The Thatcher effect or Thatcher illusion is a phenomenon where it becomes difficult to detect local feature changes in an upside down face, despite identical changes being obvious in an upright face. It is named after British former Prime Minister Margaret Thatcher on whose photograph the effect has been most famously demonstrated. This was originally created by Peter Thompson, (Thompson, 1980).
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The effect is illustrated by two originally identical photos,[1] which are inverted. The second picture is obviously altered so that the eyes and mouth are vertically flipped, though the changes are not immediately obvious until the image is viewed in normal orientation.
This is thought to be due to specific psychological processes involved in face perception which are tuned especially to upright faces. Faces seem unique despite the fact that they are very similar. It has been hypothesised that we develop specific processes to differentiate between faces that rely as much on the configuration (the structural relationship between individual features on the face) as the details of individual face features, such as the eyes, nose and mouth. When a face is upside down, the configural processing cannot take place, and so minor differences are more difficult to detect.
This effect is not present in people who have some forms of prosopagnosia, a disorder where face processing is impaired, usually acquired after brain injury or illness. This suggests that their specific brain injury may damage the process that analyses facial structures.
Rhesus monkeys also show the Thatcher effect (Adachi, et al., 2009), raising the possibility that some brain mechanisms involved in processing faces may have evolved in a common ancestor 30+ million years ago.
The basic principles of the Thatcher Effect in face perception have also been applied to biological motion. The local inversion of individual dots is hard, and in some cases, nearly impossible to recognize when the entire figure is inverted. [2]