Image caption The equipment uses lights at either side of the glasses

Psychologists say they might be able to prevent or treat post-traumatic stress disorder using glasses with a pair of flashing lights.

Dr Peter Naish of the Open University said that sufferers process the lights in their brains in the same way as people in a hypnotic state.

Sometimes they can believe that their flashbacks are real, and that real life is just a dream.

And this is echoed in how they use their brains to see the lights.

Dr Naish said that some trauma survivors seem to be in a hypnotic state most of the time, and can suffer from extreme hallucinations.

In the case of one of the survivors of the London tube bombings of 2005, he said: "In her flashbacks, she thought she was in one of the trains still and was going to die, and in the much longer moments of lucidity, she thought she was lapsing into unconsciousness and her brain was playing tricks."

For this woman, this state of confusion went on for a few days, and for at least a year clear flashbacks of the trauma continued.

Glasses

Dr Naish and his colleague Dr Ksenja da Silva have developed a piece of equipment that may be able to work out who is at risk of the disorder.

They use an ordinary pair of sunglasses, with a light at each side, on the outside corner of the lens.

People put on the glasses and the lights are flashed one after another. The subject is then asked to tell the person running the experiment which light came on first.

The brain is divided into right and left hemispheres which carry out different functions. The psychologists were able to tell which side of the brain people were using most to tell which light came on first.

Normal subjects were using both hemispheres of the brain to tell which light flashed first. When there was a strong difference, it tended to be that people were using the left side of their brains to process the information.

But when some subjects were hypnotised, they tended to use the right sides of their brains to tell which light flashed first.

Prevention

After a traumatic event, debriefing by psychologists can prevent post-traumatic stress disorder, or PTSD, but only 30% of people who have been in a disaster suffer from the disorder.

"Post-event, you don't want to rush in and treat everybody," said Dr Naish. He added that for people who are not likely to go on to suffer from PTSD, it may make things worse.

Dr da Silva tested 20 PTSD sufferers who were refugees in Slovenia, and found that they appeared to have brains that worked as if they were hypnotised all of the time.

They saw the light on the glasses first with the right sides of their brain, even though they were not under hypnosis.

The right half, or hemisphere, of the brain is believed to deal with overall or "global" information, rather than small details.

It is this side of the brain that Dr Naish thinks PTSD sufferers are relying on. He said: "It's the sense that, I'm stressed out in my mind, don't fuss me with details."

Another group of Slovenian refugees who did not have post-traumatic stress disorder had brains that worked as normal - not using their right brains, as if they were not hypnotised.

Therapy

The researchers hope that they may be able to use these brain patterns to help groups of people after a traumatic event, as well as improving therapy for individuals with PTSD.

Dr Naish said that it might be possible to use the flashing lights to try and "drag people back" if they were using their right brains too much. "There would also be other ways of encouraging the use of the other hemisphere," he added.

The researchers presented their findings at the British Science Association Festival at Aston University in Birmingham. Together with colleagues, they are looking at hallucinations in people who do not have schizophrenia.

Professor Richard Abadi, of the University of Manchester, said that another group that commonly hallucinate are people with severe age-related blindness. These people see objects in their blind patches but they are very confident the objects are not real.

Many people who hallucinate think that the "visions" or the voices they hear are actually real, but the people he has talked to are pretty clear they are just "seeing something".

Now he is particularly interested in talking to people who have severe visual problems but are quite young, under 30 years old, to see if they too experience hallucinations.