The 21-year-old systems administrator in Philadelphia isn't talking about an allergic reaction or the aftermath of a wild night of drinking. Nope, this is his thanks for creeping around abandoned missile silos and evading ruthless assassins in Half-Life's Black Mesa Federal Research Facility or emerging victorious from a death match in Unreal Tournament. These fast-action games make him sick. Literally.

Paul Tomblin, 39, faces similar problems. A programmer and small-plane pilot, Tomblin overcame the motion sickness that plagued him during flight training, but when he's careening through the virtual terrain of games like Half-Life and Quake III, he still gets nauseous. "Sometimes, after a long session of playing, I have to go lie down -- I definitely feel like I'm going to throw up," he says. "I also get bad headaches and a feeling like my eyeball muscles are tired."

These two men are afflicted with "simulator sickness." Once used to describe the motion sickness that many astronauts experience in flight simulators, the term is now commonly used by gamers whose equilibrium is upset by the realistic graphics in 3-D computer games or other virtual reality environments. The U.S. military has conducted extensive research into "sim sickness" since the 1950s, finding that 60 percent of military pilots using a flight simulator suffered at least one symptom. But as yet, there are no similar figures quantifying the number of affected gamers, and scant formal research about sim sickness and gaming.

The topic does pop up periodically on gaming sites and message boards like Slashdot.

Yet, oddly, sim sickness is rarely acknowledged by game makers, much less addressed in gaming technology. As Steve Polge, a programmer for Epic Games, which made the Unreal titles, admits: "I don't think it's a focus of game development [throughout the industry]. When play testers complain of simulator sickness, we try to address it if we understand how to alleviate the problem, but it's not an issue we spend a lot of time on."

Maybe they should. It "really is a problem," says Henry Duh, a research associate at the University of Washington's Human Interface Technology Lab (HIT Lab) and one of the few researchers looking into the effects of sim sickness on gamers. Most of the lab's work focuses on virtual reality environments that involve the user wearing a head-mounted display, but the HIT Lab is also studying computer technology that creates realistic-looking, 3-D graphic worlds -- similar to what most of the latest video games do.

Duh's group is looking into the psychological and physiological effects that people experience while being in a virtual 3-D environment. "Currently, we are doing research to answer the following questions: How do people get sick in a virtual environment; what psychological mechanism is behind simulator sickness; and how [can we] alleviate it," he says.

There are several theories about what causes sim sickness. "The well-known and widely accepted one is the 'sensory conflict theory,'" Duh says. "In the simulated 3-D environment, you receive visual information telling you that you are moving. On the other hand, you receive information from your vestibular system [in your inner ear] telling you that you are stationary." The mixed signals confuse your mind, which can't decide if you're really moving or not, and quickly make you dizzy.

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