The 'uncatchable smile' illusion in Da Vinci's Bella Principessa portrait depends on the viewing angle

The Mona Lisa is the most-visited, most written about and most parodied work of art in the world. However, the ‘uncatchable smile’ that makes Leonardo da Vinci’s Mona Lisa so special is not unique. In previous research (Pickard & Soranzo, 2012) we found that the technique which would later give his most famous subject her mysterious allure was first executed in the lesser-known painting by the Renaissance master: La Bella Principessa. Specifically, we found a gaze-dependent illusion: When viewed directly the slant of her mouth appears to turn downwards, but when viewed in peripheral vision the edges of her mouth take an upward turn. Unlike the Mona Lisa, the Bella Principessa's portrait was painted in profile and the present research discovered that the magnitude of the 'uncatchable smile' illusion depends on the viewing position: when viewed 40◦ from the left, the Principessa smiles to the viewer, but when it is viewed 40◦ from the right, she maintains a neutral expression. The question arises as to whether Leonardo created this subtle illusion to enhance the Principessa's portrait before reproducing it from a frontal position in the Mona Lisa