In the posterior parietal cortex, some neurons respond primarily to visual or somatosensory stimul, others respond mostly to current or future movements, and still others respond to a complicated mixture of the stimulus and the upcoming response
. (Shadlen & Newsome, 1996) You might think of the posterior parietal cortex as keeping track of the position of the body relative to the world. (Snyder, Grieve, Brotchie, & Anderson, 1998) Contrast the effects of posterior parietal damage with those of occipital or temporal damage. People with posterior parietal damage can accurately describe what they see, but they have trouble converting their perception into action
. Although they can walk toward something they hear, they cannot walk toward something they see, nor can they reach out to grasp something -- even after describing its size, shape, and angle. They seem to know what it is but not where it is. In contrast, people with damage to parts of the occipital or temporal cortex have trouble describing what they see, but they can reach out and pick up objects, and when walking, they step over or go around the objects in their way (Goodale, 1996; Goodale, Milnor, Jakobson, & Carey, 1991) In short, seeing what is different from seeing where, and seeing where is critical for movement.