Memory over time: Neural mechanisms for preserving memory when experiences repeat
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Date
2024-12-19
Authors
Zou, Futing
Journal Title
Journal ISSN
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Publisher
University of Oregon
Abstract
Much of our everyday life involves repeated experiences. The fact that humans are able to form and retain separate memories for experiences that repeat over time is not only important for everyday behavior, but it is foundational to theories of episodic memory. While the effect of repetitions on memory has been the subject of extensive research over decades, the mechanisms that preserve memory across repeated experiences are not well understood. Across three studies, this dissertation explores how the human brain represents and preserves memories when experiences repeat over time, using measurements of behavior as well as brain activity using functional magnetic resonance imaging (fMRI). In these studies, human participants viewed thousands of visual stimuli that repeated at lags ranging from seconds to many months and then later completed different types of memory probes. In the first chapter, we test how spacing between repeated stimulus encounters with a stimulus influences memory and corresponding neural similarity. We show that the spacing increases the similarity of neural activity patterns in the ventromedial prefrontal cortex and, critically, these increases in neural similarity parallel and predict behavioral benefits of spacing. We demonstrate that these spacing benefits-in brain and behavior-reflect the re-encoding of memories for past experiences. In the second chapter, we characterize the role of neural representations in medial temporal lobe (MTL) subregions in preserving memory for when individual events occurred. We show that the ability to remember the temporal context in which a stimulus originally occurred is predicted by re-expression of previously-evoked patterns of activity in CA1 and ERC across repeated stimulus encounters, suggesting temporal context reinstatement as a mechanism that protects temporal memories when stimuli repeat. In the final chapter, we develop a novel approach for directly measuring temporal context reinstatement. We show that re-encountering a stimulus reinstates semantic information of temporally-adjacent stimuli that putatively ‘compose’ the temporal context in which the stimulus was originally encountered. Further, we show that neural similarity in CA1 across repeated stimulus encounters positively relates to the degree of temporal context reinstatement. Collectively, these data highlight the multi-faceted impact of repeated experiences on memory and provide novel insight into the neural mechanisms that preserve memories when experiences repeat across time.