FREQUENCY EFFECT OF 0.35-1.0 HZ HORIZONTAL TRANSLATIONAL OSCILLATION ON MOTION SICKNESS AND THE SOMATOGRAVIC ILLUSION

Background: Low frequency translational oscillation can provoke motion sickness in land vehicles, ships and aircraft. Hypothesis: Nauseogeni city should decrease towards the higher frequencies. Methods: Some 12 subjects were exposed to horizontal sinusoidal motion (3.6 m.s(-2) pea k) at four different frequencies 0.35, 0.50, 0.70, and 1.00 Hz, at 1-w eek intervals, latin square order. Subjects were seated in the upright position; motion was through the head-body X-axis. Motion was stopped (motion endpoint) at moderate nausea or after 30 min. Results: The pr oportion of subjects experiencing moderate nausea decreased towards th e higher frequency: 9/12 at 0.35 Hz, 3/12 at 0.50 Hz, 0/12 at 0.70 Hz, and 2/12 at 1.00 Hz. The mean time to motion endpoint increased signi ficantly (p < 0.001) towards the higher frequency: 17.4 min 0.35 Hz, 2 6.0 min 0.50 Hz: 30 min 0.70 Hg 28.3 min 1.00 Hz. Differences between frequencies were significant (0.001 < p < 0.05) except for 0.70 Hz to 1.00 Hz. At all frequencies tested, horizontal stimuli were more nause agenic than predicted by mathematical models based on the frequency an d intensity of vertical oscillation. Somatogravic illusion (SCI) was r eported by 9/12 subjects (mean illusory tilt angles 15.6 degrees forwa rd, 14.1 degrees back). SCI tended to diminish at the higher frequenci es, but there was no relationship between SGI and motion sickness. Con clusions: These results confirm previous findings (9), and furthermore indicate that horizontal X-axis translational oscillation has greatly reduced nauseogenic potential at frequencies greater than 0.5 Hz. A m athematical model is proposed to predict motion sickness intensity and incidence due to this stimulus, which may also be applicable to equiv alent Y-axis motion.