If you are basing your answer on the Mona Lisa this would refer to that one artist and when Mona Lisa was painted by Leonardo Da Vinci he wanted to show a neutral emotion in her face meaning neither happy or sad; kind of like an in between emotion. He gave her a type of smile that would be mysterious and difficult to understand. Actual some studies were done on this painting by scientist who believed that Mona Lisa could one minute appear radiant while another time serious and so forth and they explained this variation on what cells in the retina (eye) pick up the image (what we see) and transmit that image to our brain. Like they say "beauty is in the eye of the beholder".
