The cause is not acute sensitivity to Helvetica or skeuomorphic withdrawal. Rather it's motion sickness. A small number of users are complaining that the background parallax effect -- which creates the illusion of depth in device screens -- and the app transition animations are making them feel ill.
The complaints, which appear to number over a hundred across various Apple support forum threads, run something like this: "I've experienced nausea and motion sickness and headaches since last week when I installed iOS 7, all due to the apps zooming in and out animation. I had zero problems with iOS 6."
[ On the fence about upgrading? Read 10 iOS 7 Features You'll Want. ]
Apple did not respond to a request for comment, but the company does offer a way to tame the iOS 7 parallax effect. In the Settings > General > Accessibility menu, there's a Reduce Motion option that does just what its name suggests, though it does not alter the app transition animation.
Eric Muth, professor of psychology at Clemson's School of Business and Behavioral Science, has conducted research on technologically-induced motion sickness arising from use of head-mounted displays and expressed skepticism at the suggestion that Apple's iOS 7 deserves blame for the issues being reported.
"The idea that a device with a small field of view like an iPhone is making people sick is probably a stretch," Muth said in a phone interview. "It's very unlikely you'll get any sense of motion, though that's what [iOS 7] is trying to do."
Muth confirmed that video games can contribute to motion sickness. He recounted an experiment in which he had people play Xbox games while in the back of a moving car with blacked-out windows. Doing so, he said, "was much more sickening than when playing while stationary."
If people are feeling ill after using iOS 7, Muth suggests that the animation effects might only be one factor among other more significant ones, like using an iPhone in a moving vehicle or while walking. "The person may blame the device, but they've just layered on another challenge to their sensory systems," he said.
Shun-nan Yang, director of research at the Vision Performance Institute, College of Optometry, Pacific University, said in a phone interview that although he would need to perform tests to confirm the role of iOS 7 in motion sickness, he allowed that it is possible the software could contribute to symptoms, particularly for people predisposed to motion sickness.
"There are situations where moving or flicking stimuli can heighten the symptoms [of motion sickness]," Yang said. "But it all depends on the nature of the stimuli."
High-contrast, flickering stimuli, Yang said, can induce a certain physiological response linked to our brain structure, although it's inherently short-term in nature. He pointed to a Pokemon cartoon broadcast in Japan in 1997 that displayed red and blue flickering lights for about 12 seconds, making hundreds of children feel sick.
"The potential danger is there but a TV screen is very different than a phone screen," Yang said. "The phone screen has a very small area, so your visual field will see a lot of other things."
Nevertheless, he added, this is a phenomenon companies should be aware of. "We don't want the user to experience any discomfort," Yang said.
The issue might have more to do with people's predispositions than with Apple's design choices. Within the same Apple forum, there are reports of nausea associated with iMacs from 2009 and MacBook Pros from 2006. Some individuals might simply be hypersensitive to motion sickness.