[Parallel processing of motion vision].
Several neuropsychological studies have reported dissociation between motion vision and object vision. One patient with motion blindness had a bilateral MT/V5 lesion and could see objects, but could not see the motion of the objects (Zihl et al, 1983). By contrast, some blindsight patients with primary visual cortex lesions cannot see objects but can see their movement (e.g. Riddoch, 1917). These results imply that movement vision and form vision rely on independent mechanisms. However one patient with motion blindness had controversial symptoms concerning motion vision. She could not perceive the movement of objects, although she could walk without colliding with obstacles and could catch incoming objects. It has also been reported that patients with a bilateral parietal lesion had well-preserved primary motion vision, but had problems walking and catching a ball (Vaina, 1998). Therefore, motion for vision and motion for action might have independent mechanisms. Such dissociation has also been noted in patients with Bálint syndrome. Some patients behave like a blind person but can walk and catch a ball, while other patients bump into obstacles while walking. These results indicate that the neural bases of motion vision are distinct from those of form vision and that there are subdivisions of motion vision.