Contact Lenses, the Reverse Pulfrich Effect, and Anti‑Pulfrich Monovision

www.nature.com/scientificreports OPEN Contact lenses, the reverse Pulfrich efect, and anti‑Pulfrich monovision corrections Victor Rodriguez‑Lopez1,2, Carlos Dorronsoro1,3,6 & Johannes Burge2,4,5,6* Interocular diferences in image blur can cause processing speed diferences that lead to dramatic misperceptions of the distance and three‑dimensional direction of moving objects. This recently discovered illusion—the reverse Pulfrich efect—is caused by optical conditions induced by monovision, a common correction for presbyopia. Fortunately, anti‑Pulfrich monovision corrections, which darken the blurring lens, can eliminate the illusion for many viewing conditions. However, the reverse Pulfrich efect and the efcacy of anti‑Pulfrich corrections have been demonstrated only with trial lenses. This situation should be addressed, for clinical and scientifc reasons. First, it is important to replicate these efects with contact lenses, the most common method for delivering monovision. Second, trial lenses of diferent powers, unlike contacts, can cause large magnifcation diferences between the eyes. To confdently attribute the reverse Pulfrich efect to interocular optical blur diferences, and to ensure that previously reported efect sizes are reliable, one must control for magnifcation. Here, in a within‑observer study with fve separate experiments, we demonstrate that (1) contact lenses and trial lenses induce indistinguishable reverse Pulfrich efects, (2) anti‑Pulfrich corrections are equally efective when induced by contact and trial lenses, and (3) magnifcation diferences do not cause or impact the Pulfrich efect. Monovision is a widely prescribed correction for presbyopia, the age-related loss of focusing ability, impacting 2 billion people worldwide1,2. Monovision corrections serve as an alternative to reading glasses, bifocals, and progressive lenses for millions of people3,4. With monovision, each eye is ft with a lens that sharply focuses light from a diferent distance. One eye is corrected for ‘far vision’, while the other eye is corrected for ‘near vision’. Monovision thus intentionally induces diferences in image blur between the eyes. Te eye corrected for near vision will form sharper images of near objects like books or mobile phones, and blurrier images of far objects like mountains or street signs. Similarly, the eye corrected for far vision will form blurrier images of near objects and sharper images of far objects. Burge et al.5, recently reported that interocular diferences in optical blur, like those induced by monovision corrections, have the potential to cause large errors in estimating the distance and 3D direction of moving objects. Under some conditions, the perceptual errors may be large enough to impact public safety 5. For example, the distance to a cyclist in cross-trafc may be overestimated by nearly 9 f, the width of a narrow lane of trafc (Fig. 1A). Te illusion occurs because the image in the blurrier eye is processed more quickly by a few milliseconds than the image in the sharper eye. Blur removes high spatial frequencies (i.e. fne details) from the image6–8, and high spatial frequencies are known to be processed more slowly9–14. Tus, the blurry image is processed more quickly because the high frequencies in the sharp image slow its processing down5. For moving targets, this interocular mismatch in processing speed causes a neural disparity, which results in the misperception (see “Results”). Te new illusion—the reverse Pulfrich efect—is closely related to the classic Pulfrich efect, a well-known stereo- motion phenomenon that was frst discovered 100 years ago. Te classic Pulfrich efect is induced by an interocular diference in retinal illuminance. However, darkening the image in one eye has the opposite efect of blur15. Te darker image is processed more slowly rather than more quickly16–18. Te response properties of neurons in both the retina and early visual cortex are thought to 1Institute of Optics, Spanish National Research Council (IO-CSIC), Madrid, Spain. 2Department of Psychology, University of Pennsylvania, Philadelphia, PA, USA. 32Eyes Vision SL, Madrid, Spain. 4Neuroscience Graduate Group, University of Pennsylvania, Philadelphia, PA, USA. 5Bioengineering Graduate Group, University of Pennsylvania, Philadelphia, PA, USA. 6These authors jointly supervised this work: Carlos Dorronsoro and Johannes Burge. *email: [email protected] SCIENTIFIC REPORTS | (2020) 10:16086 | https://doi.org/10.1038/s41598-020-71395-y 1 Vol.:(0123456789) www.nature.com/scientificreports/ Figure 1. Reverse Pulfrich efect, classic Pulfrich efect, and anti-Pulfrich monovision corrections. (A) Interocular blur diferences like those induced by monovision corrections cause the ‘reverse Pulfrich efect’, a substantial misperception of the distance of moving objects. If the lef eye is sharp and the right eye is blurred, an object moving from lef to right will be misperceived as farther away than it actually is, and vice versa. Te blurry image is processed faster than the sharp image, causing a neural disparity which leads to the depth misperceptions. In some scenarios, the distance misestimates can be substantial. Burge et al.5 reported that, for an individual observer with a typical monovision correction strength of 1.5D, a cyclist moving lef to right at 15 mph at a distance of 16 f may be estimated to be at 25 f. Tis overestimation of 9 f is approximately the width of a narrow lane of trafc. Tese misperceptions occur because the blurrier eye is processed more quickly by only a few milliseconds (see “Results”). (B) Interocular luminance diferences cause the classic Pulfrich efect. When both eyes are sharp, the darker eye is processed slower. If the lef eye is bright and the right eye is dark, and both eyes are sharply focused, the distance to the same cyclist will be underestimated, because the darker eye is processed more slowly by a few milliseconds. (C) Anti-Pulfrich monovision corrections can eliminate the misperceptions by darkening the blurring lens. Te reverse and classic Pulfrich efects cancel each other out. underlie the classic Pulfrich efect19,20. Te resulting illusions are thus similar to the illusions associated with the reverse Pulfrich efect, except that the classic Pulfrich efect causes distance underestimation where the reverse Pulfrich efect causes distance overestimation, and vice versa (Fig. 1B). Burge et al.5 also demonstrated that anti-Pulfrich monovision corrections can eliminate the depth and motion misperceptions associated with the reverse and classic Pulfrich efects. Te logic behind anti-Pulfrich corrections is simple. Blurry images are processed more quickly than sharp images (i.e. the reverse Pulfrich efect). Dark images are processed more slowly than bright images (i.e. the classic Pulfrich efect). Tus, if the blurry image is darkened appropriately, the two diferences in processing speed cancel each other out and eliminate the misperceptions (Fig. 1C). To date, however, the efcacy of anti-Pulfrich monovision corrections has been demonstrated only with interocular diferences in optical power induced by trial lenses 5. Monovision prescrip- tions are most commonly prescribed with contact lenses21. (Surgically implanted interocular lenses are second most common22–24.) Hence, it is important to demonstrate that anti-Pulfrich corrections are efective when implemented with the ophthalmic corrections that are most relevant to clinical practice. Another reason to study the reverse Pulfrich efect and anti-Pulfrich corrections with contact lenses is that, unlike trial lenses, contacts introduce negligible retinal magnifcation diferences between the eyes. Te retinal magnifcation induced by a lens increases with its optical power and its distance from the eye (see “Methods”). Trial lenses, like eyeglasses, are positioned 10–14 mm from the eye. As a consequence, trial lenses of diferent powers induce both blur and magnifcation diferences. Te original demonstration of the reverse Pulfrich efect thus leaves open the possibility that the reverse Pulfrich efect could be due to interocular diferences in magnifcation, rather than to interocular diferences in blur. Although theoretical considerations and previously performed control experiments make this possibility unlikely, it should still be established empirically that dif- ferences in image magnifcation play no role in driving the efect. A typical monovision correction induces a 1.5D diference in optical power between the eyes21. For contact lenses, which are ft directly on the cornea, this power diference translates into an interocular magnifcation diference of 0.4% (Fig. 2A; see “Methods”). For eyeglasses, which are typically positioned 10–14 mm from the cornea, the same power diference translates into a magnifcation diference of between 1.5 and 2.1% (Fig. 2B). Magnifcation diferences of this size are thought to cause visual discomfort and other clinical issues 25. Tis is the reason that monovision is most ofen implemented with contact lenses, with surgically implanted interocular lenses, and with laser corneal surgery, all of which induce negligible magnifcation diferences. Measuring the reverse Pulfrich efect with contact lenses has the beneft of (1) isolating the impact of blur from the potential impact of magnifcation on processing speed, (2) testing for misperceptions in the optical SCIENTIFIC REPORTS | (2020) 10:16086 | https://doi.org/10.1038/s41598-020-71395-y 2 Vol:.(1234567890) www.nature.com/scientificreports/ Monovision with MonovisioMonovisionnw

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