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Abstract

It is often assumed that the primate brain processes visual information in two different streams, one for visual awareness (or perception) and one for motor performance. Previous reports that the Ebbinghaus illusion deceives perception but not grasping, seemed to provide strong evidence for this perception versus action hypothesis. The dichotomy between an action stream and a perceptual stream appeared to be fundamental enough to be reflected in the overt behavior of non neurological, healthy humans. Contrary to this view, we show that the Ebbinghaus illusion affects grasping to the same extent as perception. We also show that the grasp effects cannot be accounted for by non perceptual obstacle avoidance mechanisms, as was recently suggested (Haffenden amp;amp; Goodale, 2000; Haffenden, Schiff, amp;amp; Goodale, 2001). Instead, even subtle variations of the Ebbinghaus illusion affect grasping in the same way as perception. Our results suggest that the same signals are responsible for the perceptual effects and for the motor effects of the Ebbinghaus illusion. This casts doubt on one line of evidence which has often been counted as being especially strong in favor of the perception versus action hypothesis.