Hypnosis has become a common medical tool, used to reduce pain, help people stop smoking and cure them of phobias.

But scientists have long argued about whether the hypnotic “trance” is a separate neurophysiological state or simply a product of a hypnotized person’s expectations.

A study published on Thursday by Stanford researchers offers some evidence for the first explanation, finding that some parts of the brain function differently under hypnosis than during normal consciousness.
The study was conducted with functional magnetic resonance imaging, a scanning method that measures blood flow in the brain. It found changes in activity in brain areas that are thought to be involved in focused attention, the monitoring and control of the body’s functioning, and the awareness and evaluation of a person’s internal and external environments.

“I think we have pretty definitive evidence here that the brain is working differently when a person is in hypnosis,” said Dr. David Spiegel, a professor of psychiatry and behavioral sciences at Stanford who has studied the effectiveness of hypnosis.

Functional imaging is a blunt instrument and the findings can be difficult to interpret, especially when a study is looking at activity levels in many brain areas. Still, Dr. Spiegel said, the findings might help explain the intense absorption, lack of self-consciousness and suggestibility that characterize the hypnotic state.

He said one particularly intriguing finding was that hypnotized subjects showed a decreased interaction between a region deep in the brain that is active in self-reflection and daydreaming and areas of the prefrontal cortex involved in planning and executing tasks.

That decreased interaction, Dr. Spiegel said, suggested an explanation for the lack of self-consciousness shown by hypnotized subjects. “That’s why the stage hypnotist can get a football coach to dance like a ballerina without feeling self-conscious about what he’s doing,” Dr. Spiegel said. He added that it might also explain, at least in part, why hypnosis is an effective tool in psychotherapy for getting people to look at a problem in a new way.

The researchers screened more than 500 potential subjects for susceptibility to hypnosis and then compared brain activity in 36 who scored very highly on tests measuring susceptibility to hypnosis and 21 who had very low scores on those tests.

Brain activity during hypnosis was also compared with activity during resting periods and during a memory task, for both high and low susceptibility groups.
In the hypnosis task, the subjects were guided through two procedures for hypnotic inductions: in one, they were instructed to imagine a time when they felt happiness; in the other, they were told to remember or imagine a vacation.

All the subjects were asked in the study to rate how deeply hypnotized they felt during the inductions. Although some researchers continue to argue that hypnosis is a state produced by people’s expectations, not by biology, Dr. Spiegel said, “At some point, I just think it becomes a kind of self-fulfilling word game.” “I see hypnosis as a kind of app you haven’t used on your cellphone,” he said. “It’s got all kinds of capacity that people are just figuring out how to use, but if you haven’t used it the phone doesn’t do that.”