Scientists have resolved another old mystery of the brain: what causes the sensation of body motion? Is it that we have two separate systems - one that commands the motion and one that checks whether the command has been fulfilled - or is it that the command itself causes the feeling of motion?

Scientists were inclined to believe the first idea, and to assume that receptors in the skin, joints and muscles are creating the feeling of movement. Nonetheless, they have constantly failed to detect which area of the brain is responsible for the checking job and, implicitly, which parts of the nervous system allow the body to sense its own position in space. They had been struggling over this issue for over a century. And now, it has been shown that the raison why they have failed to find this brain area is simply because it does not exist! The motor command itself triggers the feeling that the command has been fulfilled.

In order to prove this, Simon Gandevia of the Prince of Wales Medical Research Institute in Sydney and his colleagues devised a simple experiment. They have either anaesthetized the forearm and hand of eight volunteers, or have restricted the flow of blood to the limb. In both cases the participants reported the sensation of having a "phantom" hand with fingers clenched - although in fact their fingers were fully extended. Then, they were asked to flex or extend their wrists. Obviously, they couldn't actually do it. Nonetheless, they all reported that the position of their hand had changed, and when they were asked to increase their efforts, the perceived change also increased.

Thus, they literally had the illusion they succeeded to move their arm. This proves that the brain only has to send a command to a limb in order to create the sensation of an actual movement.

"We have used a simple method to show definitively that the motor command on its own can produce a dramatic illusion in every subject," Gandevia says. "Volunteers consistently reported that the position of their hand had changed, even though it did not move."

This gives a very interesting insight on the more general problem: How do we get the sensation of our own position in space? It appears that, besides the visual sense, we develop this sensation based on what motor commands have to be given. Thus, the brain doesn't seem to have a static representation of the body configuration in space; it has a dynamic representation of what has to be constantly done in order to maintain the equilibrium, to compensate for various external factors, and to achieve certain desired goals.

This also provides a clear explanation of the "phantom limb" phenomenon - when people continue to sense the existence of a missing limb.

Image credits: Sidilarsen 'Emotion Num�rique', altered album cover