Aging reduces shared event structure at encoding, weakening memory reinstatement in the Posterior Medial Network

Episodic memory involves forming coherent event representations during ongoing experience and reinstating those representations during recall. Although aging is thought to reduce memory specificity, it remains unclear whether age-related reductions in neural reinstatement reflect degraded event representations at encoding, impaired reactivation at retrieval, or both. Here, we used fMRI to measure activity in the posterior medial network (PMN) while younger and older adults watched a 26-minute television episode and later freely recalled it. We assessed three complementary measures: Within-participant pattern reinstatement, within- and cross- age group intersubject correlation (ISC) to measure shared temporal dynamics across individuals, and event-level intersubject pattern similarity (ISPS) to measure shared spatial representations across individuals. ISPS was computed for encoding-to-encoding, encoding-to-recall, and recall-to-recall event pairs, and was evaluated within- and cross-age groups. Older adults recalled the episode less specifically and showed weaker event-specific reinstatement than younger adults. During encoding, older adults exhibited reduced ISC and ISPS within their age group, while cross-age group comparisons showed no differences. Across both age groups, encoding-to-encoding ISPS was the most consistent predictor of reinstatement. In younger adults, reinstatement was additionally predicted by ISC and encoding-to-recall ISPS. Encoding-to-recall ISPS among older adults revealed a suggestive dissociation preferring alignment with younger adults’ encoding patterns over those of other older adults. This dissociation occurred even though encoding-to-recall and recall-to-recall ISPS were comparable across age groups, arguing against a retrieval-only account of age-related memory decline. Instead, the stability and shared structure of event representations during encoding may constrain memory reinstatement, shifting the focus toward encoding as a key contributor to age-related memory change.