What seems to be there, but isn’t?

As Dr. Oliver Sacks explains in Hallucinations, his latest collection of absorbing essays, “Hallucinations, beyond any other waking experience, can excite, bewilder, terrify, or inspire, leading to folklore and the myths (sublime, horrible, creative, and playful) which perhaps no individual and no culture can wholly dispense with.”

Hallucinations, which differ starkly from dreams and imagination, are often associated with wild visions induced by fever, madness or drugs. They come in much greater variety, however, and include hearing voices, music or noises, feeling things or smelling odors—none of which exist. This multitude of illusions has a grand litany of causes, including injury, illness, migraines, trauma, epilepsy and more.

As always, Sacks, the best-selling author of Awakenings and The Man Who Mistook His Wife for a Hat, describes a fascinating cast of patients, starting with a blind, elderly woman named Rosalie, who suddenly began seeing a parade of people in colorful “Eastern” dress and animals, and later a group of somber men in dark suits, and finally, crowds of tiny people and children climbing up the sides of her wheelchair. These crowded, complex visions rolled before Rosalie’s unseeing eyes like a movie, sometimes amusing, and at other times boring or frightening. Sacks diagnosed Rosalie with a fairly rare condition called Charles Bonnet syndrome, which causes visually impaired people to hallucinate.

In addition to Rosalie, he shares stories about a patient who keeps hearing Bing Crosby’s “White Christmas” play repeatedly, a Parkinson’s patient who watches a group of women trying on fur coats in her doctor’s waiting room, a man who feels peach-like fuzz covering everything he touches and a narcolepsy patient who sees the road rise and hit her in the face as she drives. Hallucinations, we learn, can range from terrifying to inspirational, from annoying to entertaining. One of Sacks’ elderly patients greatly looked forward to her visit each evening from “a gentleman visitor from out of town.”

In these 15 essays, Sacks clearly explains and categorizes an amazing assortment of hallucinations, trying to make sense of phenomena that seem to defy logic. He shares his own tale of a voice he heard when alone on a mountain and suffering from a dislocated knee. Just when he was tempted to lie down and sleep, a voice commanded, “You can’t rest here—you can’t rest anywhere. You’ve got to go on. Find a pace you can keep up and go on steadily.”

No doubt his many avid readers are deeply grateful that the good doctor followed the orders of this life-saving hallucination.