APOE and Alzheimer disease: AD: Alzheimer disease; DN: Default Network; MCI: mild cognitive impairment; APOE4: apolipoprotein epsilon 4; FC: functional connectivity; ADAS-Cog: Alzheimer’s Disease Assessment Scale-Cognitive; RAVLT: Rey Auditory Verbal Learning Test; SUVR: standard uptake value ratio; fMRI: functional magnetic resonance imaging.