Consider the following image:

If you stare at the black cross in the center for a short time, you will likely see two things happen. First, a green circle may appear to be rotating around the image. Second, the lilac circles may fade and even disappear.

This is called the Troxler Effect and Brad Walters at Cortical Hemming and Hawing has the story of how this may play into the Bloody Mary legend. It’s based on a paper in the journal Perception by Giovanni B Caputo entitled Strange-face-in-the-mirror illusion (PDF).

Caputo asked fifty people to stare at a mirror in a dimly lit room for 10 minutes and record what they saw.

The descriptions differed greatly across individuals and included: (a) huge deformations of one’s own face (reported by 66% of the fifty participants); (b) a parent’s face with traits changed (18%), of whom 8% were still alive and 10% were deceased; (c) an unknown person (28%); (d) an archetypal face, such as that of an old woman, a child, or a portrait of an ancestor (28%); (e) an animal face such as that of a cat, pig, or lion (18%); (f ) fantastical and monstrous beings (48%).

Caputo suggests that part of this result is explained by the Troxler effect, and the rest by what he calls the ‘multiple-faces’ phenomenon, which Walter’s describes:

When black and white photographs of familiar faces are viewed so that the face is centered on a blind spot, people have reported seeing different features and even different faces (i.e. white eyes, facial hair that’s not present, upside down faces, the subject’s own face, other faces than what is shown, etc.). Many of these characteristics were similar to what was reported in the “strange face in the mirror illusion”, and many of the same conditions appear to be necessary for both illusions to work. For example, the “multiple faces phenomenon” works much better with black and white photographs than with color photos, while the “strange face in the mirror” illusion relies on low level lighting that makes it difficult for subjects to perceive color information.