da vinci exotropia
© Coldcreation/Wikimedia CommonsIt's believed that Leonardo da Vinci used his own likeness in his Salvator Mundi painting (shown).
If Leonardo da Vinci had a good eye doctor, he might not have become such a great artist. At least that's what an analysis of paintings and sculptures believed to be modeled after da Vinci suggests.

Visual neuroscientist Christopher Tyler of the City University of London examined six pieces of art, including Salvator Mundi and Vitruvian Man. Five of the pieces depict an eye misalignment consistent with a disorder called exotropia that can interfere with three-dimensional vision, Tyler reports online October 18 in JAMA Ophthalmology.

scuptlure king david del verrocchio
Leonardo da Vinci's teacher, Andrea del Verrocchio, may have used da Vinci as a model for this sculpture of King David. The difference in the angles between the left and right pupils suggests the teacher captured da Vinci's exotropia.
Exotropia, in which one eye turns slightly outward, is one of several eye disorders collectively called strabismus. Today, strabismus, which affects 4 percent of people in the United States, is treated with special glasses, eye patches or surgery.

Tyler calculated the differences in eye alignment using the same sorts of measurements that an optometrist does when tailoring a pair of glasses. Most of the portraits showed the eyes misaligned, but Vitruvian Man by da Vinci himself did not. As a result, da Vinci may have had intermittent exotropia, present only some of the time and perhaps controllable, Tyler suspects. "The person [with intermittent exotropia] can align their eyes and see in 3-D, but if they're inattentive or tired, the eye may droop," he says.

If da Vinci could control his exotropia, Tyler speculates that it would have been an artistic advantage. "The artist's job is to paint on a 2-D surface," he says. "This can be difficult when you view the world three-dimensionally." Both eyes need to focus on the same subject for 3-D vision. Many artists shut one eye when viewing their subjects to more easily translate details into two dimensions. But with intermittent exotropia, da Vinci could have switched from 3-D to 2-D and back again with ease.