Balance control threshold to vestibular stimuli.

Bipedalism renders our erect posture unstable, requiring the integration and processing of multisensory information to remain upright. To understand how each sense contributes to balance, perceptual thresholds to isolated sensory disturbances while standing are typically quantified. Perception, however, is distinct from balance control. Both processes rely on distinct internal body representations, and participants can misattribute the consequences of self-generated balance-correcting actions as an external perturbation. Here, we used signal detection theory to quantify non-perceptual balance control thresholds to isolated vestibular stimuli given the role of vestibular cues in generating balance-correcting responses. We exposed participants standing on force plates to electrical vestibular stimulation (EVS) at varying amplitudes (0.2, 0.4, 0.6 mA) and frequencies (0.1, 0.2, 0.5, 1 Hz). Stimuli delivered at 0.2 mA (0.1-0.5 Hz) and 0.4 mA (0.1, 0.2 Hz) remained unperceived but evoked whole-body responses above the sensorimotor noise underlying balance control. Balance control thresholds ranged from 0.09 to 0.57 mA; they increased with EVS amplitude and decreased with frequency. The physiological mechanisms underlying these EVS amplitude and frequency effects involved a decrease in response gain with increased stimulus amplitude and a reduction in response variability with increased stimulus frequency. Our findings demonstrate that balance responses to isolated vestibular stimuli can be quantified below perceptual thresholds and highlight the dynamic regulation of response gain and the influence of whole-body motion variability in the vestibular control of balance. Our results also open the door to assessing the isolated vestibular contributions to postural control in people with balance impairments. KEY POINTS: Upright balance control relies on sensory information from multiple sensory systems, but balance control thresholds to isolated sensory stimuli remain largely unknown because these stimuli, or their associated responses, can be perceived. We applied isolated electrical vestibular perturbations and used signal detection theory to quantify balance control thresholds to unperceived sensory stimuli. Vestibular stimuli delivered at 0.2 mA (0.1-0.5 Hz) and 0.4 mA (0.1 and 0.2 Hz) remained unperceived but evoked balance-correcting responses above the sensorimotor noise underlying the control of standing. Balance thresholds increased with current amplitude (0.2-0.6 mA) and decreased with stimulus frequency (0.1-1 Hz) and were linked to decreased gain of lateral force and reduced lateral force variability as current amplitude and frequency increased, respectively. These results pave the way for uncovering the sensory contributions to the non-perceptual mechanisms regulating balance-correcting motor commands essential for bipedalism and their potential role in balance impairments.