Abstract
Previous studies found a systematic bias in depth perception: providing more depth cues increases the magnitude of perceived depth. Here we asked whether this bias is an artifact of cognitively driven perceptual judgements or reflects hardwired mechanisms of 3D processing which should also affect visually guided actions (e.g., reach-to-grasp movements). In the latter case, we expect an intuitive result: adding depth cues to an object that is initially grasped accurately should yield a grasping error. Importantly, this error would trigger correction mechanisms unbeknownst to the subject, giving rise to sensorimotor adaptation. We tested this hypothesis in an experiment where subjects repeatedly reached for and grasped a rendered 3D gaussian bump while receiving independently controlled haptic feedback. In the first block of trials, the 3D bump was only visually specified by binocular disparity. The participants’ movements rapidly adapted to this stimulus until the grasp stabilized. In the second block, when we added monocular cues (texture and shading) to the display, we detected the unintuitive result: the planned grip aperture was initially overestimated and then rapidly diminished in subsequent trials. This clear evidence of sensorimotor adaptation was confirmed in the third block where removing the monocular cues elicited an aftereffect. In the fourth block, we reintroduced again the monocular cues but reduced the rendered depth, so that the (overestimated) visual stimulus perceptually matched the haptic stimulus. As expected, in spite of the visuo-haptic mismatch, we did not detect any sensorimotor adaptation. Strikingly, the results of this novel and implicit test show that biased depth estimates drive both perception and action.