Effect of frequency of horizontal linear oscillation on motion sickness and somatogravic illusion.

BACKGROUND: Low frequency linear (translational) oscillation is an important stimulus in provoking motion sickness in a variety of modes of transport. HYPOTHESIS: Lower frequencies of horizontal linear oscillation would be more nauseogenic. The somatogravic illusion (SGI) would be affected by frequency. METHODS: Subjects (n = 12) were exposed to horizontal sinusoidal motion (3.6 m.s-2 peak) at three different frequencies (0.205 Hz, 0.350 Hz, 0.500 Hz) at one week intervals, with order randomized between subjects. Subjects were seated in the upright position, with motion through the X head body axis. RESULTS: The mean (+/- S.D.) motion exposure time required to produce moderate nausea decreased 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 nauseogenic potential with increasing frequency, if t1/2 were to be used as in the standard "motion dose" models. The SGI was reported by 5/12 subjects (mean illusory tilting angles 16.2 degrees forward, 14.3 degrees back) but there was no relationship between SGI and motion frequency or motion sickness. CONCLUSIONS: Horizontal motion with subjects seated upright was more nauseogenic than would be predicted by mathematical models based on vertical oscillation, and the relationship of frequency to nauseogenicity for horizontal motion was significantly less steep than that previously reported for vertical motion.