One day in the nineteen-eighties, a woman went to the hospital for cancer surgery. The procedure was a success, and all of the cancer was removed. In the weeks afterward, though, she felt that something was wrong. She went back to her surgeon, who reassured her that the cancer was gone; she consulted a psychiatrist, who gave her pills for depression. Nothing helped—she grew certain that she was going to die. She met her surgeon a second time. When he told her, once again, that everything was fine, she suddenly blurted out, “The black stuff—you didn’t get the black stuff!” The surgeon’s eyes widened. He remembered that, during the operation, he had idly complained to a colleague about the black mold in his bathroom, which he could not remove no matter what he did. The cancer had been in the woman’s abdomen, and during the operation she had been under general anesthesia; even so, it seemed that the surgeon’s words had lodged in her mind. As soon as she discovered what had happened, her anxiety dissipated.

Henry Bennett, an American psychologist, tells this story to Kate Cole-Adams, an Australian journalist, in her book “Anesthesia: The Gift of Oblivion and the Mystery of Consciousness.” Cole-Adams hears many similar stories from other anesthesiologists and psychologists: apparently, people can hear things while under anesthesia, and can be affected by what they hear even if they can’t remember it. One woman suffers from terrible insomnia after her hysterectomy; later, while hypnotized, she recalls her anesthesiologist joking that she would “sleep the sleep of death.” Another patient becomes suicidal after a minor procedure; later, she remembers that, while she was on the table, her surgeon exclaimed, “She is fat, isn’t she?” In the nineteen-nineties, German scientists put headphones on thirty people undergoing heart surgery, then, during the operation, played them an abridged version of “Robinson Crusoe.” None of the patients recalled this happening, but afterward, when asked what came to mind when they heard the word “Friday,” many mentioned the story. In 1985, Bennett himself asked patients receiving gallbladder or spinal surgeries to wear headphones. A control group heard the sounds of the operating theatre; the others heard Bennett saying, “When I come to talk with you, you will pull on your ear.” When they met with him, those who’d heard the message touched their ears three times more often than those who hadn’t.

As a teen-ager, Cole-Adams was diagnosed with scoliosis. She came to dread the dangerous surgery she might someday need to correct the curvature of her spine; in middle age, she grew increasingly stooped and realized that the surgery was inevitable. She began researching “Anesthesia” in 1999, perhaps as a means of mastering her fear, and, after nearly twenty years’ work, has written an obsessive, mystical, terrifying, and even phantasmagorical exploration of anesthesia’s shadowy terra incognita. In addition to anesthesia, the book describes Cole-Adams’s childhood, her parents, a number of love affairs, and various spiritual experiences and existential crises—a drifting, atemporal assemblage meant to evoke the anesthetized mind. Cataloguing her many forgotten experiences and unfelt feelings, she wonders to what extent we already live in an anesthetized state.

Anesthesiologists speak of patients descending through “the planes of anesthesia”—from the “plane of disorientation” through the “plane of delirium” toward the “surgical plane.” While we go under, they monitor our brain waves, titrating their “anesthetic cocktails” to make sure that we receive neither too little sedation nor too much. (A typical cocktail contains a painkiller, a paralytic, which prevents muscles from flinching at the knife—the early paralytics were based on curare, the drug South American warriors put on the poison-tipped arrows with which they shot Europeans—and a “hypnotic,” which brings unconsciousness.) But even as they operate the machinery of anesthesia with great skill, anesthesiologists remain uncertain about the drugs’ underlying mechanisms. “Obviously we give anesthetics and we’ve got very good control over it,” one doctor tells Cole-Adams, “but in real philosophical and physiological terms we don’t know how anesthesia works.” The root of the problem is that no one understands why we are conscious. If you don’t know why the sun comes up, it’s hard to say why it goes down.

In her attempts to understand what going under anesthesia really entails, Cole-Adams encounters what Kate Leslie, an Australian anesthesiologist, calls “spooky little studies”—odd, suggestive, and often unreplicable experiments. In one such study, from 1993, Ian Russell, a British anesthesiologist, ties a tourniquet around the forearms of thirty-two women undergoing major gynecological surgery. He administers his anesthetic cocktail—the hypnotic drug midazolam, along with a painkiller and a muscle relaxant—then, by tightening the tourniquet, prevents the muscle relaxant from entering each woman’s hands and wrist. During surgery, a recorded message plays through headphones in which Russell addresses each patient by name. “If you can hear me, I would like you to open and close the fingers of your right hand,” he says. If the woman moves her hand, Russell lifts one of the earpieces and asks her to squeeze his fingers; if she squeezes, he asks her to do it again if she is in pain. Of the thirty-two patients Russell tested, twenty-three squeezed to suggest they could hear, and twenty squeezed again to say they were in pain. Although Russell was supposed to test sixty patients, he was so unnerved by these results that he ended the trial early. It’s possible, he suggests, that the women were conscious and suffering on the operating table. If that’s the case, then general anesthesia might be better described as “general amnesia.” (Afterward, none of the women recalled hearing Russell’s voice or squeezing his hand.)

Could Russell have failed to administer enough anesthetic? (He says he used as much as he would in any normal operation.) Could he have been feeling movements that weren’t there or that weren’t significant? (Cole-Adams attends an operation with Russell, during which he again employs his “isolated forearm technique”; this time, when the patient grips his fingers, he deems it a meaningless “reflex movement.”) It’s possible that the patients were aware, but only partially—aware enough to squeeze Russell’s hand, but not enough to know their own names, for instance, or to recall anything about their lives. Daniel Dennett, the philosopher of mind, argues that consciousness is not a binary state but a gradual one; it’s possible to be “sort of” conscious and, during that time, to have a “sort of” self. Every year, thousands of people have colonoscopies under so-called conscious sedation: they are drowsily awake and can communicate with their doctors, but remember little or nothing about the procedure afterward. If you don’t remember the pain, does it still count? Did it happen to “you”? Maybe being “sort of” aware during surgery isn’t so bad.

There are, Cole-Adams finds, no perfect studies of awareness under anesthesia. Studies like Russell’s, which use real patients, tend be poorly designed; those that use volunteers don’t involve real surgery. Investigating anesthetized awareness without surgery, she writes, “is a bit like testing your windshield wipers without rain.” “A surgical incision has a galvanizing effect even on an anesthetized patient,” she explains. “As the scalpel enters, her heart beats faster, her blood pressure rises, sometimes she jerks. She might edge closer to consciousness.” Another approach, of course, is simply to ask large numbers of people what they remember after they emerge from surgery. A study published in The Lancet in 2000 surveyed twelve thousand patients who had undergone surgery at two Swedish hospitals. The researchers found eighteen people whom they could be confident had been awake. The patients were surveyed at different times—just after the operation and at various intervals thereafter. Some remembered their experiences right away; others had no recollections at first but recalled the surgery after a week or two. One remembered the surgery in detail only twenty-four days afterward.

We tend to think that being anesthetized is like falling asleep. Cole-Adams concludes that the truth is stranger—it’s more like having your mind disassembled, then put together again. An American anesthesiologist named George Mashour tells her that “the unconscious mind is not this black sea of nothingness,” but an “active and dynamic” place; one might imagine the anesthetized mind as a concert hall in which the conductor is missing but the orchestra still performs. The systems of the brain continue to operate, but they don’t synch up. Perhaps because everyone’s mind devolves into cacophony differently, people have a bewildering array of experiences while anesthetized. An anesthesiologist in Melbourne recalls a patient who found himself awake during bypass surgery; although the man experienced his “chest being sawn open and pulled apart,” he didn’t feel pain, and “was amazed by it, not terrified by it.” (“He was a really easygoing sort of bloke,” the anesthesiologist recalls.) Another doctor recalls a patient waking from surgery looking “very pleased with herself”; when asked why she was so happy, she said, “You won’t believe it, but I’ve just had a half-hour orgasm!”

Not everyone is so lucky. At the center of “Anesthesia” is the story of Rachel Benmayor, an Australian woman who, twenty-five years ago, found herself paralyzed but capable of sensation during her Cesarean section. (Benmayor's doctors had intended for her to go under general anesthesia.)* At first, she didn’t know where she was. Then she felt extraordinary, mounting pain, and a feeling as though a truck were driving back and forth across her midsection. (“When you open up the abdominal cavity, the air rushing onto the unprotected internal organs gives rise to a feeling of great pressure,” Cole-Adams explains.) She felt that she wasn’t breathing. (A ventilator was doing it for her.) Only when she heard the doctors talking to her husband—“Glenn, look, you’ve got a little girl!”—did she realize that she was awake during her operation. Now fully aware, she began to panic. She felt that the pain and paralysis would drive her mad. She decided to try to go “into” the pain. Instead of fleeing from the experience, she tells Cole-Adams, “I consciously turned myself around, and started feeling the pain and going into the pain, and just letting the pain sort of enclose me.” She felt herself descending into the agony—then, suddenly, although she could still feel the surgery, she found herself in a library. “It was like I was in the presence of everything that has been ever known by man and everything that ever will be,” she recalls.