Brain Spot for Body Size Perception Identified

By Anne Harding

NEW YORK — UK researchers have identified the place in the brain where we process perceptions about the size and shape of our body parts.

The spot, within the brain’s left parietal lobe, is about the size of a quarter and located at the top back of the head, Dr. Henrik Ehrsson of the University College London Institute of Neurology told Reuters Health.

The findings, Ehrsson said, could one day be used to better understand conditions that involve misperception of body image — or example anorexia, in which people see themselves as too fat even when they’re skeletal.

As they report in the journal PloS Biology, Ehrsson and his team identified the brain area by employing a technique that produces an effect known as the "Pinocchio illusion." It involves vibrating the skin over the tendons in a joint, which produces the illusion that the joint is bending or extending depending on where the stimulus is placed. For example, a person who placed his finger at the end of his nose could be made to feel that his finger was extending, giving him the perception that his nose growing.

In the current experiment, volunteers placed their hands at their waists. The skin over the tendon responsible for bending the wrists inward was then vibrated, producing the illusion that the waist was shrinking.

Brain scans during the test found that activity within the parietal region mirrored participants’ illusion of a whittled waist. The stronger the illusion, the more intense the brain activity.

The body has receptors for figuring out elementary sensations like movement, heat and pressure, Ehrsson noted. For something more complex like the perception of body size, he added, "this is something the brain has to sort of figure out by computing different signals."

Ehrsson and his team are now planning experiments in which they will directly stimulate the brain area to determine the effect on body size perception.

SOURCE: Public Library of Science Biology, December 2005.