Calibrating balance perturbation using electrical stimulation of the vestibular system

Background: Supra-threshold galvanic vestibular stimulation (GVS) can be used to challenge the balance control system by disrupting vestibular inputs. The goal of this study was to propose an objective method to assess variability across subjects in the minimum safe GVS level that causes maximum balance degradation. New method: Thirteen healthy young subjects stood on a compliant foam surface with their eyes closed and tried to maintain a stable upright stance. Variables related to the stability of the trunk and whole body were quantified to characterize the relationship between postural responses and GVS at amplitudes from 0 to 4.5 mA in 0.5 mA increments. The relationship between decrements in postural responses and GVS was linear up to a minimum GVS level (called KNEE). An increase in the stimulation level above that did not lead to any further degradation of balance performance. The KNEE was determined by iteratively performing linear fits to the performance measure at different stimulation levels. Results: There were individual differences in KNEE; it was in the range of 1-2.5 mA across subjects. GVS caused an average performance decrement of 27-99% across six variables at the KNEE level compared to a no-stimulus condition. Comparison to existing methods: We propose a method to consistently attain the maximum level of impairment across subjects using the minimum current intensity, to minimize all types of adverse effects usually observed at high intensities. Conclusions: Individual differences in the disruption of posture control in response to GVS have important implications for testing and training paradigms.