Planning and executing an action in Parkinson's disease

We evaluated the possible impairment in planning and executing an action in patients with Parkinson's disease (PD). The action considered in the prese nt study was formed by two successive motor acts: reaching-grasping an obje ct (first target) and placing it on a second target of the same shape and s ize. We examined the effects of extrinsic properties of the second target ( that is, distance) on the various kinematic phases of reaching-grasping mov ements. Distance, position, and size of both stimuli were randomly varied a cross the experimental session. Movements were executed with and without vi sual control of both targets and arm. The performance of six patients with PD was compared with an age-matched control group. The kinematics of the in itial phase of reaching was influenced by position and size of the first ta rget and by distance of the second target in both patients and control subj ects. In particular, peak acceleration was higher for farther position of t he second target. However, in the subsequent phase patients, differently fr om control subjects, removed the effects of the second target distance by m odifying their reaching kinematics. This was obtained by varying the durati on of the acceleration phase. In summary, the patients reprogrammed the rea ching component by taking into account only the properties of the first tar get. The decreasing influence of second-target distance on reaching kinemat ics of patients was more evident during movements executed under visual con trol. Moreover, their movements executed without visual control were slowed down from the beginning. The second target affected the grasping kinematic s only of the control subjects. Globally, these results indicate that PD pa tients are able to compute the general program of an action that takes into account extrinsic properties of the final target. However, the finding tha t PD patients reprogrammed the movement during its execution suggests a dec ay of the program during its time course, that is, basal ganglia can be inv olved in storing the plan of an action and in controlling its correct execu tion.