Thatcher effect

The second picture is altered so that the eyes and mouth are vertically flipped, though the changes are not immediately obvious until the image is viewed upright.

There is evidence that rhesus monkeys[3][4] as well as chimpanzees[5] experience the Thatcher effect, raising the possibility that some brain mechanisms involved in processing faces may have evolved in a common ancestor more than 30 million years ago.

The local inversion of individual dots is hard, and in some cases, nearly impossible to recognize when the entire figure is inverted.

Typically, experiments using the Thatcher illusion look at the time required to see the inconsistent features either upright or inverted.

[8] By looking at the intermediate angles between upright and inverted, studies have explored the gradual or sudden appearance of the illusions.

The Thatcher effect, shown here on a photograph of Margaret Thatcher . The two upside-down images both appear superficially correct as faces. When these images are rotated, however, it becomes clear that the face on the right had its eyes and mouth inverted.