REORGANIZATION OF PREHENSION COMPONENTS FOLLOWING PERTURBATION OF OBJECT SIZE

This study provides a kinematic comparison of the response of 12 older persons (ages 60-70) and 12 young persons (ages 20-26) to a perturbat ion of object size during a reach-to-grasp movement. For 80 out of 100 trials, each participant reached to grasp an illuminated cylinder of either small or large diameter (0.7 and 8.0 cm, respectively). For 20 out of 100 trials, a visual perturbation occurred at movement onset. T his perturbation consisted of a switch of illumination from 1 to the o ther cylinder. For the switch from large to small cylinder, participan ts changed the distal grasp from whole-hand prehension to precision gr ip. For the opposite switch, participants changed from precision grip to whole-hand prehension. The older participants successfully adapted to these perturbations but showed a more conservative approach. Genera lly the approach time as the hand neared the object was prolonged, and the coordination between transport and manipulation components was ma intained when comparing perturbed with nonperturbed trials. Young part icipants showed a more flexible pattern with a decrease or loss of tem poral coupling between the components. It is hypothesized that the mor e rigid movement pattern of older participants to unanticipated motor requirements could be a contributory factor to the higher incidence of accidents for this group.