Perceiving limb position in normal and abnormal control: An equilibrium point perspective

Two potential sources of information allow the nervous system to determine limb position in egocentric space: (i) limb kinaesthesis and (ii) extraretinal cues to direction and distance. We argue that both of these information sources arise from essentially similar physiological origins – copies of the central commands issued by the nervous system together with afferent discharge fed back from the relevant peripheral effectors. A number of studies have demonstrated that judgements of limb position are more accurate when limb kinaesthesis is combined with extraretinal cues. We review some recent studies which suggest that the weighting attached to these two sources of information varies as a function of the amount of retinal detail present within the environment. We conclude by considering how the perception of limb position may be affected in cases of abnormal movement control.