Theories of visual awareness.

The past decade has provided a wealth of data for theorists of visual awareness. Two empirical approaches, both seeking to dissociate conscious from unconscious neural processes, have been particularly fruitful. The first has focused on the neural correlates of changes in experience occurring in the absence of change in external stimuli, for example during binocular rivalry; the second has investigated the neural correlates of unconscious processes such as blindsight. Several of the theories based on these data propose that visual consciousness arises from interactions between thalamo-cortical modules whose independent operation is unconscious; popular candidate 'modules' include visual regions in the 'ventral' visual pathway and parieto-frontal regions associated with action planning. These theories can be tested against recent findings from patients in the vegetative state, a state of 'wakefulness without awareness', which can follow major insults to the brain. The findings indicate that stimulus-evoked cortical activity occurs in the vegetative state, but tends to be limited in extent, is often restricted to primary sensory areas, and is poorly integrated with activity elsewhere in the cerebrum. The theories of visual awareness reviewed previously predict that such activity should not give rise to visual experience. This prediction is reassuring, but can we be sure that it is correct? Reflection on the indirect nature of the evidence available to theorists of visual awareness makes it doubtful that we can confidently specify the minimum conditions for awareness, unless we are prepared to modify our everyday concept of consciousness. O'Regan and Noe have recently proposed a sophisticated redefinition of visual awareness along these lines. Progress at this frontier of visual neuroscience requires that scientists and philosophers join forces to clarify the concepts of experience and consciousness.