YOU may think you know the back of your hand like, well, the back of your hand. But scientists have found that our brains contain distorted representations of the size and shape of our hands. The work could have implications for how the brain unconsciously perceives other parts of the body and may help explain the underpinnings of certain eating disorders in which body image becomes distorted.

Neuroscientists at University College London asked more than 100 volunteers to place their left hand palm-down on a table. The researchers covered the volunteers' hands with a board and then asked them to indicate where they thought landmarks such as fingertips lay underneath. This data was used to reconstruct the “brain's image” of the hand.

The results showed a consistent overestimation of the width of the hand. Many of the volunteers estimated their hand was about 80 per cent broader than it really was. “It's a dramatic and highly consistent bias,” said Matthew Longo of UCL's Institute of Cognitive Neuroscience, who led the work. “It was the same with estimation of finger lengths.”

The brain uses several ways to work out the location of different parts of the body. This includes feedback from muscles and joints and also some sort of internal model of the size and shape of each part. “Previously it has been assumed that the brain uses an accurate model of the body,” said Longo. Instead, Longo's work shows that the brain's internal models can be incorrect.

Regions of high sensitivity in the skin get a correspondingly larger proportion of the brain's territory. Longo said this sensitivity was mirrored in the relative size of the fingers in the maps of perceived positions. “You find the least underestimation for the thumb and more underestimation as you go across to the little finger.”

He added that the research showed how the brain's ability to distort its representation of the body might underlie psychiatric conditions such as anorexia nervosa. “What we find for the hand is that the representation seems to be 'too fat'. If there's an implicit default representation of the brain to perceive the body as overly wide, then that could potentially account for the pattern you get with eating disorders.”

— The Guardian, London