A dissociation between perception and action in open-loop smooth-pursuit ocular tracking of the Duncker Illusion.

The visual system serves two distinct functions. The information acquired by it is used to both create a percept of the external world and to guide motor actions. In recent years there has been considerable debate regarding whether the same information is utilized and processed in the same manner in the two tasks. The Duncker Illusion, also known as induced motion, is an illusion of motion that results when there is background movement, and it neatly dissociates true from perceived motion. We examined the open-loop period of smooth eye tracking of a target that had an illusory component to its trajectory due to background motion. It was found that the eyes moved in the direction of background motion and of true target motion but not in the illusory direction, despite the strong percept of illusory motion experienced by all subjects. This finding lends support to the hypothesis of separate neurological pathways for the processing of visual information used to create the conscious percept and visual information used to drive motor actions.