In this study, the investigators will follow patients who have had stimulators implanted, at their usual clinic follow-up appointments 3, 6, 9 and 12 months after surgery. It is typical at these appointments for patients to be off medication and for the stimulators to be turned off to observe disease progress and test stimulator effectiveness. Also as part of standard clinical practice, stimulator settings are adjusted for optimal benefit to motor symptoms. Only patients who already have implants will be invited to participate in this study, and no changes to stimulator settings are made for the purposes of this study. Stimulator settings are changed based on clinical evaluation of motor symptoms, and this study has no bearing on how stimulators will be set nor how often they will be set.