viz.

Image Credit: premiumblend.net

The side of the Charlie Chaplin mask on the left is convex; the one on the right is concave. If you can see the concavity on the right, 2009 research suggests that you may be schizophrenic. Luckily for me, I can't see the concavity, but watching videos of the hollow face illusion all day has been disturbing nonetheless.