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.
Highlights
A non-physiological means of generating afferent vestibular input in the absence of head motion is the application of low amplitude low frequency (,1 Hz) electrical currents between surface electrodes over the mastoid processes, termed bilateral bipolar Galvanic vestibular stimulation (GVS)
During computerized dynamic posturography without GVS anterioposterior sway was essentially unchanged over 36 weeks
Subject performance on a computerized dynamic posturography task was significantly impaired during GVS at first exposure, but recovered to baseline over a period of 7–8 weeks (70–80 min exposure)
Summary
A non-physiological means of generating afferent vestibular input in the absence of head motion is the application of low amplitude (typically ,5 mA) low frequency (,1 Hz) electrical currents between surface electrodes over the mastoid processes, termed bilateral bipolar Galvanic vestibular stimulation (GVS). GVS application when quietly standing results in a vestibulo-spinal reflex response [9,17,18,19] manifested as a small (7 mm RMS for 5 mA peak pseudorandom GVS [9]) mediolateral displacement of the center of mass towards the anode; subjectively, the perception is of an unstable surface rather than self-motion, with the ground ‘rocking’ slightly from side to side in a random manner, as if on a boat in rough waters [20] These results are consistent with the notion that the CNS interprets the resultant sum of all vestibular afferents activated by GVS as a head tilt in the direction of the cathode [5], and generates small reflex responses towards the anode. A previous study suggests that these reflex responses do not adapt to repeated GVS exposure [21]
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