Neuropsychological studies prompted the theory that the primate visual system might be organized into two parallel pathways, the ventral stream for conscious perception and the dorsal stream to guide action. Supporting evidence in healthy subjects seemed to come from a dissociation in visual illusions: In previous studies, visual illusions deceived perceptual judgments of size, but only marginally influenced the size estimates used in grasping. This claim is tested extensively. A mathematical model is formulated that explicitly states the assumptions that are needed to compare the effects of visual illusions on perception and on grasping. Experiments show that the Ebbinghaus Illusion, the Müller-Lyer Illusion, and the Parallel-Lines Illusion affect maximum preshape aperture in grasping. The current literature is reviewed and it is concluded that the same visual signals which are used for object recognition and conscious perception are also used to guide motor actions.
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