Mona Lisa's smile a mystery no more

If you have been puzzled by Mona Lisa's smile – how she's radiant one moment and serious the next instant – then your worries are over. It happens because our eyes are sending mixed signals to the brain about her smile.

Different cells in the retina transmit different categories of information or "channels" to the brain. These channels encode data about an object's size, clarity, brightness and location in the visual field.

"Sometimes one channel wins over the other, and you see the smile, sometimes others take over and you don't see the smile," says Luis Martinez Otero, a neuroscientist at Institute of Neuroscience in Alicante, Spain, who conducted the study along with Diego Alonso Pablos.

This isn't the first time scientists have deconstructed Leonardo da Vinci's masterpiece. In 2000, Margaret Livingstone, a neuroscientist at Harvard Medical School with a side interest in art history, showed that Mona Lisa's smile is more apparent in peripheral visionMovie Camera than dead-centre, or foveal, vision. And in 2005, an American team suggested that random noise in the path from retina to visual cortex determines whether we see a smile or not. ...

Eye gaze also affects how volunteers see the smile, Otero Martinez says. His team used software to track where in the painting 20 volunteers gazed while they rated whether or not Mona Lisa's smile became more or less apparent.

With a minute to gaze at the painting, volunteers tended to focus on the left side of her mouth when judging her as smiling – further evidence that dead-centre vision picks out the smile. That can't be the whole story, though, because when volunteers had only a fraction of a second to discern her smile, their eyes tended to focus on her left cheek, hinting that peripheral vision plays a role, too.

So did Leonardo intend to sow so much confusion in the brains of viewers, not to mention scientists? Absolutely, Otero Martinez contends. "He wrote in one of his notebooks that he was trying to paint dynamic expressions because that's what he saw in the street."

via Mona Lisa's smile a mystery no more - life - 21 October 2009 - New Scientist.