampus and other cortical structures implicated in memory processing were not significant predictors of performance. As previous studies have only hinted, it seems that the size of left CA3/dentate gyrus contributes more to successful discrimination between similar mnemonic representations than other hippocampal sub-fields, MTL structures, and other neuroimaging correlates. Accordingly, the implications of aging and atrophy on lure discrimination capacities are discussed. Experiment 2. Although it is widely accepted that declarative memories are consolidated during sleep, the effects of sleep on pattern separation have yet to be elucidated. We used whole-brain, high-resolution functional neuroimaging to investigate the effects of sleep on a task that places high demands on pattern separation. Sleep had a selective effect on memory specificity and not general recognition memory. Activity in brain regions related to attention, visual acuity, and visual recall demonstrated an interaction between sleep and delay. Surprisingly, there was no effect of sleep on hippocampal activity using a group-level analysis. To further understand the role of the hippocampus on our task, we performed a representational similarity analysis. We investigated whether hippocampal activity associated with looking at novel stimuli correlated more with similar-looking (lure) stimuli or repeated stimuli. Results indicate that while a single night's sleep does not significantly impact hippocampal responses, the hippocampus does treat lure stimuli similarly as it does novel stimuli.

Degree

PhD

College and Department

Life Sciences; Physiology and Developmental Biology

Rights

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