How was Leonardo da Vinci able to depict three-dimensional scenes with such remarkable precision?
Just like the painters Rembrandt, Degas, Picasso, and Giovanni Francesco Barbieri (who was nicknamed Il Guercino, aka "the squinter"), da Vinci may have had a not-uncommon eye condition that allowed him to literally see space differently. That's according to a paper recently published in the journal JAMA Ophthalmology.
Strabismus is a binocular vision disorder that means your eyes cannot maintain proper alignment – essentially, they do not work together as a team. As one eye fixates on a particular point, the other is misaligned inwards (esotropia or cross-eyed), outwards (exotropia or wall-eyed), upwards (hypertropia), or downwards (hypotropia). This can cause double vision and so to compensate, the brain ignores any visual input entering from the misaligned eye.
Roughly 4 percent of the US population has a form of strabismus, and it can be treated with glasses, eye exercises, and/or eye muscle surgery. But the condition also comes with an unusual advantage, at least if you're a painter. That is because the inability of the eyes to work in tandem to create an integrated view of the world (stereoblindness) results in limited depth perception.
This sounds extremely problematic from an artistic point of view, but it means that people with the condition must use other clues (think: perspective and shading) to get about. Essentially, they are perceiving the space around them differently to people without the condition – which is pretty handy when you are trying to create a rendering of a 3D object onto a 2D platform. It may also be why strabismus is more common among artists than the general population.
Some neurobiologists believe that Rembrandt was wall-eyed and that this characteristic played into his idiosyncratic use of light and darkness. Now, researchers at City University of London believe they have evidence to suggest that da Vinci had a similar condition.
The first major hurdle the researchers faced was da Vinci's aversion to self-portraits. But while he might not have many "confirmed" portraits, the researchers selected a "progression of portraits" that are likely da Vinci from the time he was an artist's apprentice (Young Warrior by del Verrocchio) to his old age (a self-portrait as an elderly man). They then fit circles and ellipses to the pupils, irises, and eyelid apertures on each image to measure their mean relative alignments.
A person with normal vision would have a positive mean alignment score, but each of the six portraits (two sculptures, two oil paintings, and two drawings) came back negative. Estimates came back as -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. From this, they suggest that da Vinci probably had intermittent exotropia (wall-eyed) with an extropic tendency of roughly -10.3° when relaxed and orthotropia when attentive, allowing him to switch between monocular vision and normal vision.
There are limitations to the study, the most obvious being that the portraits (except for the self-portrait) are not necessarily of Leonardo himself. Even the old man self-portrait has sparked debates over his identity, with many saying he looks too old to be the 63-year-old artist. Additionally, the team didn't measure the eye drawings Leonardo made of other people to see whether he painted like that in general or whether it was more likely a reflection of himself.