"<TAB>MRI scans: Subjects are asked to participate in an MRI study on one of several scanners to measure blood flow in the brain, concentrations of certain chemicals in the brain, or magnetic properties of the brain. MRI uses a strong magnet and radio waves to obtain pictures of the brain. The subject lies still on a narrow bed with a metal coil close to the head. For this study, subjects may be asked to wear a special coil on the neck to help measure blood flow. They may be asked to watch a screen presenting images or to do a task in which they respond to pictures or sounds and may be asked to return for additional scans.