Movement accuracy constraints in Parkinson's disease patients.

This study examined the hypothesis that the kinematics of movements performed by PD (Parkinson's disease) patients are differentially affected depending on whether or not the aiming movement has an accuracy constraint. The aiming movements required elbow extension in the horizontal plane on a digitizer. There were two movement conditions: (1) one having a spatial accuracy requirement in which the subjects moved to the defined target and stopped on it; and (2) one requiring the subjects to move toward the defined target without stopping precisely on it. Subjects were instructed to make their movements as fast and as accurate as possible in response to the auditory imperative signal. PD patients modified the movement speed and kinematics depending on the two accuracy conditions. However, when the accuracy constraint was imposed, movement slowness observed in the patients was much more pronounced. The most revealing result was localized to the deceleration phase, particularly as the target was approached. The patients also were found to make a higher number of acceleration zero crossings from negative to positive to reach the target, indicating that the movements were more irregular. For the patients, the first acceleration zero crossing from negative to positive occurred much earlier in the movement than that for the controls. In addition, when movement accuracy was constrained, the number of zero crossings was accentuated. These data show that when PD patients make aiming movements to a target, their deceleration phase becomes longer and more variable.