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Visual awareness is considered by many neuroscientists as the appropriate starting point to study the neural correlates of consciousness. Here, I put forward a novel model of visual awareness, with a particular emphasis on the distinction between visual information processing that invoke qualia (Jackson 1982, Chalmers 1995) and those which do not. I show how this distinction is crucial in streamlining the seemingly incompatible physiological evidence on the neural correlates of visual awareness.
Currently, there are two streams of opinion on the neural correlates of visual awareness.
One stream puts forward the view that visual awarensss correlates with the "explicit" representation of visual features in the higher visual areas. Crick and Koch (1995) put forward the view that only neural activities in those visual areas directly connected with the prefrontal and parietal cortices correlate with visual awareness. By this criterion, they specifically exclude the primary visual cortex (V1) from the correlates of visual awareness. Recent experiments on binocular rivalry (Logothetis 1998, Lumer et al. 1998, Tononi et al. 1998) has correlated the shift in perceptual dominance with activity changes in the prefrontal and parietal cortices. These neurophysiological data seem to be compatible with the Crick and Koch hypothesis.
On the other hand, the neurology of blindsight (Weiskrantz et al. 1995, Cowey & Stoerig 1995) implicates that the loss of V1 results in a loss of visual awareness. The residual information passed on to the higher visual areas via alternative routes gives the subject the ability to discriminate visual information (such as the direction of movement) without the subject consciously seeing the stimulus. Thus, blindsight seems to suggest that the neural activities in V1 is crucial for invoking visual awareness, in contradiction with the Crick and Koch hypothesis.
I argue that the Crick and Koch hypothesis and the V1 hypothesis can be made compatible by noting that explicit representation of visual features in higher visual areas do not necessarily result in qualia. The contours of illusory figures are represented explicitly only in the higher visual areas. However, the completion of illusory contours are often done amodally (Davis and Driver 1997, Shimojo and Nakayama 1990) invoking no change in the visual qualia. When we see Kaniza's triangle, for example, the completion of the contours of the illusory triangle is done amodally, unless there is a perceived luminance change in the interior of the triangle. Thus, it can be argued that the explicit representation that Crick and Koch equate with visual awareness does not necessarily invoke visual qualia corresponding to that representation. This realization leads to a model of visual awareness in which the activity in V1 is essential in invoking visual qualia, while activities in higher visual areas give explicit representation of visual features but do not necessarily result in qualia. I show with such a model that it is possible to explain amodal completion and binocular rivalry on a common basis, giving new insights to the nature of visual awareness.
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