Orientation of the opposition axis in mentally simulated grasping

Five normal subjects were tested in a simulated grasping task. A cylindrica l container filled with water was placed on the center of a horizontal moni tor screen. Subjects used a precision grip formed by the thumb and index fi nger of their right hand. After a preliminary run during which the containe r was present, it was replaced by an image of the upper surface of the cyli nder appearing on the horizontal computer screen on which the real cylinder was placed during the preliminary run. In each trial the image was marked with two contact points which defined an opposition axis in various orienta tions with respect to the frontal plane. The subjects' task consisted, once shown a stimulus, of judging as quickly as possible whether the previously experienced action of grasping the container full of water and pouring the water out would be easy, difficult or impossible with the fingers placed a ccording to the opposition axis indicated on the circle. Response times wer e found to be longer for the grasps judged to be more difficult due to the orientation and position of the opposition axis. In a control experiment, t hree subjects actually performed the grasps with different orientations and positions of the opposition axis. The effects of these parameters on respo nse time followed the same trends as during simulated movements. This resul t shows that simulated hand movements take into account the same biomechani cal limitations as actually performed movements.