Background: Earlier work with neurological patients has shown that the visual perception of object size and orientation depends on visual pathways in the cerebral cortex that are separate from those mediating the use of these same object properties in the control of goal-directed grasping. We present evidence suggesting that the same dissociation between perception and action is evident in the visual processing of object shape. In other words, discrimination between objects on the basis of their shape appears to be mediated by visual mechanisms that are functionally and neurally distinct from those controlling the pre-shaping of the hand during grasping movements directed at those same objects. Results: We studied two patients with lesions in different parts of the cerebral visual pathways. One patient (RV), who had sustained bilateral lesions of the occipitoparietal cortex, was unable to use visual information to place her fingers correctly on the circumference of irregularly shaped objects when asked to pick them up, even though she had no difficulty in visually discriminating one such object from another. Conversely, a second patient (DF), who had bilateral damage in the ventrolateral occipital region, had no difficulty in placing her fingers on appropriate opposition points during grasping, even though she was unable to discriminate visually amongst such objects. Conclusions: This double dissociation lends strong support to the idea that the visual mechanisms mediating the perception of objects are functionally and neurally distinct from those mediating the control of skilled actions directed at those objects. It also supports the recent proposal of Goodale and Milner that visual perception depends on a ventral stream of projections from the primary visual cortex to the inferotemporal cortex, whereas the visual control of skilled actions depends on a dorsal stream from the primary visual cortex to the posterior parietal cortex.
