Subjects referred for the study underwent prior cognitive testing at either the Massachusetts Alzheimer's Disease Research Center or the Boston University Alzheimer's Disease Center. Subjects with MCI met the 2004 MCI Working Group Criteria [20] for amnestic MCI and had a global clinical dementia rating (CDR) of 0.5. Subjects with AD met both Diagnostic and Statistical Manual of Mental Disorders, 4th Edition (DSM-IV) [21] diagnostic criteria for dementia and National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimer's Disease and Related Disorders Association criteria [22] for probable AD. Subjects with AD had a global CDR of either 1.0 or 2.0. Control subjects had normal cognition and a global CDR of 0.0. All diagnoses were made by a multidisciplinary consensus diagnostic conference. The Institutional Review Boards at the Massachusetts Eye and Ear Infirmary (MEEI), Massachusetts General Hospital, and the Boston University Medical Center approved this study. Written informed consent to participate was obtained from each subject before the ophthalmoscopic examinations that were performed at the MEEI. The investigators performing the retinal assessments were blind to the cognitive status of the subjects.