Resilience of otolithic reflex to gravitational changes.

Otoliths within the vestibular system detect the magnitude and orientation of gravity and provide a reference frame for spatial orientation. Although vestibular processing has been studied in altered gravity, little is known about the initial minutes of exposure, when space motion sickness emerges. This study investigated whether otolithic detection is modified in different gravities or by sensory conflicts. Otolithic function was assessed using vestibular-evoked myogenic potentials (VEMPs) recorded from three muscles, reflecting utricular and saccular activity. VEMPs were measured during parabolic flight to assess modified gravity and longitudinal effects associated with otolitho-canalar conflict, and before and after a virtual reality exposure inducing visuo-vestibular conflict. Otolithic reflexes were largely resistant to gravity changes, sensory conflicts, and motion sickness. VEMP amplitudes remained stable, while oVEMP latencies increased ( ~ 1 ms in microgravity; ~0.4 ms post-flight). No differences were observed between motion-susceptible and non-susceptible participants. Baseline otolithic sensitivity predicted cyber-sickness susceptibility but not space-sickness.