 Motor stereotypy is a common movement disorder observed in children and adolescents but whose pathophysiology is unknown. Therefore, our overall objective is to study cerebral activity related to stereotypies. First, we want to define whether motor stereotypies are preceded by a cortical potential, as it is observed before voluntary movements. We hypothesize that no pre-movement potentials will precede the stereotypies. However, if a pre-movement potential is observed, our secondary objective is to compare this potential to cortical potentials that precede voluntary movements. We also aim to study cortical potentials preceding voluntary movements in healthy children and to compare them with the potentials observed in children with stereotypies. 