Perceiving an entire object and grasping only half of it.

A single case study is presented of an unusual dissociation between the sensory perception of an object and the grasping action towards the same object. The patient IW was found at age 74 to have spent all his life without the left parietal lobe, as a result of a congenital peri-natal insult. IW does not show any signs of sensory dysfunction, but he has a persistent motor bias that arises during movement towards objects. When IW was required to reach and grasp stationary and rotating objects, he consistently grasped the objects to the left side. Thus, it appears that information that is available at a sensory level can nevertheless be unavailable at a motor level. Our findings not only help to clarify the functions of the parietal lobe, but also show that lesions in this area are linked specifically to a process of response performance, which is fundamental for our understanding of visuomotor control.