Studies have revealed reduced left CA2, 3 and dentate gyrus (CA23DG) activity in cognitively intact APOE ε4 carriers, which may suggest that reduced neural activity in hippocampal subregions may underlie the compensatory increase in extrahippocampal activity in people with a genetic risk for AD prior to the onset of cognitive deficits (84). Here, APOE is linked to Alzheimer disease.