The whiz kids at io9 have put together a compilation of research that explains why we see imaginary, monstrous faces when we look in the mirror for an elongated period of time. This is the very basis for sleepover games like Bloody Mary, designed to spook kids into wetting themselves while surrounded by their jerkwad friends.
Here’s the jist:
The brain, when faced with a lot of stimulation, only some of which is considered relevant, will tune out the non-relevant parts, filling in what it can from the general area. It’s a little like how the blind spot works, except this is a dynamic process. The brain will zoom in on a desired area, and the rest of the space will fade away. This is called the Troxler Effect, or Troxler Fading.
The post goes on to discuss a study where respondents looked in a mirror for 10 minutes and reported all manner of horrors staring back at them by the end of it. 10% explained seeing dead parents with key elements of their face rearranged.