James Cameron sees Avatar as the beginning of a revolution in 3D movies, but not everyone will be able to join in. Some viewers are sickened, in some cases to the point of vomiting, by 3D viewing. Adults with vestibular problems—related to the nerves and canals inside the ear—can experience motion sickness while watching the film, explains MSNBC. About 40% of adults older than 40 have such problems to some degree.
But even those without a history of vestibular trouble, especially children, also can experience what's known as "sensory conflict." The illusion of depth provided by 3D images disrupts the brain's sense of orientation, tricking it "into thinking the body has been poisoned," says one researcher. And when the body thinks it's been poisoned, it tries to throw up.