Full Idea
The human perceptual system appears unable to represent a nose as concave rather than convex. If you look at the concave side of a mask, you see the features as convex.
Gist of Idea
We are unable to perceive a nose (on the back of a mask) as concave
Source
Roy Sorensen (Vagueness and Contradiction [2001], 4.3)
Book Reference
Sorensen,Roy: 'Vagueness and Contradiction' [OUP 2004], p.74
A Reaction
I don't think that is quite true. You wouldn't put a mask on if you thought it was convex. It is usually when seen at a distance with strong cross-lighting that the effect emerges. Nevertheless, it is an important point.