Stimulus-dependent changes in the vestibular contribution to human postural control

Humans maintain stable stance in a wide variety of environments. This robust behavior is thought to involve sensory reweighting whereby the nervous system adjusts the relative contribution of sensory sources used to control stance depending on environmental conditions. Based on prior experimental and modeling results, we developed a specific quantitative representation of a sensory reweighting hypothesis that predicts that a given reduction in the contribution from one sensory system will be accompanied by a corresponding increase in the contribution from different sensory systems. The goal of this study was to test this sensory-reweighting hypothesis using measures that quantitatively assess the relative contributions of the proprioceptive and graviceptive (vestibular) systems to postural control during eyes-closed stance in different test conditions. Medial/lateral body sway was evoked by side-to-side rotation of the support surface (SS) while simultaneously delivering a pulsed galvanic vestibular stimulus (GVS) through electrodes behind the ears. A model-based interpretation of sway evoked by SS rotations provided estimates of the proprioceptive weighting factor, W_p, and showed that W_p declined with increasing SS amplitude. If the sensory-reweighting hypothesis is true, then the decline in W_p should be accompanied by a corresponding increase in W_g, the graviceptive weighting factor, and responses to the GVS should increase in proportion to the value of W_g derived from responses to SS rotations. Results were consistent with the predictions of the proposed sensory-reweighting hypothesis. GVS-evoked sway increased with increasing SS amplitude, and W_g measures derived from responses to GVS and from responses to SS rotations were highly correlated.