Participants will undergo MRI and MRS. MRI uses a strong magnetic field and radio waves to visualize brain anatomy and chemistry. For this study, the subject lies on a stretcher, which is moved into a strong magnetic field (the MRI scanner). Earplugs are worn to muffle loud thumping noises caused by the electrical switching of the radio frequency circuits. During the study, the subject lies still during each scan, for 1 to 8 minutes at a time. Total scanning time varies from 20 minutes to 2 hours, with most examinations lasting between 45 and 90 minutes. The subject can speak through an intercom with the staff member performing the study at all times during the procedure. Up to 5 studies may be performed.