An ageing woman waits calmly on her hospital bed for her regular "gentleman visitor from out of town". She urges her relatives away. She knows they can't possibly see him, and it's his company she wants. Another, while attending a clinic, notices five women trying on fur coats on a hot summer's day. A man walks past a French bakery. He hears the sounds of "Alouette, gentille alouette" coming out of its doors. He is alone to hear it. The song is only for him and it's there every time he passes.

Driving along a highway, a woman detects a weird smell: it resembles "shit, puke, burning flesh and rotten eggs". It pursues her for miles and days.

Trapped in a crevasse after falling from an ice ledge in the Andes, his leg broken, a mountain climber hears "a clean and sharp and commanding" voice which obliterates all the chatter in his own mind: if he obeys it, he knows he will be all right. He is.

I have long thought of the neurologist Oliver Sacks as the greatest living ethnographer of those fascinating tribes who live on the outer and still largely uncharted shores of the land of Mind-and-Brain. Here, inhabitants suffer from strange talents: they mistake their wives for hats, can communicate better with animals than humans, have phantom limbs, outlandish tics or prodigious memories, artistic and musical skills. Other more familiar abilities may be compromised, but for all that their humanity is great.

Sacks's reports from these climes are filled with scintillating detail. He has an ability to evoke on the page the fullness of a life that includes yet moves beyond whatever "peculiarity" it may also contain. In this he is like the best of 19th-century doctors: a humane physician interested in a full clinical case, a patient history, and not merely the location of a surface symptom to be classified as a disease and somehow eradicated. Sacks himself cites that earlier neurologist, Sigmund Freud, and the wonderfully named Edward Liveing, author of an 1873 treatise on migraine, as inspirations.

An anthropologist on Mars, to cite the title of an earlier collection of seven case studies, Sacks is also that paradoxical character, the "participant observer". Not only does he shadow his patients' lives outside the consulting room to observe their condition fully and clearly, but like a novelist he also empathises so markedly with them that he develops or finds kindred states in himself. In Hallucinations he charts the many strange forms that hallucinating  this reality-mimicking misperception, whether visual, olfactory or auditory  may take. He is keen to destigmatise hallucination: hearing voices and seeing what isn't there is not necessarily a sign of schizophrenia or mental aberration but part of the grand vista of common enough human experience. Many hallucinators are not persecuted by their "visions" or even uncomfortable in their presence. In past times hallucinations were not even considered aberrant: take Joan of Arc or the hauntings of peasant lore.

All kinds of people, Sacks shows, experience hallucinations: ordinary sleepers, migraine sufferers, epileptics, alcoholics, drug-takers or those deprived of them too abruptly (and we're not talking just street drugs), the delirious, and that cohort with whom Sacks now often works, the ageing. As the old lose their sight or hearing they may begin to hallucinate, but they're wary of admitting it for fear of being registered as demented.

Though neuroscience can still only shed a partial light on why we might perceive what isn't there, the mapping technologies of MRI and fMRI have shown us where in the brain such hallucinatory activity takes place and how it is related to common perception.

But Sacks is less interested here in neurological investigation or even a complicated phenomenology of individual conditions. The book could almost be renamed "100 Varieties of Hallucinatory Experience: From Dancing Patterns to Post-Traumatic Stress Disorder", in which terrifying nightmare scenes intrude themselves on the sufferer's consciousness and won't be put to rest, or to language. There are also altered states and visions of that perhaps biologically necessitated other "presence", who is sometimes a doppelganger, sometimes God.

Some of this material, like that culled from William James's Varieties of Religious Experience, or Breuer and Freud's Case of Anna O, the "hysteric" who gave birth to the talking cure, will be well known; some less so. In one of the liveliest sections of this hallucinator's smorgasbord, Sacks narrates his own early days as an LSD, mescalin and amphetamine user  an activity he only discontinued after he was taken over by a panic-inducing set of visions. These included seeing a huge proboscidean head, a busload of glittering insect eyes and flapping buildings. Like Alice in her Wonderland, these generous annals make us all hallucinators of the most vivid kind.

Lisa Appignanesi's books include Mad, Bad and Sad and All About Love. Her King's College London open series runs until April

In undertaking this wide-ranging, compassionate and ultimately revelatory survey of the strange terrain of humans' delusional capability, Oliver Sacks says he has been inspired by William James's example  in The Varieties of Religious Experience and elsewhere  to create a sort of "natural history or anthology" of the hallucinatory. "My own favourite definition" of an hallucination, Sacks writes in one of his characteristically compendious footnotes, "is that given by William James in his 1890 Principles of Psychology: 'An hallucination is a strictly sensational form of consciousness, as good and true a sensation as if there were a real object there. The object happens not to be there, that is all' (his italics)."

So, following this plan  which appeals to common sense by eliding tricky questions about the reality or otherwise of the external world with a strictly phenomenological approach  Sacks introduces us to a realm of compelling oddities. He begins with Charles Bonnet Syndrome, which affects a significant proportion of blind and visually impaired people. It can give rise to hallucinations that range from simple geometrical patterns and phosphenes (rings or blots of colour), through galleries of faces  both realistic, cartoon-like and even horribly grotesque  to entire moving scenes, often featuring figures in "exotic dress". CBS hallucinations differ markedly from our conception of normal mental imagery in having far greater complexity; and when those experiencing them are observed, their sightless eyes do indeed saccade across and then focus on these objects that happen not to be there.

From his earliest writings Sacks has managed to convey in relatively accessible prose (I say "relatively", because of necessity he employs a wide range of specialist neurological and other medical terminology) the forensic nature of his discipline. Following the founding father of modern neurology AR Luria  whose Mind of a Mnemonist and The Man with a Shattered World he took as his templates  Sacks has presented individual case histories, linked them to diagnostic criteria, and then shown how particular psychic phenomena may well have a discoverable organic basis. Luria's patients were Soviet second world war casualties, often with pinpoint wounds that had produced localised brain damage, and, like him, Sacks proceeds from the palpable effect of a given impairment, such as blindness, to hypotheses concerning the functional organisation of the brain. So, with CBS, the tendency of these hallucinations to consist of strongly defined images, although not determinate things (nothing, for example, that the hallucinator remembers from their sighted experience), suggests to Sacks that "at some lower level, in the early visual system, there is a categorical dictionary of images or part images  of generic 'noses', for example, or 'headwear'." It follows that in the absence of expected visual stimuli, the brain gets to work generating its own phantasmal world, using this lexicon of innate types. Sacks delves deeper into this phenomenon, quoting William Burke, a neurophysiologist who has himself experienced the geometric hallucinations associated with CBS and who hypothesises that the actual imagery may well reflect the very anatomy of the visual cortex itself.

CBS hallucinations, while they may be extremely fanciful and fantastical, produce remarkably little emotional impact on those who experience them, and Sacks is at pains to distinguish these from other phenomena  such as the hectoring, persecutory voices endured by schizophrenics or the classic "bad trip" occasioned by LSD  that may lead to lasting psychological disturbance. With CBS, Sacks was surprised, given his own clinical experience, by how little prevalence the literature indicated historically. Contemporary studies suggest that complex hallucinations may be experienced by up to 15% of blind and partially sighted people, while 80% may be subject to the basic geometric type. Sacks thinks that the failure to report these hallucinations  and many others that have no implications for mental pathology  may be a function of people not wishing to appear crazy.

Certainly, once the whole panoply of the hallucinatory has been brought into plain view  from the sharp "tears" in the visual field and multicoloured swirls associated with migraine, to rare Lilliputian hallucinations in which people see tiny humanoid figures  it seems hard to resist Sacks's contention that these phenomena have always "had an important place in our mental economy", influencing traditional art, folklore and even  for example, in the case of the distinctive transcendent "aura" that precedes an epileptic fit  "generating our sense of the divine".

Divine some hallucinations may be, but most of those recounted by Sacks seem on a sliding scale between the mundane and the terrifying: a sufferer from the herpes simplex virus, which can attack the olfactory nerve, found himself assailed first by the smell of wet cardboard and then  for an entire year!  by that of rotting fish. When this stench finally blew through his mind it took with it his ability to discriminate smell (and by extension, taste), leaving behind only the cruel taunt of an olfactory hallucination as a precursor to his next attack of neuritis. Another sufferer  this time from Parkinson's  began to notice tactile hallucinations soon after being diagnosed: "the surfaces of various objects were covered by a film of fuzz, like peach fuzz, or the down in a pillow." Another Parkinsonian, almost certainly suffering from "Lewy bodies" (abnormal aggregates of protein inside the nerve cells), saw myriad little figures  "Chuckys", she called them  running around her apartment, as well as horror scenes enacted in front of her: "I saw my son murdered right in front of my eyes."

For those unacquainted with Sacks's earlier work, Hallucinations is a perfectly respectable place to start, but for those already familiar with such classic works as The Man Who Mistook his Wife for a Hat, Awakenings and the more recent Musicophilia, there is pleasure to be gained from the way he cross-references, with one case history  say, in the case of the latter, of musical hallucination (some people have both auditory hallucinations of music they've never heard, and also "see" equally bizarre musical notation plastered all over their visual field)  reappearing, and being re-examined from a different angle. This general inspissation of the Sacks worldview can seem both stimulating and disturbing, and about of a third of the way into Hallucinations, I began to feel that the entire world must be full of people furiously hallucinating; while my own phenomenological oddities  migraine, sleep paralysis, quite a lot of major psychotropic-induced visions  seemed curiously diminished.

There was also  for me  a paradoxical sense of distancing from Sacks himself. He's never afraid of putting himself in his own narratives as a character. He has also written a rather wonderful "scientific memoir" of his childhood, Uncle Tungsten, and he always  and with exemplary humanity  makes it clear when he's writing about his own patients. But for all this, the sheer strangeness of his material has, over the years, tended to make his undoubtedly compassionate involvement seem nonetheless a little bit creepy. I was getting this quite familiar sensation from Hallucinations, when, in the section on the visions engendered by intoxicants, the whole book caught fire. Writing without a scintilla of sensationalism, Sacks relates his experiences in the late 60s with LSD, mescaline, cannabis, amphetamine, chloral hydrate and even injectable morphine, culminating in the astonishingly complex and extreme hallucinations occasioned by his taking 20 Artane pills (a synthetic drug related to belladonna).

Suggested to him by his pals on Muscle Beach (a casual aside in the text, but I happen to know that Sacks was a champion weightlifter in his youth), the Artane provoked completely veridical hallucinations of friends coming to visit him in his LA home, his parents arriving by helicopter, and even a conversation with a philosophic spider who's opening comment was: "did I think that Bertrand Russell had exploded Frege's paradox?" Normally such wacky reminiscences are a turn-off for the enquiring reader, but in the context of Sacks's natural history of hallucinations they are revelatory, making of the author not simply an interpreter of others' dreams, but one who experiences them as well. The last two-thirds of the book gain immeasurably in force and depth from the centrality he himself has taken in the narrative; indeed, for seasoned Sacks-watchers, I would say that Hallucinations is really the keystone of the amazing edifice that is this remarkable thinker's oeuvre; a body of work that sets out to do nothing more or less than examine the totality of human being from the perspective of neurology.

 Will Self's Umbrella is published by Bloomsbury.