Intended and spontaneous motor behavior under a 3D perspective visual illusion

Abstract

OBJECTIVE: To test the hypothesis that top-down visual processes affect our visual percepts and leak into our sensory-motor system. In particular, we ask whether reaches toward a target embedded in a 3D scene are governed by the real or the perceived geometry.

METHODS: 3D stimuli: a proper- and a reverse-perspective. (1) In the proper ("forced") perspective, the perspective-painted cues were congruent with the bottom-up signals of binocular disparity and motion parallax. (2) In the reverse-perspective, the painted cues competed with the bottom-up signals, thus eliciting bistable percepts: (a) The veridical depth percept. (b) The illusory reverse-depth percept in which concave parts are perceived as convex and vice versa; as a result of the illusory percept, the perceived 3D orientation of surfaces is affected drastically. Subjects viewed the stimuli and either pointed to or grabbed at planar disk targets at instructed fast or slow speeds while we recorded their movements.

RESULTS: Hand trajectories intended toward targets remained invariant to changes in speed according to various geometric measures. However, the hand-paths were significantly affected by the illusion, particularly with respect to their lengths and curvature. These effects were even stronger when comparing the endpoint accuracy of the reverspective-veridical and reverspective-illusory percepts. Additionally, strong differences were found between the reaches intended toward the target and the spontaneous transitions retracting the hand to its resting position. These significant differences in the moving hand transferred to the resting hand, thus suggesting a type of motor-overflow with the increase in cognitive load that cannot be explained by the instructed changes in speed alone.

CONCLUSIONS: Overall we find compelling evidence that top-down visual processes affect both our visual percepts and our sensory-motor systems. These effects are separable according to sensory-motor patterns of variability that change the noise levels between intended and spontaneous actions.

Meeting abstract presented at VSS 2012