Postural instability precedes motion sickness

We evaluated the hypothesis that postural instability precedes the onset of motion sickness. Subjects standing in a "moving room" were exposed to near ly global oscillating optical flow. In the experimental condition, the opti cal oscillations were a complex sum-of-sines between 0.1 and 0.3 Hz, with a n excursion of 1.8 cm. This optical motion was of such low frequency and ma gnitude that it was sometimes not noticed by subjects. However, in two expe riments, exposure to the moving room produced significant increases in scor es on a standard motion sickness questionnaire. In addition, approximately half of subjects reported motion sickness. Analysis of postural motion duri ng exposure to the moving room revealed increases in postural sway before t he onset of subjective motion sickness symptoms. This confirms a key predic tion of the postural instability theory of motion sickness. (C) 1999 Elsevi er Science Inc.