I am about to have my left leg paralysed, my arm taken over by an alien force and, quite possibly, be made blind. I confess I'm a bit nervous. But also, strangely, I hope it all works.

These insults to my body will not be inflicted with a scalpel, but instead induced using hypnosis. The effects, if they occur, will only be temporary, my hypnotist, David Oakley, reassures me.

It's all being done in the interest of science. Oakley is an emeritus professor at University College London. He is one of a handful of researchers who hope that by taking hypnosis seriously, they will help to dispel its quackery-tainted image. "I think hypnosis is underaccepted and undervalued," says psychologist Irving Kirsch at the University of Hull, UK. "Partly because of lurid tales published in books and movies, which lead to views of hypnosis as a strange and unbelievable state. Still many people scoff."

Aside from improving the reputation of hypnosis, Oakley also aims to better understand some of the strangest neurological conditions out there. The idea is to use hypnosis to induce symptoms in otherwise healthy people. This creates "virtual patients" with symptoms that can literally be switched on and off with a snap of the fingers, making it easier to study the abnormal brain activity that causes them. "It's like reverse engineering," says Peter Halligan, a neuropsychologist at Cardiff University, UK, who works with Oakley. "It's only when things break down that you appreciate the mechanism involved."

For their experiments, Halligan and Oakley have focused on a range of rare and bizarre conditions. They include hysterical blindness (the person cannot see but has no perceptible damage to their eyes or brain), hysterical paralysis (an inability to move a part of the body despite having no physical injury - the same limb may move while the person is asleep), prosopagnosia (an inability to recognise faces despite having good sight), alien limb syndrome (the feeling that an arm or leg is acting of its own accord), visual neglect (total lack of awareness of half of the visual field) and Capgras syndrome (a delusional belief that a loved one has been replaced by an imposter).

These are all conditions that the researchers believe can be recreated in healthy people using hypnosis. Many of them are somatoform disorders, in which people develop physical symptoms in the absence of an identifiable physical cause. All are rare, and when they occur it is often in people with other problems, such as depression or schizophrenia, making them hard to study.

To get a sense of what these conditions are like and how it feels to be hypnotised I have come, with some trepidation, to University College London's psychology department. As we sit in an ordinary office, Oakley guides me through a relaxation procedure, starting by asking me to imagine that I am in a happy and familiar place. I soon feel much as I do at the end of a yoga class - calm and relaxed.

I expected to feel dizzy or drunk, or perhaps to forget where I was, but the street traffic is still audible and I feel normal. Then Oakley suggests that my left leg is unable to move, and my leg starts to tingle with pins and needles. I am surprised by this, but go with it. When he asks me to lift my leg, I can do it, but only slowly, awkwardly and with difficulty.

This means the procedure has partially worked; I'm probably slightly above average on the hypnotisability scale, says Oakley. Had I been in the 10 per cent of people who are highly hypnotisable I would have ceased to hear the traffic and been unable to lift my leg at all.

Hypnotic elite

Julia Russell, a schoolteacher and former student of Oakley's, is one of the hypnotic elite. Russell participated in one of Oakley's recent alien limb experiments. She describes one experience as "quite extraordinary". During hypnosis, Russell's arm was moved in a number of ways, first manually. Then the idea was suggested to her that an experimenter would enter the room who wanted her arm to move. When the experimenter appeared, Russell says she felt a bizarre sensation. "I was being bypassed. She wanted my hand to move, so it did, and I wasn't involved in the movement at all," she says. "It was as if her thoughts were affecting my behaviour directly, without my thoughts intervening."

Can the activity in a hypnotised brain really mimic what's going on in someone with a real disorder? And, if it can, what can it teach us?

Oakley and Halligan are convinced their virtual patients are experiencing some of the same brain changes as people with genuine disorders. Halligan tells how they once induced a case of visual neglect in a volunteer by suggesting that the left side of his visual field would cease to exist. They then asked him to copy a picture with a dozen objects scattered on the page. Most hypnotised people given this instruction would copy only the objects on the right hand side of the page, as most people with visual neglect do. But this volunteer drew the right-hand side of every object on the page. "That's not something you would intuitively expect," says Halligan, "but it is also seen in real patients."

The similarities reach into the brain, he says. Halligan and colleagues put 12 highly hypnotisable students under and then either suggested that their left leg was paralysed, or told them to merely pretend that their left leg was paralysed, with the promise of a reward if they managed to fool an investigator. The investigators, unaware of which group the participants were in, couldn't tell who was faking paralysis - until they saw scans of the volunteers brains. There were clear differences in brain activity. One of the brain areas that was highly active, or "lit up", in the hypnotically paralysed volunteers was the right orbitofrontal cortex - a region thought to be involved with emotional inhibition, and which has also been seen to light up in hysterical paralysis.

Yann Cojan at the University of Geneva in Switzerland recently tried a similar experiment where volunteers' left hands were "paralysed" by hypnosis. He and his colleagues also found that brain scans distinguished those under hypnotic suggestion from the fakers.

Same, but different

However, both groups also saw some differences in brain activity between hypnotic paralysis and people with genuine cases of hysterical paralysis. Cojan - who saw greater differences than Halligan - thinks this means that, while interesting as a research tool, hypnosis is unlikely to produce clinically relevant results for people with real disorders.

Nevertheless, their studies have lead them to some interesting conclusions. For example, the work suggests that brain areas normally associated with the intentional inhibition of movement are not active in people with hysterical paralysis nor hypnotised volunteers, suggesting that it really is the case that they cannot, rather than will not, move.

Whether such studies will help to develop treatments for patients remains to be seen. Perhaps the biggest impact will simply be to convince people that the conditions are real. "These patients can run into aspects of the healthcare system that are very pejorative - they're constantly told 'it's all in your head'," says Anthony Feinstein of the Sunnybrook Health Sciences Center in Toronto, Canada. "We say 'yes, it's all in your head, because your brain is in your head'." Being able to show both doctors and patients that the conditions are being taken seriously is a big step forward.

At present, treatments for the disorders include talking therapy, hypnotic suggestion to relieve symptoms and even physiotherapy - which sometimes gives a person an "excuse" to get better. In many cases, these approaches are unsuccessful. A better understanding of the pathways in the brain that have gone awry should help to direct future therapies. "If we understand the normative system, then we understand how to treat it," says Halligan.

Back in the hypnotist's chair, Oakley moves on to the next virtual disorder. He induces in me a sensation that my right hand is being lifted by a balloon, a gentle approximation of alien limb syndrome. My wrist inches upwards. He next suggests that when I open my eyes, I will not be able to see anything in the left of my visual field, an analogue of visual neglect. I open them slowly, fascinated to see what this will be like, but my vision is normal. I feel strangely disappointed, but Oakley tells me that even highly hypnotisable people only rarely experience this effect.

The only thing I seem to have lost permanently is a little bit of time; when Oakley asks me how long I think I have been under, I reply, "about 20 minutes". He grins when he tells me it was over an hour. Time compression is a common side effect of hypnosis, he says.

Although I failed to experience full alien limb syndrome or visual neglect, my thinking on hypnosis has been changed. I am more convinced of the power of the phenomenon after experiencing it for myself. Though I can't help suspecting that Oakley planted the thought in my head while I was under...

What is hypnosis?

Some say hypnosis is nothing more than people acting how they think they are meant to act when hypnotised. In some instances this may be true, but to dismiss hypnosis vastly underestimates its power, much like dismissing a placebo as "just a sugar pill" glosses over the fact that it can help someone get better.

Severe burn patients, some of whom are driven psychotic from pain even when given morphine, have been made calm through hypnosis, says David Oakley, an emeritus professor at University College London. When Michael Nash, a psychologist at the University of Tennessee in Knoxville, goes to the dentist for some minor drilling work, he shuns anaesthesia in favour of self-hypnosis. "The dentist is always impressed," he says.

Since 1996, the US National Institutes of Health in Bethesda, Maryland, have deemed hypnosis an effective method of pain relief, and more recent meta-analyses have confirmed its usefulness in surgery. Some studies have shown that hypnosis can have a greater pain-relief effect than placebo pills.

Amir Raz of McGill University in Montreal, Canada, has demonstrated another powerful effect, which often silences critics who say hypnotised volunteers are just faking it. In general, if you show somebody the name of a colour written in a different colour - "red" written in blue, for example - and ask them the colour of the ink, they pause. This is a very deeply ingrained effect, but Raz has showed that using hypnosis to suggest a volunteer cannot read can eliminate this delay. "When scientists try to measure these funky things, it's really important to have objective measures," says Raz.

Whether there is a pattern of brain activity characteristic of hypnosis is still up for debate. Some recent research hints that hypnotised volunteers have less "mind wander" when in a resting state. But it's hard to say whether this is due to hypnosis, or the relaxation techniques used to instigate it.

How hypnotisable are you?

Hold your arms out straight in front of you, palms facing each other. Then close your eyes and imagine a magnetic force pulling your hands together, and really concentrate for a minute on what that might feel like. Now open your eyes. Have your arms moved together? They do for about 70 per cent of people. This group is considered suggestible, and so probably hypnotisable.

The best way to assess someone's susceptibility to hypnotism is simply to hypnotise them and see what happens. There are various defined scales - such as the Stanford hypnotic susceptibility scale - that rank a person's hypnotisability based on their response to a dozen suggested tasks under hypnosis - from feeling your hands being drawn together as if by magnets, to feeling your arm being lifted by a balloon. A strong indicator of how someone will perform on such tests is simply how susceptible they are to suggestion under normal circumstances.

People who lose themselves so completely in movies or books that they believe the characters to be real, who go into a kind of trance when doing endurance sports, or who had imaginary friends as children, are usually hypnotisable. The motivation to be hypnotised also helps.

Some studies hint that our hypnotisability is deeply ingrained. When 50 people were re-tested on the Stanford scale after a 25-year gap, their scores stayed remarkably consistent. James Horton at the University of Virginia in Charlottesville has even found physical brain differences: highly hypnotisable participants had a 30 per cent bigger rostrum, a part of the brain thought to help focus attention. A few studies have shown that hypnotisability may be hereditary, and some researchers - including Amir Raz of McGill University in Montreal, Canada - are trying to track the genes involved.

Curiously, most of the scientists interviewed for this piece categorised themselves as only mediocre on the scale of hypnotisability. "In general, researchers tend to dissect and criticise and re-evaluate," says hypnosis researcher David Spiegel of Stanford University, California. "And that's not a very hypnotic state of mind."