Whether you feel fat, thin or something in between has little to do with the reality of the situation, suggests a new study led by the University College London (UCL) and published in the journal Public Library of Science Biology. A person's self-image is an illusion constructed in the brain, the researchers say.

Dr. Henrik Ehrsson of the UCL Institute of Neurology and colleagues used a trick called "the Pinocchio illusion" to give study volunteers the sensation that their waists were shrinking. They used functional magnetic resonance imaging (fMRI) to scan the participants' brains during the experience and observe which parts of the brain are involved in body image.

The results may shed some light on anorexia, an eating disorder, and body dysmorphic disorder. People with the latter condition typically are overconcerned about a small or imagined defect in their body, and they frequently overestimate or underestimate their actual body size.

Brain Creates a Map of the Body

A vibrating device placed on each study volunteer's wrist served to stimulate the tendon and create the sensation that the joint was flexing, even though it remained stationary. When their hands touched their waists, the volunteers felt their wrists bending, creating the illusion that their waists were shrinking.

During the tendon exercise, all 17 participants felt that their waist had shrunk by up to 28 percent. The researchers found high levels of activity in the posterior parietal cortex, an area of the brain that integrates sensory information from different parts of the body. Volunteers who reported the strongest shrinking sensation also showed the strongest activity in this area of the brain.

"We process information about our body size every day, such as feeling thin or fat when we put our clothes on in the morning, or when walking through a narrow doorway or ducking under a low ceiling," says Dr. Ehrsson.

"However -- unlike more elementary bodily senses such as limb movement, touch and pain -- there are no specialized receptors in the body that send information to the brain about the size and shape of body parts. Instead, the brain appears to create a map of the body by integrating signals from the relevant body parts, such as skin, joints and muscles, along with visual cues," Dr. Ehrsson adds.

Alice in Wonderland Syndrome

"Other studies have shown that people with injuries in the parietal cortex area of the brain experience the feeling that the size and shape of their body parts have changed. People who suffer from migraine with aura can sometimes experience a phenomenon called the 'Alice in Wonderland syndrome,' where they feel that various body parts are shrinking," notes. Dr. Ehrsson.

"This could also be linked to the same region of the brain," he points out.

"In addition, people with anorexia and body dysmorphic disorder who have problems with judging the size of their body might similarily have a distorted representation of their body image in the parietal cortex. These are areas which would be worth exploring in future research, to establish whether this region of the brain is involved in anorexia and the rare but peculiar shrinking symptoms of some migraines," Dr. Ehrsson concludes.