Visual extinction and awareness: The importance of binding dorsal and ventral pathways

Abstract

Patients with visual extinction were tested on three tasks involving stimulus iden-tification and localization. In the first experiment, in which patients were to iden-tify and localize stimuli, they demonstrated high levels of contralesional omissions. This primarily occurred under conditions of double simultaneous stimulation (DSS), consistent with the character of extinction. In contrast, when patients had to simply localize or count stimuli in Experiment 2, their contralesional omissions were very low. Similarly, when patients were to iden-tify stimuli without localizing them (Experiment 3) they again showed very low contralesional error rates. These results support the view (Baylis, Driver, & Rafal, 1993) that visual extinction may be greatest when information about loca-tion cannot be bound to information about identity. Our visual experience has the compelling quality of being unitary in nature. That is, we are aware of a single, unified visual world in which we act. Although this unitary nature is of obvious value, even a necessity given the singular nature of the world we live in, it is unclear how our brains ensure that it is achieved. Very early on, the visual input is directed to a large number of visual areas that are specialized for the processing of different visual features such as colour and motion. For example, for an observer to be aware of a red ball in a particular location they must be simultaneously aware of the shape, the colour, and the location. Please address all correspondenc e to G.C. Baylis, Attention and Perception Laboratory,