Hypnosis has a "very real" effect that can be picked up on brain scans, say Hull University researchers.

An imaging study of hypnotised participants showed decreased activity in the parts of the brain linked with daydreaming or letting the mind wander.

The same brain patterns were absent in people who had the tests but who were not susceptible to being hypnotised.

One psychologist said the study backed the theory that hypnosis "primes" the brain to be open to suggestion.

Hypnosis is increasingly being used to help people stop smoking or lose weight and advisers recently recommended its use on the NHS to treat irritable bowel syndrome.

It is not the first time researchers have tried to use imaging studies to monitor brain activity in people under hypnosis.

But the Hull team said these had been done while people had been asked to carry out tasks, so it was not clear whether the changes in the brain were due to the act of doing the task or an effect of hypnosis.

In the latest study, the team first tested how people responded to hypnosis and selected 10 individuals who were "highly suggestible" and seven people who did not really respond to the technique other than becoming more relaxed.

The participants were asked to do a task under hypnosis, such as listening to non-existent music, but unknown to them the brain activity was being monitored in the rest periods in between tasks, the team reported in the journal Consciousness and Cognition.

Default mode

In the "highly suggestible" group there was decreased activity in the part of the brain involved in daydreaming or letting the mind wander - also known as the "default mode" network.

One suggestion of how hypnosis works, supported by the results, is that shutting off this activity leaves the brain free to concentrate on other tasks.

Study leader Dr William McGeown, a lecturer in the department of psychology, said the results were unequivocal because they only occurred in the highly suggestible subjects.

"This shows that the changes were due to hypnosis and not just simple relaxation. "Our study shows hypnosis is real."

Dr Michael Heap, a clinical forensic psychologist based in Sheffield, said the experiment was unique in showing brain patterns supporting the theory that hypnosis works by "priming" the subject to respond more effectively to suggestions.

"Importantly the data confirm that relaxation is not a critical factor.

"The limited data from this experiment suggest that this pattern of activity then dissipates (at least to some extent) once the subjects start to engage in the suggestions that follow."

But he said the small study, which needed repeating in other populations, did not prove that people being hypnotised were in an actual "trance".