Segment difficulty in two-stroke movements in patients with Parkinson's disease.

In the horizontal plane on a digitizer tablet, subjects made an elbow-extension, two-stroke movement away from the trunk to a first target and then on to a second target. If the two segments of the movement were executed in an integrative manner, the accuracy constraint on the first segment should have produced changes in kinematic features not only of that segment but also of the second segment. Two-stroke movements of ten Parkinson's disease (PD) patients and ten controls were studied to examine whether a high-accuracy constraint on the first segment influences the performance of the second, when the second target has either a high- or low-accuracy requirement. When the accuracy requirement of the second segment was low, both PD patients and controls showed that changing the first target size from large to small influenced the performance of not only the first segment but also the second segment. For the first segment, movement time, acceleration time, and deceleration time increased when moving to the small first target as compared to the large first target. The peak velocity and peak acceleration also decreased as the first target size decreased. For the second segment, similar patterns of kinematic changes in relation to the first segment were observed in all of these parameters. When the accuracy requirement of the second segment was high, the controls showed similar changes in the first and second segments in relation to the change of first target sizes. In contrast, the PD patients showed that the target size that defined the first movement mainly influenced the performance of that segment. Among kinematic parameters tested for the second segment, only acceleration time increased as the first target size decreased. Other parameters in general did not change, regardless of whether movement of the first segment was made to the small or large target. These results indicate that the two-stroke movements of PD patients showed little evidence that they were planned and organized in an integrative manner when there was a high-accuracy constraint imposed on the second segment. On the other hand, control subjects performed two-stroke movements in a manner that suggested the two segments were planned and organized together regardless of an accuracy constraint imposed.