n articulate, liberally educated neurologist at the University of Edinburgh, Adam Zeman has written columns for The Times of London and is an occasional commentator for the BBC and the co-author of a book on ethical problems in neurology. His new book covers many aspects of consciousness for general readers. His treatment of the disorders of knowledge is superb. If you were intrigued with ''The Man Who Mistook His Wife for a Hat,'' you'll appreciate the buildup to what Oliver Sacks described in that work. Zeman's much more subtle examples give you some appreciation for how seeing and describing can become disconnected from recognition and other forms of knowledge.

There have been a number of fine books on consciousness in the last dozen years, starting with Daniel Dennett's ''Consciousness Explained,'' which was written from the standpoint of a philosopher well versed in cognitive sciences and evolution. I am also fond of ''The Feeling of What Happens,'' by Antonio Damasio. Like Zeman, Damasio is a neurologist steeped in both literature and philosophy. But Zeman's ''Consciousness'' is the broader book, the one that could be used in an undergraduate humanities or psychology course to fill in the neuroscience background for readers coming to it for the first time. Indeed, Zeman first introduces his subject and then spends a hundred pages on neurobiology and human evolution before returning to consciousness. Readers impatient for consciousness per se can skim these chapters without losing the thread, though they are relatively painless introductions to what consciousness is built atop of.

Consciousness implies both awake and aware. ''Sleep, like wakefulness, is organized from the brainstem,'' Zeman writes. ''It has a hidden structure of its own: in the course of the night we cycle repeatedly from light sleep to deep, from deep to dreaming sleep. The brainstem continues to generate these rhythms after the destruction of the hemispheres, as, for example, in the 'vegetative state.' By contrast, death of the brainstem is almost always followed, within hours or days, by death, pure and simple.'' The nerves controlling the entire body pass through the brainstem, and some brainstem strokes injure these connections while leaving the patient surprisingly alert and aware. ''In these circumstances awareness may survive while almost all means of expressing it are lost, an unhappy state of affairs known as the 'locked-in syndrome.' Sufferers from this disorder usually retain the ability to make voluntary up and down movements of their eyes, and can use these to communicate.'' He adds that we can't deny ''the disturbing possibility'' that this disorder may affect more people than we can now properly recognize as being afflicted by it.

A couple of chapters after I read this, my 91-year-old mother suffered a similar brainstem stroke. There were periods when she could communicate only by moving her eyes and eyelids. Because she was sometimes able to get a few words out, it was obvious she was tracking what we said and was thinking ahead as usual; her consciousness was trapped in a body that would no longer obey her. We knew her wishes about such situations quite well, her sister having lingered five years in a similar state, but my mother was still competent to make her own decision if we could frame it for her limited ability to communicate. So her doctor told her that her situation was unlikely to improve, that we proposed doing nothing except comfort drugs and that she probably would die within a few days. Was that what she preferred? We thought she would have to communicate by blinking her eyelids. But she burst forth with her longest utterance since her stroke: ''You're the best doctor I ever had.'' Those turned out to be her last words. She died two days later.

Consciousness is, however, more than just the minimum requirements of awake and focused. Zeman explains that ''sensation becomes conscious only when it undergoes some further process -- when it encounters past associations, or is used to govern future action, or becomes the object of reflection or is felt to impact upon the self.'' There is an ''important link between consciousness and volition. . . . Willed or voluntary acts are those with aims of which we are conscious and are -- usually -- prepared to acknowledge.'' Consciousness, he concludes, ''bridges perception and action, the events we perceive and the ones we bring about.''

But consciousness is fragile, and ''however magical it may be, it is a physical affair: mundane requirements for oxygen and glucose, electrical equilibrium, clean blood and adequate sleep must be met in the brain -- or consciousness fails. . . . However coherent our experience and behavior may appear, they are prone to fragment under stress. . . . Faints, fits and intoxication all reveal that perception, memory, movement and speech are separable capacities.'' He adds: ''It may be arrogant to deny that consciousness can ever slip its moorings in the brain -- after all, much of the world's population believes firmly that it can -- but the evidence in favor of this happening is tenuous at best.''

Though the book's subtitle proclaims it a ''user's guide,'' the phrase is not easy to find in these pages, making one suspect it was tacked on by the publisher to make the book sound approachable and instructive -- which, fortunately, it is. However, it is not tuned in to two important issues that one would expect to find in a user's guide.

The first concerns how aspects of consciousness develop in childhood. As a child, you eventually come to realize that someone else may not know what you know. This much is covered well. But such structured thinking begins as a 2-year-old makes the transition from two-word utterances to speaking long sentences that cannot be understood without some structuring principles. They are usually called grammar or syntax, and we use them to create past and future tenses and to nest phrases inside clauses and vice versa. Children understand structured sentences long before they can produce them -- if they can hear long sentences or watch them being signed. Certainly, one of life's major tragedies occurs when a child is not recognized as being deaf until well after the major windows of opportunity for soft-wiring the brain in early childhood have closed (much of structuring is not hard-wired instinct). We know how essential this tune-up period is for normal adult consciousness from the short-sentence, present-tense-only adult abilities of deaf children of hearing parents who failed to provide an environment during preschool years that was adequately rich in sign language. (Deaf children of signing deaf parents do fine.)

That suggests that a child born both deaf and blind has little opportunity to soft-wire a brain capable of structured consciousness. But what about Helen Keller and what Zeman calls ''her rich inner life?'' Zeman, alas, makes the usual mistake of describing her as ''born blind and deaf,'' when in fact she probably had 19 months of normal exposure to language before being stricken by meningitis (by 18 months, some children start to express structured sentences, showing that they had been understanding them even earlier). So she probably soft-wired her brain for structured stuff like syntax before losing sight and sound.

A user's guide also needs something about how we create a plan or utterance of high quality -- something better than our dreams, where we see cognitive processes freewheeling without much quality control. They provide us with a nightly experience of people, places and occasions that do not fit together. Fortunately our movement command centers are inhibited during most dreams, so we don't get into trouble acting on nonsense. Awake, we are always searching for coherence, trying to shape combinations that ''hang together'' well enough to act on. When our quality control fails and incoherence is the best thing our consciousness has available during waking hours, it tends to be called hallucination, delusion or dementia.

A great deal of our consciousness -- indeed, our intelligence -- involves guessing well, as we try to make a coherent story out of fragments. Zeman lumps this under a search for meaning, but his description is memorable: ''Eye and brain run ahead of the evidence, making the most of inadequate information -- and, unusually, get the answer wrong. . . . Our knowledge of the world pervades perception: we are always seeking after meaning. Try not deciphering a road sign, or erasing the face of the man in the moon. What we see resonates in the memory of what we have seen; new experience always percolates through old, leaving a hint of its flavor as it passes. We live, in this sense, in a 'remembered present.' ''