Ed. note: Elements of this post were published previously as part of our Scientific American Special Issue on Illusions
This weeks pick was inevitable in light of the news that Margaret Thatcher died on Monday. The Margaret Thatcher Illusion, discovered by vision scientist Peter Thompson of York University in the UK, is one of the most famous illusions of all time.
When Thompson developed this illusion in 1980 (Thatchers second year as the UK Prime Minister), scientists already knew that faces were difficult to recognize upside-down. But the assumption was that, since the brain always sees faces right-side up, the face-recognition cells were optimized for right-side up faces. This assumption was partially true, but the Thatcher illusion showed further that the brain doesnt simply process and store representations of whole faces per se, but rather individual face-features (mouth, eyes, etc.) in isolation of each other.
The top and bottom rows of Thatchers are identical to each other, but flipped vertically. The top row looks like two upside-down Thatchers, no problem there. But the bottom row looks like a Thatcher on the left, and a horrible mutant on the right. The reason is that, whereas the left column are normal faces (though the upper face is upside-down), the right column are Frankensteinish composites of Thatcher with only the eyes and mouths flipped vertically. The top right Thatcher doesnt freak you out because the eyes and mouth are right-side up (though the overall face is upside-down), and your face-perception neurons therefore see them as normal (even though they dont match the rest of the face). The bottom right image, to the contrary, is creepy because the eyes and mouth are upside down and thus all wrong, despite the fact that the face as a whole is right-side-up.
Thompson,P.(1980) Margaret Thatcher: a new illusion. Perception 9 483-4.