Participants in this 4-year, observational study will undergo a series of tests and interviews. Participants may choose to complete all procedures over 2 days or three appointments. Procedures will include a 20-minute medical interview, a 4-hour neuropsychological evaluation, a 5-minute functional ability questionnaire, blood and urine collection (approximately 15 minutes), and a 1-hour magnetic resonance imaging (MRI) test of the head. The neuropsychological evaluation will test participants' memory, concentration, reasoning, and speed of thinking. All procedures will be completed over approximately 6 hours.