Picturing unilateral spatial neglect: viewer versus object centred reference frames.

Vision theorists postulate that knowledge of objects in space is shaped by different spatial coordinate systems. An object may be represented by its location in relation to the viewer, or it may be represented by its own intrinsic spatial properties. When patients with left sided neglect fail to respond to stimuli on the left, it is not clear whether "left" refers to a viewer or an object centred reference frame. To uncouple these two reference frames, eight patients with neglect were asked to centre lines and objects in photographs. Viewer centred neglect would result in images appearing on the right side of photographs and object centred neglect would result in images appearing on the left. Four patients demonstrated viewer centred neglect and three demonstrated object centred neglect. One patient had variable performance, perhaps resulting from competing effects of both viewer and object centred neglect. Stimuli characteristics did not affect the spatial coordinate system in which neglect occurred. These results suggest that viewer centred and object centred reference frames are functionally dissociable, and that patients may have spatial neglect predominantly in either coordinate system.