Aging shifts hippocampal reactivation from selective reinstatement to category-level misbinding during episodic memory.

Episodic memory is the ability to bind elements of an experience into coherent associations. This ability declines with age, often producing errors that can interfere with daily life. Memory distortions might arise from unstable hippocampal representations that cannot be maintained across encoding, storage, and later retrieval. However, whether aging disrupts the temporal continuity of these representations across memory phases has not been directly tested. Here, adults aged 20-74 completed functional magnetic resonance imaging (fMRI) scanning during an associative memory task involving face-object and face-scene pairings, followed by post-encoding rest and a cued retrieval test. Using multivoxel correlation structure analyses, we quantified hippocampal pattern similarity across each memory stage. Increasing age was associated with increased category errors and reduced representational similarity across all transitions. Higher encoding-retrieval similarity predicted better associative accuracy in younger adults and was less predictive in middle-aged and diminished in older adults. Uniquely among older adults, greater cross-phase similarity predicted elevated category-level binding errors, suggesting a shift from selective reactivation to misbinding with age. Baseline hippocampal organization and encoding-specific connectivity supported accurate reinstatement only in younger adults. Aging disrupts hippocampal stability, possibly representing a core mechanism of age-related decline in associative memory across the adult lifespan.