IT'S ALL IN YOUR HEAD:

The famous Stroop test offers some helpful evidence. This test measures how difficult people find it to identify the colour a word is written in when that word is the name of another colour. For instance, picture the word "red" written in blue ink. It takes people longer to say that the ink is blue than when the ink is the corresponding red colour (You can take the test for yourself here.)

When hypnotised participants were told they were no longer able to read, the letters became meaningless shapes - and so they got quicker at identifying the colour of the mismatched words, because they were no longer distracted by the words on the page.

There also appear to be differences in brain activity when someone is asked to "fake it", compared with when they are experiencing an involuntary response. In one small experiment, researchers studied 12 healthy participants in a positron emission tomography (PET) scanner, to measure metabolic activity in parts of the brain. In one set of tests, they were given the instruction to fake being unable to move their leg. In another set of tests, the same people were hypnotised and given the suggestion that their leg was paralysed. The brain imaging studies showed distinct brain regions were activated in each of the two conditions.

A later study expanded on the same hypnotism vs. faking it question, this time using an MRI scanner, which gives more detail when looking at soft tissues. This time, the researchers saw the motor cortex - part of the brain which controls body movements - showed activity in the patients under hypnosis. This suggests the hypnotised people were really preparing to try to move their limb, despite achieving no more movement than the group who were faking limb paralysis.

So, are there any hallmarks of the hypnotised brain that can explain the peculiar sensation and experiences of a hypnotic response? It's an emerging area of research, but there are a couple of candidates.

Part of the story can be found in the brain's salience network, says Spiegel. This network helps us identify which aspects of our environment are worth paying attention to - sifting out relevant information from the swathes of sensory data that our brains are inundated with every second of the day. In one experiment, he and his colleagues hypnotised both "highs" and "lows" while scanning their brains. The highs had lowered activity in the salience network during hypnosis. "When that happens, you're less worried about what else might be going on," says Spiegel. "It allows you to disconnect from the rest of the world."

That might go some way to explaining the feeling of intense focus during hypnosis, but what about the strange sensation that your body is doing things of its own accord?

The best evidence points to the brain's default mode network, says Terhune, a set of brain regions that are most active when we are at rest. "It's believed to be integrally involved in self-related mentation - daydreaming, mind-wandering and so on," says Terhune.

One part of this network in particular - the anterior medial pre-frontal cortex - is thought to play a crucial role in hypnosis. "This region seems to be involved in self-related processing, metacognition [thinking about thinking], and the ability to control your own thoughts," says Terhune. "Those are processes that might be dampened in response to hypnotic induction."

With temporarily impaired activity in the default node network, it may become harder to think about yourself as a conscious agent. This might be at the root of the remarkable sensation that you are not an entirely autonomous over your own body.

The relevance of this part of the default mode network in hypnosis has been found in many studies, but Terhune adds a note of caution: "Sometimes we don't know what the causal ingredient is." For example, the medial pre-frontal cortex is also involved making inferences about other people's mental states. It could be that while you're being hypnotised, you also happen to be thinking about the experimenter and what they're thinking.

"But that's the best line of evidence," Terhune sums up. "It's a reduction in self-related processing and metacognition."

While academic experimenters tease out the details of why hypnosis works the way it does, clinicians are making use of its effects - as they have been doing for centuries.

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