The purpose of NOMEM is to learn more about how blood sugar and other factors relate to the brain and mental abilities of persons living in Northern Manhattan. We are seeking your help to conduct this study. Participation will include these activities: 1. Questionnaires 2. Blood tests 3. A brain MRI 4. A brain PET scan with contrast. We will compensate your time for participating in these 4 activities with $350. We will also give you the results of important blood tests.