Time constraints improve reaching movements in an ataxic patient.

We report on a patient (AM) with a post-traumatic ataxia who has uncoordinated reaching movements to resting targets, but is able to catch moving objects. AM participated in three experiments to identify factors responsible for the favorable effect of object motion on her performance. In the first experiment, the task was to catch an object that moved away from AM. The speed of the object to be grasped (target object) varied. In experiment 2, the effect of time constraints on reaching performance was examined. AM had to reach for and grasp a stationary object and was allowed either 600 ms or 2000 ms to perform the task. In the third experiment, liquid crystal shutter glasses were used to manipulate the time that the subject was able to view the stationary target object and her reaching movements (vision-on time). While increased speed of the object, tighter time constraints, and short vision-on time hardly affected the performance of AM's unaffected left hand, they greatly improved her right-hand performance. These results are discussed in light of the hypothesis that the brain mechanisms controlling externally triggered movements differ from those controlling internally regulated movements.