ERROR PROCESSING IN POINTING AT RANDOMLY FEEDBACK-INDUCED DOUBLE-STEPSTIMULI

Two experiments were conducted to determine the spatial and temporal o rganization of the arm trajectory in human subjects as they pointed to single- and double-step target displacements. Subjects pointed either without (Experiment 1) or with (Experiment 2) vision of their moving hand throughout the trial. In both experiments, target perturbation oc curring in double-step trials was clearly perceived by the subjects an d was randomly introduced either at the onset or at peak velocity of h and movement. Regardless of whether or not visual reafference from the pointing hand was available, subjects corrected the trajectory of the ir moving hand to accommodate the double-step. Moreover, asymmetrical velocity profiles were observed for responses to both types of target, with or without vision of the moving hand. The acceleration phase was a fixed pattern independent of the type of step stimulation. However, a clear dissociation, both in the deceleration phase and accuracy of responses to double-step targets, emerged according to the timing of t arget perturbation. When targets were perturbed at the onset of hand m ovement, subjects modulated the deceleration phase of their response t o compensate for 88 to 100% of the second target displacement. In cont rast, when targets were perturbed at peak velocity of hand movement, s ubjects were unable to modulate the deceleration phase adequately and compensated for only 20 to 40% of the perturbation. These results sugg est that motor error is dynamically evaluated during the acceleration phase of a movement toward a perturbed target, allowing amendments to the trajectory to be performed during the deceleration phase. This mai n corrective process appears to be basically independent of visual rea fference from the moving hand.