Psychiatrists have been using hypnosis on patients for decades — to help them reduce their pain or kick a smoking habit, among other reasons.

But what, exactly, is happening to the patients’ brains when they are in a hypnotic state?

To tackle that question, David Spiegel, a psychiatrist at Stanford University School of Medicine, and his colleagues recently decided to scan patients’ brains and see if hypnosis left a mark. It did.

In a new study, published Thursday in the journal Cerebral Cortex, Spiegel and his colleagues report that they’ve found a distinctive signature in the brain when a patient has undergone hypnosis. What’s more, the changes in brain activity give scientists hints about what happens to the mind of hypnotized subjects. The discovery may ultimately lead to new insights about how to make hypnosis more effective.

To run the new experiment, Spiegel and his colleagues screened 545 healthy college students, giving them a test to see how easily they could be hypnotized. The researchers ended up with 36 highly hypnotizable subjects — whom Spiegel nicknamed “the highs.” They also selected 21 low-scoring subjects who couldn’t fall under the spell of hypnosis.

The scientists then had each of the subjects lie in an fMRI scanner while wearing headphones. They were told to close their eyes, take a deep breath, let the breath out, and imagine their body floating. In some cases, the scientists asked their subjects to imagine a time when they were happy; in another, they imagined themselves on vacation.

To compare the hypnotic state with other kinds of brain activity, the scientists also asked the subjects to think back on their day, in order to switch on memory-related circuits. They also scanned the volunteers as they thought of nothing in particular, so they could measure the spontaneous activity of the brain.

After the experiment, the scientists looked at how the brain activity of the highs and the lows changed with each task. They found that during hypnosis, the highs displayed a distinct pattern of brain activity — one that was not found in the control group, and one that was not found when the highly hypnotizable people were in a resting state or using their memory.

“What the highs did in hypnosis is different,” Spiegel said. “This is strong evidence that it’s a different brain state.”

During hypnosis, the scientists found, a region of the brain called the dorsal anterior cingulate cortex became less active. Studies have found that that region helps people stay vigilant about their external environment. Spiegel noted that highly hypnotizable people are also more likely to lose themselves in a sunset or a movie. The activity in their dorsal anterior cingulate cortex may account for that behavior, too.

The scientists also found that the connections between other regions of the brain became stronger. Some of these regions strengthen the bond between the mind and body, regulating how much pain we feel, for example. Those connections could help make hypnotizable people more suggestible.

At the same time, some regions of the brain became disconnected. Those regions normally are important for self-consciousness. By weakening those connections, hypnosis may allow people to do things they might never do in their normal life.

“That’s why stage hypnotists can get football coaches to dance like a ballerina,” said Spiegel.

Because the study is the first of its kind, it remains to be seen if more experiments will uncover the same patterns. And while the experiment offers clues to what happens during hypnosis, it’s still an open question why the highs are so prone to slip into the state.

Spiegel noted that some studies have found that highs are unusually likely to have experienced stress early in life.

“People who have traumatic experiences tend to disconnect,” said Spiegel. On a neurological level, that experience may have strengthened and weakened connections between certain regions of the brain — changes that made them able to be hypnotized later in life.

But stress may not be the only path to hypnosis, Spiegel cautioned. Studies have also found that people who used their imagination a lot early in life — such as being read stories to by their parents — also appear to be more hypnotizable when they grow up.

“What I think we’re seeing is the revised wiring diagram of the brain based on early experiences,” said Spiegel.

In any case, Spiegel hopes that brain scans will help him come up with better ways to use hypnosis for his patients. By looking at how the brain responds to different techniques, he may be able to reduce more pain in some people, for example, or help people who normally can’t be hypnotized slip into the state.