The purpose of this protocol is to improve understanding of the pathophysiology of dystonia by performing an electrophysiological study using plasticity induction protocols based on dual-site transcranial magnetic stimulation (TMS). We hypothesize that dystonic patients have enhanced responsiveness to plasticity induction in the parieto-motor network. In another exploratory study, we will also explore the responsiveness to plasticity induction of the cerebello-motor network. The clinical significance of such an enhanced plasticity will be evaluated by correlating the plasticity measurements with subjects' performance on two tasks engaging high-order motor processing and involving the parietal cortex and the cerebellum.