consciousness

consciousness The twentieth-century British psychologist Stuart Sutherland once defined consciousness as ‘a fascinating but elusive phenomenon: it is impossible to specify what it is, what it does, or why it evolved. Nothing worth reading has been written about it.’ Consciousness is indeed hard to define, but most people have an intuitive idea of what it is. It encompasses two different concepts: the notion of a self, and the feelings of which the self is aware, especially qualia — our raw sensory experiences.

Although some philosophers (panpsychists) have believed that all things, including inanimate objects such as chairs and umbrellas, are conscious, most people agree that consciousness is associated with brains (and, some would argue, with inanimate machines that work, in some crucial respect, like brains). Stated simply, then, the problem is how does the activity of nerve cells in the brain give rise to our subjective mental life? Neurons — specks of jelly in the brain, with their electrical impulses and their little squirts of neurotransmitter — seem so utterly different from the redness of red or the flavour of Marmite on toast.

The riddle of qualia is best illustrated with a thought experiment. Imagine a neuroscientist in some future century, who has complete knowledge of the workings of the brain — including the mechanisms of colour vision — but who happens to be colour blind and cannot herself distinguish between red and green. She uses the latest scanning techniques to generate a total description of all the electrical and chemical events in the brain of a normal human as he looks at a red object. The functional account may seem complete, but how could it be so without an explanation of the nature of the unique experience of red, which the scientist herself has never had? There is a deep epistemological gulf between descriptions of physical events in the brain and the personal, subjective experiences that we presume to be associated with those events.

Is consciousness a property of the entire brain — does it ‘emerge’ when the brain reaches a certain level of complexity? Or are only some parts of the brain conscious? (After all, if we argue that only brains and not other organs are conscious, why not imagine that only some parts of the brain are involved?) Indeed, neurological evidence suggests that we are unconscious of most of the activity in our brains — not just the below-stairs business of running the heart, digestion, posture, and so on, but also the pre-perceptual processing of information from the senses, and the complex task of selecting and controlling the individual muscles that carry out actions.

A rare disorder, aptly called ‘blindsight’, strikingly demonstrates a dissociation between conscious and unconscious visual processing. It results from damage restricted to the primary visual cortex in the cerebral hemispheres, which classically causes ‘cortical blindness’ in a corresponding part of the visual field. Although the patient denies seeing, say, a small spot of light presented in the blind part of the field, he or she can fairly accurately point towards the spot. Moreover, if a moving spot or a line is shown, the patient can ‘guess’ the direction of movement or the angle of the line, all the time unaware that it exists! This amazing paradox is explained by the fact that there are two main pathways of interconnected nerve cells from the eyes and through the brain. One goes to the primary visual cortex, and on into the lower parts of the temporal lobe, which is responsible for the identification of objects and the laying down of personal memories. The other projects via the reflex visual centres of the midbrain (which control eye movements) and thence up to the parietal lobes of the cortex, where the information is used to guide hand movements. Since the latter pathway is still intact in the patient with blindsight, he or she can use it for reaching for the object. But the other pathway into the temporal lobe seems to be intimately involved in conscious perception. A more subtle dissociation can occur in patients who have extensive damage to the temporal lobe, which does not interfere with basic visual functions but can cause agnosia — an incapacity to distinguish consciously between different objects and shapes. However, such patients can correctly shape their hands to pick up different objects that they cannot perceptually distinguish. It is almost as though there is an unconscious ‘zombie’ inside the heads of such patients, ‘seeing’ the world and guiding the hands but not troubling consciousness with what it is doing.

Unconscious vision is not just a neurological anomaly — it occurs even in normal people. If you are driving a car while talking to the person next to you or on a mobile telephone, many parts of your brain are processing enormous amounts of visual information to enable you to negotiate the traffic. Yet little of it reaches consciousness so long as your attention is focused on the conversation. Interestingly, it is hard to imagine the opposite scenario — of having a conversation unconsciously while paying attention to the traffic. At any instant, we seem to be fully aware of only a minute fraction of the things that we could be aware of. As you stand chatting to a friend at a party, you are unaware of the content of the other conversations around you — unless you deliberately eavesdrop out of the ‘corner of your ear’. Equally, our embarrassingly poor ability to recall the detail of a visual scene if the lights are suddenly switched off indicates that we are genuinely aware of only a tiny fraction of the flood of information that pours into our brains from our eyes. Only the focus of current attention seems fully represented in our consciousness, in the sense that it can be remembered. This all suggests that there is a link between consciousness, attention, and memory, and also that we cannot use language creatively without being conscious.

This raises the so-called ‘Zombie problem’. If we are able to do so much without being aware of it, what purpose does consciousness serve, and how did it evolve? Imagine an unconscious zombie that looks exactly like a person and does all the things a conscious human does, but without being conscious. There seems to be nothing logically impossible about this. Indeed, we have no way of knowing, for sure, that machines, animals, or even other human beings are truly conscious in the way that we feel ourselves to be. Some philosophers, most notably Gilbert Ryle, have argued that concepts of mind, such as self and intention, are merely ‘category mistakes’ — muddles that arise from the misuse of language. Such virtuosic philosophical argument reinforces the ‘Zombie problem’, but is deeply unsatisfactory. We know that we are conscious. Indeed, as René Déscartes pointed out, knowing that we are aware is the only thing that we are really sure about — ‘cogito, ergo sum’.

In parallel with Ryle's attempt to explain away the ‘Ghost in the Machine’, the school of psychology called behaviourism also argued that consciousness does not (or need not) exist and that science should confine itself to an attempt to explain externally observable behaviour. To behavioural psychologists, it has indeed been valuable to view the brain objectively, merely seeking accounts of behaviour without the baggage of common-language concepts such as will, intention, and need. However, it is difficult for most people — even brain researchers — to accept the extreme notion of ‘eliminative materialism’, namely that words such as ‘love’, ‘want’, and even ‘red’ have the same logical status as the once universal but now arcane view that living things have some kind of ‘vital essence’, which distinguishes them from the inanimate world.

More intriguing is epiphenomenalism. Just as the shadow of a running horse appears to run along with it but plays no causal role in the running, consciousness may simply accompany certain brain events but not itself have a function. Can it really be true that when you feel that you are choosing to pick up a cup, it is not the conscious intention that initiates the picking up? In fact, there is growing evidence that our subjective impressions of events in the world and of our intended actions are a kind of post-hoc ‘commentary’ on things that have already happened. Disturbing though it is, our conscious lives may be a plausible but illusory tale, a translation of the zombie world into the domain of subjectivity. But why should we have such a self-deluding system in the brain? How did it evolve? What could its value be?

Faced with such philosphical conundrums, many neuroscientists, with Francis Crick as their standard-bearer, have argued that we should simply aim to define the ‘neural correlate of consciousness’ — the parts of the brain and the nature and activity of nerve cells that implement conscious states. Once we have a clear understanding of the neural activity that is both necessary and sufficient for subjectivity, perhaps many of the philosophic problems will disappear.

The pragmatic advantage of this approach is that it transforms consciousness into an empirical problem that is approachable experimentally. Instead of asking ‘What is consciousness?’, one asks ‘What parts of the brain are active, or in what special way are they active, when someone does something consciously?’ One experimental approach that is proving fruitful is to monitor the activity of different parts of the cerebral cortex (with microelectrodes in animals, or with imaging techniques in human beings) while the retinal image is unchanging but the content of consciousness changes. For instance, how does activity in the brain change as a person or animal shifts attention from one thing to another? What happens when they view ambiguous visual images that can appear, at one moment, to be one thing, but, at another instant, to be something else?

That other mysterious aspect of subjectivity — the feeling of ‘free will’ and intention — is more difficult to study. However, fingers of evidence point towards the anterior cingulate cortex, a region on the inner surface of the frontal lobe. Patients with damage here sometimes feel that their own actions occur without being intended — alien hand syndrome. Conversely, they may be fully conscious but feel that they don't want to do anything at all — akinetic mutism.

The early decades of the twenty-first century will undoubtedly see great advances in our understanding of the neural correlate of consciousness. What is less certain is whether such empirical observations will take us any closer to resolving what philosopher David Chalmers has called the ‘Hard Problem’, that is, what really is the nature of subjectivity? We may be forced to admit that consciousness, like infinity and the particle-wave concepts in quantum mechanics, is a property that cannot be made intuitively straighforward. Consciousness, like gravity, mass, and charge, may be one of the irreducible properties of the universe for which no further account is possible.

V. S. Ramachandran, and Colin Blakemore

Bibliography

Churchland, P. M. (1996). The engine of reason, the seat of the soul. MIT Press, Cambridge MA.
Crick, F. H. C. (1993). The astonishing hypothesis. Charles Scribner, New York.
Ramachandran, V. S. (1998). Phantoms in the brain. William Morrow, New York.
Searle, J. (1994). The rediscovery of the mind. MIT Press, Cambridge MA.
Weizkrantz, L. (1986). Blindsight. Oxford University Press.

See also brain; colour blindness; illusions; imaging techniques; perception; vision.