Scientists don't have all of the answers when it comes to motion sickness. What they widely believe is that the ailment strikes when there's a discrepancy between your body's motion sensors.
There are three different parts of the body that sense motion and relay that information to your brain: your eyes (they see motion), your inner ear (it senses motion, acceleration and gravity) and the sensory receptors in your skin, muscles and joints (they determine if you're moving your muscles and which parts of your body are touching the ground). If there isn't harmony between these sensors, motion sickness occurs [sources: WebMD, KidsHealth].
So how can these sensors get scrambled? Let's say you're on a boat. Your inner ear senses the way the waves are gently rocking it, but your eyes aren't detecting any movement. This can cause motion sickness. Or perhaps you're watching the IMAX movie "Avatar." Your eyes are seeing motion as the movie makes it appear you're flying over the great land of Pandora. But your inner ear and sensory receptors are telling your brain you're sitting still. This causes confusion and can upset your body. Typical motion sickness symptoms include nausea, headache, dizziness, pale skin, cold sweats, increased salivation, fatigue and vomiting [source: University of Maryland Medical Center].
While scientists are pretty confident these sensory discrepancies cause motion sickness, they don't know why. The theory that our senses being at odds with one another is what causes these physical symptoms is mainly conjecture.
Some people are definitely more sensitive to motion than others. Studies indicate your heightened sensitivity could be genetic. Scientists also know motion sickness can only occur if we have our inner ear, the organ that senses motion. And while vision is often a major component in whether or not someone suffers from motion sickness, blind people can become motion-sick, so sight is not a critical factor [source: Kraft].
Another issue of study in the field of motion sickness is our stomachs. It appears reasonable that conflicting motion sensors can cause us to become dizzy. But why should that conflict cause us to feel nauseated and puke? One theory is that upchucking is tied into an evolutionary means humans developed to protect themselves from being poisoned. Some toxins that they consumed in the past attacked the inner ear. So, the theory goes, if our inner ear is feeling discombobulated, our bodies may have evolved to fear this means we ate something poisonous, so its built-in response is to make us throw up to get it out of our systems [source: Beck].
Besides the "senses out of harmony" theory, the other main idea as to what causes motion sickness is the "sway theory," which we'll look at on the next page.