Grasping visual illusions: No evidence for a dissociation between perception and action

Neuropsychological studies motivated the theory that the primate visual system might be organized into two parallel pathways, one for conscious perception and one to guide action (Milner Goodale, 1995). Supporting evidence in healthy humans seemed to come from a dissociation in visual illusions: Aglioti, DeSouza, and Goodale (1995) reported that the Ebbinghaus (or Titchener) Illusion deceived perceptual judgments of size, but only marginally influenced the size estimates used in grasping. Here we show that identical effects of the illusion are found if the perceptual and grasping tasks are appropriately matched. We show that the differences found by Aglioti et al. (1995) can be accounted for by a hitherto unknown, super additive effect in the illusion. We conclude that the illusion does not provide evidence for the existence of two distinct pathways for perception and action in the visual system