Virtual reality, long the stuff of sci-fi movies and expensive, disappointing gaming systems, appears poised for a breakout. Facebook CEO Mark Zuckerberg spent $2 billion in 2014 to acquire Oculus VR and its Rift virtual-reality headsets. Google now sells a boxy cardboard viewer that lets users turn their smartphone screens into virtual- reality wonderlands for a mere $15. And YouTube just introduced live, 360-degree streaming video.

There's a big barrier to the widespread use of this technology, though: Virtual reality often makes people sick.

Virtual-reality sickness isn't a new problem. It's been known as long as test pilots, test drivers and potential astronauts have been practicing their skills in mock vehicles, though it was called simulator sickness in those cases. Not unlike motion sickness or seasickness, VR sickness has its roots in the mismatch between the visual and vestibular systems, said Jorge Serrador, a professor of pharmacology, physiology and neuroscience at Rutgers New Jersey Medical School.

(Our sister site, Tom's Guide, put together a great primer on all of the VR headsets on the market right now: The Best VR Headsets)

How VR sickness works

Imagine standing below decks in a boat on choppy seas. The entire cabin is moving, so your eyes tell you you're standing still. But you feel the movement — up, down, pitching side to side. You start to feel clammy. Your head aches. You go pale and reach for a trash basket to retch into.

The problem starts in the vestibular system, a series of fluid-filled canals and chambers in the inner ear. This system includes three semicircular canals, all lined with hair cells, so named for their hair-like projections into the liquid-filled channels. As the head moves, so too does the fluid in the canals, which in turn stimulates the hair cells. Because each canal is situated differently, each sends information on a different type of motion to the brain: up/down, side to side and degree of tilt.

Connected to the semicircular canals is the utricle, a sac containing fluid and tiny calcium carbonate particles called otoliths. When the head moves, so too do the otoliths, sending the brain signals about horizontal movement. Next door, a chamber called the saccule uses a similar setup to detect vertical acceleration.

This system typically works in tandem with the visual system and with the proprioceptive system, integrating sight and sensations from the muscles and joints to tell the brain where the body is in space. A virtual-reality environment hammers a wedge between these systems.

Simulator sickness

Unlike seasickness or car sickness, virtual-reality sickness doesn't require motion at all. It was first reported in 1957 in a helicopter-training simulator, according to a 1995 U.S. Army Research Institute report on the topic. One 1989 study found that as many as 40 percent of military pilots experienced some sickness during simulator training — an alarming number, according to the Army report, because military pilots are probably less likely than the general population to have problems with "motion" sickness.

Because of simulator sickness, early simulator developers started to add motion to their models, creating plane simulators that actually pitched, rolled and moved up and down a bit. But sickness still occurs, according to the Army report, because the computer visualization and the simulator motion might not line up completely. Small lags between simulator visuals and motion remain a problem today, Serrador said.

"You go into a simulator and [the movements] don't match exactly the same as they do in the real world," he said. "And all the sudden, what you'll find is you just don't feel right."

Typically, the bigger the mismatch, the worse the sickness. In one 2003 study published in the journal Neuroscience Letters, Japanese researchers put people in a virtual-reality simulator and had them turn and move their heads. In some conditions, the VR screen would turn and twist twice as much as the person's actual head movement. Unsurprisingly, the people in those conditions reported feeling a lot sicker than those in conditions where the movement and the visual cues matched up.

Combating the nauseating effects of VR

No one really knows why vestibular and visual mismatches lead to feelings of nausea. One theory dating back to 1977 suggests that the body mistakes the confusion over the conflicting signals as a sign that it's ingested something toxic (since toxins can cause neurological confusion). To be on the safe side, it throws up. But there's little direct evidence for this theory.

People have different levels of susceptibility to virtual-reality sickness, and they can also adapt to situations that initially turn them green around the gills. The Navy, for example, uses a swivel chair called the Barany chair to desensitize pilots to motion sickness. Over time, the brain figures out which cues to pay attention to and which to ignore, Serrador said. At some point, even the act of putting on a virtual reality headset will trigger the brain to go into a sort of virtual-reality mode, he said.

"There's lots and lots of data that show that your brain will use the context cues around it to prepare itself," Serrador said.

Virtual-reality developers are working to combat the nauseating side effects of their products. Oculus Rift, for example, boasts a souped-up refresh rate that helps prevent visual lags as the user navigates the virtual world. And Purdue University researchers invented a surprisingly simple fix: They stuck a cartoon nose (which they call the "nasum virtualis") in the visual display of a virtual-reality game. Their results, presented in March 2015 at the Game Developers Conference in San Francisco, showed that this fixed point helped people cope with virtual-reality sickness. In a slow-paced game in which players explored a Tuscan villa, the nose enabled users to go 94.2 seconds longer, on average, without feeling sick. People lasted 2 seconds longer in an almost intolerably nauseating roller-coaster game. The nose seems to give the brain a reference point to hang on to, said study researcher David Whittinghill, a professor of computer graphics technology at Purdue.

"Our suspicion is that you have this stable object that your body is accustomed to tuning out, but it's still there and your sensory system knows it," Whittinghill said in a statement.

Still Interested in VR?

Out sister-site, Tom's Hardware, has a great primer on how virtual reality has evolved since the 1950s and Wired just published an amazing article on the science and future of virtual reality. Lastly, if you're in the market for VR, check out Tom's Guide's virtual reality headset recommendations.

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