Effects of Healthy Aging on Hippocampal and Rhinal Memory Functions: An Event-Related fMRI Study

TLDR

Memory for past events can be based on recollection, which depends on the hippocampus, or familiarity, which depends on the rhinal cortex, and healthy aging disproportionately affects recollection, yet the neural correlates of these effects are poorly understood. The study aimed to isolate recollection‑ and familiarity‑related neural activity by differentiating linear and quasi‑exponential perceived‑oldness functions derived from recognition confidence. Event‑related fMRI was employed to examine how healthy aging affects hippocampal and rhinal memory functions, isolating recollection and familiarity activity via linear and quasi‑exponential perceived‑oldness functions derived from recognition confidence. Older adults show a double dissociation in medial temporal lobe activity—hippocampal activity decreases with age while rhinal activity increases—along with age‑related changes in parietal and posterior midline regions, altered functional connectivity (reduced hippocampal–retrosplenial/parietotemporal, increased rhinal–frontal), suggesting compensatory reliance on rhinal cortex that may have implications for early Alzheimer’s disease.

Abstract

Event-related functional magnetic resonance imaging was used to study the effects of healthy aging on hippocampal and rhinal memory functions. Memory for past events can be based on retrieval accompanied by specific contextual details (recollection) or on the feeling that an event is old or new without the recovery of contextual details (familiarity). There is evidence that recollection is more dependent on hippocampus, whereas familiarity is more dependent on the rhinal cortex, and that healthy aging has greater effects on recollection than on familiarity. However, little evidence is available about the neural correlates of these effects. Here, we isolated activity associated with recollection and familiarity by distinguishing between linear and quasi-exponential "perceived oldness" functions derived from recognition confidence levels. The main finding was a double dissociation within the medial temporal lobes between recollection-related activity in hippocampus, which was reduced by aging, and familiarity-related activity in rhinal cortex, which was increased by aging. In addition, age dissociations were found within parietal and posterior midline regions. Finally, aging reduced functional connectivity within a hippocampal–retrosplenial/parietotemporal network but increased connectivity within a rhinal–frontal network. These findings indicate that older adults compensate for hippocampal deficits by relying more on rhinal cortex, possibly through a top–down frontal modulation. This finding has important clinical implications because early Alzheimer's disease impairs both hippocampus and rhinal cortex.