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

Neuropsychological studies prompted the theory that the primate visual syst em might be organized into two parallel pathways, one for conscious percept ion and one for guiding action. Supporting evidence in healthy subjects see med to come from a dissociation in visual illusions: In previous studies, t he Ebbinghaus (or Titchener) illusion deceived perceptual judgments of size , but only marginally influenced the size estimates used in grasping. Contr ary to those results, the findings from the present study show that there i s no difference in the sizes of the perceptual and grasp illusions if the p erceptual and grasping tasks are appropriately matched. We show that the di fferences found previously can be accounted for by a hitherto unknown , non additive effect in the addition. We conclude that the illusion does not pro vide evidence for the existence of two distinct pathways for perception and action in the visual system.