Mona Lisa’s smiles in Leonardo’s drawings

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, Soranzo & Newberry (‎‎2015, VR) found that the technique which would later give his most famous subject ‎her mysterious allure was first executed in the lesser-known painting, La Bella Principessa, recently claimed to be a "Leonardo" (Kemp & Cotte,2010). ‎Soranzo & Newberry suggested that most of the "mysterious allure" of both Mona Lisa and La Bella Principssa can be explained by an illusion of direction of the mouth of the portrayed subjects which is spatial frequency dependend: When viewed directly the slant of the ‎mouth of the subjects appears to turn downwards, but when viewed from far away the edges ‎of the mouth appear to take an upward turn. At its turn, this perceived change in the mouth slant, generate a perceived change in the facial expression. We have now extended this line of research and discovered that a similar illusion is present in different Leonardo's paintings. By asking participants to judge how much a figure painted by Leonardo is smiling, we found that the ratings depend on the spatial frequency: when judged from far away or in blurring conditions, the figures are smiling more. This illusion is present in some of the women, angels and babies painted by the Master but we couldn't find it in any of the males. This discovery support to the hypothesis that Leonardo worked on "ambiguity" in the expression of the portrayed subjects over the years before producing it in the most famous masterpiece, Mona Lisa and leaves the open question of why he did not want to portray this illusion on male figures.