Grasping visual illusions: consistent data and no dissociation

The finding that the Ebbinghaus/Titchener illusion deceives perception but not grasping is usually seen as strong evidence for Goodale and Milner's (1992) notion of two parallel visual systems, one being conscious and deceived by the illusion (vision-for-perception) and the other being unconscious and not deceived (vision-for-action). However, this finding is controversial and led to studies with seemingly contradictory results. We argue that these results are not as contradictory as it might seem. Instead, studies consistently show similar effects of the illusion on grasping. The perceptual effects are strongly dependent on the specific perceptual measure employed. If, however, some methodological precautions are used, then these diverse perceptual results can be reconciled and point to a single internal size estimate that is used for perception and for grasping. This suggests that the Ebbinghaus illusion deceives a common representation of object size that is used by perception and action.