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The brain thinks hands are wider and stubbier than they actually are


picture of hand is identifiable but volunteers thought thier own hands were wider and shorterTo function well in the world, people need a good sense of where their body is in space and how it's postured. This "position sense" helps us coordinate high-fives, boot a soccer ball or pick up the remote. But that doesn't seem to mean that our brains have an accurate sense of our body's precise proportions. A new study found that people tend to have rather inaccurate mental models of their own hands.

When asked to estimate where the fingertips and knuckles of their hidden hands were, study volunteers were way off. But they were all incorrect in the same directions, guessing that their hands were both shorter and wider than they actually were. The findings come from a study led by Matthew Longo of the Institute of Cognitive Neuroscience at University College London and were published online June 14 in Proceedings of the National Academy of Sciences.

"Our results show dramatic distortions of hand shape, which were highly consistent across participants," Longo said in a prepared statement.

He and his coauthor, Patrick Haggard, had subjects place their left hand on a platform (using different orientations in different groups), which was then covered with a board to obscure the hand. The subjects were asked to use their free right hand point with a baton to the location of each knuckle and fingertip of their left hand. The process was filmed and compared to before and after pictures of the hand.

On average, the volunteers judged their hands to be 27.9 percent shorter and 69 percent wider than they were measure to be. Underestimation of each finger length, from the thumb to the pinky, increased by about 7 percent in each finger, rendering the little finger quite a bit littler that it really was.

This trend "mirrors similar gradients of decreasing tactile acuity," the authors pointed out, and the results seem to back up models of the human body, such as the somatosensory homunculus, constructed from the amount of sensory cortex dedicated to various body parts. In these models the hands and face are disproportionately large in comparison to most of the body. But Longo and Haggard are still not sure why the brain has such a distorted perception of our hand proportions.

"Of course, we know what our hand really looks like, and our participants were very accurate in picking out a photo of their own hand from a set of photos with various distortions of hand shape," Longo said. "There is clearly a conscious visual image of the body as well. But that visual image seems not to be used for position sense."

Longo speculated that these disproportions might occur in other parts of the body as well. "These findings may well be relevant to psychiatric conditions involving body image such as anorexia nervosa, as there may be a general bias toward perceiving the body to be wider than it is," Longo said. "Our healthy participants had a basically accurate visual image of their own body, but the brain's model of the hand underling position sense was highly distorted. This distorted perception could come to dominate in some people, leading to distortions of body image."

Given the findings, the researchers suggested, you might now want to question someone who says, "I know this town like the back of my hand."

Image courtesy of iStockphoto/leaf

The views expressed are those of the author and are not necessarily those of Scientific American.

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