APOE and hippocampal atrophy: The mechanism by which APOE e4 may have its effects is currently unclear, but the presence of the allele has been linked to neural characteristics including reduced cerebral blood flow (Thambisetty, Beason-Held, An, Kraut, & Resnick, 2010), hippocampal atrophy (den Heijer et al., 2002), and a thinner frontal cortex (Fennema-Notestine et al., 2011).