EFFECT OF FREQUENCY OF HORIZONTAL LINEAR OSCILLATION ON MOTION SICKNESS AND SOMATOGRAVIC ILLUSION

Background: Low frequency linear (translational oscillation is an impo rtant stimulus in provoking motion sickness in a variety of modes of t ransport. Hypothesis: Lower frequencies of horizontal linear oscillati on would be more nauseogenic. The somatogravic illusion (SGI) would be affected by frequency. Methods: Subjects (n = 12) were exposed to hor izontal sinusoidal motion (3.6 m.s(-2) peak) at three different freque ncies (0.205 Hz, 0.350 Hz, 0.500 Hz) at one week intervals, with order randomized between subjects. Subjects were seated in the upright posi tion, with motion through the X head body axis. Results: The mean (+/- S.D.) motion exposure time required to produce moderate nausea decreas ed significantly (p < 0.01) towards the lower frequency: 24.4 +/- 19.3 min. at 0.500 Hz, 12.0 +/- 9.5 min, at 0.350 Hz, 7.8 +/- 6.2 min. at 0.205 Hz. The linear fit of time (t) to nausea with frequency was -7.4 dB/octave. This was equivalent to a -3.7 dB/octave decrease of nauseo genic potential with increasing frequency, if t(1/2) were to be used a s in the standard ''motion dose'' models. The SGI was reported by 5/12 subjects (mean illusory tilting angles 16.2 degrees forward, 14.3 deg rees back) but there was no relationship between SGI and motion freque ncy or motion sickness. Conclusions: Horizontal motion with subjects s eated upright was more nauseogenic than would be predicted by mathemat ical models based on vertical oscillation, and the relationship of fre quency to nauseogenicity for horizontal motion was significantly less steep than that previously reported for vertical motion.