 At the 6 months visit, a cross-over is scheduled: group A will receive sham stimulation and group B will receive stimulation of the motor cortex for the next 6 months. In group A, the stimulation of the motor cortex will be resumed before the end of the 6 month sham stimulation, when the clinical status of the patient will come back to the status quo ante UPDRS score equal to baseline pre-implant score. 