 - Second visit: Participating children will spend 1 day in a room at the NIH Clinical Center Pediatric Day Hospital. During the visit, participants will wear a portable EEG unit to measure brain activity. For the first hour of the visit, researchers will perform movement tests to study the brain activity related to normal movements. For the rest of the day, participants may play games, watch television or movies, read, or nap, while continuing to wear the EEG to monitor brain activity related to the stereotypic movements. 