It is fitting for the author of a new book about hallucinations to have had so many himself. "A spider on the wall said hello to me," recalls neurologist Oliver Sacks. "And for some reason it didn't seem too surprising that a spider should say hello. We had a conversation about analytic philosophy, a rather technical conversation."

Sacks is describing his experience with a drug called artane in the 1960s. One Sunday morning when he was working as a neurology resident in California, he took a large dose of the drug, which is sometimes used to treat Parkinson's disease and has a similar chemistry to the toxin from the deadly nightshade plant. He hallucinated two close friends arriving, chatted to them while taking their breakfast orders – and then found they were nowhere to be seen. Then the spider started talking. "I thought I'd better take myself in hand," says Sacks in an interview with the Nature podcast.

When the brain perceives things that are not really there, we call them hallucinations. They can be seen, heard, or even felt, and they occur in a wide range of neurological conditions – migraine, epilepsy, blindness – and of course after taking hallucinogenic drugs.

Much of Hallucinations is dedicated to Sacks's stock-in-trade – richly descriptive case histories of patients whom he has seen or who have written to him over the decades he has been practising. He is as respectful with their stories as he is honest about his own.

Among the multitudes are Rosalie, a blind woman who saw processions of people in brightly coloured "Eastern dress" parade in front of her; Marlon, the man who was convinced that a cast of shadowy characters inhabited his New York flat; and Toni, whose migraines provoke vivid displays of black and white zigzags in her peripheral vision.

Sacks is convinced that his own hallucinatory experiences have helped him to empathise with those having them for neurological reasons. "Patients would mention phenomena which I think I would not have taken seriously had I not experienced similar things myself."

One patient complains of hallucinations where motion is broken into a series of still images – a similar experience to Sacks's on acid, he says. "So I knew what it was like for her. Though I cannot pretend that was my primary motive [for taking the drug]."

It's this sense of empathy and humanity that makes Hallucinations such a pleasure to read, and allows Sacks to succeed in one of his most urgent aims: to challenge the perception that hallucinations portend madness. "The vast majority of hallucinations have no such dark implications," he writes.

And yet, some of the most delightful parts of our conversation are just a little bit mad. Take the artane-induced spider, for instance. "Years later I mentioned this philosophical spider to an entomologist friend of mine," Sacks tells me, "and he nodded and said, 'yes, I know the species'."

Hallucinations by Oliver Sacks is published by Picador in the UK from 8 November 2012

Kerri Smith is a writer and podcast presenter at Nature. You can listen to her interview with Sacks here © Guardian News and Media 2012

[Collage of human head, digits via]