The lenses in human eyes lose some ability to focus as they age. Monovision—a popular fix for this issue—involves prescription contacts (or glasses) that focus one eye for near-vision tasks such as reading and the other for far-vision tasks such as driving. About 10 million people in the U.S. currently use this form of correction, but a new study finds it may cause a potentially dangerous optical illusion.

Nearly a century ago German physicist Carl Pulfrich described a visual phenomenon now known as the Pulfrich effect: When one eye sees either a darker or a lower-contrast image than the other, an object moving side to side (such as a pendulum) appears to travel in a three-dimensional arc. This is because the brain processes the darker or lower-contrast image more slowly than the lighter or higher-contrast one, creating a lag the brain perceives as 3-D motion.

Johannes Burge, a psychologist at the University of Pennsylvania, and his colleagues recently found that monovision can cause a reverse Pulfrich effect. They had participants look through a device showing a different image to each eye—one blurry and one in focus—of an object moving side to side. The researchers found that viewers processed the blurrier image a couple of milliseconds faster than the sharper one, making the object seem to arc in front of the display screen. It appeared closer to the viewer as it moved to the right (if the left eye saw the blurry image) or to the left (if the right eye did). “That does not sound like a very big deal,” Burge says, but it is enough for a driver at an intersection to misjudge the location of a moving cyclist by about the width of a narrow street lane (graphic).

Credit: Brown Bird Design

Burge and his colleagues had expected the opposite: that the brain would process the blurry image more slowly because of its lower contrast, similar to the traditional Pulfrich effect. They resolved this paradox by showing that blur reduces the contrast of fine details more than that of coarse ones. Because the brain takes more time to process fine details, the blurry image is processed faster. The researchers published their study in August in Current Biology.

Douglas Lanska, a retired University of Wisconsin neurologist who has studied the Pulfrich effect and was not involved in the study, calls the findings “intriguing” but says, “My guess is that the modeling overestimated the real-world impact some.” The reverse Pulfrich effect should be tested outside the laboratory, Lanska adds.

Burge and his team found they could correct the effect by tinting the blurrier lens, creating a classic Pulfrich effect that cancels out the reverse one. The brain may also compensate for the limitations of monovision—but further study is needed, Burge says. These misperceptions are rare, he notes, suggesting that “under normal circumstances, our visual systems are exquisitely well calibrated.”