Asymmetry of balance responses to monaural galvanic vestibular stimulation in subjects with vestibular schwannoma.

OBJECTIVE: We investigated the potential of galvanic vestibular stimulation (GVS) to quantify lateralised asymmetry of the vestibulospinal pathways by measuring balance responses to monaural GVS in 10 subjects with vestibular schwannoma and 22 healthy control subjects. METHODS: Subjects standing without vision were stimulated with 3 s, 1 mA direct current stimuli delivered monaurally. The mean magnitude and direction of the evoked balance responses in the horizontal plane were measured from ground-reaction forces and from displacement and velocity of the trunk. Vestibular-evoked myogenic potentials (VEMPs) to 500 Hz air and bone-conducted tones were also recorded. RESULTS: In healthy subjects, the magnitudes of the force, velocity and displacement responses were not significantly different for left compared to right ear stimulation. Their individual asymmetry ratios were always < 30%. Subjects with vestibular schwannoma had significantly smaller force, velocity and displacement responses to stimulation of the affected compared with non-affected ear. Their mean asymmetry ratios were significantly elevated for all three measures (41.2 ± 10.3%, 40.3 ± 15.1% and 21.9 ± 14.6%). CONCLUSIONS: Asymmetry ratios of balance responses to monaural GVS provide a quantitative and clinically applicable lateralising test of the vestibulospinal pathways. SIGNIFICANCE: This method offers a more clinically relevant measure of standing balance than existing vestibular function tests which assess only vestibuloocular and vestibulocollic pathways.

The human semicircular canal model of galvanic vestibular stimulation.

A vector summation model of the action of galvanic stimuli on the semicircular canals has been shown to explain empirical balance and perceptual responses to binaural-bipolar stimuli. However, published data suggest binaural-monopolar stimuli evoke responses that are in the reverse direction of the model prediction. Here, we confirm this by measuring balance responses to binaural-monopolar stimulation as movements of the upper trunk. One explanation for the discrepancy is that the galvanic stimulus might evoke an oppositely directed balance response from the otolith organs that sums with and overrides the semicircular canal response. We tested this hypothesis by measuring sway responses across the full range of head pitch. The results showed some modulation of sway with pitch such that the maximal response occurred with the head in the primary position. However, the effect fell a long way short of that required to reverse the canal sway response. This indicates that the model is incomplete. Here, we examine alterations to the model that could explain both the bipolar and monopolar-evoked behavioural responses. An explanation was sought by remodelling the canal response with more recent data on the orientation of the individual canals. This improved matters but did not reverse the model prediction. However, the model response could be reversed by either rotating the entire labyrinth in the skull or by altering the gains of the individual canals. The most parsimonious solution was to use the more recent canal orientation data coupled with a small increase in posterior canal gain.

Non-linear vector summation of left and right vestibular signals for human balance.

The left and right vestibular organs always transduce the same signal of head movement, and with natural stimuli can only be activated simultaneously. To investigate how signals from the left and right vestibular organs are integrated to control human balance we electrically modulated the firing of vestibular afferents from each labyrinth independently and measured the resulting balance responses. Stimulation of one side at a time (monaural) showed that individual leg muscles receive equal inputs from the two labyrinths even though a single labyrinth appeared capable of signalling 3-D head motion. To deduce principles of left-right integration, balance responses to simultaneous stimulation of both sides (binaural) were compared with responses to monaural stimuli. The binaural whole-body response direction was compatible with vector summation of the left and right monaural responses. The binaural response magnitude, however, was only 64-74% that predicted by the monaural sum. This probably reflects a central non-linearity between vestibular input and motor output because stimulation of just one labyrinth revealed a power law relationship between stimulus current and response size with exponents 0.56 (force) and 0.51 (displacement). Thus, doubling total signal magnitude either by doubling monaural current or by binaural stimulation produced equivalent responses. We conclude that both labyrinths provide independent estimates of head motion that are summed vectorially and transformed non-linearly into motor output. The former process improves signal-to-noise and reduces artifactual common-mode changes, while the latter enhances responses to small signals, all critical for detecting the small head movements needed to control human balance.