How multiple retrievals impact neural reactivation in young and older adults.

Abstract

OBJECTIVES\nAging can reduce the specificity with which memory episodes are represented as distributed patterns of brain activity. It remains unclear, however, whether repeated encoding and retrieval of stimuli modulate this decline. Memory repetition is thought to promote semanticization, a transformative process during which episodic memory becomes gradually de-contextualized and abstracted. Because semantic memory is considered more resilient to aging than context-rich episodic memory, we hypothesized that repeated retrieval would impact cortical reinstatement differently in young versus older adults.\n\n\nMETHODS\nWe re-analyzed data from young and older adults undergoing fMRI while repeatedly viewing and recalling short videos. We derived trial-unique multivariate measures of similarity between video-specific brain activity patterns elicited at perception and at recall, which we compared between age groups at each repetition.\n\n\nRESULTS\nWith repetition, memory representation became gradually more distinct from perception in young adults, as reinstatement specificity converged downward toward levels observed in the older group. In older adults, alternative representations that were item-specific but orthogonal to patterns elicited at perception became more salient with repetition.\n\n\nDISCUSSION\nRepetition transformed dominant patterns of memory representation away and orthogonally from perception in young and older adults, respectively. Although distinct, both changes are consistent with repetition-induced semanticization.