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See Inside April 2009

Finding Balance: A Novel Theory on Seasickness

Is poor posture control the real cause of motion sickness?

To avoid a potentially nasty cleanup, students are deprived of food for four hours before entering the experimental chamber. Inside, they step onto a force-sensing platform and stare at a paper map of the U.S. The walls begin sliding back and forth on a track a mere 1.8 centimeters with each cycle. At the right frequencies, this movement triggers a tugging sensation that begins somewhere in the brain and mysteriously travels to the belly. But before the full effects of motion sickness set in, the subjects will typically turn away and beg for mercy.

At that point, the students may be thinking that the course credit they will receive for participating in Thomas Stoffregen's sensory funhouse may not be worth it. But for the University of Minnesota psychologist, every student he lures inside is another data point that he believes will overturn the dogma about the cause of motion sickness. If he is right, the findings could lead to new ways of identifying people susceptible to motion sickness before they get sick and may provide designers of simulators and video games with ways to keep controllers in the hands of potentially woozy players.

For the past century, scientists have believed that motion sickness derives from a conflict among our senses. Our inner ears contain sensors for both angular motion (the semicircular canals) and linear motion (the otoliths). When these sensors disagree with the information we expect to receive from our eyes and muscles, motion sickness manifests itself. And yet, Stoffregen says, our senses constantly provide different channels of information: redundancy is an essential part of the sensory system, and our brains do not compare our senses in any direct manner. Moreover, because it is impossible to determine which of the conflicting senses is interpreted as being "wrong" by an individual's brain, Stoffregen has branded the conflict theory with the highest-order insult a scientist can muster: unfalsifiable.

Indeed, researchers have long wondered why some individuals and certain classes of people children and pregnant women are more susceptible to motion sickness than others. What is more, experiments conducted since the dawn of the Space Age, when NASA wanted to prevent its astronauts from falling ill, can predict who will succumb to motion sickness with only about 30 percent accuracy. Finally, Stoffregen notes the puzzling observation that, whereas people may feel sick on the deck of boat, they rarely get the urge to vomit when fully immersed in water.

Stoffregen instead argues that motion sickness comes from the brain's persistent inability to modulate the body's movements in a challenging environment. Postural instability the inability to maintain balance was considered a symptom of motion sickness. Not so, Stoffregen says. Although postural control relies on sensory feedback, motion sickness is really a sign that the motor-control system is going haywire.

His alternative theory, first published in 1991, landed with a dull thud, and his papers garner just a handful of citations each year. But experts have been muttering privately about Stoffregen ever since and lately some have grudgingly begun to accept him. "It was a very, very different theory," says Larry Hettinger, a longtime motion sickness researcher now at defense contractor Northrop Grumman. "I clearly remember people thinking, 'This is nonsense, this is crazy foolishness.'"

The growing acceptance of Stoffregen's view has much to do with experiments conducted over the past two decades. Inside the moving room, he has found that volunteers can significantly reduce motion sickness simply by widening their stance an observation, he says, not predicted by sensory conflict theory. Students standing with their feet five centimeters apart tend to get motion sickness about 60 percent of the time. Spreading their legs to 30 centimeters increases the stability of the head and torso and decreases the incidence of motion sickness to about 20 percent. Stoffregen says that by monitoring body sway, he can predict the onset of motion sickness with 60 percent accuracy. If swaying is just a symptom of motion sickness, it would be detectable only after participants reported feeling sick.

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