The control of an action in Parkinson's disease

We studied, in Parkinson's disease (PD) patients and healthy control subjects, the kinematics of the action formed by two successive motor acts: reaching-grasping an object (first target) and placing it on a second target. We examined the effects of extrinsic (i.e., distance) and intrinsic (i.e., size) properties of the second target on the various kinematic phases of reaching-grasping. We randomly varied distance and size of both stimuli across the experimental session. The kinematics of the reach initial phase of both patients and controls was influenced by the distance of both the first and the second target. In particular, peak acceleration increased for farther position of the second target. However, in the subsequent phase, patients, differently from controls, modified their reaching kinematics, removing the effects of second target position. These results were due neither to a visual interference effect of the second target on reaching-grasping nor to the complexity of movement sequence. Finally, the size of the second target did not affect grasp kinematics of both patients and controls. The results of the present study support the hypothesis that PD patients are able to compute the general program of an action in which extrinsic properties of both the actual and the final target are computed. However, PD patients re-program movement during its execution. This suggests a decay of the motor program. That is, basal ganglia can be involved in storing the plan of an action and in controlling its correct execution.