Effects of disease progression and L-dopa therapy on the control of reaching-grasping in Parkinson's disease

The present study aimed to determine whether the bradykinesia of Parkinson's disease (PD) patients during the execution of reaching-grasping movements (i) is related to an impaired implementation of movement parameters and (ii) selectively involves the control of reach and/or grasp movements. We compared the kinematics of reaching to grasp of differently sized objects placed at different positions, among PD patients in the early stage of disease (ESPD), in the advanced stage of disease (ASPD) without L-dopa medication (off-state), and in healthy controls. In addition, we analysed the effects of L-dopa replacement therapy by comparing the kinematics of the patients in the advanced stage of disease after L-dopa administration with those of the other groups. Bradykinesia increased with disease progression, but only in the initial phases of the reach and grasp components. However at both stages of the disease, the kinematics of reaching and grasping responded to extrinsic and intrinsic object properties just as in controls. L-dopa administration improved the performance of PD patients, though this was more evident for the reach than for the grasp. We suggest that the basal ganglia (BG) are involved in implementing kinematic parameters, but neither (or only marginally) in the initial movement parameterization itself, nor in the on-line control of movement. Specifically, the BG dysfunction in PD induces a slowed implementation of movement parameters. The lack of effect of L-dopa administration on grasp kinematics may be because the motor control of distal effectors is less represented in the motor circuitry formed by the supplementary motor area (SMA), thalamus and BG.