GRASPING VERSUS POINTING AND THE DIFFERENTIAL USE OF VISUAL FEEDBACK

The purpose of this study was to evaluate the speed at which visual fe edback about changes in target position could be used to amend reachin g movements when subjects pointed to targets or grasped objects. Subje cts were required to point to or grasp a target which sometimes change d position unexpectedly upon hand-movement initiation. We found no evi dence of early (before peak velocity) trajectory corrections when subj ects pointed towards targets. When subjects were grasping perturbed ta rgets, however, peak velocity was lower and was achieved more quickly than it was on trials in which the target did not change position. The se results suggest that subjects were amending their grasping movement s very early in the trajectory, indicating that visual information abo ut changes in target position is used in a different manner when subje cts intend to grasp an object as opposed to point to it. These results are discussed in terms of the function of reaching and grasping.