This illusion by Peter Thompson of York University (UK) was a critical discovery in our understanding of face perception. When the illusion was discovered in 1980, 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 went further to show that the brain doesn’t simply process and store representations of whole faces per sé, but rather individual face-features (mouth, eyes, etc) in isolation of each other. The top and bottom row of Thatchers in the accompanying slide 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 doesn’t 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 don’t 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. Harvard neuroscientists Winrich Freiwald, Doris Tsao, and Margaret Livingstone have now found neurons in the brain that are selective to specific face-features such as mouths and eyes, confirming the predictions that were made from this illusion several decades earlier.