Mechanisms of positional and episodic memory : grid cells, boundary proximity, and aging trajectories

Abstract

Grid cells establish a fundamental spatial metric that supports navigation and memory. Located primarily in the entorhinal cortex of rodents, non-human primates, and humans, these cells exhibit a unique hexadirectional firing field that covers the environment, providing constant and uniform representations of visited locations. Environmental geometry and boundaries can disrupt the regular firing fields of grid cells, possibly affecting positional memory. In Project One, we investigated the increased linear perspective cues in trapezoidal environments that negatively influence positional memories in young adults. While positional memory performance was comparable across environments, participants demonstrated better memory on the narrow side of the trapezoidal environments than on the broad side, regardless of the linear perspective manipulation. Additionally, memory performance improved for targets closer to boundaries, regardless of the environment, suggesting a boundary-related influence on positional memory rather than linear perspective. Aging is another factor that also degrades grid-like activity in humans, impairing navigation. In Project Two, we examined whether grid-like activity differentially supports episodic memory domains in younger and older adults. Younger adults showed superior episodic memory performance and higher grid-like activity compared to older adults. Regardless of age groups, the grid-like magnitudes predicted ‘what’ memory performance but not ‘where’ domain and path integration. Subsequently, to determine whether older adults with exceptional episodic memory – SuperAgers - retain their grid-like activity, Project Three explored episodic memories and grid-like activities of SuperAgers in aged-matched and younger-older adult groups. SuperAgers did not outperform control groups in episodic memory, path integration, and grid-like activity. However, consistent with Project Two, grid-like activity predicted ‘what’ memory performance across all groups. Taken together, this thesis offers new insights into how environmental boundaries influence positional memory and highlights how grid-like activities may support episodic memory across the lifespan.