This illusion highlights a flaw in how our brains work – we can’t process an upside-down face because we are programmed to recognize faces the right way up.
We create a mental map by recognizing the face in pieces – eyes, mouth, and nose.
So when we’re presented with an upside-down, Thatcherised image, it’s not processed properly.
We know it’s upside down, but because we so rarely encounter upside-down faces, we haven’t evolved to interpret the expressions on them.
The facial features look fine, so our brains assume the rest of the face is as well.
That’s why we don’t see anything out of the ordinary until we turn the face the ‘right way’ up.