Abstract
Monovision corrections are a popular treatment for presbyopia. Each eye is fit with a lens that sharply focuses light from a different distance, causing differential blur in the two eyes’ images. Approximately 12.5 million people in the United States have a monovision correction, but little is known about how differential blur affects motion perception. Accurate motion perception is critical for many daily tasks. We investigated by measuring the Pulfrich effect, a stereo-motion phenomenon first reported nearly 100 years ago. When a target oscillating in the frontoparallel plane is viewed with unequal retinal illuminance or contrast in the two eyes, the target appears to follow an elliptical trajectory in depth. 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? To investigate, we used trial lenses to induce interocular blur differences up to 1.0D, and a haploscope for dichoptic presentation of targets undergoing sinusoidal motion. Then, as a function of onscreen interocular delay, we measured how frequently human observers perceived ‘front right’ motion. Remarkably, the results show that 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 i) that blur reduces the contrast of high-frequency image components more than low-frequency image components, and ii) that high spatial frequencies are processed more slowly than low spatial frequencies, all else equal. Thus, this new version of a 100-year-old illusion is explained by known properties of the early visual system. Implications for the spatial frequency binding problem and for public safety will be discussed.
Acknowledgement: NIH: R01-EY028571