Life

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 vision 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.

Visual pathways

To get a fuller picture of the reasons behind Mona Lisa's vanishing smile, Martinez Otero and Alonso Pablos varied different aspects of the Mona Lisa that are processed by different visual channels, and then asked volunteers whether they saw a smile or not.

To start with, the duo asked volunteers to look at the painting in varying sizes from varying distances. When standing far away or when viewing a tiny reproduction of the portrait, the volunteers had trouble making out any facial expression.

When they moved in closer, or viewed a larger copy of the painting, they began to see the smile – and the larger the picture more likely they were to see it. This suggests that retinal cells that process dead-centre vision convey information about the smile just as well as the cells that contribute to peripheral vision.

Next, Martinez Otero's team compared how light affects our judgement of Mona Lisa's smile. Two kinds of cells determine the brightness of an object relative to its surroundings: "on-centre" cells, which are stimulated only when their centres are illuminated, and allow us to see a bright star in a dark night; and "off-centre" cells, which fired only when their centres are dark, and allow us to pick out words on a printed page.

Light and darkness

Martinez Otero jammed these channels by showing another set of volunteers either a black or white screen for 30 seconds followed by a shot of the Mona Lisa. Volunteers were more likely to see Mona Lisa's smile after they had been shown the dark screen. This would have muted the off-centre cells, leading Martinez Otero to conclude that it is these the on-centre cells that sense the Mona Lisa's smile.

Eye gaze also affects how volunteers see the smile, Martinez Otero 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, Martinez Otero 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."

The research was presented at the Society for Neuroscience's annual meeting in Chicago this week.

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