Grasping Visual Illusions: No Evidence for a Dissociation between Perception and Action

• Published in: Psychological science Vol. 11; no. 1; pp. 20 - 25
• Main Authors: Franz, V. H., Gegenfurtner, K. R., Bülthoff, H. H., Fahle, M.
• Format: Journal Article
• Language: English
• Published: Los Angeles, CA Blackwell Publishing 01.01.2000; SAGE Publications; SAGE PUBLICATIONS, INC
• Subjects: Circles; Consciousness; Dissociation; Eyes & eyesight; Hands; Humans; Illusion; Illusions; Mental objects; Mental Processes; Modeling; Neurology; Neuropsychology; Object recognition; Optical Illusions; Psychology; Sensory perception; Visual Pathways - physiology; Visual Perception; Visual system
• ISSN: 0956-7976; 1467-9280
• DOI: 10.1111/1467-9280.00209

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 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.