Hippocampal subfields
The distinct anatomical subregions of the hippocampus, such as CA1, the subiculum and the CA2, CA3 and dentate gyrus area, which can be measured separately on high-resolution brain scans.
The hippocampus is a curved structure in the temporal lobe, central to forming new memories and to spatial navigation. It is not uniform: it divides into anatomically and functionally distinct subregions including the dentate gyrus, the CA fields CA1 to CA3, and the subiculum.
Those subfields do different jobs. The dentate gyrus is associated with pattern separation, keeping similar memories distinct, and it is one of the few regions where new neurons are thought to be generated in adults. CA1 and the subiculum handle much of the output to the rest of the brain.
They matter because they are affected differently by different conditions. Alzheimer's disease affects some subfields earlier than others, chronic stress and depression are associated with volume changes in specific regions, and normal ageing does not touch them uniformly.
High resolution MRI now allows them to be measured separately, which is why they appear in recent research rather than in older work, and why studies increasingly report subfield volumes rather than whole hippocampal volume.
The finding most relevant here is that aerobic exercise appears to affect hippocampal volume in older adults, with trials reporting increases in anterior hippocampal volume alongside improvements in memory measures. The literature is genuinely mixed and the effect sizes are modest, so this belongs in the promising rather than the settled column.
The mechanisms proposed include increased blood flow, growth factor signalling and improved sleep, all of which are plausible and none of which is proven to be the operative one in humans.
In practice, this is a research level term. The actionable version is unchanged and well supported for other reasons: regular aerobic exercise is good for the ageing brain.