Into the Silent Land: Travels in neuropsychology by Paul Broks

Paul Broks sits in a small seminar room surrounded by "rows of display jars containing specimens of human brain, each suspended in a liquid the colour of watery piss". Among them are three which once belonged to the participants in a murderous love triangle. "I love the stillness of this place," he writes, "and the hum of the void - the sense of worlds dissolved and dissipated passions. It fills me with a sense of being. I am not yet pickled meat."

It also causes him to reflect on his work. "My area of supposed expertise, neuropsychology, is the subject about which I feel the most profound ignorance ... Wouldn't it be absurd for an airline pilot to deny knowledge of the principles of flight, or for a physician to claim ignorance of the basics of human physiology and anatomy? Yet I can give no satisfactory account of how the brain generates conscious awareness. Worse still, I find myself edging towards a doubt that it means anything at all to say that the brain generates consciousness."

This, of course, is a celebrated philosophical conundrum usually known as "the mind-body problem": our brains are obviously necessary for consciousness, but what is the precise link between the "flesh and blood and bone and brain" inside our heads and our rich inner life of thought, feeling and fantasy? Some have argued that it is a "category mistake" to try and explain one in terms of the other. A different school, sometimes derided as "Mysterians", believes that we humans will simply never be smart enough to understand the connections. Many have tried to test their views by means of thought experiments featuring "the brain in the vat, the brain transplant, and other science fiction fantasies about teleportation and mind duplication".

Broks's wonderful book is like an extended, rather anguished argument with himself on these issues. A key claim is that some of the people he sees in his clinic, and particularly the split-brain patients, are "thought experiments made flesh". He acknowledges the influence of Oliver Sacks (and Sacks's own model Alexander Luria) in his commitment to a "romantic science" which does justice to the individual patient as well as the neurological syndrome. But he also worries whether "neuro-gothic" tales, whatever their scientific value, owe their appeal to "the intrinsic fascination of the aberrant and the bizarre".

Many of the damaged personalities are very curious indeed. Michael wonders aloud: "Why does raw meat give me a hard on?" Martin is in his late forties but, asked what he's been up to, yells back to the assembled Senior Common Room: "I've been masturbating quite a lot." Maggie is incapable of feeling fear in real life, but reacts violently to mild aggression in a soap opera.

Anthony loses his ability to "read between the lines" in conversation and finds he can deal more easily with straight-talking Australians than the elliptical English. Jeanne draws a complete blank when asked to list some four-legged animals: "For some reason I can only think of three-legged animals."

Perhaps strangest of all are the people suffering from dysexecutive syndromes. One kept stopping, mid-handshake, to pick up a milk bottle, straightened up, turned towards Broks and then bent down again in an endlessly repeating pattern. Another suffered a kind of moral equivalent, a guilt so intense over "a single, weedy act of marital infidelity, a long time ago" that his whole head of hair fell out over a single weekend.

All this makes Broks a worthy claimant to the much contested title of "the new Oliver Sacks", although the flavour of his work is very different. It is hard to imagine Sacks having a meeting with a "distractingly beautiful" expert on body mutilation and then going home to his wife to tell her: "I'm thinking of having my penis tattooed [with the words] Wolverhampton Wanderers. Or maybe just Wolves."

A more important difference is that all the startling case histories keep circling round the same difficult questions of selfhood and consciousness. Additional illumination comes from anecdotes, autobiographical reflections, slightly arch philosophical fables, urban legends about the fate of Einstein's brain, and an account of the "Little People" Robert Louis Stevenson claimed inspired his best work while he dreamed.

Broks's professional view is brisk: "Like the surface of the Earth, the brain is pretty much mapped. There are no secret compartments inaccessible to the surgeon's knife or the magnetic gaze of the brain scanner... you will search in vain for any semblance of a self within the structures of the brain: there is no ghost in the machine. It is time to grow up and accept this fact."

But although he can say this, Broks is also very exercised by the fact that all our intuitions work the other way. Dualism - the belief in a clear distinction between physical and mental - feels right, even if we can't really justify it. When we look at someone's face, we sense there must be a self behind it. Evolution has made us all natural mind-readers, and those who can't interpret other people's mental states or interpret them in wildly unreliable ways (such as autistics and schizophrenics) tend to lose out. It is hard to doubt that even dogs have feelings.

It is here that split brain patients prove so disconcerting. When Naomi had each hemisphere briefly suppressed in turn, "Ms Left-brain was talkative and cheerful. Ms Right-brain was unsettled, mute, morose." (Her only semi-coherent words were "Watafam dooneer", which Broks later deciphered as "What the fuck am I doing here?")

Afterwards, when the drugs were no longer active, "Ms Left-brain spoke for the whole person. 'It was a breeze,' she said. There was no recollection of Ms Right-brain's discomfort." This is fairly typical. The left hemisphere is not a neutral reporter but a "confabulator" which "edits our conscious experiences, makes them comprehensible and palatable. It is the brain's spin doctor."

What this implies is even more disturbing: "One might think that the self is divided in such circumstances, but this would be to swallow the illusion of unity... From a neuroscience perspective we are all divided and discontinuous. The mental processes underlying our sense of self - feelings, thoughts, memories - are scattered through different zones of the brain. There is no special point of convergence. No cockpit of the soul."

This may be a factual claim, but it has huge moral consequences. Another patient Broks describes had fallen down a lift shaft and emerged with a head looking like "the shell of a hard-boiled egg cracked with a spoon". He could growl, grunt, dribble and launch a torrent of obscenities but not speak properly. He became much calmer when his mother was around, but afterwards Broks "sensed a seeping away of the boy's self".

"In such circumstances," he asks movingly, "how are we to distinguish failure of empathy from valid observation? Perhaps they amount to the same thing." This haunting question is typical of a book full of wonders and unsettling new perspectives. It gives us lots of information about neuroscience, but it also helps us understand the deep human and ethical dilemmas of those who work in "the silent land" of other people's brains.