Leonardo da Vinci gave the world the Mona Lisa and The Last Supper.
Now, a British researcher suggests an untreated eye ailment may have helped the Renaissance genius perfect these and other masterpieces.
After analysing a series of paintings and sculptures that are thought to depict the likeness of Italian painter da Vinci (1452-1519), investigator Christopher Tyler concluded that the artist appears to have suffered from chronic, if intermittent, "lazy eye" due to strabismus, a misalignment of the eyes.
But rather than undermine his ability to render lifelike imagery on a flat surface, the condition was probably a creative boon, Tyler theorised.
Why? Lazy eye would have forced da Vinci to occasionally rely on just one eye, compromising normal 3-D vision. But in the world of 2-D vision - which is what one eye would see - that would have supercharged his ability to render multiple 2-D applications of paint across a flat canvas.
The ironic result: richly layered masterpieces jam-packed with subtlety.
"This condition has minimal impairment, because stereo acuity is usually quite good when the eyes are aligned, [and] they only misalign when inattentive or tired, or allowed to do so by relaxing attention," Tyler explained.
"So there is a voluntary aspect [to this], in that it can be used at will if a monocular view is required, and he might well have been well aware of this effect."
Tyler is a professor in the division of optometry and vision sciences with the school of health sciences at the City University of London.
And while looking at a canvas with mono-vision would cut down on the ability to see depth, it would also increase da Vinci's ability to perceive the intricate building blocks of depth.
"One of the things he is most famous for is his 3-D modeling [in which he adds] up to 30 layers of shading to get the subtle gradients," Tyler noted. "This is the kind of cue you don't notice if you have full 3-D vision, but [it] can become more apparent if you shut - or shut off - one eye."
Exceptionally talented artist
According to the American Association for Pediatric Ophthalmology and Strabismus (AAPOS), the condition Tyler believes da Vinci suffered stems from a misalignment of the eyes, which is typically due to a neuromuscular abnormality that undermines the brain's ability to properly control eye movement.
The problem often affects children who are otherwise healthy. But it can also develop in both children and adults, either as a complication of trauma or due to a range of diseases, including cerebral palsy, brain tumors or stroke.
The AAPOS estimates that roughly 4% of Americans have strabismus. Most commonly that means "crossed eyes" (esotropia), in which one or both eyes turn inward; "lazy eye" (exotropia) in which one or both eyes turn outward; or hypertropia, in which the visual center of one eye is higher than the other.
Dr Steven Brooks is chief of pediatric ophthalmology at Columbia University's College of Physicians and Surgeons and medical director of Jonas Children's Vision Care in New York City. He explained that the visual experience of those who have strabismus varies considerably.
"Some seem virtually completely unaware of the deviation," Brooks noted. "Others describe an occasional pulling sensation or strain, others get intermittent double vision, [and] some describe intermittent blurring."
Tyler said many famous artists are believed to have struggled with strabismus, including Rembrandt, Picasso, Degas and Durer.
To see whether the same was true of da Vinci, Tyler studied the pupil alignment depicted in six representations believed to be of the artist: two sculptures, two oil paintings and two drawings.
In the end, he diagnosed da Vinci with intermittent "lazy eye" in his left eye. And it was likely a persistent problem, given that modern long-term cures - such as an operation or botulinum injections - were not options in the 15th century.
Brooks pointed out that the vision complication that da Vinci allegedly suffered from is considered to be "a significant problem." And he added that eye doctors now "make every effort to correct it at as early an age as is possible."
But Brooks cast doubt on the notion that the condition might have given the artist a visual leg up.
"I'm not aware of any visual advantages to strabismus, even intermittent strabismus," he said.
"My guess," added Brooks, "is that da Vinci was just an exceptionally talented artist.