Predicting any arm movement feedback to induce three-dimensional illusory movements in humans.

Our sense of body posture and movement is mainly mediated by densely packed populations of tiny mechanoreceptors present in the muscles. Signals triggered in muscle spindles by our own actions contribute crucially to our consciousness of positions and movements by continuously feeding and updating dynamic sensorimotor maps. Deciphering the coding rules whereby the nervous system integrates this proprioceptive information perceptually could help to elucidate the mechanisms underlying kinesthesia. The aim of the present study was to test the validity of a "propriomimetic method" of predicting the proprioceptive streams emitted by each of the muscles involved in two- (2D) and three-dimensional (3D) arm movements. This method was based on the functional properties of muscle spindle populations previously recorded microneurographically in behaving humans. Ia afferent patterns mimicking those evoked when the "arm-forearm" ensemble is drawing straight lines, graphic symbols, and complex 3D figures were calculated. These simulated patterns were then delivered to the main elbow and shoulder muscle tendons of motionless volunteers via a set of vibrators. Results show that the simulated proprioceptive patterns applied induced, in passive subjects, illusory 2D and 3D arm movements, the trajectories of which were very similar to the expected ones. These simulated patterns can therefore be said to be a substitute for the Ia proprioceptive feedback evoked by any human arm movement and this method can certainly be extended to other musculoskeletal ensembles. The illusory movements induced when these proprioceptive patterns are applied to muscle groups via sets of vibrators may provide useful tools for sensorimotor rehabilitation purposes.