Mona Lisa's smiles in Leonardo's drawings

‘Mona Lisa’ (1503-05) is the most-visited, written about and parodied work of ‎art in the world. However, the ambiguous allure it features is not unique. Soranzo & Newberry (‎‎2015) found a similar display of ambiguity in the lesser-known painting ‘La Bella Principessa’. They suggested that most of the ambiguity of both portraits can be explained in terms of a spatial frequency contingent illusion concerning the direction of the mouth. When viewed closely, the slant of the ‎mouth appears to turn downwards, but when viewed from afar, or when the image is blurred, the edges ‎of the mouth appear to take an upward turn. This apparent modification in mouth slant results in a change of facial expression. The ambiguity may therefore be explained by the perceptual instability of the mouth slant. We have now extended this line of research and discovered that a similar illusion of direction is also present in two Leonardo's drawings: La Scapigliata (1508) and another Female Head (1470-76). This discovery supports the suggestive hypothesis that Leonardo studied the generation of ambiguity in the expression of portrayed subjects as matter to ‘moti mentali’, i.e. what we may now identify as micro expressions.