You may think you know the back of your hand like, well, the back of your hand. But think again. Scientists have found that our brains contain highly distorted representations of the size and shape of our hands, with a strong tendency to think of them as shorter and fatter than they really are.
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 people's body image becomes distorted.
In the study, 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 on it where they thought landmarks such as fingertips and knuckles lay underneath. This data was used to reconstruct the "brain's image" of the hand.
The results, published today in the Proceedings of the National Academy of Sciences, showed a consistent overestimation of the width of the hand. Many of the volunteers estimated their hand was around 80% 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," he said, where the bias was to judge them as shorter than the reality: "When you get to the ring finger, with the largest bias, it's 30-40% underestimation."
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 body part.
"Previously it has been assumed that the brain uses a perfectly accurate model of the body and it's not mysterious where that might come from," said Longo. "We see our body all the time and it wouldn't be surprising if the brain had developed an accurate representation of the body."
Instead, Longo's work shows that the brain's internal models can be hopelessly wrong. The errors could partly be explained because of the way the brain allocates its processing capacity, said Longo. Regions of high sensitivity in the skin, such as the fingertips and the lips, get a correspondingly larger proportion of the brain's territory.
Longo said that 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. You see the same pattern if you measure tactile sensitivity."
The research was carried out on the hand because there were obvious landmarks for the volunteers to point out, which could then be used by the researchers to draw the brain's image of the hand. But the results might be applicable across the body.
"It would be very surprising if there was a distorted representation of the hand but an accurate representation of the complete rest of the body. That would be a bizzare finding, so my guess is that there would be similar sorts of biases, perhaps bigger ones, on other parts of the body," said Longo.
He said the research showed how the brain's ability to distort its representation of the body might underlie certain psychiatric conditions involving body image such as anorexia nervosa.
"It's interesting to note that 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."
He added: "Our healthy participants had a basically accurate visual image of their own body, but the brain's model of the hand's underlying position sense was highly distorted. This distorted perception could come to dominate in some people, leading to distortions of body image as well, such as in eating disorders."
