The Thatcher Effect

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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, who is a professor at the University of York, England (Thompson, 1980).

... the changes [in the altered photograph] 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. This effect is not present in people who have some forms of prosopagnosia, a disorder where face processing is impaired, usually 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), indicating that the brain mechanisms involved in processing faces may have evolved in a common ancestor 30+ million years ago.

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