The eyes lose their ability to focus up close, as people grows older. The condition called presbyopia, and it is both extremely common and relatively easy to fix; with solutions like reading glasses, bifocals or progressive lens. Another common correction, called mono-vision, solves the problem with different lenses in each eye; one that focuses nearby, the other that focuses far away.

In order to adjust for the blur differences caused by wearing different lenses; the brain suppresses the blurrier image and preferentially processes the sharper image, effectively enhancing the depth of field. Ten million people in the United States currently use mono-vision to correct their presbyopia; and as the population ages that number is likely to grow.

A team from the University of Pennsylvania and the Institute of Optics in Madrid; Spain, recently discovered that mono-vision can cause dramatic misperceptions of the distance and 3-D direction of moving objects. What is more, the farther away the objects are and the faster they move, the larger the misperceptions become.

Fogged-up lens

“Imagine you are riding in a car, pulling up to an intersection,” Burge says. “A cyclist in cross-traffic is going by at 15 miles per hour. If you calculate it out, the misperception of depth will be about nine feet. That’s a big deal that’s the width of a traffic lane.”In general, Burge’s lab studies how the human visual system processes the images that fall on the back of the eye when they are sitting in a room or walking down the street.

Burge is particularly interested in understanding what enables people to perceive motion, depth, and blur. “It seems really simple. We open our eyes and see,” he explains. “But as with most things, when you look under the hood to see how it actually works, it turns out to be a lot more complicated.” This new line of research is closely related to the Pulfrich effect, a 100-year-old perceptual illusion named for German physicist Carl Pulfrich. To understand this effect, picture a clock pendulum swinging from side to side.

Viewing the pendulum with one eye darkened, through a pair of sunglasses with one lens missing for example, the pendulum will not appear to move side to side, but rather in an elliptical trajectory that changes depths. The same effect occurs for images with different contrast, like looking through a pair of glasses with one fogged-up lens.

Neural binocular disparity

The illusion happens because the brain processes the darker (or less contrast-y) image milliseconds more slowly than the brighter (or more contrast-y) image. For moving images, the processing delay causes what’s called a “neural binocular disparity,” meaning the actual location of the image on the back of the eye does not match where the visual system estimates it to be. A similar principle explains how stereo-3-D movies work.

Brightness and contrast variation between the eyes cause the classic Pulfrich effect. Monovision induces blur differences between the eyes, and blur reduces contrast, so Burge and colleagues hypothesized that monovision would also lead to the Pulfrich effect. Using an apparatus called a haploscope essentially, a laboratory version of a 3-D movie house they measured the effect of different monovision-like corrections.

Surprisingly, participants experienced a reverse Pulfrich effect rather than a classic one. Instead of being processed more slowly, the blurry image was processed milliseconds faster than the sharp image.  “As the image goes out of focus, first you lose the pinstripes in my shirt and the hairs of my eyebrow. Then you lose the medium details. And finally, the coarse details,” he explains. “Neuroscience has shown that the brain processes fine details more slowly than coarse details, all else being equal.