Eye, head and skeletal muscle spindle feedback in the elaboration of body references

Evidence is presented to support the notion that the sensory feedback originating in muscles is of major importance in the central elaboration of motor representation. The muscle spindle messages during movement and postural performance may be processed in order to elaborate continuously updated static and dynamic body references. These may then form the basis for the interpretation of retinal information in terms of spatial coordinates. The main arguments supporting this view are as follows. Experimental manipulation of muscle spindle proprioceptive feedback by tendon vibration induced segmental or postural kinaesthetic illusory movements in the direction of stretch of the vibrated muscles. By modifying the spatial distribution (agonists and/or antagonists), the frequency and the duration of the vibratory stimuli it is possible to induce simple or complex kinaesthetic illusions the parameters of which may be predicted. Microneurographic recordings confirmed that vibration rather selectively excited spindle Ia afferents, eliciting 1:1 driving up to 80-100 Hz. Moreover painless vibration, applied at increasing frequency (from 10 to 80 Hz) to either the medial, lateral, superior or inferior rectus of a subject's eye, was found to induce directional perceptual and motor effects which were closely related to the postural context. Likewise, the subjects reported illusory directional shifts of a visually fixed target in darkness during extraocular muscle vibration. These data suggest that extraocular proprioception contributes to the coding of eye, head and body position in relation to postural and environmental conditions. As with eye muscle stimulation, directional visual and postural effects were induced by vibration of neck and/or ankle postural muscles. These effects were found to summate when vibrations were applied simultaneously to the eye, neck and ankle muscles. The likely involvement of extraocular proprioception in interrelating body space with extrapersonal space in oriented behaviour is discussed.