Monovision and the Misperception of Motion

Abstract

Monovision is a common prescription lens correction for presbyopia [1]. Each eye is corrected for a different distance, causing one image to be blurrier than the other. Millions of people have monovision corrections, but little is known about how interocular blur differences affect motion perception. Here, we report that blur differences cause a previously unknown motion illusion that makes people dramatically misperceive the distance and three-dimensional direction of moving objects. The effect occurs because the blurry and sharp images are processed at different speeds. For moving objects, the mismatch in processing speed causes a neural disparity, which results in the misperceptions. A variant of a 100-year-old stereo-motion phenomenon called the Pulfrich effect [2], the illusion poses an apparent paradox: blur reduces contrast, and contrast reductions are known to cause neural processing delays [3-6], but our results indicate that blurry images are processed milliseconds more quickly. We resolve the paradox with known properties of the early visual system, show that the misperceptions can be severe enough to impact public safety, and demonstrate that the misperceptions can be eliminated with novel combinations of non-invasive ophthalmic interventions. The fact that substantial perceptual errors are caused by millisecond differences in processing speed highlights the exquisite temporal calibration required for accurate perceptual estimation. The motion illusion-the reverse Pulfrich effect-and the paradigm we use to measure it should help reveal how optical and image properties impact temporal processing, an important but understudied issue in vision and visual neuroscience.

Keywords: adaptation; anti-Pulfrich correction; binding problem; defocus; disparity; monovision; motion-in-depth; spatial frequency.

Figure 1.. Classic and reverse Pulfrich effects.

A Classic Pulfrich effect. A left-eye neutral density filter causes horizontally oscillating frontoparallel motion to be misperceived in depth (i.e. ‘front left’; clockwise motion from above). The image in the eye with lower retinal illuminance (gray dot) is delayed relative to the other eye (white dot), causing a neural disparity. B Reverse Pulfrich effect. A left-eye blurring lens causes illusory motion in depth in the other direction (i.e. ‘front right’). The blurrier image (gray dot) is advanced relative to the other eye (white dot), causing a neural disparity with the opposite sign. C Neural image positions across time for the classic Pulfrich effect, no Pulfrich effect, and the reverse Pulfrich effect.

Figure 2.. Reverse, classic, and anti-Pulfrich conditions: Psychophysical data.

A Binocular stimulus. The target was a horizontally moving 0.25×1.0° white bar. Arrows show motion speed and direction, and dashed bars show bar positions during a trial; both are for illustrative purposes only and were not in the actual stimulus. Observers reported whether they saw three-dimensional (3D) target motion as ‘front right’ or ‘front left’ with respect to the screen. Fuse the two half-images to perceive the stimulus in 3D. Cross- and divergent-fusers will perceive the bar nearer and farther than the screen, respectively. B Points of subjective equality (PSEs) for one observer, expressed as onscreen interocular delay relative to baseline. Interocular differences in focus error (bottom axis, white circles) cause the reverse Pulfrich effect. Interocular differences in retinal illuminance (top axis, gray squares) cause the classic Pulfrich effect. Appropriately tinting the blurring lens (light gray circles) can eliminate the motion illusions and act as an anti-Pulfrich correction. (In the anti-Pulfrich conditions, optical density was different for each observer and focus difference.) Shaded regions indicate bootstrapped standard errors. Best-fit regression lines are also shown. C Psychometric functions for seven of the reverse Pulfrich conditions in B. Arrows indicate raw PSEs. See also Figure S1.

Figure 3.. Spatial frequency filtering: Psychophysical data.

A Original stimuli were composed of adjacent black-white (top) or white-black (bottom) 0.25°×1.00° bars. B High-pass or low-pass filtered stimuli (shown only for black-white bar stimuli). High- and low-pass filtered stimuli were designed to have identical luminance and contrast (see Figure S2). C Resulting interocular delays. High-pass filtered stimuli are processed slower, and low-pass filtered stimuli are processed faster than the original unfiltered stimulus. Negative cutoff frequencies indicate that the left eye was filtered (high- or low-pass). Positive cutoff frequencies indicate that the right eye was filtered. D Effect sizes for each human observer in multiple conditions, obtained from the best-fit regression lines (see Figures 2B,3C). Two manipulations resulted in reverse Pulfrich effects (white bars): blurring one eye (left) and low-pass filtering one eye (right). Two manipulations resulted in classic Pulfrich effects (gray bars): darkening one eye (left) and high-pass filtering one eye (right). A fifth manipulation—appropriately darkening the blurring lens (left, small light gray bars)—eliminates the Pulfrich effect and acts as an anti-Pulfrich correction.

Figure 4.. Monovision corrections and real-world misperceptions of depth.

A Illusion size as a function of speed for an object moving from left to right at 5.0m, with different monovision corrections strengths (curves). Monovision correction strengths (interocular focus difference, ΔF ) typically range between 1.0D and 2.0D[1]. Shaded regions show speeds associated with jogging, cycling, and driving. Illusion sizes are predicted from stereo-geometry, assuming a pupil size (2.1mm) that is typical for daylight conditions[39], and interocular delays that were measured from observer S1 (see Figure 2B). The predictions assume that the observer can focus the target at 5.0m in one eye[40]. B The distance of cross traffic moving from left to right will be overestimated when the left eye is focused far (sharp) and the right eye is focused near (blurry). C The distance of left-to-right cross traffic will be underestimated when the left and right eyes are focused near and far, respectively. See also Figure S3.