Toward a theory of visual consciousness

The visual brain consists of several parallel, functionally specialized pro cessing systems, each having several stages (nodes) which terminate their t asks at different times; consequently, simultaneously presented attributes are perceived at the same time if processed at the same node and at differe nt times if processed by different nodes. Clinical evidence shows that thes e processing systems can act fairly autonomously. Damage restricted to one system compromises specifically the perception of the attribute that that s ystem is specialized for; damage to a given node of a processing system tha t leaves earlier nodes intact results in a degraded perceptual capacity for the relevant attribute, which is directly related to the physiological cap acities of the cells left intact by the damage. By contrast, a system that is spared when all others are damaged can function more or less normally. M oreover, internally created visual percepts-illusions, afterimages, imagery , and hallucinations-activate specifically the nodes specialized for the at tribute perceived. Finally, anatomical evidence shows that there is no fina l integrator station in the brain, one which receives input from all visual areas; instead, each node has multiple outputs and no node is recipient on ly. Taken together, the above evidence leads us to propose that each node o f a processing-perceptual system creates its own microconsciousness. We pro pose that, if any binding occurs to give us our integrated image of the vis ual world, it must be a binding between microconsciousnesses generated at d ifferent nodes. Since any two microconsciousnesses generated at any two nod es can be bound together, perceptual integration is not hierarchical, but p arallel and postconscious. By contrast, the neural machinery conferring pro perties on those cells whose activity has a conscious correlate is hierarch ical, and we refer to it as generative binding, to distinguish it from the binding that might occur between the microconsciousnesses. (C) 1999 Academi c Press.