Does depth-cue combination yield identical biases in perception and grasping?

Grasping critically depends on stereo information. We previously found that binocular disparities yield a distorted visual space, in which objects close to the observer are grasped and perceived as if they were more elongated than farther objects. Such lack of shape constancy results from the inaccurate estimate of the viewing distance, which affects the estimated depth-to-width ratio of an object. This is because (1) depth from binocular disparities scales with the square of the distance and (2) width from retinal size scales linearly with distance. Conversely, depth from monocular cues (i.e., motion and texture gradients) scales linearly with distance, hence the overall shape from these signals should not be affected by errors in egocentric estimates of object location. We therefore reasoned that adding these cues to stereo information should improve shape constancy. Contrary to expectations, in four experiments we found that stereo-texture and stereo-motion stimuli appeared even more distorted than stereo stimuli. More remarkably, results revealed that grasping execution showed identical biases, which were corrected only late in the movement through online control mechanisms, but only if both grasping digits could be visually guided on their respective contact locations. On the contrary, when the index was occluded by the object, biases in shape estimation continued to affect grasping execution until movement completion. Moreover, while the initial part of the grasp showed evidence of collision avoidance, a control experiment suggested that the above biases could have emerged as early as at movement planning. consistent with previous evidence.