Stimulate STN (at 0, 5, 10, 15, 20, 25, 30, 40, 70 and 130 Hz) in chronically implanted patients at rest and during simple and complex motor tasks while recording the steady state evoked potential over the cortex using EEG over the 19 sites of the classical 10/20 system and a channel to record artifact over the stimulator. Experiments will be carried out both in the OFF medication and ON medication condition.