Here's an experiment you can try (right now if you're sitting at a desk or table): take your left hand (or right hand if you're left-handed) and place it palm towards the floor beneath the table surface. Now place a piece of paper on top where your hand is. Draw 10 dots representing where you think your fingertips end and where the knuckles at the base of each finger or thumb are located. Connect the five knuckle dots and draw lines connecting each knuckle to the fingertip. Compare this drawing to your own hand. Did you get it right?
Chances are, your drawing is very distorted from how your hand is actually shaped. When scientists from University College London tried a similar experiment (their results appeared last week in PNAS), participants drew their hands as being much wider than reality and their fingers much shorter. The Guardian explains:
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. ... 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 says that it is likely that we have similar distorted perceptions of other parts of our bodies and that the brain's ability to do this may be a factor in psychiatric conditions that relate to body image, such as anorexia.