The hippocampus is necessary for binding and reconstituting information in relational memory. These essential memory functions are supported by the distinct cytoarchitecture of the hippocampal subfields. Magnetic resonance elastography is an emerging tool that provides sensitive estimates of microstructure vis-à-vis tissue mechanical properties. Here, we report the first in vivo study of human hippocampal subfield viscoelastic stiffness and damping ratio. Stiffness describes resistance of a viscoelastic tissue to a stress and is thought to reflect the relative composition of tissue at the microscale; damping ratio describes relative viscous-to-elastic behavior and is thought to generally reflect microstructural organization. Measures from the subiculum (combined with presubiculum and parasubiculum), cornu ammonis (CA) 1–2, and CA3-dentate gyrus (CA3-DG) were collected in a sample of healthy, cognitively normal men ( n = 20, age = 18–33 years). In line with known cytoarchitecture, the subiculum demonstrated the lowest damping ratio, followed by CA3-DG and then combined CA1–CA2. Moreover, damping ratio of the CA3-DG—potentially reflective of number of cells and their connections—predicted relational memory accuracy and alone replicated most of the variance in performance that was explained by the whole hippocampus. Stiffness did not differentiate the hippocampal subfields and was unrelated to task performance in this sample. Viscoelasticity measured with magnetic resonance elastography appears to be sensitive to microstructural properties relevant to specific memory function, even in healthy younger adults, and is a promising tool for future studies of hippocampal structure in aging and related diseases.

Advanced age and vascular risk negatively affect episodic memory. The hippocampus (HC) is a complex structure, and little is known about the roles of different HC regions in age-related memory declines. Using data from an ongoing longitudinal study, we investigated whether memory functions are related to volumes of specific HC subregions (CA1-2, CA3-4/dentate gyrus, and subiculum). Furthermore, we inquired if arterial hypertension, a common age-related vascular risk factor, modifies age-related differences in HC regional volumes, concurrent memory performance, and improvement in memory over multiple administrations. Healthy adults ( n = 49, 52–82 years old) completed associative recognition and free recall tasks. In grouped path models, covariance structures differed between hypertensive and normotensive participants. Whereas larger CA3-4/dentate gyrus volumes predicted greater improvement in associative memory over repeated tests regardless of vascular risk, CA1-2 volumes were associated with improvement in noun recall only in hypertensive participants. Only among hypertensive participants, CA1-2 volumes negatively related to age and CA3-4/dentate gyrus and CA1-2 volumes were associated with performance at the last measurement occasion. These findings suggest that relatively small regions of the HC may play a role in age-related memory declines and that vascular risk factors associated with advanced age may modify that relationship.