Instead of bifocals, some try monovision. But depth perception can suffer, Penn scientist says.

While bifocal eyeglasses allow near and far vision in both eyes, some people opt to split the difference — correcting one eye with a contact lens to sharpen the focus on far-away objects, the other eye with a lens to see things close up.

Called monovision, this way of seeing the world is not for everyone, in part because it can interfere with depth perception.

It turns out that when the object is moving, the impact on depth perception is much worse than is commonly appreciated, according to a new study led by a University of Pennsylvania scientist.

The reason for this optical illusion takes a bit of explanation, but it is significant enough that it could pose a public safety hazard, said neuroscientist Johannes Burge, an assistant professor in Penn’s psychology department.

Picture a motorist who slows down at an intersection because a cyclist is riding from left to right in cross traffic. If the driver is wearing this type of lenses and the bike is traveling 15 miles per hour, it will seem to be as much as 9 feet farther away from the car than it actually is, Burge and his colleagues reported in Current Biology.

That illusion could lead the motorist to engage in what the study authors termed “casual braking” rather than coming to a full stop, hitting the cyclist as a result. What’s more, the illusion becomes even greater when the bike (or other vehicle in cross traffic) is traveling at higher speeds.

“This illusion is new to science,” Burge said. “It has the potential to cause a pretty serious public safety issue.”

The findings were based on hours of measurements made with a haploscope, a finely calibrated contraption in Burge’s lab that consists of eyepieces, mirrors, and computer screens. He was joined in the research by two scientists from the Institute of Optics in Madrid, Carlos Dorronsoro and graduate student Victor Rodríguez-López.

An estimated 10 million people in the United States have their eyesight corrected to achieve monovision, typically with contacts or lens implants that are inserted during cataract surgery. Presidents Ford, Carter, and Reagan all did so.

Monovision lenses have long been known to cause a slight decline in depth perception because, as the name suggests, they compromise the person’s ability to see in stereo. That’s because a given object appears sharp in one eye and blurry in the other, so the brain suppresses the blurry image to some degree. The viewer is therefore less able to “triangulate,” and as a result is less precise at telling how far away an object is.

The phenomenon described in the study occurs on top of that loss of precision, but only when the object is in motion. To a person with such lenses, a sideways-moving object appears significantly nearer or farther than it actually is, depending on whether it is traveling left to right or right to left.

So a pendulum in a grandfather clock, for example, will appear to be traveling along the path of an oval — weirdly seeming to swing forward and backward as it moves from side to side.

The reason for this illusion is far from intuitive, but it has to do with the fact that the brain can process blurry images several milliseconds faster than it handles sharply focused images, the study authors said.

Wills Eye Hospital surgeon Robert S. Bailey Jr., who was not involved with the research, agreed that it was a novel contribution to the field of vision science.

He said the type of illusion the researchers identified may be fairly subtle, as none of his patients with monovision lenses had ever mentioned it. Instead, some say it is hard to get used to having objects appear blurry in one eye and focused in the other, said Bailey, chief of the cataract and primary eye care service at Wills Eye.

“It can be hard for the brain to adjust to the fact that you’ve got two different focal points,” he said. “Most people can get used to it."

Andi Coyle, a resident of Philadelphia’s Bella Vista neighborhood who had monovision lenses implanted during cataract surgery, likewise said she noticed no loss in depth perception. If there is any impact, she said, it is worth it because she does not have to worry about eyeglasses.

“I think it depends upon the individual and how well you can or can’t adapt to it,” she said. “It is so incredibly convenient not having to put your glasses on and off.”

The impact on depth perception is nevertheless unmistakable in the controlled setting of Burge’s lab.

Rodríguez-López, who is conducting research in the lab while visiting from Spain, set up a computer program one day in July to display a white bar on a screen. To the naked eye, the bar appeared to be moving from side to side. But when seen through monovision lenses, it seemed to emerge and recede as it traveled sideways.

Monovision’s impact on depth perception has been blamed in at least one serious accident. In October 1996, a Delta flight struck a light tower on approach to LaGuardia Airport, in part because the pilot was wearing that type of contacts, federal investigators found.

Was the problem that the lenses made his depth perception less precise, as ophthalmologists at the time were aware? Or was it the result of the added optical illusion reported in July by Burge and his colleagues? Hard to say.

If it was the latter, Burge has an idea to prevent any future such accidents.

Previous research has shown that the brain processes dark images more slowly than light images. So if one of the lenses used to achieve monovision were tinted by just the right amount, that would cancel out the brain’s faster processing of blurry images, Burge said. The optical illusion would disappear.

He and his colleagues already have demonstrated that this concept works in the lab, and have filed for a provisional patent.

But the pure science of discovering the illusion was enough to delight the Penn researcher. Since the optical illusion is caused when the brain processes the image from one eye a few milliseconds slower than the other, that suggests that under ordinary conditions, the brain processes the two images simultaneously.

“The fact that we typically see things quite accurately is evidence that under normal circumstances, they’re calibrated to within less than a millisecond,” he said. “That I find kind of wonderful.”