If you have been puzzled by Mona Lisa鈥檚 smile 鈥� how she鈥檚 radiant one moment and serious the next instant 鈥� 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 鈥渃hannels鈥� to the brain. These channels encode data about an object鈥檚 size, clarity, brightness and location in the visual field.
鈥淪ometimes one channel wins over the other, and you see the smile, sometimes others take over and you don鈥檛 see the smile,鈥� says , a neuroscientist at Institute of Neuroscience in Alicante, Spain, who conducted the study along with Diego Alonso Pablos.
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This isn鈥檛 the first time scientists have deconstructed Leonardo da Vinci鈥檚 masterpiece. In 2000, , a neuroscientist at Harvard Medical School with a side interest in art history, showed that Mona Lisa鈥檚 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鈥檚 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 鈥� 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鈥檚 team compared how light affects our judgement of Mona Lisa鈥檚 smile. Two kinds of cells determine the brightness of an object relative to its surroundings: 鈥渙n-centre鈥� cells, which are stimulated only when their centres are illuminated, and allow us to see a bright star in a dark night; and 鈥渙ff-centre鈥� 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鈥檚 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鈥檚 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鈥檚 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 鈥� further evidence that dead-centre vision picks out the smile. That can鈥檛 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. 鈥淗e wrote in one of his notebooks that he was trying to paint dynamic expressions because that鈥檚 what he saw in the street.鈥�
The research was presented at the this week.
