TIME TO CONTACT AND THE CONTROL OF MANUAL PREHENSION

In the present study, a kinematic analysis was made of unconstrained, natural prehension movements directed toward an object approaching the observer on a conveyor belt at one of three constant velocities, from one of three different directions (head-on or along the fronto-parall el plane coming either from the subject's left or right). Subjects wer e required to grasp the object when it reached a target located 20 cm directly in front of the hand's start position. The kinematic analysis revealed that both the transport and grasp components of the movement changed in response to the experimental manipulations, but did so in a manner that guaranteed that, for objects approaching from a given di rection, hand closure would begin at a constant time prior to object c ontact (regardless of the object's approach speed). The kinematic anal ysis also revealed, however, that the onset of hand closure began earl ier with objects approaching from the right than from other directions - an effect which would not be predicted if time to contact was the k ey variable controlling the onset of hand closure. These results, then , lend only partial support to the theory that temporal coordination b etween the transport and grasp components of prehension is ensured thr ough their common dependence on time to contact information.