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The new software also produces the impression of zooming in and out when a user switches between applications, which may confuse the brain into thinking that the person is moving.
"Visually, the input is indicating that the person is moving, but all the other senses indicate the person is not moving — or, even worse, with these phones, is moving in a different way," Bonato said.
Such conflicting information can cause dizziness, headaches and nausea — effects that can also sometimes be caused by IMAX movie theaters and flight simulators. Although phone screens are much smaller than movie-theater screens, phone screens are placed closer to the eyes, which means the visual input dominates the brain, experts said.
"With the iPad, it's even worse, because it's larger and is covering more of your field of view," said Andrea Bubka, who researches cybersickness at Saint Peter's University in Jersey City, N.J.
Some users may also experience eyestrain due to the parallax effect.
"It looks three-dimensional, but it's actually two-dimensional. When that happens, your eye doesn't know exactly where to focus," Bonato said.
Higher-resolution and sharper images in iOS 7 may make the visual input look less like it is coming from a display, and more like a piece of the real world.
"I'm not surprised to hear this about iOS 7," said Thomas Stoffregen, a professor of kinesiology who studies motion sickness at the University of Minnesota. "As imaging technology develops across platforms, and we get greater frame rates and resolution, we find an increasing tendency for it to make people sick."
The iOS 7 software comes pre-installed on all new iPhone 5c and 5s models. For people with earlier-version devices who upgraded to iOS 7, going back to iOS 6 is not an option. This has prompted users to ask why this problem was not detected in testing.
"It hurts my eyes and makes me dizzy — so annoying that we can't downgrade!!!!" one user said.
"I can't believe someone, when testing iOS 7 didn't say, 'Hey, this animation makes me nauseous.' I find that incredible," another said.
One solution that may reduce some of the motion on the screen is to go to Settings, then General and then Accessibility. There, users can turn on Reduce Motion.
Although the effects may make symptoms worse for people who have disorders of the inner ear that affect their movement and balance, for most people, this is not going to be a big issue, Bubka said.
"There are little tricks you can do to adapt. You just have to look elsewhere for a little while," Bubka said.
--By Bahar Gholipour, LiveScience
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