Difficulty in terminating the preceding movement/posture explains the impaired initiation of new movements in Parkinson's disease.

To determine whether the difficulty of initiating volitional movements in Parkinson's disease is primarily due to impaired termination of preceding movement/posture or to impaired initiation of new movement, patients with Parkinson's disease and age-matched controls were first asked to visually fixate a stationary spot and simultaneously align wrist position accurately with it. They were then requested to make rapid movements of eyes and wrist to a test stimulus presented in the peripheral visual field. We analyzed latencies of ocular and manual movements to the test stimulus in two conditions; in the overlap task the stationary spot remained on during illumination of the test stimulus requiring subjects to terminate fixation and wrist positioning themselves to initiate new movements. In the gap task, the stationary spot was turned off 200 ms before illuminating the test stimulus. Latencies of ocular and manual movements were prolonged in the overlap task than those in the gap task. Effects of fixation/wrist positioning on the latency of new movement were evaluated by the difference in latencies between the overlap and gap tasks normalized by the latency difference of the controls. These ratios increased exponentially as Parkinson's stage increased, suggesting the latency prolongation in patients with stage III and IV Parkinson's disease under the overlap condition primarily reflected the contribution of difficulty to terminate existing fixation/wrist positioning.