Alan Bennett once commented, probably thinking of the sad experiences of his mother and aunt, that common mental problems don't attract much interest, and that you get more attention if you do something bizarre like mistaking your wife for a hat. This might be a fair criticism of some of Sacks' earlier books, but I have to suspend judgment, as it's some time since I read the book in question.
In Hallucinations, Sacks casts his net far and wide. Some of the types of hallucination he describes are rare and exotic , e.g. Charles Bonnet syndrome in which people who were once sighted but are now blind experience vivid visual hallucinatuions. Others, however, are associated with well-known conditions, such as migraine or Parkinson's, and there is even a chapter on hallucinations which any of us could experience ("On the threshold of sleep"). We tend to think of hallucinations as visual, but Sacks covers the other senses too; for example, with hearing he covers tinnitus and then moves on to more obscure conditions.
Sacks is able to draw on his own personal experiences in two of the topics he covers. One is migraine, as he has suffered from this. The other is use of hallucinogenic drugs. He describes, with remarkable candour, how in the mid 1960s, during the postgraduate phase of his career, he would "spend the whole weekend so high that images and thoughts would become rather like controllable hallucinations." His motivation was a mixture of scientific research and opening "the doors of perception". [It must have been a tough job, but someone had to do it ...] An older psychoanalyst friend suggested to him that this behaviour "surely testified to some intense inner needs or conflicts", and this led Sacks to see a therapist.
I found the chapter on Parkinson's particularly poignant, as it accords completely with the experiences of an elderly friend who is a long-term sufferer. The title of this chapter, "illusions of Parkinsonism", relies on a distinction which recurs throughout the book: "illusions" which the subject knows to be unreal, and "hallucinations" which at the time appear genuine perceptions.
There is a fascinating contrast between those who enjoy, and even yearn for, the images produced by their condition (for someone with Charles Bonnet syndrome, you can fully understand this, as it is their only chance of reliving their sense of sight) and those who tend through shame or pride to censor the experience. For example:
"By the late 1980s, physicians had begun to realise (only in response to careful enquiry, for patients are often reluctant to admit it) that perhaps a third or more of those being treated for Parkinson's experienced hallucinations."
I have only one minor gripe: Sacks defines the technical terms used, but sometimes the definition occurs several chapters after the term's first use.
As always, Sacks, now pushing 80, manages to inform and entertain in equal measure.