To reach for and grasp an object, its distance, shape and size must be known. In principle, the combination of disparity and motion information could be used to provide this information as the perception of object shape from disparity is biased and the perception of object size from motion is indeterminate. Here we investigate whether the visual system can take advantage of the simultaneous presence of both cues in the control of reaching and grasping. For both real and virtual objects, peak grip aperture scaled with object size and peak wrist velocity scaled with object distance. Kinematic indices, which reflect distance reached and perceived size, showed clear and systematic biases. These biases may be interpreted as arising from the biases in the use of binocular disparity, and the indeterminacy of the information provided by motion. Combining disparity and motion information improved estimates of the width, but not the depth or distance of objects. Overall, these results suggest that accurate metric depth information for the control of prehension is not available from binocular or motion cues, either in isolation or in combination.
Prehension; Binocular disparity; Distance perception; Size perception
