Monovision and the Misperception of Motion

Monovision corrections are a common treatment for presbyopia. Each eye is fit with a lens that sharply focuses light from a different distance, causing the image in one eye to be blurrier than the other. Millions of people in the United States and Europe have monovision corrections, but little is known about how differential blur affects motion perception. We investigated by measuring the Pulfrich effect, a stereo-motion phenomenon first reported nearly 100 years ago. When a moving target is viewed with unequal retinal illuminance or contrast in the two eyes, the target appears to be closer or further in depth than it actually is, depending on its frontoparallel direction. The effect occurs because the image with lower illuminance or contrast is processed more slowly. The mismatch in processing speed causes a neural disparity, which results in the illusory motion in depth. What happens with differential blur? Remarkably, differential blur causes a reverse Pulfrich effect, an apparent paradox. Blur reduces contrast and should therefore cause processing delays. But the reverse Pulfrich effect implies that the blurry image is processed more quickly. The paradox is resolved by recognizing that: i) blur reduces the contrast of high-frequency image components more than low-frequency image components, and ii) high spatial frequencies are processed more slowly than low spatial frequencies, all else equal. Thus, this new illusion—the reverse Pulfrich effect—can be explained by known properties of the early visual system. A quantitative analysis shows that the associated misperceptions are large enough to impact public safety.