A double dissociation between action and perception in the context of visual illusions: opposite effects of real and illusory size.

The idea that there are two distinct cortical visual pathways, a dorsal action stream and a ventral perception stream, is supported by neuroimaging and neuropsychological evidence. Yet there is an ongoing debate as to whether or not the action system is resistant to pictorial illusions in healthy participants. In the present study, we disentangled the effects of real and illusory object size on action and perception by pitting real size against illusory size. In our task, two objects that differed slightly in length were placed within a version of the Ponzo illusion. Even though participants erroneously perceived the physically longer object as the shorter one (or vice versa), their grasping was remarkably tuned to the real size difference between the objects. These results provide the first demonstration of a double dissociation between action and perception in the context of visual illusions and together with previous findings converge on the idea that visually guided action and visual perception make use of different metrics and frames of reference.