Abstract
To assess vestibular influences on recovery of balance during sudden falls, we measured the postural responses of five healthy subjects to a hold and release perturbation coupled with galvanic vestibular stimulation (GVS). Two electrode pairs were located with the anterior electrode of each pair over the mastoid process and the posterior electrode over the trapezius muscle on the same side. Bilateral unipolar GVS was generated 60 ms after a holding force against the sternum was released by individually driving left and right electrode pairs as cathode or anode at 1 mA for 12 s or 2 mA for 6 s. We computed the frequency and damping parameters of a multi-link inverted pendulum model of the body which best fit the transient postural oscillations after release for each subject. These parameters did not differ significantly across conditions indicating the GVS did not modify the preset overall strategy of postural recovery. The intensity and polarity of GVS significantly biased both the postural lean during the oscillatory period and the resting postural stance achieved during stimulation, deviating them forward for cathodal stimulation and backward for anodal. The residuals of the multi-link fit, the frequency spectra of the actual body sway ripples about the modeled sway, were different across conditions. Because GVS affected postural bias but not dynamics, it is likely that it provided erroneous velocity signals leading to vestibulospinal compensations in segmental stiffness and damping mechanisms. Our findings are consistent with theoretical analyses of the influence of GVS on the semicircular canals and otolith organs of the inner ear.