A number of varieties of hallucinatory experience are described, but this is by no means an exhaustive study, as such experiences are more common than is generally realised. Dr Sacks has made extensive use of the experiences of his patients, and of his own experiences.
It was not until he was thirty that Dr Sacks started to experiment with drugs. By then he had the excuse of being a neurologist who needed to know a lot about their effects. He started with cannabis and LSD, then friends suggested he should try Artane, a synthetic drug allied to belladonna, used in small doses for the treatment of Parkinson's disease.
He took a large dose one Sunday morning but nothing seemed to happen until his friends Jim and Kathy came round, as they often did on Sunday mornings, and as they sat in the living room he cooked breakfast for them and chatted to them as he could hear them from the kitchen. However, when he walked into the living room with their breakfast they were not there and he soon realised that he had hallucinated the entire episode. A bit later he heard the sound of a helicopter which he believed to be bringing his parents to visit him, but this also turned out to be illusory. Finally, he noticed a spider on his kitchen wall which said "Hello!" He started a conversation with it "mostly on rather technical matters of analytical philosophy".
Sleep deprivation, exhaustion and extreme physical stress can also cause hallucinations. Sacks quotes from an account of an experience of Michael Shermer (a well-known sceptic), published in Scientific American (292:34), which occurred when he was taking part in a bike marathon. A large craft with bright lights overtook him and alien beings emerged and abducted him. This experience seemed quite real at the time, but it was triggered by the support vehicle which pulled up alongside him with its crew urging him to take a sleep break.
Most people think of hallucinations as being visual, but readers will find a chapter devoted to auditory hallucinations. At one time, psychologists, as well as society in general, regarded "hearing voices" as evidence of insanity. In 1973, however, there was a furore when the journal Science published an article describing an experiment, organised by the psychologist David Rosenhan, in which eight "pseudopatients" with no history of mental illness presented themselves to hospitals across the United States, saying that they "heard voices". Apart from this, they behaved normally, but one of them was diagnosed with "manic-depressive psychosis" and the other seven were diagnosed as schizophrenic.
There are discussions of the work of researchers into hallucinatory experiences involving sleep, including hypnagogic and hypnopompic hallucinations, which are associated with the processes of falling asleep or awakening, as well as nightmare-like hallucinations associated with sleep paralysis. Sacks notes that these were called "Old Hag" in Newfoundland, but the researches he refers to do not include David J. Hufford's well-known work, The Terror That Comes in the Night (University of Pennsylvania Press, 1982). This could be because Hufford wrote as a folklorist rather than as a psychologist.
Sacks remarks that the beings seen in hallucinations are remarkably similar in different cultures, with local variations. "When traditional figures - devils, witches, or hags—are no longer believed in, new ones—aliens, visitations from "a previous life" - take their place".
A more controversial matter concerns the role of neurological disorders in experiences involving apparent religious inspiration. One example given is Joan of Arc. "The early hypotheses of divine (or diabolic) inspiration have given way to medical ones, with psychiatric diagnoses vying with neurological ones." It is thought that Joan may have had "temporal lobe epilepsy with ecstatic auras". He also quotes from a case where a non-religous man became religious after an epileptic seizure, but after another seizure a few years later he lost his religious faith.
The vivid descriptions of different kinds of hallucinatory experiences are presented with the author's usual clarity of style. I can thus highly recommend it to non-specialist readers who are interested in this subject, who will find plenty of material for interesting discussions and arguments. -- John Harney