Customizing galvanic vestibular stimulation amplitude: An objective technique using sway indices to account for individual sensitivity

Galvanic vestibular stimulation (GVS) uses transcutaneous direct current stimulation to either improve balance function at low intensities or provide a sensory disruption at higher intensities. The present study sought to establish an objective procedure for determining individual sensitivity levels for GVS using postural sway. GVS thresholds were obtained in healthy young adults over three sessions while standing with feet together and eyes closed using sinusoidal stimuli at intensity amplitudes ranging between 0.1-0.7 mA. Sway amplitudes from inertial motion sensors mounted on the head and torso were derived from sinusoidal fits over each 20 sec trial. Individual sensitivities were determined using amplitude indices (lowest level that induced sway greater than sway without stimulation), critical indices using logistic analysis of sway amplitudes across the entire stimulus range, and subjective indices (lowest level that subjects perceived a sense of motion). The sensitivity indices based on sway reflected greater intersubject variability than subjective indices. Sensitivity indices for head and torso were strongly correlated, as were the corresponding amplitude and critical indices, and indices across multiple sessions. We conclude that a sinusoidally evoked postural sway sensitivity index provides an objective measure that can be used to optimize GVS stimulation levels by accounting for intersubject sensitivity.