THE REACHING MOVEMENTS OF PATIENTS WITH PARKINSONS-DISEASE UNDER SELF-DETERMINED MAXIMAL SPEED AND VISUALLY CUED CONDITIONS

Two-dimensional kinematic analysis was performed of the reaching movem ents that six subjects with Parkinson's disease and six healthy subjec ts produced under self-determined maximal speed and visually cued cond itions. Subjects were required to reach as fast as possible to grasp a ball (i) that was fixed stationary in the centre of a designated cont act zone on an inclined ramp (self-determined maximal speed condition) , or (ii) that rolled rapidly from left to right down the incline and into the contact Zone (visually cued condition). Parkinson's disease s ubjects displayed bradykinesia when performing maximal speed reaches t o the stationary ball, but not when they reached for the moving ball. In response to the external driving stimulus of the moving ball, Parki nson's disease subjects showed the ability to exceed their self-determ ined maximal speed of reaching and still maintain a movement accuracy that was comparable to that of healthy subjects. Thus, the bradykinesi a of Parkinson's disease subjects did not seem to he title result of a basic deficit in their force production capacity or to be a compensat ory mechanism for poor movement accuracy. Instead, bradykinesia appear ed to result from the inability of Parkinson's disease subjects to max imize their movement speed when required to internally drive their mot or output. The occasional failure of Parkinson's disease subjects to s uccessfully grasp the moving ball suggested errors of coincident antic ipation and impairments in grasp performance rather than limitations i n the speed or accuracy of their reaches. These results are discussed in relation to the notion that the motor circuits of the basal ganglia play an important role in the modulation of internally regulated move ments.