When they stop reading and get up to do something else, they might suddenly lose vision in the eye they’ve been using to read their smartphone, said Plant, senior author of a paper on the phenomenon.
Luckily, it’s a temporary condition that lasts for several minutes, with no risk of permanent damage, he added.
Plant said he wrote the paper because these people might think they’ve suffered a stroke or some other medical emergency.
“I have seen a dozen or so similar cases,” Plant said. “The reason I wish to make this known is because it leads to anxiety and unnecessary investigation because the patients — and their doctors — think they have had a transient ischemic attack.” A TIA is a temporary loss of blood circulation in the brain that can serve as a warning sign for stroke.
The optical trick results from the eye’s ability to adapt to dark conditions, Plant said. It’s similar to how your vision may become dim when you move from a very bright space to a very dark location.
“The patients are looking at their phone in the dark lying on their side,” he said. “If on their left side, the left eye is occluded by the pillow and they are viewing the phone with the right eye. The left eye is adapted to the dark and the right eye is adapted to the light.”
When they switch off the phone, Plant said, they can’t see with the light-adapted eye in the dark, as it takes several minutes to adjust to the dark. However, they can see with the dark-adapted left eye, and so they think that they have lost vision in the right eye, he explained.