Oliver Sacks, the neurologist best known for “The Man Who Mistook his Wife for a Hat,” talks about his new book, “Hallucinations.”

What is the difference between hallucination and imagination?

I think you recognize that what you imagine is your own, whereas with hallucinations there is no sense of you having produced them. One feels “What’s that? Where did it come from?”

I saw this very clearly many years ago in an old lady who started to hear Irish songs in the middle of the night. She thought a radio had been left on but couldn’t find the radio. She then thought that a tooth filling was somehow acting as a [transmitter]. Finally, when certain tunes kept repeating themselves, all tunes that she knew, she wondered if it was a sort of radio inside her head, a mechanism not under her control and apparently not related to what she was thinking or feeling or doing. That way of putting things is very common in people with musical hallucinations.

You experimented with LSD and other hallucinogenics. Have those experiences informed your work as a neurologist?

I think it made me more open to some of my patients’ experiences. For example, there is something which I think of as stroboscopic vision, or cinematic vision, where, instead of seeing a scene continuously, you see a series of stills. I’ve had that myself on LSD, I’ve had it in migraine, and my patients taking L-dopa sometimes describe it, too. So rather than saying nonsense, or closing my ears, I am open to these descriptions. Whether these psychedelic drugs made much difference to me otherwise, I don’t know. I’m glad I had the experience. It taught me what the mind is capable of.

One time you had a conversation with a spider . . .

With the spider, I should have known that it’s impossible. That’s one of the few times when I was completely taken in. The business of believing and being converted by hallucinations worries me. For example, a book has just been published by a neurosurgeon who had a so-called near death experience and is convinced that he saw heaven. I want to say, strongly, hallucinations aren’t evidence of anything, let alone heaven.

You talk about how hallucinations can result from loss of hearing or vision.

Normally, there’s a system of checks and balances in the brain, to prevent any particular region [from] taking off autonomously. If one loses these constraints — for example, if one is blind or even blindfolded — then the visual brain may take off on its own and utilize memory and imagination to give one hallucinations. I work especially in old-age homes and see elderly people — I’m now more elderly than many of them — with hearing and visual impairments but quite clear mentally. I’ve been struck by their tendency to have hallucinations as the sense of perception is diminished.

You have a visual impairment. Does this influence whether you have hallucinations?

I have low-level hallucinations all the while. I see geometrical patterns and proto-letters everywhere. For example, looking up at the ceiling, I see angled forms which look like letters or words. They form and re-form very rapidly. Gradually it’s got more pronounced. But I can and do ignore it, just as I ignore my tinnitus, which goes with my deafness. I’m getting like my patients with auditory and visual impairment. I hope there’s no mental impairment yet.

This article was produced by New Scientist magazine and can be read in its entirety at www.newscientist.com.