Watch this slideshow but keep your eyes trained on the white cross between the pictures. Nothing has been done to alter the portraits of the people who appear. And yet, very quickly, your brain will turn them into grotesque monsters.
It's something that researchers are calling the 'Flashed Face Distortion Effect' until they come up with something better. By flashing ordinary portraits aligned at the eyes, the human brain begins to compare and exaggerate the differences, causing the faces to seem hideous and ogre-like. The effect was discovered quite by accident when an an undergraduate student, Sean Murphy, was working alone in a lab on a set of faces for one of his experiments. He aligned a set of faces at the eyes and started to skim through them. After a few seconds, he noticed that some of the faces began to appear highly deformed and grotesque. He looked at the especially ugly faces individually, but each of them appeared normal or even attractive.