Motion-transparent inducers have different effects on induced motion and motion capture

To assess the relationship among the underlying mechanisms of induced motion, motion capture, and motion transparency, directions of the former two illusions in the presence of motion-transparent inducers were examined. Two random-dot patterns (inducers) were superimposed upon a stationary disk (target), and moved in orthogonal directions. Either a high-contrast target (for induced motion) or a low-contrast target (for motion capture) was used. The task was to report the perceived direction of the target. The depth order of inducers was controlled either by adding binocular disparity or by asking the subject to report subjective depth order. For induced motion, the target appeared to move in the direction opposite to the inducer that had a disparity closer to the target; when there was no difference in disparity, induced motion occurred oppositely to the 'vector sum' of the inducers' directions. For motion capture, the target was captured by the inducer that subjectively appeared behind. These results suggest that the underlying mechanism of motion capture utilizes the output from the process for motion transparency, whereas induced motion has no clear relationship to the output of the process for motion transparency.