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We are deeply saddened by the loss of Oliver Sacks, a neurologist and one of the great public intellectuals of our time. A man of immense narrative skill, Sacks displayed a unique ability to translate brain science to lay people, authoring over a dozen books, including The Man Who Mistook His Wife for a Hat, Musicophilia: Tales of Music and the Brain, and Awakenings, which was adapted into an Oscar-nominated film starring Robert DeNiro and Robin Williams. In 2009, Oliver Sacks delivered the Robert B. Silvers lecture. For this week's episode of the New York Public Library Podcast, we're remembering Sacks's talk on hallucinations.

Before discussing hallucinations, Sacks spoke of the importance of libraries in his life as a learner. He framed the library as a place where he could enter a discourse of all written texts, authors both alive and dead:

"When I grew up in London, libraries were my home and my education. I was rather bad at school. I—I’m not good at being taught. I need to feel out information and knowledge and understanding in my own way, and libraries were crucial to me, and books allowed a communion with the dead and with everyone, and, in particular, everything I’ve written, if it has stemmed partly from clinical experience or personal experience, has stemmed equally from books and often old books. "

Comparing sensory deprivation or impairment to sleep, Sacks noted that when stripped of a sense, individuals may develop non-psychotic hallucinations:

"The old notion used to be that when you are asleep, the brain shuts down. On the contrary, it is more active in dreaming than at any other time, but it’s a different sort of activity. And this is also true of the perceptual systems. If there’s no visual input, the visual system doesn’t shut down, it becomes hungry, it wants activity, it has to keep going, and it will start to generate images or hallucinations of its own. And one can show, that with visual deprivation or visual impairment, the visual parts of the brain become hyperactive, and in particular those parts of the brain which— which would perceive particular things. There are different parts of the brain involved, say, in the perception of faces, of animals, of landscapes, of lighting, and of this and that, and if one does imaging, as one can do, on someone, while they are experiencing hallucinations, when they say, 'I see a face,' or whatever, or maybe like Rosalie, the greatly enlarged teeth on one side, you will find activity, a sudden surge of activity, in that particular part of the brain. There’s also quite recently been a description of particular cells in the brain which are involved in the perception of, or recognition of, faces, landscapes, and these too become active."

The brain activity associated with hallucinations is nothing to be afraid of, according to Sacks. With characteristic empathy, he spoke of the need for patients to be reassured that their hallucinations are not necessarily frightening and definitely are not shameful:

"As a neurologist, I encounter people, people come to me with stories of hallucination, or I elicit stories—hallucinations can occur in many other conditions, in Parkinson’s disease and Alzheimer’s and various—various medications—with people who are on steroids, my function as a neurologist is first to clarify and reassure. Very, very crucial. There are hundreds of thousands of people who are having visual hallucinations who are terrified of it and really they need to be reassured. As a scientist or physiologist, I am fascinated at this window into the brain, into a primitive, image-forming part of the brain and as it were seeing the brain at play, a primitive part of the brain at play. Beyond that, I can’t help wondering how much this gets into common discourse as folklore and as belief in spirits, and apparitions, and so forth. I am—very particularly common in the hallucinations are Lilliputian hallucinations, little people, and in every culture, you have elves, fairies, gnomes, leprechaun sprites. I think the little people of the world may come from hallucinations. I think this is also so of angels and aliens, but I would hesitate to tell that to the eighty percent of the population who believe in them."