THE ROLE OF LEARNED PICTORIAL CUES IN THE PROGRAMMING AND CONTROL OF GRASPING

Binocular information has been shown to be important for the programmi ng and control of reaching and grasping. Even without binocular vision , people are still able to reach out and pick up objects accurately - albeit less efficiently. It remains unclear, which of the many availab le monocular depth cues humans use to calibrate manual prehension when binocular information is not available. In the present experiment, we examined whether or not subjects could use a learned relationship bet ween the elevation of a goal object in the visual scene and its distan ce to help program and control the required grasp. The elevation of th e goal object was systematically varied with distance in some blocks o f trials by presenting the object at different positions along a horiz ontal plane 35 cm below eye level. In other blocks of trials, elevatio n did not vary with distance because the objects were always presented along the subject's line of sight. When subjects viewed these two dis plays monocularly, they showed fewer on-line adjustments in the trajec tory of the limb and the aperture of the fingers when the elevation of the target object in the visual scene could be used to help program t he required movements. No such difference between performance on the t wo arrays was seen when subjects were allowed a full binocular view. T his study confirms that subjects are indeed able to use a learned rela tionship between the elevation of an object and its distance as a cue for programming grasping movements when binocular information is not a vailable. Together with evidence from work with neurological patients who have difficulty perceiving pictorial cues, these findings suggest that the visuomotor system might normally ''prefer'' to use binocular cues, but can fall back on learned pictorial information when binocula r vision is denied.