Prolegomenon to any study of consciousness

The most puzzling aspect of vision and visual perception is that it gives rise to conscious seeing: to the vivid sense of a hot pink shirt, the sight of the face of a loved one, or the profound sense of depth when gingerly stepping up to the edge of a bluff. After a hiatus of half a century or more (e.g., Westheimer, 1999), the physical origins of consciousness are once again being vigorously debated in numerous books and monographs published in the past decade. What is it that we can we ascertain about the neurobiological basis of consciousness, in particular visual consciousness, and what can we reasonably assume at this point in time? Neuroscientists have made a number of working assumptions that, in the fullness of time, need to be justified more fully.

Firstly, and most importantly, is the assumption there is something to be explained—that the subjective content associated with a conscious sensation exists and has its physical basis in the brain. Philosophers refer to this as qualia, what-it-is-like, the aboutness of consciousness, and other terms. At this point in time, no even remotely satisfactory accounts of why brains can produce subjective feelings have been presented. Our strategy is to focus for now on other aspects of qualia where progress can be made.

There is general agreement that much of consciousness is private. I cannot convey to you exactly how red looks to me, even if experiments show that you and I respond to colors in a very similar way. We have claimed (Crick and Koch, 1995a) that this is because, at each stage of processing in the cortex, the information symbolized is recoded, so that the more internal neural activity is expressed only very indirectly in any motor output, such as speech. On the other hand, a person can say whether two shades of red appear identical or not. It is not surprising that much of the content of consciousness is largely private. What is mysterious is the exact nature of these internal experiences.

Secondly, consciousness is one of the principal properties of the human brain, a highly evolved system. Therefore, it must have some useful function to perform. It is extremely unlikely that the well-structured nature of conscious representations are epiphenomena without any causal function, as is sometimes asserted by philosophers. We (Crick and Koch, 1995b) assume that the function of visual consciousness is to produce the best current interpretation of the visual scene—in the light of past experiences—and to make it available, for a sufficient time, to the parts of the brain which contemplate, plan, and execute voluntary motor outputs (including language). This needs to be contrasted with on-line or zombie behaviors that bypass consciousness but can generate stereotyped motor outputs (see below).

Thirdly, at least some animal species possess some aspects of consciousness. In particular, this is assumed to be true for nonhuman primates such as the macaque monkey. Visual consciousness is likely to be similar in humans and monkeys for several reasons. (1) For most sensory tasks (e.g., for visual motion discrimination, as discussed in Chapters 81 to 83), trained monkeys behave as humans do under similar conditions. (2) The gross neuroanatomy of humans and nonhuman primates is the same once the difference in size has been accounted for. This is not to deny differences in the microcircuitry or the existence of cell types unique to humans and great apes (Nimchinsky et al., 1999). (3) As emphasized throughout this book, functional brain imaging in humans is confirming the existence of a functional organization similar to that characterized by single-cell electrophysiology in the monkey.

As a corollary, it follows that neither language nor a highly developed sense of the self or of others is necessary for consciousness to occur. Indeed, even severely autistic individuals can carry out very complex sensory discrimination tasks and show no evidence of deficits in sensory discrimination or detection tasks, conscious or not (Baron-Cohen, 2001). Of course, language does enrich consciousness considerably, but there is little hard evidence that it is necessary for conscious sensations to occur.