Following visits 1 and 2, qualified subjects will undergo Magnetic Resonance brain scans the Brain Imaging Center of McLean Hospital in Belmont, MA. Prior to MR scanning, subjects will be questioned for a second time about the presence of metallic implants and fragments or any other contraindications to MR imaging. They will be asked about recent alcohol, drug (licit and illicit), and caffeine use. They will also be asked to provide a urine sample for drug screening. Women will take a urine pregnancy test. Also, since brain GABA levels can be affected by the menstrual cycle, women will be scheduled to have their MR scans during the first portion of their menstrual cycle. Subjects will also be asked to complete the following assessments before the MR scans: