Saturday, June 16, 2012

Scientists measure size of 'phantom limbs'

Hands and feet are ultimately controlled by signals coming from the brain, which direct muscle movement. Our nervous system is very accurate in directing movement, as is visible by the incredibly precise and fine movements of our fingers. The brain cells and the required neuronal wiring for limb control do not go away after we lose one of our limbs, for example after amputation. Because of that, patients sometimes feel pain coming from an arm or leg that does no longer exist. This is called phantom pain and is caused by neuronal pathways that are still active after amputation. Scientists have been able to map what a phantom hand feels and 'looks' like, to precisely determine its size and shape, which tells us something about what such patients experience.

For their mapping study, a 38-year old female patient born without a left hand was asked by the University of London to describe the size and shape of her hands. She was asked to place her right hand beneath a board and indicate the location of her fingertips and knuckles. Naturally, she was quite able to do so with her existing right hand, but she was also asked to repeat it with her left hand, which does not exist.

Despite absence of her left hand, the patient was able to indicate where she thought her fingertips and knuckles were under the board. This allowed the scientists to map the size and shape of the phantom hand, which is an indicator for the internal representation of that particular hand in the brain. What they found is that the patient perceives both her right and left hand in the same way, even though the latter one does not exist.

Human beings have a tendency to underestimate the size of their fingers, and she also made these characteristic mistakes when describing her both hands. This is due 'errors' in brain parts that are sensitive for tactile input coming from the fingers. This is why we make the biggest error in describing our little finger, as it is the least sensitive and therefore has the smallest brain part associated with it. In the study, the description of the missing left hand matched these typical brain characteristics, showing that what she perceives is an internal representation seated in the brain.
An image made by analysing the patient's description.
This study tells us something about how the brain maps our body parts, and what patients suffering from phantom pain experience. But the scientists argue that their research could have broader implications. The whole issue with internal representation of body parts could also be relevant for other malfunctions, such as eating disorders. It could be that these patients also have a faulty 'brain map' that makes inaccurate assumptions about their body shape, which then influence eating patterns.

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