Mechanisms of human proprioception

Proprioception includes sensations of the position of the limbs in space, the velocity they are moving, the force the muscles are applying and how hard the muscles are being driven. While we do not consciously attend to this information, it is critical for the control of movement and the loss of proprioceptive sensation causes devastating disability. This thesis reviews the history of research on proprioception and presents the work from four studies that investigated proprioception in healthy human subjects. The first study considered the sense of limb position and the interaction of signals from muscle spindles and motor commands. It investigated the combined effect of voluntary muscle contraction and the history of muscle contraction on position sense at the wrist. The results show a novel interaction that suggests the brain weights the sensory information available to it according to its reliability. The second study investigated the role of centrally-generated command signals in the sense of limb velocity. By inducing experimental phantom hands in human subjects this study showed that central command signals have a role in the sense of limb velocity. The next study investigated the influence of non-cutaneous proprioceptive signals on the sense of body ownership. It is well established that cutaneous signals influence body ownership, and the results from this study are the first to show conclusively that non-cutaneous proprioceptive signals also influence the sense of body ownership. The final study investigated the detail of a model proposed by others to explain why subjects overestimate external forces when matching them with voluntary forces. The results show that there are two separate effects in play, rather than one, as previously reported, and that the previously proposed model is insufficient to explain the overestimation of external forces. Furthermore, the overestimation is not preserved at high levels of force. In summary this thesis presents several novel findings on the mechanisms underlying proprioception in human beings and emphasises the need to explore how different proprioceptive signals are combined