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Abstract

Neuropsychological studies prompted the theory that the primate visual system might be organized
into two parallel pathways, one for conscious
perception and one for guiding action. Supporting
evidence in healthy subjects seemed to come from a
dissociation in visual illusions: In previous
studies, the Ebbinghaus (or Titchener) illusion
deceived perceptual judgments of size, but only
marginally influenced the size estimates used in
grasping. Contrary to those results, the findings
from the present study show that there is no
difference in the sizes of the perceptual and
grasp illusions if the perceptual and grasping
tasks are appropriately matched. We show that the
differences found previously can be accounted for
by a hitherto unknown , nonadditive effect in the
addition. We conclude that the illusion does not
provide evidence for the existence of two distinct
pathways for perception and action in the visual
system.