High-altitude hypoxia alters the visual control of standing balance in lowlanders and Tibetan highlanders

High-altitude hypoxia affects both visual function and postural control, yet the influence of optic-flow perturbations on standing balance under hypoxic stress remains unclear. Tibetan highlanders (TH) exhibit adaptations to chronic hypoxia, but whether their visually-driven postural responses differ from those of lowlanders (LL) has not been investigated. We examined how high-altitude exposure and acclimatization influence static and dynamic visual contributions to balance by delivering sinusoidal optic-flow perturbations in virtual reality at low altitude (1,400 m) and after incremental ascent to high altitude (4,300 m) in acclimatizing LL (n=15) and TH (n=14). Anteroposterior center of pressure (AP CoP) velocity and mean power frequency (MPF) were measured during three visual-field conditions (full-, central-, peripheral-vision) and two optic-flow velocities (peak 1m/s and 8m/s at 0.25 Hz). At high altitude, both groups showed attenuated responses to optic flow compared to 1,400 m, reflected by reduced AP CoP velocity and lower MPF across visual-field conditions, consistent with reduced responsiveness to dynamic visual-motion cues under high altitude hypoxia. In contrast, during eyes-open quiet stance (no VR), TH but not LL exhibited increased AP CoP velocity and MPF at 4,300 m, and no altitude effect was observed with eyes-closed in either group. This finding indicates that TH adopt a visually-dependent postural strategy at altitude, whereas LL show minimal changes in static visual balance control but reduced responsiveness to fast dynamic motion. Together, these findings demonstrate that high-altitude hypoxia disrupts dynamic visual processing for balance control in both groups, while revealing group differences in the use of static visual cues during quiet stance.