Participants will undergo positron emission tomography (PET) scanning to study tics under three conditions- spontaneous tics, suppression of tics, and sleep-to determine which areas of the brain are responsible for generation of tics. For this procedure, the subject is injected with H215O, a radioactive substance similar to water. A special camera detects the radiation emitted by the H215O, allowing measurement of brain blood flow. Subjects will receive up to 20 injections of H215O during the scanning.