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Motion sickness-whether caused by traveling in a car or a boat or being in outer space-is the unpleasant consequence of disagreement between the brain's expectations in a given situation and the information it receives from the senses.

To retain balance, the brain synthesizes data from many sources, including sight, touch and the inner ear. The last is particularly important because it detects angular and linear motion. Most of the time, all the inputs agree. When they do not jibe with what the brain expects in that situation, however, motion sickness-with its spatial disorientation, nausea or vomiting-can occur.