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 seahorse-shaped structure deep in each temporal lobe, central to forming and retrieving memories. It is not uniform tissue: it is built from subregions with different cell types and different connections, including CA1, the subiculum, and the CA2, CA3 and dentate gyrus region often grouped together as CA23DG.
These subfields matter because they do not age or respond to disease identically. Ageing, mood disorders and neurodegenerative conditions appear to affect some subregions more than others, so a single whole-hippocampus volume can hide a difference that is concentrated in one part.
Subfield volumes are estimated from high-resolution MRI scans using automated segmentation software, then compared between groups or tracked over time. They are research measures rather than clinical tests, and volumes vary widely between healthy individuals, which is why studies rely on group comparisons rather than judging any one person's scan.