Dr. Margaret Livingstone,
a Harvard Neuroscientist, has provided a concrete explanation for popular fascination with the intriguing smile of Mona Lisa. She feels that the quality of the Mona Lisa smile is derived from the design of the Human visual system.
Her scientific explanation for the elusive smile is that an individual's center of gaze is focused on Mona Lisa's eyes with less accurate peripheral vision on her mouth.
This focus picks up shadows from the Mona Lisa's cheekbones, which suggests a curvature of a smile, but when the viewer's eyes then shift to her mouth, the shadows of her mouth elude the viewer. The smile appears present and then gone because of the visual processing. In particular, that of the peripheral area surrounding the fovea, where individuals see black and white, motion and shadows.
Therefore, Mona Lisa's smile is the outcome of one's peripheral vision based on the facial contours.
The New York Times Science Section, November 21, 2000, included a feature "What Is It With Mona Lisa Smile? It's You!" by N.Y. Times reporter Sandra Blakeslee.