Participants will be studied under 4 conditions, over a span of as few as two days to as long as two months. The four Conditions are : 1) The DBS stimulator will be turned OFF for both sides. 2) The DBS stimulator will be turned ON for the right side. 3) The DBS stimulator will be turned ON for the left side. 4) The DBS stimulator will be turned ON for both sides. The order of these conditions will be picked randomly.