Vestibulo-ocular responses in man to +Gz hypergravity.

The influence of high +Gz gravito-inertial force on the vestibular system in man was investigated in a 4-m centrifuge with a freely swinging gondola. The Gz profile was: acceleration +0.2 Gz/s, +3 Gz sustained for 3 min, deceleration -0.2 Gz/s. The subject was exposed to this profile under two conditions in randomized order: facing forward and facing backward. Under these conditions, the effective angular velocity in the plane of the vertical semicircular canals is opposed. Adding the slow phase velocity responses from these conditions yields the Gz effect only; subtracting yields the angular velocity effect only. Vertical vestibular nystagmus was analysed in five subjects. Results indicate that +3 Gz induced a subject-dependent vertical nystagmus with slow phase downwards. The average amplitude of this nystagmus reached a maximum of 27 degrees/s at 16 s from G onset, and was 11 degrees/s after 3 min of sustained +3 Gz. The vestibular stimulation by +Gz could result in false subjective perception of attitude, and play a major role in spatial disorientation in flight.