The effects of stochastic galvanic vestibular stimulation on human postural sway

Galvanic vestibular stimulation serves to modulate the continuous firing le vel of the peripheral vestibular afferents. It has been shown that the appl ication of sinusoidally varying, bipolar galvanic currents to the vestibula r system can lead to sinusoidally varying postural sway. Our objective was to test the hypothesis that stochastic galvanic vestibular stimulation can lead to coherent stochastic postural sway. Bipolar binaural stochastic galv anic vestibular stimulation was applied to nine healthy young subjects. Thr ee different stochastic vestibular stimulation signals, each with a differe nt frequency content (0-1 Hz, 1-2 Hz, and 0-2 Hz), were used. The stimulati on level (range 0.4-1.5 mA, peak to peak) was determined on an individual b asis. Twenty 60-s trials were conducted on each subject - 15 stimulation tr ials (5 trials with each stimulation signal) and 5 control (no stimulation) trials. During the trials, subjects stood in a relaxed, upright position w ith their head facing forward. Postural sway was evaluated by using a force platform to measure the displacements of the center of pressure (COP) unde r each subject's feet. Cross-spectral measures were used to quantify the re lationship between the applied stimulus and the resulting COP time series. We found significant coherency between the stochastic vestibular stimulatio n signal and the resulting mediolateral COP time series in the majority of trials in 8 of the 9 subjects tested. The coherency results for each stimul ation signal were reproducible from trial to trial, and the highest degree of coherency was found for the 1- to 2-Hz stochastic vestibular stimulation signal. In general, for the nine subjects tested, we did not find consiste nt significant coherency between the stochastic vestibular stimulation sign als and the anteroposterior COP time series. This work demonstrates that, i n subjects who are facing forward, bipolar binaural stochastic galvanic sti mulation of the vestibular system leads to coherent stochastic mediolateral postural sway, but it does not lead to coherent stochastic anteroposterior postural sway. Our finding that the coherency was highest for the 1- to 2- Hz stochastic vestibular stimulation signal may be due to the intrinsic dyn amics of the quasi-static postural control system. In particular, it may re sult from the effects of the vestibular stimulus simply being superimposed upon the quiet-standing COP displacements. By utilizing stochastic stimulat ion signals, we ensured that the subjects could not predict a change in the vestibular stimulus. Thus, our findings indicate that subjects can act as "responders" to galvanic vestibular stimulation.