Time constraints improve reaching movements in an ataxic patient

We report on a patient (AM) with a post-traumatic ataxia who has uncoordina ted reaching movements to resting targets, but is able to catch moving obje cts. AM participated in three experiments to identify factors responsible f or the favorable effect of object motion on her performance. In the first e xperiment, the task was to catch an object that moved away from AM. The spe ed 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 2 000 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 vi sion-on time hardly affected the performance of AM's unaffected left hand, they greatly improved her right-hand performance. These results are discuss ed in light of the hypothesis that the brain mechanisms con trolling extern ally triggered movements differ from those controlling internally regulated movements.