Italian psychologist Giovanni Caputo published an article in 'Perception' about this very phenomenon. In order to see an illusion, you need to stare at your reflection in a large mirror. (Remember? It was always someone's bathroom mirror—usually the biggest in the house.) The room must be dimly lit. (The bathroom lights had to be off!) Then you have to stare directly at your reflection, and in less than a minute (is that why we had to say "Bloody Mary" three times? To kill time?), You will perceive a shift in your own image which could sometimes be interpreted as someone else looking back at you.
Creepy? Maybe not. All of the participants in Caputo's study were required to stare into a mirror under the above conditions for ten full minutes and then report what they had perceived visually and emotionally. All of them saw some form of face-shifting and perceived appearances of anything from a deceased loved one to an old woman to the face of a monster. All of them, likewise, experienced strong emotional responses ranging from joy to anxiety to fear. Interestingly, all of them also experienced what Caputo calls "dissociative identity effect"—the feeling that they were staring at someone or something other than themselves, and that person or other being was watching them back.
He credits this to a combination of basic visual distortions that happen whenever someone looks at the same object for an extended period of time (just like how a word you hear over and over can start to sound odd) and the way our brains are wired for facial recognition. This is why we tend to see faces in clouds or trees or emoticons. So the next time you're at a slumber party and someone wants to play "Bloody Mary," let your friends in on the science behind the effect. Just don't be surprised when you wake up in the morning and your bra is in the freezer.
