Timothy C. Hain, a professor of neurology, otolaryngology, and physical therapy/human movement science at Northwestern University Medical School, and Charles M. Oman, director of the Man Vehicle Laboratory at Massachusetts Institute of Technology and leader of the neurovestibular research program at the NASA National Space Biomedical Research Institute, explain.
In order for a person to estimate his location, the brain combines information from a variety of sources, including sight, touch, joint position, the inner ear and its own expectations. The inner ear (see image below) is particularly important because it contains sensors for both angular motion (the semicircular canals) and linear motion (the otoliths). These sensors are called the vestibular system. Under most circumstances, the senses and expectations all agree. When they disagree, however, conflict arises and motion sickness can occur. Motion sickness usually combines elements of spatial disorientation, nausea and vomiting.
Consider the situation when one is reading in the back seat of a car. Your eyes, fixed on the book with the peripheral vision seeing the interior of the car, say that you are still. But as the car goes over bumps, turns, or changes its velocity, your ears disagree. This is why motion sickness is common in this situation. If you have this sort of reaction it is usually helpful to stop reading and look out the window. The driver of the car is generally least likely to suffer from motion sickness, because he not only has accurate sensory information from his ears, eyes and touch, but he is also controlling the car and can therefore anticipate turns, accelerations and decelerations. This position allows him to better calibrate his expectations of movement with the car's actual movement.
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Motion sickness also routinely occurs on boats. Seasickness usually develops when you are below decks and cannot see out. If you stay on deck, where you have a good view of the horizon, you usually don't feel so bad. Once your balance system learns to properly anticipate the motion of the boat--in other words, when you get your "sea legs"—your susceptibility to seasickness disappears. Of course, when you finally go ashore, you can often feel your own body anticipating the movement of the boat a few hours or even days after hitting dry land. This can make some people feel sick, even though they are standing still.
Finally, sensory conflict can also be produced by space flight. After a lifetime of living with the effects of Earth's gravity on our bodies, suddenly going into weightlessness creates potential for many different types of sensory conflict. When astronauts tilt their heads, signals from the inner ear angular motion sensors (the semicircular canals) and body joints confirming the tilt are no longer corroborated by signals from the inner ear otolith organs. The tiny stones in the otoliths can no longer ascertain "down," because there is no longer any physical "down." Some crew members say they feel continuously "upside down," no matter how their bodies are oriented in the spacecraft. The confusion makes 75 percent of astronauts experience motion sickness on their first flight.
An interesting, though as yet unproven, theory as to why sensory conflict--particularly when it involves the workings of the inner ear--causes sickness is that the vestibular system is very sensitive to small changes in the body environment. This extreme sensitivity might provide an early warning system for poisoning. According to this hypothesis, when the vestibular system starts to disagree with the other sources of orientation information, nausea and vomiting follow, thereby removing the cause of disturbance.