If you’re older than forty, chances are that reading texts or playing with your smart phone is now harder than it used to be. Such difficulty with near focusing is usually the result of presbyopia, the hardening of the lens of the eye that starts to take place in middle age. From eyeglasses to refractive surgery, many available solutions allow GenXers and baby boomers to read small print and conduct other near-vision tasks to their hearts’ content.

The problem is, one of the most prevalent treatments for presbyopia could make you less safe on the road.

Broadly, people suffering from presbyopia can opt for eyeglasses, contact lenses or surgery. Eyeglasses include reading glasses (used for close-up vision only), as well as glasses with bifocal, multifocal or progressive lenses (which are worn all day and allow vision at a range of distances). Contact lens correction can work just like with eyeglasses, but it also offers an alternative solution for presbyopia, called monovision. In monovision, one eye is corrected for close-up viewing, and the other eye for long-distance viewing. Thus, at any distance (near or far), at least one eye offers clear vision even when the image from the other eye is blurred. Eventually, the brain learns to suppress the blurred images and rely on the crisp images only, so people can enjoy clear vision at all distances. Finally, those with presbyopia can opt for refractive eye surgery, including monovision LASIK, which typically corrects the nondominant eye for near vision while leaving the dominant eye able to see long distance.   

Among baby boomers, monovision is the most popular contact lens correction for presbyopia, and monovision LASIK is also on the rise for eligible people over the age of 40. Yet, according to new research by Johannes Burge, Victor Rodriguez-Lopez, and Carlos Dorronsoro at the University of Pennsylvania, monovision corrections could present previously unidentified safety concerns, especially while driving. The reason is related to a century-old illusion called the Pulfrich effect.

Named after the German physicist Carl Pulfrich, the Pulfrich effect is a type of motion misperception that occurs when one eye receives a dimmer image than the other eye (for instance, via a tinted lens). Because dimmed images produce delayed neural signals, the brain’s visual system ends up with a several millisecond gap between the information arriving from one eye versus the other. As a result, objects moving towards the dimmed or delayed eye seem closer to the viewer than in reality. That is, if the right eye is dimmed, objects moving to the right will seem closer than objects moving to the left.

In laboratory experiments, Burge and his colleagues found that monovision conditions produce a reverse Pulfrich effect, where the brain processes the blurred image more quickly than the nonblurred image. This speeding up takes place because blurring eliminates the detailed visual information from an image (thereby reducing processing time). The perceptual consequence is a substantial mislocalization of moving objects, the team reported.

Credit: Johannes Burge and Current Biology

Though the researchers point out that it would be premature to abandon monovision based on their lab results, they recommend that monovision should be avoided while playing sports that require accurate perception of rapidly moving targets (such as tennis, baseball, and other ball sports).

The scientists also propose a few adjustments to make monovision safer on the road. Specifically, one should correct the right eye for far distances in countries where motorists drive on the right side of the road, such as in the US. The reason is that cars and cyclists approaching in the near lane of cross traffic move from left to right; therefore, “placing the far lens in the left eye will cause distance overestimation, which may result in casual braking and increase the likelihood of traffic accidents.” Conversely, in countries where motorists drive on the left side of the road, such as the UK, one should correct the left eye for far distances instead.