Monovision Contact Lenses Raise Risk for Traffic Accidents

Laird Harrison

May 22, 2020

Monovision contact lenses, a common prescription, corrects each eye for a different distance, causing one image to be blurrier than the other and prompting some to misjudge the distance of a moving object, researchers report.

A typical monovision prescription could cause users to overestimate the distance of an object moving at 15 miles per hour by about 3 meters, warned Victor Rodríguez-Lopez, from the Institute of Optics–Spanish National Research Council in Madrid and the University of Pennsylvania in Philadelphia. "It can be dangerous," he told Medscape Medical News.

But the illusion can be eliminated with a dark filter over one eye, he reported during his virtual presentation at the Association for Research in Vision and Ophthalmology 2020 Annual Meeting.

Researchers have long known that monovision can compromise a person's depth perception of an object at rest, but this effect is rarely considered a safety issue.

However, it becomes more significant when the object is moving, said Rodríguez-Lopez. He and his colleagues refer to their monovision discovery as "the reverse Pulfrich effect," because the illusion created is opposite of the Pulfrich psychophysical percept. They theorize that the effect occurs because the brain processes blurry images more quickly.

In 1922, German physicist Carl Pulfrich discovered that a dark filter placed over one eye can result in a misperception of the distance of a moving object. This happens because the eye with the filter processes the image more slowly than the eye without the filter. If the left eye is dimmed and an object is moving from left to right, it will appear closer than it really is, and vice versa. The phenomenon can be used to create the appearance of three dimensions in movies.

In a recent study, investigators examined the case of a motorist whose left eye is corrected for near vision — and therefore blurry at a distance — looking at a bicyclist crossing her path from left to right at a distance of 8 meters and speed of 15 miles per hour. The motorist would perceive the bicyclist as being 8 meters away when, in fact, the bicyclist is only 5 meters away, which could result in a collision.

But they demonstrated that corrective trial lenses can cancel out the reverse Pulfrich effect.

We can cancel this reverse Pulfrich effect by dimming the eye that is blurred with contact lenses.

Now, they have examined whether the effect they measured with trial lenses also applies to contact lenses, and whether it might be caused by magnification in a study funded by the National Eye Institute the Office of Social and Behavioral Science at the National Institutes of Health, the Spanish Government, and La Caixa Foundation.

The team used passive circular polarizers to measure the Pulfrich effect in four people with myopia by changing the onscreen interocular delay of a moving target: a white bar on a 3D television at a viewing distance of 2 meters. The polarizers covering one eye caused the bar to appear to be in front of the screen when moving one direction, and behind the screen when moving the other direction.

The white bar oscillated from left to right (or right to left) while the observer fixated on a central dot. The researchers manipulated the onscreen interocular delay of the target images. When the onscreen delay was zero, the bar appeared to move in the plane of the screen. If the onscreen delay was greater than zero, the target appeared to move on an elliptical trajectory outside the plane of the screen.

The researchers asked the patients to indicate whether the target was moving right or left when it appeared to be in front of the screen. They had to choose either right or left; they couldn't choose "neither." By changing the onscreen delay and averaging the subjects' responses to the "left or right" question, the researchers could calculate the change in neural processing speed.

They tested the reverse Pulfrich effect using contact lenses with interocular blur differences that ranged from 0 to 1.50 D in both eyes, replicating their trial-lens study. They measured an increased processing speed in the blurred eye of 1.9 ms with contact lenses and 2.1 ms with trial lenses.

Next, they measured the classic Pulfrich effect with interocular luminance differences onscreen of ±0.15 optical density, and found that the dimmed eye processed the images 1.6 ms slower.

Then they tried dimming the blurry eye, and found that the perturbed eye processed the images just 0.1 ms slower. In effect, this confirmed that dimming the blurry eye could cancel the reverse Pulfrich effect.

Finally, they tried onscreen magnification differences of ±3.6%. The processing speed difference was 0.0 ms, so they concluded that magnification was not an important factor in the phenomenon.

"We can cancel this reverse Pulfrich effect by dimming the eye that is blurred with contact lenses," said Rodríguez-Lopez.

"The next step is to clearly test this in more populations, to see the prevalence and so on, and also to test with real images," he explained. "We have tested this with lab stimuli, but we would like to see what happens with real stimuli."

In the meantime, he suggested that ophthalmologists and optometrists tell their patients about the reverse Pulfrich effect when prescribing monovision lenses.

"I would say this is potentially important," said Stephen Macknik, PhD, from SUNY Downstate Health Sciences University in Brooklyn, New York. "This is a really good preliminary study, and it makes me think it's worth doing a bigger study."

Research needs to be done with more diverse subjects, not just with people with myopia, and in a variety of lighting conditions, he told Medscape Medical News.

But he wonders if dimming one eye is a practical solution to the reverse Pulfrich effect. Dimming the eye corrected for near vision might work well for activities that require distance vision, such as driving, but what about other activities?

"If you have a person who is using monovision lenses and they spend most of their day reading a book, maybe that's going to cause problems," Macknik said. And "I think there are potential plasticity issues, where one eye has impoverished vision. You can get side effects from that."

Association for Research in Vision and Ophthalmology (ARVO) 2020 Annual Meeting.

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