I was called in to see one patient, Rosalie, a blind lady in her 90s, when she started to have visual hallucinations; the staff psychiatrist was also summoned. Rosalie was concerned that she might be having a stroke or getting Alzheimer’s or reacting to some medication. But I was able to reassure her that nothing was amiss neurologically. I explained to her that if the visual parts of the brain are deprived of actual input, they are hungry for stimulation and may concoct images of their own. Rosalie was greatly relieved by this, and delighted to know that there was even a name for her condition: Charles Bonnet syndrome. “Tell the nurses,” she said, drawing herself up in her chair, “that I have Charles Bonnet syndrome!”

Rosalie asked me how many people had C.B.S., and I told her hundreds of thousands, perhaps, in the United States alone. I told her that many people were afraid to mention their hallucinations. I described a recent study of elderly blind patients in the Netherlands which found that only a quarter of people with C.B.S. mentioned it to their doctors — the others were too afraid or too ashamed. It is only when physicians gently inquire (often avoiding the word “hallucination”) that people feel free to admit seeing things that are not there — despite their blindness.

Rosalie was indignant at this, and said, “You must write about it — tell my story!” I do tell her story, at length, in my book on hallucinations, along with the stories of many others. Most of these people have been reluctant to admit to their hallucinations. Often, when they do, they are misdiagnosed or undiagnosed — told that it’s nothing, or that their condition has no explanation.

Misdiagnosis is especially common if people admit to “hearing voices.” In a famous 1973 study by the Stanford psychologist David Rosenhan, eight “pseudopatients” presented themselves at various hospitals across the country, saying that they “heard voices.” All behaved normally otherwise, but were nonetheless determined to be (and treated as) schizophrenic (apart from one, who was given the diagnosis of “manic-depressive psychosis”). In this and follow-up studies, Professor Rosenhan demonstrated convincingly that auditory hallucinations and schizophrenia were synonymous in the medical mind.

WHILE many people with schizophrenia do hear voices at certain times in their lives, the inverse is not true: most people who hear voices (as much as 10 percent of the population) are not mentally ill. For them, hearing voices is a normal mode of experience.

My patients tell me about their hallucinations because I am open to hearing about them, because they know me and trust that I can usually run down the cause of their hallucinations. For the most part, these experiences are unthreatening and, once accommodated, even mildly diverting.

David Stewart, a Charles Bonnet syndrome patient with whom I corresponded, writes of his hallucinations as being “altogether friendly,” and imagines his eyes saying: “Sorry to have let you down. We recognize that blindness is no fun, so we’ve organized this small syndrome, a sort of coda to your sighted life. It’s not much, but it’s the best we can manage.”

Mr. Stewart has been able to take his hallucinations in good humor, since he knows they are not a sign of mental decline or madness. For too many patients, though, the shame, the secrecy, the stigma, persists.