Even blindfolded we can sense where our hands and other body parts are.© Getty

To sense where the various parts of our body are, we sometimes rely on signals that originate in our brain rather than in our fingers or toes, a new study shows.

The so-called sixth sense, known as proprioception, is essential to many basic actions, including walking without having to look at your feet or touching your nose with your eyes closed. But scientists have long pondered how this sense works. It is generally accepted that sensors in the skin, muscles and joints send information to the brain that is crucial to sensing limb position. Now researchers in Australia have evidence of the importance of outgoing messages from the brain.

"This will provoke debate, because the idea that the sense of position is mostly the result of the sensory receptors is well-entrenched," says Timothy Miles, a physiologist at the University of Adelaide, Australia, who is independent of the study.

Janet Taylor from the Prince of Wales Medical Research Institute in Sydney and her colleagues strapped the forearm and hand of volunteers into a covered apparatus that prevents the subject from seeing the position of their hand. Under normal conditions, the subject could accurately tell if and how their hand had been moved by an experimenter or by themselves.

Dead hand

Things changed when their forearm and hand were paralysed by a restriction of blood flow and anaesthetized by injection. Volunteers then could not say where their hand had been moved to when it was re-positioned by an experimenter.

But they still felt they could move their hand when directed to do so. In fact they were adamant that they had moved their hand even when it was prevented from moving. The harder they tried, the more they felt it had moved. In the absence of signals coming in from the outside world, from feeling or sight, the brain's motor commands dominated the volunteer's sense of where their hand was.

"We were very surprised that the results were so obvious and so consistent across the subjects," says Taylor. They report the work in the Journal of Physiology¹.

In and out

The results may shift researchers' understanding of the importance of motor commands in proprioception. "It seems there is some sort of comparison between outgoing and ingoing signals that result in the final sense of position," says Miles.

The work also helps to explain the phenomenon of 'phantom limb syndrome', where amputees have a sense that their limb still exists.

But there is still much work to be done. "We don't know how big the contribution of this out flowing signal is to the sense of proprioception under normal conditions," says Taylor. Researchers also don't know what the neural pathways are that transmit the outgoing signal from the brain.

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• References

  1. Gandevia S. C., Smith J. L., Crawford M., Proske U.& Taylor J.L. J Physiol, 571.3 . 703 - 710 (2006).