Visitors to the Louvre Museum in Paris inevitably make their way up the stairs to the gallery where one of the most famous paintings in western art hangs: the Mona Lisa. People over the centuries have delighted in the woman's coy smile. In this Science Update, you'll hear what the Mona Lisa's been hiding all these years.
Hiding a smile in the shadows. I'm Bob Hirshon and this is Science Update.
Leonardo da Vinci's famous painting has inspired romantic songs and art lovers the world over, who become captivated by the Mona Lisa's enigmatic smile.
The portrait also intrigued Margaret Livingstone, a neurobiologist at Harvard Medical School who studies the human visual system. When she looked at the painting a few years ago, Livingstone noticed the Mona Lisa's changing expression.
It was very clear to me that when I looked at her mouth, she wasn't smiling as much as when I looked at her eyes.
Later, she realized what was happening. Livingstone says our peripheral vision sees blurry images while our central vision sees fine detail.
When Livingstone blurred the face with a filter, the Mona Lisa looked as if she were smiling cheerfully. But homing in on the fine detail gave her a more demure expression.
So Livingstone says that in his painting, da Vinci achieved an unusual effect: the Mona Lisa's smile changes depending on where you look.
For the American Association for the Advancement of Science, I'm Bob Hirshon.
Making Sense of the Research
Artists make a career out of manipulating our perception. Usually, the manipulation is on an emotional and psychological level. But often, artists manipulate what we see in strictly physical ways as well: for example, by carefully creating the illusion of 3-D perspective on a flat surface. And as this study shows, artists can sometimes manipulate our perception in ways that even they aren't aware of.
Leonardo da Vinci had a great scientific mind, but even he couldn't have known the real secret behind the Mona Lisa's smile. The proper understanding of the human visual system was still centuries away. Still, he knew that he pulled some kind of neat trick; the painting was one of his personal favorites and he gave it to the king of France as a gift.
Livingstone's research is based on differences in spatial frequency perception within the eye. Spatial frequency is basically a measure of how detailed an image is. A good example of spatial frequency is right on your computer. Images on a computer screen are made up of pixels (tiny dots of colored light). Pictures with higher spatial frequency (in other words, more pixels crammed into every square inch) are sharper and more detailed than pictures with lower spatial frequency.
The tricky concept here is that different parts of your eye are actually "tuned in" to different spatial frequencies. Anything you look at has both high and low spatial frequency patterns, layered on top of each other—and what you see depends on how you look at it.
To get a sense of this, pick an object in the room, like a picture. Look at it out of the corner of your eye. It's kind of blurry, right? Now look straight at it. It snaps into focus. That's because you can see high spatial frequencies (fine details) with your central vision, but not low spatial frequencies (broad, blurry patterns). The opposite is true of your peripheral vision (the stuff outside the center of your gaze).
The secret behind the Mona Lisa is that the "happy" part of her smile is actually buried in a low spatial frequency pattern. So if you're not looking directly at her mouth, her smile looks cheerful. But when you look directly at her smile, parts of it disappear into the background. As a result, you're never quite sure if she's smiling or not.
If this is hard to understand, another kind of art might help. Find a painting that uses a lot of little dots or brushstrokes to make the picture (anything by Claude Monet or George Seurat would work). The picture is actually clearer if you stand back or look at it a little off-center. If you stare straight at it up close, it breaks down into too much detail, and it's harder to recognize what you're looking at. That's what happens when you look at low spatial frequencies through a high-frequency filter (your central vision).
Livingstone actually thinks that our perception of real-life smiles might be affected in a similar way. She says that the detailed expressions created by our facial muscles are actually blurred by the fat under our skin. So the best way to figure out what someone's feeling may not be to look right at them, but instead to look slightly away.
Now try and answer these questions:
- What is spatial frequency? What role does it play in our vision?
- How does our perception of spatial frequency affect our perception of the Mona Lisa?
- Can you think of other things in real life that are easier seen from a distance, or from an off-center view, then from a closer look?
- What other techniques might artists use to affect the way we perceive things?
The Joy of Visual Perception, an online book by Peter Kaiser of York University in Canada, includes an optical illusion exercise based on spatial frequency.
Vision and Art, by Hanover College psychology professor John Krantz, discusses a number of ways in which artists manipulate what we see in their work.
Purdue University's Visual Perception Online Laboratory has a number of online experiments and demonstrations that you can try.