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Nothing can ruin a good movie faster than the sudden feeling that you're about to throw up. But for many people, the images in 3-D or IMAX movies look so real that they mess up the brain's ability to sort out the signals coming in from the senses, and trigger that queasy feeling.

Researchers who study this type of nausea call it cybersickness.

Dizziness, headaches and nausea happen while watching 3-D or IMAX movies because the brain receives conflicting information from the senses, said professor Andrea Bubka, who researches cybersickness at St. Peter's College in Jersey City, N.J.

When vivid images play out on-screen, the eyes send signals that tell the brain the body is in motion. Yet inside the inner part of the ear, where the movement of fluid is used to sense motion and balance, no change in the body's position is detected. The eyes tell the brain the body is moving, but the ears say it's not, and this is a recipe for nausea, Bubka told Life's Little Mysteries.

This is the opposite of what happens during motion sickness. For example, when reading a book while riding in a car, the eyes are focused on the page and do not sense that the body is moving. But the fluid within the ears senses the movements of the car, and the difference between the information coming in from these two senses causes the brain to trigger nausea and dizziness, Bubka said.

Vomitator tests

In order to better understand why 3-D movies so easily trigger nausea, Bubka and her colleague Frederick Bonato developed a series of tests in their lab. In a device called the optokinetic drum, which has been nicknamed the Vomitator, subjects are seated in the center a large cylinder that is open on the top and bottom. The drum spins, and images on the inside of the drum move across the subject's visual field while the subject remains still.

Sooner or later, everyone who has been in the drum feels sick.

"Within about 20 seconds, they feel like they're moving in the opposite direction," from the way the drum is spinning, Bonato explained. And after a few minutes, people start to feel nauseated.

The key finding of this research, Bonato said, is that the more complex the images inside the drum, the faster people start to feel sick. Subjects get sick about 75 percent faster when they watch a black and white checkerboard pattern whirl around them inside the drum than they do when they watch simple black and white lines. And colored lines also make people sick faster than black and white lines.

"It's a very big effect," Bonato said. "More complex pictures make people sick faster, and their symptoms are worse."

And it happens to almost everyone, Bonato said.  Other studies have shown that people with birth defects in their inner ears have absolutely no capacity to feel motion sickness, and it is likely that only those people are completely immune to cybersickness as well. They can't sense motion, so there is no conflict between the senses, Bonato said.

Like poison

The reason for this universal experience may be rooted in the body's response to the feeling of being poisoned. The conflict between the two senses mimics the effect of some poisons, and the brain is evolutionarily programmed to initiate behaviors that will rid the body of the poison. Upchucking your popcorn is a way to do just that.

More complex images – such as the virtual world of planet Pandora seen in "Avatar" or the surreal experience of "Alice in Wonderland" – can prompt the brain to react as if the body has received a megadose of poison.

Hormones and genetics may both play a role in the severity of the sickness, according to Bonato, who found that women seem to be more susceptible than men, and that different ethnic groups vary in their responses.

Although the research has led to a better understanding of why movies can make us nauseated, the researchers' goal is to develop ways to help people who work in environments where sensory conflicts are a constant problem. About 70 percent of astronauts get sick in space, and although they cannot simulate weightless conditions in their lab, Bubka and Bonato have developed a virtual reality program that lets users don a visor and enter a zero-gravity room. They are testing ways to help people adapt their brains to incoming information so that sickness is less of a problem.