Monkeys ‘get’ the Thatcher effect

The ‘Thatcher effect’, named after a illusion using Margaret Thatcher's face, is created by inverting specific facial features such as eyes or mouth. When the entire face, including the inverted features, is upside down, it’s very difficult to detect the oddities, even though they are glaringly obvious when the face is right side up.

It’s no surprise that people quickly notice ‘thatcherized’ pictures of other people. When the same procedure is done with pictures of other primates, both the normal and thatcherized photos seem fine to most people. What is more surprising, is that the reverse is true for monkeys.

Christian Wallraven of Korea University plus colleagues from the Max Planck Institute and the University of Manchester tested rhesus macaque monkeys to see if they noticed anything unusual about thatcherized pictures of either other macaques or of humans. The researchers used a combination of eye tracking and habituation (how long the monkeys seemed interested) to assess the monkeys’ opinions of the pictures.

Like humans, the macaques could recognize altered pictures of members of their own species, but not of other species. The researchers suspect that all primates use the same mechanisms for facial recognition, but only of their own species.

The Thatcher illusion: Local changes in facial features are hardly noticeable when the whole face is inverted (rotated 180°), but strikingly grotesque when the face is upright.

(Credit: Christoph Dahl)