Central adaptation to repeated galvanic vestibular stimulation: implications for pre-flight astronaut training

Abstract

Healthy subjects (N = 10) were exposed to 10-min cumulative pseudorandom bilateral bipolar Galvanic vestibular stimulation (GVS) on a weekly basis for 12 weeks (120 min total exposure). During each trial subjects performed computerized dynamic posturography and eye movements were measured using digital video-oculography. Follow up tests were conducted 6 weeks and 6 months after the 12-week adaptation period. Postural performance was significantly impaired during GVS at first exposure, but recovered to baseline over a period of 7-8 weeks (70-80 min GVS exposure). This postural recovery was maintained 6 months after adaptation. In contrast, the roll vestibulo-ocular reflex response to GVS was not attenuated by repeated exposure. This suggests that GVS adaptation did not occur at the vestibular end-organs or involve changes in low-level (brainstem-mediated) vestibulo-ocular or vestibulo-spinal reflexes. Faced with unreliable vestibular input, the cerebellum reweighted sensory input to emphasize veridical extra-vestibular information, such as somatosensation, vision and visceral stretch receptors, to regain postural function. After a period of recovery subjects exhibited dual adaption and the ability to rapidly switch between the perturbed (GVS) and natural vestibular state for up to 6 months.

Figure 1. Posturography and ocular torsion data from a typical subject.

(A) Anterioposterior sway plots without (upper row blue trace) and with (lower row red trace) 5 mA GVS for SOT 5 (eyes closed and sway-referenced support surface). The subject fell during initial exposure to GVS, but anterioposterior sway decreased to baseline levels over the 12 weeks of GVS exposure, and this recovery was maintained 6 months after adaption. (B) Torsional (roll) eye position traces without (upper row blue trace) and with (lower row red trace) 5 mA GVS. In contrast to the posturography data, the vestibulo-ocular reflex response to GVS was unchanged by repeated GVS exposures.

Figure 2. Computerized dynamic posturography data from all 10 subjects (mean and 95%CI) without (blue trace) and with (red trace) 5 mA GVS.

(A) The composite equilibrium score was significantly lower when first exposed to GVS, but recovered to baseline after 7 weeks (70 min total) exposure, and this recovery was retained for up to 6 months post adaptation. (B) The somatosensory index, and (C) the visual index, were unaffected by GVS exposure. (D) In contrast, the vestibular index (SOT5/SOT1 ratio), was significantly affected by GVS exposure, illustrating the almost exclusive impact of GVS on the vestibular component of postural control. The vestibular index recovered to baseline by week 8, following 70 min of cumulative GVS exposure, and this recovery was maintained 6 months post-adaptation.

Figure 3. Torsional (roll) eye position data from all 10 subjects (mean and 95%CI) without (blue trace) and with (red trace) 5 mA GVS.

There was a significant increase in ocular torsion during GVS relative to baseline, which was unaffected by repeated GVS exposure.