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Motion Sickness

Motion sickness is caused when signals from the sense of equilibrium do not mesh with signals from the sense of vision. In a moving car, the sense of sight indicates motion, but the kinesthetic sense tells the brain that the body is sitting still. The disagreement between visual and equilibratory signals causes motion sickness in some people. Sea sickness, a variety of motion sickness, is caused the same way.

What causes motion sickness?

One cure for motion sickness is to fixate one's gaze on the horizon. In an airliner or on a cruise ship, one can get motion sickness by staying in a room or cabin that feels and looks motionless. The sense of equilibrium responds to up and down motions of the craft, but this does not match the cues from the sense of vision that show a motionless room. If one stares at the horizon -the line in the distance where the earth or sea meets the sky-the visual system receives accurate information about how the body is tilted. Then the input from visual system matches the input from the sense of sense of equilibrium, and motion sickness should go away.

What is an evolution-based theory of motion sickness?

Why does the body have such a peculiar response to mismatched signals from the senses of sight and balance? Triesman (1977) proposed an "evolutionary hypothesis," noting that poisonous substances disturb the delicate coordination of vision and balance. Therefore the body interprets the mismatched signals as evidence of poisoning. By becoming sick, one attempts to clear one' system of the poisonous substance.

Whatever the reason, illusions involving the sense of equilibrium provoke a "gut reaction". This was demonstrated in the 1860s with an amusement park novelty called the haunted swing. As described in by R.W. Wood:

I was much interested in the curious sensations produced by the swing at the Midwinter Fair in San Francisco. On entering the building, we found ourselves in a spacious cubicle room, furnished with sofa, table, chairs, a massive iron safe, and a piano, together with other minor articles. But the most conspicuous object was the huge swing, capable of holding forty or more persons, which hung in the center, suspected from an iron cylinder passing through the center of the room. Each vibration of the swing caused those peculiar "empty" sensations that one feels in an elevator...

...We were then told to hold on tightly as the swing was going clear over, and sure enough, so it did.... Many persons were actually made sick. Many said that they could scarcely walk out of the building because of dizziness and nausea....

How did the "haunted swing" work, and what effect did it have on passengers?

The device was worked in the following way: The swing proper was practically at rest, merely being joggled a trifle, while the room itself was put in motion, the furniture being fastened down to the floor, so that it could be completely turned over....

The curious and interesting feature however was, that even though the action was fully understood, as was my case, it was impossible to quench the sensations of "goneness within" with each apparent rush of the swing. (In Cohen, 1969)


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