Thought of 'I' remains a mystery

What does it mean to be a user of consciousness? Embarking on Consciousness: A User's Guide , I was longing to know what Adam Zeman, a consultant neurologist at the University of Edinburgh, would have to say on this most contentious of issues. Does consciousness really do anything? Can it be used? And if so, who or what is the user?

Zeman begins by exploring the origins of the word consciousness and its relatives, such as conscience, derived from the Latin conscio meaning "I know together with". Contrary to some popular claims, he finds equivalent words in widely separated modern languages and is reassured to learn that people everywhere have felt the need to discuss awareness. He distinguishes different modern uses of the term, including self-consciousness and self-awareness, and emphasises that the real issue for contemporary neuroscience concerns consciousness as subjective experience, or "what it's like to be". This leads to the "hard problem" of consciousness; that is, how can our private subjective experiences arise from, or even be, the workings of a physical system, albeit a very complex one?

At this point, Zeman abandons the philosophical difficulties and embarks instead on a detailed tour of that highly complex neural mechanism. Most of the book is not, therefore, directly about the problem of consciousness at all, but about the structure, function and pathology of complex nervous systems. So those seeking the latest arguments in consciousness studies will be disappointed. However, as overviews of neuroscience go, this one is excellent. Many popular books describe the workings of the brain but this is undoubtedly one of the best. Zeman writes clearly, adds interesting examples from his own practice, and sets the whole discussion within a broad evolutionary context, even taking us back to the Big Bang and the origins of life on earth.

Beginning with an overview of the nervous system, he moves on to sleep-wake cycles, control of arousal, the effects of drugs and varieties of coma and anaesthesia. There is a large and informative section on vision and pathologies of vision, and another on the evolution of nervous systems and the origins of awareness. Along the way we meet some of the cases that have earned a special place in the science of consciousness, such as blindsight, split brains, multiple personality and the agnosias. Blindsight, in particular, offers a challenge to theories of consciousness. Patients with blindsight typically have a large scotoma, or blind area, caused by damage to primary visual cortex. The oddity of blindsight is that if a stimulus is presented in their blind field, patients will deny consciously seeing anything at all, but if they are presented with one of two stimuli, such as vertical or horizontal stripes, and forced to guess which it is, they do far better than chance. They say they are blind but behave as though they can see.

Blindsight brings all the problems of consciousness into sharp focus. The most intuitively appealing interpretation is that the patient has lost "conscious vision" but retained "unconscious vision". This in turn suggests that if we can find where the damage is, we might be able to locate consciousness itself and even find out how the brain produces it. But then we come right back to the central mystery. Even if we found the place, we could still not understand what it means for one part of the brain (or type of firing, or neural assembly) to produce consciousness while all the rest do not. In the voluminous literature on consciousness, the obvious interpretation has been torn apart and blindsight has been used to bolster and to reject just about every theory of consciousness we have. Yet Zeman's brief review does not really engage with the problem. From blindsight and other strange phenomena, he concludes merely that "both the location and the quality of brain activity influence its chances of reaching consciousness".

Its chances of "reaching consciousness"? This is just one of many examples that expose Zeman's underlying assumptions. He belongs to that large majority of thinkers who are what the philosopher Daniel Dennett calls Cartesian materialists. Cartesian materialism is "the view that nobody espouses but almost everybody tends to think in terms of" ( Consciousness Explained ). It is the assumption that somewhere in the mind or brain there is a place where consciousness happens; a "theatre of the mind" in which "I" experience the contents of my "stream of consciousness". It is the view that there is a crucial finishing line after which things "enter consciousness". All this has to be false, claims Dennett. Not only does it fail to fit with much of the evidence, but the ordinary view stumbles straight into the hard problem and an insoluble mystery. Something magical must happen to those plain old neural firings that suddenly transforms them into personal private experiences as they "enter consciousness". But what? Cartesian materialism is intuitively appealing, and endlessly popular, but leads straight into mystery.

Zeman leaves the mystery intact. He describes those areas of the brain that "supply the contents of consciousness". He describes some types of processing as leading to consciousness while others are disconnected from it. And he describes his own overview of the visual system as following "the chain from the arrival of light in the eye to the moment of visual awareness". And what happens in that special moment? Zeman does not ask.

The book concludes with evolutionary functions of consciousness, social theories of the origin of introspection, and the implications for human freedom and responsibility. Yet Zeman does not seriously consider the possibility that there may be no user of consciousness at all, or that the ordinary view of consciousness as a power within us might be completely wrong. On the function of consciousness, he says "it is intuitively unlikely, to say the least, that it has none" - and he concludes that consciousness helps "us to select appropriate actions in an unpredictable world". So information flows in through our senses, arrives "in consciousness", and this helps us to select our actions? I think not.

Susan Blackmore's most recent book is Consciousness: An Introduction .