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Within seconds, they could actually feel their waists had begun to shrink. It would have been a great advance in the world of weight loss ' if only it had been real. But the shrinking feeling was just an illusion, created by scientists who wanted to study how the brain creates body image or an individual’s perception of his or her own size and shape.

The researchers, led by Dr H. Henrik Ehrsson of University College London, fooled 17 people into feeling as if they were getting skinnier by outfitting them with gadgets that stimulated a tendon in each wrist to create the false sensation that both hands were moving inward.

The subjects wore blindfolds and placed their hands at their waists, and then the stimulators were turned on, while an MRI scanner measured activity in different parts of the brain. For the subjects, the feeling that their wrists were flexing inward was so powerful that they felt their waists had to be getting smaller.

Their study was published in the current issue of the journal Public Library of Science Biology (www.plosbiology.org).

The technique is a variation on the Pinocchio illusion, an experiment first done ' and named ' in 1988 by another researcher, James R. Lackner of Brandeis University, in which stimulation of wrist tendons convinced blindfolded people who were touching their noses that their noses were growing.

Ehrsson said he had tried the tricks on himself.

“It’s a very funny sensation,” he said. “The nose is my favourite. It really feels like your nose is getting longer, and it’s very striking. You start giggling.”

The first time people feel the illusions, “they’re very surprised and shocked,” Ehrsson said in a telephone interview. “They say, ‘Wow, oh my God’. It’s quite strong.”

“The illusion happens as a result of a conflict between senses,” Ehrsson said, explaining that the wrists feel as if they are moving, but the palms of the hands, resting against the waist, do not. “The brain has to interpret the conflicting sensory information. The brain hates ambiguity. It always tries to come up with an explanation.”

Not everyone experiences the illusions, he said; some people feel nothing, but no one knows why.

The point of his experiment was to use the illusion as a scientific tool, to force the brain to change its perception of body size so that the scientists could use an MRI scanner to spy on the mental work in progress. This kind of mapping to find the origins of body image had not been done before, Ehrsson said.

“It’s a little bit of a forgotten sense,” he said. “We know about touch, pain and position, but the sense of size of body parts has been a mystery. There are no receptors in skin or muscle that tell the brain the size of body parts, so the brain has to figure it out by comparing signals.”

He said the research might shed light on anorexia, in which people can be emaciated and wasting away, and yet still see themselves as fat.

“They seem to have a perception deficit,” Ehrsson said. “We don’t know if it’s the cause of anorexia, probably not, but it’s clearly part of the problem.”

The study showed that while subjects were experiencing the shrinking-waist illusion, they had high levels of activity in the posterior parietal cortex, a region of the brain that processes sensory information from different parts of the body.

The people who felt the most shrinkage in their waists had the highest levels of parietal activity.

Lackner said the new study was of “major physiological significance”, and proof that the brain was continuously mapping and remapping body dimensions. “I was very happy to see this result and thought it was very imaginative,” he said.

Ehrsson’s finding was not surprising, since other studies had shown that people with brain injuries in that area had strange sensations suggesting that the sizes and shapes of parts of their bodies had changed.

Also, in rare cases, people with migraine headaches suffer from the “Alice in Wonderland syndrome”, in which they feel as if parts of their bodies are shrinking.

Future studies, Ehrsson said, may include blind people and people with anorexia, to find out how their brains process the illusions.

NYTNS