Loneliness, Chronic Conditions, and Healthcare Engagement in Japan: A Nationwide Cross-Sectional Study.

Episodic memory is the ability to encode, store, and retrieve associations among multiple elements of past experiences and critically depends on the hippocampus (; ). fMRI studies demonstrate that patterns of hippocampal activation observed during encoding re-occur during both post-encoding rest and retrieval, possibly reflecting neural reinstatement processes (; ). The present study investigated how functional connectivity in the hippocampus during these memory stages contributes to both stabilized and distorted memory traces in young, middle-aged, and older adults. Prior work in young adults has revealed similarity in hippocampal multivoxel patterns between encoding and post-encoding rest (; ; ; ). These similarity patterns often predict subsequent associative memory performance, suggesting that spontaneous reactivation of newly encoded representations during rest supports memory consolidation. Likewise, encoding–retrieval neural similarity has been linked to memory performance, indicating that reinstating encoding-related activation can support successful memory reconstruction (; ). However, the role of the post-encoding rest to retrieval transition has not been assessed (). This transition may not simply reflect the passage of time, but how representations that undergo offline processing during rest are subsequently expressed during retrieval. Thus, post-encoding rest–retrieval similarity provides a novel window into whether representations reactivated during post-encoding rest are preserved, reorganized, or transformed before being accessed at retrieval. Examining this transition therefore allows us to determine whether memory-related representations remain stable across phases or evolve in ways that may contribute to memory errors. Aging provides a natural test for examining these mechanisms. Aging is associated with hippocampal atrophy and network reorganization that can alter the contributions of hippocampally mediated memory processes (). Specifically, the atrophy–compensation hypothesis proposes that age-related structural decline may be accompanied by compensatory changes in brain function to support cognition (). Older adults also exhibit reduced distinctiveness of neural representation and weaker hippocampal engagement during memory processing (; ). For example, older adults show reduced encoding–retrieval neural similarity, suggesting diminished reinstatement of encoding-related representations during retrieval (; ).