The influence of gravitoinertial force level on oculomotor and perceptual responses to sudden stop stimulation.

Our goal was to determine whether the vestibular response to vertical, z-axis body rotation in the dark is influenced by the magnitude of gravitoinertial force. We measured the nystagmus and the duration of illusory self-motion elicited in blindfolded subjects by cessation of such rotation during the free-fall, high, and terrestrial force phases of parabolic flight maneuvers. Both measures were significantly lower in zero G than in 1 G, and lower to a smaller extent in 1.8 G. The decreased intensity of nystagmus was due specifically to a decrease in the time constant of slow phase velocity decay with no decrement in peak velocity. This pattern of findings is consistent with the responses we had observed earlier to constant levels of Coriolis, cross-coupled stimulation during parabolic flight maneuvers both in terms of the mode of nystagmus suppression and the effect of G-level. Attenuation of the vestibular response to rotary acceleration in free-fall causes sensory-motor mismatches during natural head movements in orbital flight that may be important factors in the evocation of space motion sickness.