Only in the past 40 years have scientists been equipped with instruments and methods for discerning the facts of hypnosis from exaggerated claims. But the study of hypnotic phenomena is now squarely in the domain of normal cognitive science, with papers on hypnosis published in some of the most selective scientific and medical journals. Of course, spectacles such as "stage hypnosis" for entertainment purposes have not disappeared. But the new findings reveal how, when used properly, the power of hypnotic suggestion can alter cognitive processes as diverse as memory and pain perception.

Wheat from the Chaff

Scoring on the Stanford scales ranges from 0, for individuals who do not respond to any of the hypnotic suggestions, to 12, for those who pass all of them. Most people score in the middle range (between 5 and 7); 95 percent of the population receives a score of at least 1.

What Hypnosis Is

A person's responsiveness to hypnosis also remains fairly consistent regardless of the characteristics of the hypnotist: the practitioner's gender, age and experience have little or no effect on a subject's ability to be hypnotized. Similarly, the success of hypnosis does not depend on whether a subject is highly motivated or especially willing. A very responsive subject will become hypnotized under a variety of experimental conditions and therapeutic settings, whereas a less susceptible person will not, despite his or her sincere efforts. (Negative attitudes and expectations can, however, interfere with hypnosis.)

Several studies have also shown that hypnotizability is unrelated to personality characteristics such as gullibility, hysteria, psychopathology, trust, aggressiveness, submissiveness, imagination or social compliance. The trait has, however, been linked tantalizingly with an individual's ability to become absorbed in activities such as reading, listening to music or daydreaming.

Under hypnosis, subjects do not behave as passive automatons but instead are active problem solvers who incorporate their moral and cultural ideas into their behavior while remaining exquisitely responsive to the expectations expressed by the experimenter. Nevertheless, the subject does not experience hypnotically suggested behavior as something that is actively achieved. To the contrary, it is typically deemed as effortless--as something that just happens. People who have been hypnotized often say things like "My hand became heavy and moved down by itself" or "Suddenly I found myself feeling no pain."

Many researchers now believe that these types of disconnections are at the heart of hypnosis. In response to suggestion, subjects make movements without conscious intent, fail to detect exceedingly painful stimulation or temporarily forget a familiar fact. Of course, these kinds of things also happen outside hypnosis--occasionally in day-to-day life and more dramatically in certain psychiatric and neurological disorders.

Using hypnosis, scientists have temporarily created hallucinations, compulsions, certain types of memory loss, false memories, and delusions in the laboratory so that these phenomena can be studied in a controlled environment.

As scientists discover more about hypnosis, they are also uncovering evidence that counters some of the skepticism about the technique. One such objection is that hypnosis is simply a matter of having an especially vivid imagination. In fact, this does not seem to be the case. Many imaginative people are not good hypnotic subjects, and no relation between the two abilities has surfaced.

The imagination charge stems from the fact that many people who are hypnotizable can be led to experience compellingly realistic auditory and visual hallucinations. But an elegant study using positron emission tomography (PET), which indirectly measures metabolism, has shown that different regions of the brain are activated when a subject is asked to imagine a sound than when he or she is hallucinating under hypnosis.

In 1998 Henry Szechtman of McMaster University in Ontario and his co-workers used PET to image the brain activity of hypnotized subjects who were invited to imagine a scenario and who then experienced a hallucination. The researchers noted that an auditory hallucination and the act of imagining a sound are both self-generated and that, like real hearing, a hallucination is experienced as coming from an external source. By monitoring regional blood flow in areas activated during both hearing and auditory hallucination but not during simple imagining, the investigators sought to determine where in the brain a hallucinated sound is mistakenly "tagged" as authentic and originating in the outside world.

Szechtman and his colleagues imaged the brain activity of eight very hypnotizable subjects who had been prescreened for their ability to hallucinate under hypnosis. During the session, the subjects were under hypnosis and lay in the PET scanner with their eyes covered. Their brain activity was monitored under four conditions: at rest; while hearing an audiotape of a voice saying, "The man did not speak often, but when he did, it was worth hearing what he had to say"; while imagining hearing the voice again; and during the auditory hallucination they experienced after being told that the tape was playing once more, although it was not.

The tests showed that a region of the brain called the right anterior cingulate cortex was just as active while the volunteers were hallucinating as it was while they were actually hearing the stimulus. In contrast, that brain area was not active while the subjects were imagining that they heard the stimulus. Somehow hypnosis had tricked this area of the brain into registering the hallucinated voice as real.

Another objection raised by critics of hypnosis concerns its ability to blunt pain. Skeptics have argued that this effect results from either simple relaxation or a placebo response. But a number of experiments have ruled out these explanations. In a classic 1969 report, Thomas H. McGlashan and his colleagues at the University of Pennsylvania found that for poorly hypnotizable people, hypnosis was as effective in reducing pain as a sugar pill that the subjects had been told was a powerful painkiller. But highly hypnotizable subjects benefited three times more from hypnosis than from the placebo. In another study, in 1976, Hilgard and Stanford colleague Éva I. Bányai observed that subjects who were vigorously riding stationary bicycles were just as responsive to hypnotic suggestions as when they were hypnotized in a relaxing setting.