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Although a number of conditions can produce motion sickness, it is generally thought that it is evoked from a mismatch in sensory cues between vestibular, visual, and proprioceptive signals (Yates, Miller, & Lucot, 1998). For example, reading a book in a car on a winding road can produce motion sickness, whereby the accelerations experienced by the vestibular system do not match the visual input. However, if one looks out the window at the scenery going by during the same travel, no sickness occurs because the visual and vestibular cues are in alignment.

Sea sickness, a form of motion sickness, appears to be a special case and arises from unusual vertical oscillatory and roll motion. Human studies have found that low frequency oscillations of 0.2 Hz and large amplitudes (such as found in large seas during a storm) are most likely to cause motion sickness, with higher frequencies offering little problems. At frequency oscillations of 2 Hz or above, motion sickness ceases completely, although discomfort and injury can still occur. It’s possible that high frequency oscillations happen so quickly that the vestibular system doesn’t have time to properly function to send the proper signals to the brain, so no motion sickness occurs as the visual and vestibular cues are not clashing, but much more research is needed to determine a proper explanation to this phenomena.

It is often difficult to predict who will experience motion sickness and who won’t. One thing that predicts motion sickness is postural instability. Normally, the muscles in our legs and our core body muscles automatically contract and relax, in response to sensory cues from proprioceptors in the muscles and tendons and pressure sensors on our feet, to automatically adjust our posture when the floor tilts. However, if the floor is moving in an unpredictable pattern, it is more difficult for us to keep stable and we sway. Tom Stoffregen at the University of Minnesota has shown that the amount a person sways in response to unpredictable motion of the floor predicts whether or not that person will experience motion sickness. The nature of any causal link between postural stability and motion sickness is not yet known; nor do we know whether this connection is mediated by cue conflict like the above examples. But it is one more clue to help us understand why people experience motion sickness.