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.