OCT Angiography and NRAI in Dementia

Inclusion Criteria for dementia subjects: * Physician-confirmed diagnosis of probable Alzheimer's disease * Mild dementia, as defined by score of 20 or greater on the Mini-Mental State Exam, or score of 15 or greater on the Montreal Cognitive Assessment, or Clinical Dementia Rating Scale score of 1. * Age older than 55 years. * Able to comply with study procedures * Corrected visual acuity at least 20/400 in either eye. * Has a legally authorized representative who can sign study consent form and accompany the participant to the OCT study visit. Inclusion Criteria for dementia-free controls: * Age older than 55 years * Able to comply with study procedures * Able to maintain stable fixation for OCT imaging * Corrected visual acuity of at least 20/40 in either eye * Dementia-free, as defined by score of 24 or greater on the Mini-Mental Status Exam, or a score of 18 or greater on the Montreal Cognitive Assessment, or Clinical Dementia Rating of \<1.0. Exclusion Criteria for both dementia and dementia-free subjects: * Non-Alzheimer's disease related primary neurologic disease affecting the central nervous system (i.e. multiple sclerosis, Parkinson's disease) * Evidence on ophthalmological exam within the last year of other ocular diseases or pathology that would confound the assessment of dementia (e.g. glaucoma, diabetic or hypertensive retinal disease, amblyopia, etc.) * Media opacity such as cataract, corneal scar, or vitreous opacity that could interfere with retinal imaging. * Previous intraocular surgery except for uncomplicated cataract extraction with posterior chamber intraocular lens implantation * Inability to maintain stable fixation for OCT imaging or provide informed consent * Spherical equivalent refractive error greater than +3 or -7 diopters, or astigmatism magnitude of greater than 2 diopters. * Diabetes for more than 10 years or hemoglobin A1C level of \> 10 within the 180 days prior to OCT scanning. * Uncontrolled hypertension. : SBP \> 170 or DBP \> 100 * Arrhythmia: irregular pulse, or heart rate not between 50 and 110 beats per minute * Pregnancy or breast feeding.