Participants will have MRI scans while they lie still or perform certain movements, as instructed. MRI uses a strong magnetic field and radio waves instead of X-rays to show structural and chemical changes in tissues. During the scan, the subject lies on a table in a narrow cylinder containing a magnetic field. He or she can speak with a staff member through an intercom system at all times during the procedure. All participants will first have a scan to show brain structure, which will take about 30 minutes. A second scan will measure blood flow or biochemical concentration and will take from 1 to 2 1/2 hours.