There's no doubt Leonardo da Vinci was a genius. But the secret to this Renaissance artist's talent may have been in the way he saw the world.

Da Vinci may have had an eye condition that helped him depict the distance and depth of the 3D world intricately on flat surfaces, according to a new study. It was likely a form of strabismus — informally known as "crossed eyes" — a condition in which a person's eyes don't look in exactly the same direction at the same time.

Specifically, da Vinci may have had intermittent exotropia, which means one or both eyes occasionally turn outward, according to report author Christopher Tyler, a professor in the City University of London's Division of Optometry and Visual Sciences.

"Looking at his work, I noticed the pronounced divergence of the eyes in all of his paintings," Tyler told CNN.

Studying the gaze

For the study, which was published in JAMA Ophthalmology, Tyler examined six works of art that were thought to be likely portraits or self-portraits of da Vinci. In all of them, the direction of the gaze of each eye is clear.

The works were two sculptures, two drawings and two oil paintings, including "Salvator Mundi" shown above. That one was not considered to be a self-portrait but is believed to have been based somewhat on da Vinci's appearance.

Tyler drew circles on the pupils, irises and eyelids on each image and then measured their relative positions. That's how he determined that the artist showed exotropia intermittently, but was able to revert to straight alignment, likely when he was less relaxed and more focused.

"What he was looking at would look more like a flat canvas" but for others who use both eyes simultaneously would look three-dimensional, Tyler said. This made it "easier to translate things onto the canvas."

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.

Leonardo da Vinci's eye condition may have made him an exceptional artist
The eye condition intermittent exotropia may have been behind Leonardo da Vinci's artistic genius, research says.