Originally Posted by Thunderbolt
Mind. Blown. Does anyone know why that happens?
Currently scientific theory believes that it occurs because the bias of the human brain for perceiving faces as convex is so strong that it will ignore cues like lighting and shading to interpret it as such. Basically, your brain is so used to seeing faces as pushing outwards that it can't handle seeing an inverted face.
Strangely enough, if someone is asked to touch a point on an inverted mask like that, they will usually have no trouble. That is, although the center of the mask looks to be closer than the sides, most people will automatically move their fingers further than the sides of the mask to touch the dot. This points to very interesting disconnect between the different visual systems; although you consciously perceive the face as pushing out towards you, at the same time you unconsciously realize that it's actually pushing away from you.
I've recently been reading Phantoms in the Brain, which discusses stuff like this is brain-damaged patients. One woman was brain-damaged in such a way that she could not tell if a slot was vertical or horizontal, how wide or narrow it was, or even how far away it was. That is, if you asked her if it was up or down or side-to-side, or to estimate how far across the slot was, she couldn't tell you. Yet if you told her to put an envelope through the slot, she could do so with no problem. Which relates to the hollow face illusion in that there are many different visual systems in the brain analyzing different things that merely give us the perception of one, whole picture. This, among other stories bordering on absurdity, point to the very disturbing idea that our conscious minds are far less in control of ourselves than we would like.