Mona Lisa's smile: Perception or deception

What gives Mona Lisas smile such a mysterious quality? Livingstone (2000) has suggested that the portrait changes its expression depending on where on the portrait the observer looks. The mouth, which is the essential feature of Mona Lisas remarkable expression (Kontsevich & Tyler, 2004), appears to form an enigmatic smile. Due to sfumato technique (Gombrich, 2005), this impression of a smile is more prominent in the gradual luminance changes that observers perceive mainly in the periphery of their vision— that is, in low spatial-frequency ranges. It is less prominent in the fine details that observers perceive only at the center of their gaze, in high spatial-frequency ranges.1 Consequent-ly, the subtle smile one perceives while looking at Mona Lisas eyes (when her mouth appears blurred) vanishes when one attempts to verify this impression by looking a