It's a feat to bring any specialty in medicine vividly to life, and to do so without relinquishing the sensitivity and empathy that characterise the best doctors is something that few achieve. Oliver Sacks has managed it throughout his career, illustrating esoteric neurological syndromes for the lay person and explaining the patho-physiological abnormalities responsible in an enthusiastic but humane way; always pitching perfectly, maintaining an intelligent tone without descending into unintelligible jargon.
His latest book is about the ability of the mind to conjure up perceptions that are unrelated to stimuli in the external world. Sacks stresses that, although such sights, sounds, smells, tastes and sensations are not "real" in the sense that most of us consider, they are very much so to those experiencing them, and are often triggered in the same parts of the brain responsible for perceiving and interpreting true stimuli.
He investigates the likelihood that these hallucinations have contributed to many human endeavours, including art and fiction. Lewis Carroll's imagination may have been fired by Lilliputian visual hallucinations (also called Alice in Wonderland syndrome) prior to migraine; Dostoevsky's gradual move from realism to spiritualism by ecstatic pre-epileptic hallucinations; Joan of Arc's religious visions might also be explained by epilepsy.
Sacks ventures into the brains of his blind or near-blind patients with Charles Bonnet syndrome, who "see" clear images that are often more colourful and engaging than true vision, because of a hyper-excitability in the under-stimulated regions of the brain and the visual pathways. Sensory deprivation is another cause of hallucinations, again because it excites neurones in the area of the brain deprived of normal stimuli. Such hallucinations are not restricted to any one sense: there is the conviction that amputees have that they can "feel" their missing limb, for example; the voices "heard" by those deprived of auditory input; and the smells (including the vile ones caused by the olfactory dysfunction cacosmia) in those who have lost their sense of smell.
As well as a convivial host, Sacks is an entertaining one. In the chapter on altered states, he tells us of his own experiments with drugs, which led to occasions when he had conversations with friends who weren't there, and discussed Bertrand Russell with a spider. In the riveting chapter on epilepsy, Sacks relates the case of an epileptic woman who interpreted the euphoric prodromes that can occur before seizures as a message from God to run for Congress. This she did, as a Republican candidate, and amassed the support of thousands by stating in her public appearances that God had told her to run.
Affable, affectionate, respectful and smart, Sacks could be the David Attenborough of the human mind.