Converging data from rodents and humans have demonstrated an age-related decline in pattern separation abilities (the ability to discriminate among similar experiences). Several studies have proposed the dentate and CA3 subfields of the hippocampus as the potential locus of this change. Specifically, these studies identified rigidity in place cell remapping in similar environments in the CA3. We used high-resolution fMRI to examine activity profiles in the dentate gyrus and CA3 in young and older adults as stimulus similarity was incrementally varied. We report evidence for "representational rigidity" in older adults' dentate/CA3 that is linked to behavioral discrimination deficits. Using ultrahigh-resolution diffusion imaging, we quantified both the integrity of the perforant path as well as dentate/CA3 dendritic changes and found that both were correlated with dentate/CA3 functional rigidity. These results highlight structural and functional alterations in the hippocampal network that predict age-related changes in memory function and present potential targets for intervention.L ong-term memory function is commonly known to deteriorate with increasing age. One of the sites that undergo the earliest changes is the hippocampus (1, 2), which has a well-known role in learning new facts and remembering events (3). Recently, electrophysiological recording studies in aged rodents have shed light on some of the possible neural mechanisms in the hippocampus that underlie this decline (1). These studies have demonstrated "rigidity" in aged CA3 place cell firing patterns in similar environments. In contrast to young CA3 place cells, which readily remap and shift their representations in these environments, aged CA3 place cells retain their original fields despite the changes in the environment. These data strongly suggest that aging is associated with a diminished capacity for pattern separation (learning new information by decorrelating similar inputs to avoid interference) and an increased propensity for pattern completion (retrieval of previously stored information from a partial cue), and further suggest that this shift could be the result of a functional imbalance in the hippocampal dentate gyrus (DG) and CA3 network.The role of hippocampal subfields in these key processes has long been hypothesized in computational models (4-7). The models suggest that the DG granule cells are capable of performing especially strong pattern separation on the distributed representations arriving from layer II entorhinal neurons, projecting this signal onto the CA3 subfield of the hippocampus via the strong mossy fiber pathway. Empirical evidence for the involvement of the DG and CA3 in pattern separation has been demonstrated by using electrophysiological recordings (8-10), immediate-early genes (11), and high-resolution functional MRI in humans (12, 13). Ablation studies using DG-specific ibotenic acid lesions (14), as well as genetic NMDA receptor knockouts (15), have additionally shown that the DG is critical for, and the likely ...