LIFTING a finger is easy. What is far more difficult is understanding how our brains sense that it has moved.
For over a century scientists have struggled to understand which parts of the nervous system allow the body to sense its own position in space. Now Simon Gandevia of the Prince of Wales Medical Research Institute in Sydney, Australia, and colleagues have devised a technique which clearly demonstrates that the brain only has to send a command to a limb to create the sensation of movement.
Their discovery goes some way to resolving the enigma of whether motor commands or receptors in the skin, joints and muscles are more important for creating a feeling of movement.
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Gandevia鈥檚 team studied eight volunteers, by either anaesthetising their forearm and hand, or restricting the flow of blood to the limb. Both techniques deadened sensation to the extent that the volunteers felt they had a 鈥減hantom鈥� hand with fingers clenched, though in fact their fingers were fully extended. When they were asked to flex or extend their wrists, they consistently reported that the position of their hand had changed, in the direction of their efforts, even though it did not move. When they were asked to increase their efforts, the perceived change also increased.
鈥淰olunteers consistently reported that the position of their hand had changed, even though it did not move鈥�
鈥淲e have used a simple method to show definitively that the motor command on its own can produce a dramatic illusion in every subject,鈥� Gandevia says. The team鈥檚 research will be published in the Journal of Physiology.
