Predicting vestibular, proprioceptive, and biomechanical control strategies in normal and pathological head movements

Little is known of the functionality of the vestibulo-collic reflex (VCR) and cervico-collic reflex (CCR) during head and neck movements caused by perturbations of the trunk, previously, the authors formulated mathematical expressions for these neck reflexes and incorporated them into a model of horizontal plane head movements. The formalism of this neuromechanical model allowed the authors to examine separately the main components of head movement control. In the present study, they examine selected parameters within the main components of the model, and associate variations of these parameters with disease processes affecting head and neck movements, such as loss of sensory input or modification in central or motor function. The authors' simulations led them to several conclusions. First, the probable use of the VCR and CCR in yaw plane head movements is to tune the head response. In the time domain, they diminish natural head oscillations (head wobble) related to head mechanics. Equivalently, in the frequency domain, they reduce the amplitude of head wobble (resonances) around 2 Hz. Second, the authors' simulations suggest that the VCR is about ten times stronger than the CCR in normal humans. Moreover, this disproportion is associated with only very minor contributions from the CCR in yaw. Third, head oscillations (or instability) ran be generated by mechanical or neural changes in the head and neck system. Finally, readjustments of central nervous system dynamic operations could provide mechanisms to compensate for sensory and motor dysfunction caused by disease.