The vacillating emotion some people see in Mona Lisa's smile is a mindtrick caused by different kinds of visual information going to our brains, according to researchers at the Institute of Neuroscience in Alicante, Spain. Apparently, there are different visual channels that deal with data about the size, clarity, brightness, and location of an object you're looking at. In the case of the Mona Lisa, what you see in her expression is determined by which channel is dominating at that moment. From New Scientist:
"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..."Mona Lisa's smile a mystery no more"
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."