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Every day, thousands of people file through a small room in the Louvre to see the Mona Lisa, waiting for hours to jostle for a good view.

What accounts for the lasting power of Leonardo da Vinci’s masterpiece? There are many answers to that question, says Pascal Mamassian, PhD, a psychology professor at Paris Descartes University. At the time it was painted, the Mona Lisa was a groundbreaking piece — an intimate portrait from the waist up, rather than a traditional full-length treatment. It’s also an astounding example of masterly brush technique and composition, he says.

But more than that, there’s something uncanny about the Mona Lisa that’s captured people’s imagination for 500 years.

“That painting almost seems to come alive under the viewer’s gaze,” says Mamassian.

The Mona Lisa’s vividness, he says, is a quality common to all great works of art. Master painters are also keen observers of human vision and perception, Mamassian posits, and they use that knowledge to create pictures that don’t just lie flat on the wall.

“Artists are vision scientists, they just call themselves something different,” adds Margaret Stratford Livingstone, PhD, a neurobiology professor at Harvard.

Vision scientists are now playing catch-up, say Mamassian, Livingstone, and others. By studying paintings that have stood the test of time, psychologists are gleaning truths about human perception. In the process, they are explaining some of the magic behind great works of art.

“From the perspective of a vision scientist, I think it’s important to understand why artists have chosen to depict things in certain ways ... because these are critical clues to some fundamental aspects of visual processing,” Mamassian says.

The ‘what’ and the ‘where’ systems

When we look at a painting or a real-life scene, we see it simultaneously in two very different ways: One that’s like a black-and-white photocopy, the other that’s closer to a full-color picture, according to research by Livingstone and her colleagues. Starting in the retina and continuing throughout the visual system, our brain keeps information about a scene’s color and its brightness separate. In fact, the brain areas that process color are several inches away from those that process brightness, making them as physiologically distinct as sight and sound, she says.

The colorblind part of our visual system — commonly called the “where” stream — is something we have in common with all mammals, says Livingstone. We use “where” information to locate things in space, navigate our environments and track movement. The part of our visual systems that carries color information is a more recent development — the “what” stream — and it’s found only in the brains of primates. It’s this “what” stream that helps us determine what objects are.

Early in her career, Livingstone was trying to trace these two streams of information as they flowed through the brain, so she needed to create images that would send information down one stream but not the other. The images she came up with had different colors but equal brightness — they would be a uniform shade of gray if they were photocopied.

Creating these images gave Livingstone an idea: Perhaps artists, too, use “equal value” to confound the brain. Monet, in particular, is famous for creating paintings with an ineffable, eerie quality, says Livingstone. For example, his most famous painting, “Impression Sunrise,” depicts an orange sun that seems to blaze and even move in a gray sky.

With a grant from Harvard’s Mind, Brain, Behavior Institute, Livingstone traveled to the Musée Marmottan Monet in Paris to measure the luminescence of the painting’s sun and the surrounding sky. Her hunch turned out to be correct: The sun and the sky have exactly the same level of brightness, making them invisible to the parts of our brain that locate things in space. So, while we can see the sun, it’s hard to see exactly where it sits in the sky.

“A lot of the motion in Monet’s paintings comes from the fact he used equal luminescence,” says Livingstone.

This technique gives Monet’s work liveliness and a sense of interactivity with the viewer, adds Bevil Conway, PhD, an artist and neuroscience professor at Wellesley. It’s especially effective in Monet’s “Coquelicots,” where the poppies seem to wave in the breeze, perhaps because the red blooms are the same brightness as the surrounding green field, he says.

“There is a big chunk of your visual system that just can’t see those poppies, because they are encoded only with color,” he says. “That’s what makes them look shaky and unstable.”

A field of tiny dots

Pointillists also take advantage of the two-stream processing of our visual system, says Conway. Rather than painting a tree green, Georges Seurat and other pointillists composed the field of tiny dots of different colors, greens, yellows, blues and reds. The end result is a green tree that seems to shimmer and even change color.

As with the other impressionists, the pointillists’ technique works because the part of our visual system that sees color is not adept at locating items in space, says Conway. Our brains end up blending the dots into a color that’s not actually there.

“Each dot is visible to some part of our system, but not our color system, and that becomes quirky and interesting to look at,” he says. “It’s interesting from a phenomenological perspective, but from a physiological perspective, we have a good understanding of how it works.”

Another mechanism behind pointillists’ shimmer, says Conway, is that a given patch of a painting changes depending on how much of the picture you take in—which is why red looks redder on a green background. “That’s why we select red for our holiday tree decorations,” he says.

Research by Conway and others have discovered the mechanism behind these color contrasts: “double-opponent” cells in the visual cortex that fill in information they expect to see.

Other scientists, however, says pointillism’s magic has not yet been fully explained. With the above theory, the dots in a pointillist’s painting might simply average out to a single color, says Mamassian. But when you view a Seurat masterpiece, the colors shift depending on where you focus on the painting, he says. That may be the result of a visual illusion recently explained by Patrick Monnier, PhD, and Steven Shevell, PhD, in Vision Research (Vol. 48, No. 20). In their study, Monnier and Shevell showed participants with normal color vision a set of four orange rings each surrounded by different rings: one by a single, large green ring, another by a single, large purple ring and two with a series of narrow purple and green rings.

Though all the orange rings were the same color, the rings adjacent to the purple ones looked pinker than those next to the green rings. Curiously, that effect was enhanced by alternating small bands of green and purple. The effect also changed depending on the size of the concentric rings, becoming most vivid when they were an intermediate size, just a little bigger than this typeface.

Seurat seems to have carefully chosen his colors to create an effect like the one described by Monnier and Shevell, Mamassian says.

“I think the true skill of the pointillists was in the selection of the group of colors for the tiny patches,” says Mamassian. “If they are well chosen, they can increase the apparent color contrast with the background, or even produce interesting optical effects such as shimmering.”

While that may be so, Shevell doubts the Seurat’s shimmer can be completely explained by the neural process described in his paper.

“Many dots are too small at art museum viewing distance to excite the receptive fields causing those large color shifts,” he says. Rather, he agrees with Conway, saying the effect is in part caused by our brains’ inability to precisely locate pointillists’ dots in space.

Photographer Gary Rosenblum finds both arguments compelling. Rosenblum, who calls his work “pointillist photography,” photographs Kodachrome slides to capture the individual crystals in a pictures, creating a Seurat-like effect. Sometimes, he says, his images are dead on the page, but other times, the color contrast and crystal size comes out just right, creating that pointillist shimmer.

“Every once in a while, this combination of great composition and an interesting subject and the right amount of grain — where it’s not too big and it’s not too small—is just right,” he says. “The colors light up and I say, ‘Aha! Now I have something here.’”

Mona Lisa’s smile

The fact that our vision isn’t consistently sharp may also explain the enigma of Mona Lisa’s smile, says Livingstone. Our peripheral vision is only good at picking out big details, while images projected right on the center of our retinas can discern sharp details. That’s why we move our eyes as we read, she notes.

When da Vinci painted the Mona Lisa, he hinted at her smile with big brush strokes, but he put her more neutral expressions in the painting’s fine details. So if you look at Mona Lisa’s hands, for example, you might see her smiling out of the corner of your eye. But as soon as you focus in on that smile, it evaporates.

“As you move your eyes around, her smile changes,” says Livingstone.

That observation dovetails with a 2004 study by Leonid Kontsevich, PhD, and Chistopher Tyler, PhD, in Vision Research (Vol. 44, No. 13). In it, they randomly added “noise” to images of the Mona Lisa — similar to the snow on a TV with poor reception — and had participants rate how happy or sad she looked. They then averaged the noise that had randomly made Mona Lisa look happy, and the noise that participants said made Mona Lisa look sad. They found that the participants were making their judgments based entirely on her mouth area, and paying scant attention to her eyes or other parts of her face.

“The old adage that the eyes are the window to the soul doesn’t seem to hold up according to this study,” Tyler says.

Mona Lisa’s mercurial mouth, adds Tyler, gives viewers the illusion of changing emotion in her eyes. Though there are many other things to admire in the masterwork, it’s impressive that da Vinci so effectively employed a relatively simple trick — taking advantage of differences in visual acuity between our peripheral and central vision — to enliven a portrait, giving it the power to captivate audiences for around 500 years.

What artists have learned about human vision through intuition, trial and error, scientists are unraveling through experimentation, says Conway. And though scientists can’t get much hard data from studying paintings, they can get inspiration.

“Almost every single experiment I do comes out of something from art,” he says.

Further reading

  • Conway, B.R., & Livingstone, M.S. (2007). Perspectives on science and art. Current Opinion in Neurobiology, 17, 476–482.

  • Livingstone, M.S. (2002). Vision and Art: The Biology of Seeing. New York: Harry N. Abrams.

  • Mamassian, P. (2008). Ambiguities and conventions in the perception of visual art. Vision Research, 48, 2,143–2,153.

  • Shevell, S.K., & Kingdom, F.A.A. (2008). Color in complex scenes. Annual Review of Psychology, 59, 143–166.