Oliver Sacks, the pioneering neurologist who has exposed the workings of the human brain in his popular books, turns to the subject of hallucinations in his latest title.

Hallucinations, published this week, aims to defuse the stigma around the visions and restore them as a normal, occasionally helpful part of human life.

People are "fascinated and slightly frightened" by the idea of seeing things, Sacks said in an interview Friday with CBC’s Q cultural affairs show.

"I think hallucinations, the word, often has an ominous sound. It’s a portent of mental illness, of brain degeneration, which is a way people who have hallucinations often talk about them," he said.

Like his previous works, such as The Man Who Mistook His Wife for a Hat and An Anthropologist on Mars, Hallucinations includes several case histories. Among them is the story of Rosalie, a mentally sharp 90-year-old who lost her vision due to macular degeneration and suddenly began to have hallucinations.

Although neurologically she had no abnormalities, Sacks determined that the visual parts of her brain, deprived of stimulation, were compensating with a parade of mental images. The diagnosis allowed her to ignore the hallucinations, which she soon found "boring."

Hallucinations often come to people before sleep or those with a fever and it is "wrong to dismiss them as purely pathological when they may also have positive and important effects," Sacks said.

"One sees this sometimes in bereavement hallucinations. About one third of those who have lost a spouse — often after a long, happy marriage — feel this aching hole. This may be filled with seeing and hearing their loved ones and that can be seen as a normal part of the mourning process," he added.

Hallucinations also includes a chapter on Sacks’ period of experimentation with drugs when he was in his 30s.

Sacks talks to Q about the role hallucinations play in the human need for transcendence and what he discovered from discussing philosophy with a spider who sounded like Bertrand Russell.