Those who join the study will return for a second visit for magnetic resonance imaging. MRI uses a magnetic field and radio waves to produce images of the brain. For the procedure, the participant lies still on a stretcher that is moved into the scanner (a narrow cylinder containing the magnet). Earplugs are worn to muffle loud noises caused by electrical switching of radio frequency circuits used in the scanning process. The scan will last about 45 to 60 minutes, at most. Some volunteers may be asked to return for a third visit to obtain a second MRI on a different scanner.