The article claims, "When viewed directly, the slant of the mouth in the Mona Lisa (pictured) is distinctly downwards. But as the eye moves elsewhere to examine other features, the mouth appears to take an upward turn, creating a smile that can only be seen indirectly."
I don't see the mouth slant downwards, To my eye, the curve of the mouth is very subtly upward and that just blows up the whole premise of the article. What do you see?