Effects of a moving target versus a temporal constraint on reach and grasp in patients with Parkinson's disease

The reaction times and kinematics of reach and grasp were analyzed for eight subjects with Parkinson's disease (PD) and eight healthy subjects during three variations of a maximal speed prehension task: (a) grasping a stationary ball as fast as possible, (b) grasping a stationary ball within specific time constraints (520 ms and 450 ms), and (c) grasping a moving ball within the same time constraints. Subjects with PD exhibited bradykinesia when reaching for a stationary ball. When reaching for a moving or stationary ball with temporal constraints, subjects with PD moved as fast as healthy subjects. The reaction times of both groups were shorter when reaching to a moving ball than to a stationary ball, regardless of the time constraint. Subjects with PD had a slower velocity of hand opening and closing, a smaller maximal aperture, and a longer time to maximal aperture than healthy subjects in all task conditions. Thus, visual motion cues and external temporal constraints had a greater effect on reach than on grasp. The results suggest that the bradykinesia observed in individuals with PD during self-determined maximal speed prehension may reflect a strategy used to compensate for deficiencies in the grasp component of the task.