The Hollow-Face Illusion is an optical illusion where the perception of a concave object appears as a normal convex object. This usually pertains to masks and faces, but the effect can also be seen with other kinds of subjects. The human brain sees the shadows and the form and because all the massive amounts of data that we have previously acquired say that the form should be convex, we perceive it as such. It's best documented against human faces, and the more human a concave shape appears, the most likely it is to be perceived as convex. Other familiar concave objects will also appears as convex. We "know" that the object should appear a certain way, and this trumps the actual information that we are receiving.
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