Can (Optical Coherence Tomography) Pictures of the Retina Detect Alzheimer's Disease at Its Earliest Stages?

Years before someone experiences the symptoms of Alzheimer's disease, a compound called amyloid beta (Aβ) builds up in the brain. Excess Aβ - directly or indirectly - causes many of the symptoms of Alzheimer's dementia. However, recent studies of the FDA-approved drugs lecanemab (Leqembi®) and aducanumab (Aduhelm®) indicate that removing Aβ from the brain doesn't stop Alzheimer's. Clearly, there are other problems that need to be fixed. The investigators are interested in the cause of Aβ buildup. Non-neuronal support cells, called glia, keep neurons healthy by regulating water and nutrient levels for the neurons. They also help clear Aβ away from neurons. Maybe Aβ builds up when glia are unhealthy. Glia are very hard to study in the brain. Luckily, the light-sensing part of the eye - the retina - is an extension of the brain. The investigators study glia in the retina to learn about glia in the brain. To study retinal glia, the investigators take pictures of the retina with optical coherence tomography (OCT). OCT is safe, painless, and is used in many eye clinics to look at the structure of the retina. When the investigators take OCT pictures under a bright light, and compare those to OCT pictures collected in darkness, it gives the investigators information about glial function. In a study published in 2020 ("Optical coherence tomography reveals light-dependent retinal responses in Alzheimer's disease") the investigators showed that this functional OCT measurement was different in people with Alzheimer's dementia, compared to age-matched healthy adults. The goal of this observational study is to compare people at a pre-dementia stage of Alzheimer's disease to people who do not have any signs at all of Alzheimer's disease. By "pre-dementia stage", the investigators mean people who are either cognitively normal, or have mild cognitive impairment, but have had a medical test that shows the chemical beginnings of Alzheimer's disease. Members of the comparison group will also be cognitively normal, or have mild cognitive impairment, but had a medical test that shows utterly no signs of Alzheimer's disease. The main question this study, is whether functional OCT can tell these two groups apart. If so, that would: * Help build the case for glial health being important in the earliest stages of Alzheimer's, which in turn could lead to new treatment strategies, and * Suggest that functional OCT might be used as an early (pre-dementia) screening test for Alzheimer's disease Participants will: * undergo a brief eye exam (the investigators will not dilate pupils for this study) * undergo a paper-and-pencil cognitive test (to help verify "normal" or "mild cognitive impairment" status) * take brief one-page survey to collect demographic information (like age) * permit limited access to pre-existing medical or research records (to verify the presence/absence of the chemical beginnings of Alzheimer's disease) * take several OCT pictures of both eyes, in light and after 2 minutes of darkness (several rounds of images are taken) The expectation is that all study procedures will fit within 2 hours of one day.