Effects of deep brain stimulation on prehensile movements in PD patients are less pronounced when external timing cues are provided.

It has been repeatedly demonstrated that the movements of patients with Parkinson's disease (PD) are less impaired when external timing cues are provided. This suggests that the basal ganglia, which are impaired in PD, are less involved in the control of externally timed movements. In the present study, we tested this hypothesis by contrasting the effect of deep brain stimulation (DBS) in the basal ganglia (more precisely, the internal globus pallidum) on internally versus externally timed movements. Our first movement task was a standard prehensile task involving a reach-to-grasp movement. In the externally-timed condition, the target object was moving rapidly away from the subject; in the internally-timed condition, the target object was stationary. We found, that for most aspects of the prehensile movement the effect of DBS was less pronounced in the externally than in the internally timed condition. A similar reduction of the DBS effects in the externally-timed condition was also found for a second movement task, which required an isolated grasping movement. We conclude that the basal ganglia are significantly less involved in the control of externally timed movements.