Coordination of what and where in visual attention.

Performance often suffers when two visual discriminations must be made concurrently ('divided attention'). In the modular primate visual system, different cortical areas analyse different kinds of visual information. Especially important is a distinction between an occipitoparietal 'where?' system, analysing spatial relations, and an occipitotemporal 'what?' system responsible for object recognition. Though such visual subsystems are anatomically parallel, their functional relationship when 'what?' and 'where?' discriminations are made concurrently is unknown. In the present experiments, human subjects made concurrent discriminations concerning a brief visual display. Discriminations were either similar (two 'what?' or two 'where?' discriminations) or dissimilar (one of each), and concerned the same or different objects. When discriminations concerned different objects, there was strong interference between them. This was equally severe whether discriminations were similar--and therefore dependent on the same cortical system--or dissimilar. When concurrent 'what?' and 'where?' discriminations concerned the same object, however, all interference disappeared. Such results suggest that 'what?' and 'where?' systems are coordinated in visual attention: their separate outputs can be used simultaneously without cost, but only when they concern one object.