Abstract
This is a brief review of some recent experiments exploring aspects of human limb position sense. It is known that muscle vibration at 80 Hz stimulates muscle spindles to generate a sense of muscle lengthening, representing arm extension for elbow flexors. For the forearm, it was shown in a two-arm position matching task that if the subject was blindfolded or could see their indicator arm made little difference in their ability to indicate the illusion of extension of the reference arm during its vibration. However, when the indicator arm was replaced with a dummy, or the subject used a mirror image of the indicator to achieve a match, the size of the vibration illusion was significantly reduced. It was argued that these two latter conditions represented an arm pointing task rather than a matching task, and this was responsible for signalling a smaller vibration illusion. By means of the techniques of muscle conditioning and of muscle vibration it was shown that it was the difference signal from the two elbow antagonists of each arm that was important, and that the brain compared the difference signals from the two arms to determine the accuracy of arm alignment. It was argued that this mechanism, which relies on the afferent discharges of muscle spindles, could only operate over a limited range of elbow angles. One of the challenges for the future is to define those limits.
