Reaching between obstacles in spatial neglect and visual extinction.

The aim of the present studies was to investigate whether 'perception' and 'visually guided action' could be dissociated with regard to two different aspects of the neglect syndrome. In the first study we tested a group of patients with neglect in two tasks, both within the same experimental setting. One task was to bisect a space between two objects, while the other required subjects to reach between the same pair of objects en route to a target area, so that the objects became potential obstacles to the reach. In the second study we tested a patient with visual extinction to double simultaneous stimulation, using a similar reaching task. Our aim was to determine whether visual awareness of obstacles in the workspace was necessary for successful navigation. In both studies we found evidence that reaching responses took normal account of the presence and location of obstacles on the left side, despite the tendency to neglect such left-sided information in more explicit perceptual tasks. We interpret both sets of results within a theoretical framework that identifies on-line visuomotor control with the occipito-parietal 'dorsal stream' (along with associated premotor and subcortical structures), and visual perception with the occipito-temporal 'ventral stream', plus associated temporo-parietal areas.