The simultaneous tilt illusion reveals separate yet interacting visual systems for perception and action

Proponents of Milner & Goodale’s (2008) two-visual-systems hypothesis of primate vision cite the resistance of the hand’s in-flight grasp aperture to illusory target size as strong support for separate dorsal and ventral processing streams. Here we test whether this dissociation holds for target orientation, one of many object features that must be taken into account when grasping it. Participants (N=65) responded to a virtual target: the mirror-reflection of a real, three-dimensional bar set against background gratings designed to induce the simultaneous tilt illusion. Responses included natural grasping, pantomime grasping, and two-alternative perceptual judgments of bar orientation. On natural grasp trials, a copy of the bar was positioned behind the mirror to match the apparent location and orientation of the virtual one. On pantomime grasp trials, no object was positioned behind the mirror; participants reached for the virtual bar and pretended to grasp it. Participants performed natural and pantomime grasps without visual feedback, while motion-capturing equipment tracked the positions of markers attached to the index finger and thumb. In line with the two-visual-systems hypothesis, the simultaneous tilt illusion influenced perceptual judgments but not grip orientation of natural grasps. The results for pantomime grasps were more nuanced. At peak grasp aperture, the illusion bore no effect on grasp orientation, implying dorsal stream input. However, at the end of the reach, an effect of the illusion emerged that was comparable to the one on judgments. This finding implies that the visual control of naive pantomime grasp orientation transitions from a dorsal to ventral origin as the reach unfolds. An important future direction will be to test participants with greater experience in fine-motor control and pantomime skills (e.g., skilled pianists, magicians, and sign-language users), because studying these persons may allow a natural manipulation of the degree of cognitive control exerted during pantomime grasps.