Modulation of stepping balance reactions through the alignment of vestibular input with body perturbation axis

Balance reactions depend on sensing imbalances to direct recovery responses, though the role of vestibular feedback in scaling these responses remains uncertain. While vestibular input can alter anticipatory postural adjustments before a compensatory step (McMorran, Bent, and Zettel 2024), this study aimed to clarify whether this reflects vestibular-based scaling of step-recovery or if vestibular input acts as a stability reference in feedforward control of step execution. To investigate vestibular input's influence on step scaling, galvanic vestibular stimulation (GVS) was aligned to amplify or diminish the sense of perturbed postural motion in forward step recovery, through inducing the sensation of forward (FGVS) or backward (BGVS) postural motion. Effects of altered vestibular input was analyzed in terms of postural and step motion and restabilization. GVS modulated forward step responses asymmetrically, with BGVS exhibiting greater postural motion in advance of the step as indicated through forward stability, while FGVS evoked larger and faster forward steps relative to the body. Upon landing the step, these differences culminated as skewed stability according to GVS direction, with a smaller stability margin in BGVS compared to a larger one in FGVS. This stability shift continued post-recovery, with a forward-BGVS and backward-FGVS shift when re-establishing equilibrium. These results demonstrate step-recovery scaling according to the GVS direction, indicating vestibular-based modulation of compensatory stepping reactions.