They’re surprisingly common, and they don’t necessarily mean madness.

TO: Is there potential for people to follow commands they are "given" during hallucinations?
OS: It seems probable that Joan of Arc had command hallucinations of a sort. At first these would just present themselves as figures and voices and a light to one side, and only later did she feel these figures were telling her to go and do things.

TO: You also talk about how hallucinations can result from loss of hearing or vision.
OS: Normally, there's a system of checks and balances in the brain, to prevent any particular region 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.

TO: You have a visual impairment. Does this influence whether you have hallucinations?
OS: I have low-level hallucinations all the while. I see geometrical patterns and proto-letters everywhere. For example, looking up at the ceiling, as I'm doing now, 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. [Laughs]

TO: Do you worry that sharing your patients' stories somehow exploits them?
OS: I'm on this delicate boundary, and have been for 50 years or so. At one time I was my own prime accuser. Whenever I saw the word portrayal, I would misread it as betrayal. First, in addition to any formal consent, I want to be reassured from what I know of a patient that they won't be upset by anything.

TO: Do you hope that sharing these stories changes people's perceptions?
OS: I feel that if I describe things respectfully, tenderly, and truly, then this is an important thing to do. It's not voyeurism, it's not exploitation, but an essential form of knowledge. I think the detailed case history has no equal in conveying understanding, not only of what a condition is like, but of the ways in which a person may respond to a condition.

I remember when an opera was made from my book The Man Who Mistook His Wife for a Hat, I said to the librettist, you must go and see Mrs. P—the woman who was mistaken for a hat—and see how she would feel about this. I watched her watch the opera, wondering fearfully what she might be thinking. But she came up to me and the librettist and said, you have done honor to my husband. I hope in some sense I can do honor to the patients.

TO: This is your 12^th book. Does writing help you make sense of the world?
OS: I often don't feel I've come to terms with an experience unless I have had a go at describing it. This has been an almost lifelong disposition; I have notebooks going back to the age of 14.

This article originally appeared in New Scientist.

Correction, Nov. 13, 2012: This article originally gave an outdated academic affiliation for Oliver Sacks. He is a professor at New York University School of Medicine, not Columbia University.