The great Italian Renaissance artist and scientist Leonardo da Vinci may have suffered from an unusual visual tic: an intermittent outward turn of the eye, clinically known as strabismus. According to a new paper in JAMA Ophthalmology, that disorder may have helped this quintessential Renaissance man capture 3D space on a flat 2D canvas so brilliantly. 
 
 If so, Leonardo would be in very good company. Several other famous artists—including Rembrandt, Durer, Degas, and Picasso—likely also had some form of strabismus, based on analysis of eye alignment in their respective self-portraits. Because such a misalignment tends to suppress the deviating eye, this condition would have enabled Leonardo to shift between monocular vision and normal vision, giving him a distinct artistic advantage. 
 
 "Try shutting one eye," explains author Christopher Tyler, a visual neuroscientist at City University of London. "The world now looks flatter, so the spatial relations are easier to translate onto the flat canvas." 
 
 "[The soul] guides the painter's arm and makes him reproduce himself." 
 
 Tyler closely examined six likely portraits and self-portraits (two sculptures, two oil paintings, and two drawings) of Leonardo for evidence of the condition, fitting circles and ellipses around the pupils, irises, and eyelids and measuring their relative positions. Those measurements were consistent with a diagnosis of strabismus. 
 
 There are remarkably few existing portraits of Leonardo, given his preeminent status in Renaissance Italy, but Tyler argues that any of Leonardo's portraits likely would have reflected the artist's own appearance to some extent. "[The soul] guides the painter's arm and makes him reproduce himself, since it appears to the soul that this is the best way to represent a human being," Leonardo observed in his Codex Atlanticus. 
 
 For Tyler, this statement makes it far more likely that he would have used himself as the model in, for example, his famed Vitruvian Man or for the restored Salvator Mundi painting recently confirmed to be Leonardo's work (it sold last November at Christie's for a record-breaking price of $450,312,500). 
 
 The faces of Leonardo? 
 
 C. Tyler/Rufus46/Creative Commons 
 
 Christopher Tyler 
 
 Christopher Tyler 
 
 Christopher Tyler 
 
 C. Tyler/Saiko, Creative Commons 
 
 Christopher Tyler 
 
 C. Tyler/GNU Free documentation license 
 
 C. Tyler/Ser Amantio di Nicolao/Creative Commons 
 
 Admittedly, assuming all these examples really are based on the real Leonardo is a probabilistic inference, so there will naturally be some skepticism. Art scholars disagree about the exact number of Leonardo portraits that still exist. Only one—a red-chalk portrait of Leonardo as an old man—is recognized by the leading authority on the artist, Kenneth Clark, as being definitely of Leonardo. 
 
 Other scholars still harbor some doubts, since the old man in the drawing seems at least ten years older than Leonardo would have been at the time. It could be a portrait of his father or uncle, for instance. (That said, some of the artist's contemporaries remarked that he looked much older than his actual age.) 
 
 As for the other candidates, the contemporary historian Vasari asserted that Verrocchio's David sculpture was based on a young Leonardo, whom he described as having "outstanding physical beauty." Since Leonardo was Verrocchio's apprentice at the time, it would have been perfectly natural for him to serve as a model for the sculpture. Tyler himself co-authored a 2012 paper that quantitatively examined six of the most likely candidate artworks, concluding the results were consistent with Leonardo being the subject, even though the authors acknowledged that more comparisons were needed to truly reach statistical significance. 
 
 "All art critics disagree with each other, so I am not expecting them all to agree with me," says Tyler. But he is reasonably confident in his assumptions and diagnosis of strabismus, even over a span of several centuries. 
 
 DOI: JAMA Ophthalmology, 2018. 10.1001/jamaophthalmol.2018.3833 (About DOIs). ||||| "Salvator Mundi," a painting recently attributed to Leonardo da Vinci, may provide evidence that the artist had an eye condition called strabismus, according to a new paper. 
 
 Leonardo da Vinci may have literally seen the world differently. 
 
 The famed Renaissance artist may have had an eye condition that helped him better-depict the 3D world on a flat surface when drawing and painting, a new report suggests. 
 
 The report, which analyzed six works of art that are likely portraits or self-portraits of Leonardo, suggests that the artist may have had strabismus, or crossed eyes, a condition in which a person's eyes don't look in the same direction at the same time. 
 
 In some cases of strabismus, the vision in the "wandering" eye is suppressed, which gives the person 2D "monocular" vision that could be advantageous to painting and drawing, wrote report author Christopher Tyler, a visual neuroscientist and a professor in the City University of London's Division of Optometry and Visual Sciences in the United Kingdom. [Leonardo Da Vinci's 10 Best Ideas] 
 
 Thus, having strabismus "may have contributed to da Vinci's exceptional ability to capture space on the flat canvas," Tyler wrote in the Oct. 18 issue of the journal JAMA Ophthalmology. 
 
 Some studies have found that visual artists are more likely than nonartists to have problems with their stereoscopic vision (perception of depth using two eyes), such as strabismus. And research has shown that some famous painters, including Rembrandt van Rijn and Pablo Picasso, had strabismus, based on analyses of their eyes in self-portraits. 
 
 However, there are few confirmed self-portraits of Leonardo da Vinci, making it hard to assess whether he had the condition. 
 
 In the new report, Tyler analyzed six works of art that are thought to be portraits or self-portraits of the artist, or images that may reflect his likeness. 
 
 For example, the sculptures "David" and "Young Warrior" by Andrea del Verrocchio are thought to be modeled on Leonardo, who was an apprentice for the older artist. Three other artworks included in the study, "Young John the Baptist," "Salvator Mundi" and "Vitruvian Man," all by Leonardo, are not generally thought of as self-portraits of Leonardo, but they may have captured some of his appearance, Tyler said wrote. (Leonardo himself said, "[The soul] guides the painter's arm and makes him reproduce himself, since it appears to the soul that this is the best way to represent a human being.") Finally, the analysis also included a self-portrait of Leonardo in old age. 
 
 By analyzing the position of the pupils in the eyes of these artworks, Tyler found that the eyes tended to have exotropia, a type of strabismus in which one or both of the eyes turn outward. The presence of exotropia was more pronounced in portraits than in self-portraits. One explanation for this may be that Leonardo had "intermittent exotropia," meaning that the artist's eye condition was not constant, Tyler said. For example, the condition may have been more severe when Leonardo was relaxed but corrected itself when he was attentive or focused on a specific object. 
 
 It's important to note that the analysis presents a hypothesis, and it cannot prove that Leonardo had this condition. 
 
 Still, Tyler noted that having intermittent exotropia would be "rather convenient for the painter, since viewing the world with one eye allows direct comparison with the flat image being drawn or painted." 
 
 And if Leonardo's strabismus was intermittent, he could have also switched to viewing the world stereoscopically, Tyler said. 
 
 Originally published on Live Science. ||||| Key Points 
 
 Question Did Leonardo da Vinci, the preeminent artist-scientist of the Italian Renaissance, have a form of strabismus that could have facilitated his artistic work? 
 
 Findings Examination of 6 likely portraits and self-portraits of da Vinci in which the direction of gaze of each eye is identifiable shows that most paintings exhibit a consistent exotropic strabismus angle of −10.3°, supported by a similar Hirschberg angle in the recently identified da Vinci painting Salvator Mundi. 
 
 Meaning The presence of exotropia, particularly if it was intermittent, may have contributed to da Vinci's exceptional ability to capture space on the flat canvas. 
 
 Abstract 
 
 Importance Strabismus is a binocular vision disorder characterized by the partial or complete inability to maintain eye alignment on the object that is the target of fixation, usually accompanied by suppression of the deviating eye and consequent 2-dimensional monocular vision. This cue has been used to infer the presence of strabismus in a substantial number of famous artists. 
 
 Objective To provide evidence that Leonardo da Vinci had strabismus. 
 
 Design, Setting, and Participants In exotropia, the divergent eye alignment is typically manifested as an outward shift in the locations of the pupils within the eyelid aperture. The condition was assessed by fitting circles and ellipses to the pupils, irises, and eyelid apertures images identified as portraits of Leonardo da Vinci and measuring their relative positions. 
 
 Main Outcomes and Measures Geometric angle of alignment of depicted eyes. 
 
 Results This study assesses 6 candidate images, including 2 sculptures, 2 oil paintings, and 2 drawings. The mean relative alignments of the pupils in the eyelid apertures (where divergence is indicated by negative numbers) showed estimates of −13.2° in David, −8.6° in Salvator Mundi, −9.1° in Young John the Baptist, −12.5° in Young Warrior, 5.9° in Vitruvian Man, and −8.3° in an elderly self-portrait. These findings are consistent with exotropia (t 5 = 2.69; P = .04, 2-tailed). 
 
 Conclusions and Relevance The weight of converging evidence leads to the suggestion that da Vinci had intermittent exotropia with the resulting ability to switch to monocular vision, which would perhaps explain his great facility for depicting the 3-dimensional solidity of faces and objects in the world and the distant depth-recession of mountainous scenes. ||||| 
 
 A close-up of Leonardo da Vinci’s “Salvator Mundi.” (Christopher Tyler/American Medical Association) 
 
 For centuries, people have theorized about the source of Leonardo da Vinci’s artistic genius. Just how was he able to so accurately capture depth and perspective on a flat canvas? 
 
 New research suggests da Vinci’s unmatched talent may in part be the result of his ability to see the world differently — literally. 
 
 There is now evidence that da Vinci’s renowned capacity to reproduce the three-dimensional world in paintings may have been aided by an eye disorder that allowed him to see in both 2-D and 3-D, according to a study published Thursday in JAMA Ophthalmology, a peer-reviewed journal. 
 
 Da Vinci is believed to have had a condition called intermittent exotropia — commonly referred to as being “walleyed” — a form of strabismus, eye misalignment that affects about 4 percent of the U.S. population. Those with exotropia usually end up favoring one eye over the other, which means they are more likely to see the world as if it were, say, painted on a flat canvas. 
 
 “When they’re in that condition . . . they’re only seeing the world monocularly, with much reduced depth cues,” the study’s author, Christopher Tyler, a professor at City University of London and researcher at the Smith-Kettlewell Eye Research Institute in San Francisco, told The Washington Post. “The image they’re seeing is much closer to what they want to paint on the canvas.” 
 
 But in da Vinci’s case, the painter was, at times, able to control his wandering eye, which in turn provided him with an artistic advantage, Tyler said, noting that the ability to switch between the two perspectives meant that da Vinci would “be very aware of the 3-D and 2-D depth cues and the difference between them.” 
 
 Tyler, who has studied da Vinci’s life for more than 20 years, said he started noticing the disorder’s telltale sign while examining works by the artist and those done of him. 
 
 In many cases, “they had the eyes diverted,” he said. 
 
 “This is something I would notice, what I’m attuned to notice,” said Tyler, who specializes in studying binocular vision. 
 
 
 
 “David” by del Verrochio, full face. (Courtesy Christopher Tyler/American Medical Association) 
 
 Tyler set out to support his theory by conducting mathematical analyses on six works — two sculptures, two oil paintings and two drawings — believed to reflect da Vinci’s appearance. The pieces included Andrea del Verrocchio’s “David,” a bronze sculpture said to be a depiction of da Vinci as a youth, as well as da Vinci’s “Salvator Mundi,” which recently became the most expensive painting ever auctioned, selling for more than $450 million last year. 
 
 Measuring the relative positions of the pupils, irises and eyelids in each work, Tyler wrote in the study that da Vinci had “an exotropic tendency of approximately -10.3° when relaxed.” 
 
 “There’s a weight of evidence in all these portraits that tends to add up to something meaningful,” Tyler said. 
 
 Da Vinci would not be the first famous artist to have the disorder. Previous studies analyzing eye alignment in self-portraits have suggested that painters such as Rembrandt, Edgar Degas and Pablo Picasso were also strabismic. 
 
 Shira Robbins, a professor of ophthalmology at the University of California at San Diego who was not involved in the research, called Tyler’s findings “intriguing,” echoing his belief that the ability to switch between seeing out of one or both eyes “can be advantageous from an artistic perspective.” 
 
 “What happens in some people is when they’re only using one eye . . . they develop other cues besides traditional depth perception to understand where things are in space, looking at color and shadow in a way that most of us who use both eyes at a time don’t really appreciate,” said Robbins, who is also the educational director at the Ratner Children’s Eye Center within the Shiley Eye Institute in La Jolla, Calif. 
 
 But Robbins pointed out that there’s a “caveat” in the research — the “images are not 100 percent identifiable as being” da Vinci, she said. 
 
 Very few self-portraits of da Vinci exist, Tyler said, adding that he had to take “a more creative view” when identifying “likely portraits” of the famed artist. 
 
 While several of the works examined “have not generally been considered to be self-portraits,” the study said, “da Vinci himself was very clear that artists’ work is likely to reflect their own appearance.” 
 
 In the Codex Atlanticus, the largest set of da Vinci’s drawings and writings, he wrote that the soul “guides the painter’s arm and makes him reproduce himself, since it appears to the soul that this is the best way to represent a human being.” 
 
 If the works Tyler studied are in fact accurate reflections of da Vinci, Robbins said she would agree that the painter appeared to have strabismus. 
 
 The research, she noted, would have a “hugely positive” impact on the large number of people with eye-alignment disorders. The condition often leads to the misconception that people who have it are of “lesser intellect,” Robbins said, adding that research has shown those people often get passed up for jobs or aren’t as frequently invited to social gatherings. 
 
 “Anybody who has strabismus will look at this and be imbued to know that someone as brilliant as Leonardo da Vinci had a similar problem to them, and it certainly didn’t seem to hinder him in any way possible,” she said. 
 
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– Whole libraries have been written trying to explain how Leonardo da Vinci produced some of the world's most revered paintings. Now a new study suggests he had a biological edge: an eye disorder that affected his depth perception. In JAMA Ophthalmology, researchers make the case that da Vinci had a rare condition known as strabismus, which allowed him to better present a 3D world on a 2D canvas, reports Ars Technica. The condition is known as being "walleyed," per the Washington Post. As Live Science explains, it generally means that one of da Vinci's eyes wandered instead of focusing on the same thing as the other eye, which led him to favor the non-wandering eye. He would thus be "seeing the world monocularly, with much reduced depth cues," says study author Christopher Tyler of City University of London. That may not sound ideal for painters, but "the image they’re seeing is much closer to what they want to paint on the canvas," Tyler explains to the Post. Da Vinci could apparently control the wandering eye as needed, and thus "he’d be very aware of the 3D and 2D depth cues and the difference between them.” Other artists such as Rembrandt, Degas, and Picasso also were believed to have strabismus to some extent. Tyler's team figured this out by looking at six artworks, either by da Vinci or others, that are believed to be likenesses of da Vinci himself. (Among them was da Vinci's Salvator Mundi, the most expensive painting ever sold at auction.) Tyler took eye measurements of those images, and the results pointed to strabismus. (Elsewhere on the da Vinci front, it's possible that a physical condition also explains the most famous smile in art history.)