Abstract
It is argued that when normal humans make reaching movements, they move their limbs from a kinaesthetically appreciated starting position. There is a considerable amount of evidence, however, to support the contention that this ‘felt position’ of the limb is highly labile, notwithstanding the apparently contradictory evidence for the existence of receptors which directly determine joint angle.
It is proposed that the lability of ‘felt’ position sense in limbs is a transient emergency response of the human localizing system. It is suggested that such a response enables adequate localization to occur subsequent to an error of localization being made when none was intended. (This does not exclude the possibility of lability being associated with other conditions.)
Lability is also shown to be a factor in an individual's appreciation of direction of gaze, thus simple experimental protocols can lead to visual misperception of the direction of distal objects. The evidence supports the idea that the visual control system is reset by means of cross modal checking, namely seeing limb extremities to be where they are felt to be. There is also evidence which indicates that kinaesthesis can be altered and stabilized by using visual or auditory position information. This circularity, that vision can modify kinaesthesis and that kinaesthesis can modify apparent direction of gaze, leads to a problem as to the nature of the fixed referent for the human localization system.
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