The Thatcher effect or Thatcher illusion is a phenomenon where it becomes more 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. The effect was originally created by Psychology Professor Peter Thompson in 1980.
Local changes in facial features are usually hardly noticeable when the whole face is inverted (rotated 180°), but strikingly grotesque when the face is upright. In the image above two faces of an individual are presented. One picture is normal, while the seems grotesque by an upright presentation, but not if the faces are rotated 180°. Moreover, this illusion disappears if faces of another species are manipulated in the same way (see monkey faces).
Here’s two great video clips illustrating the illusion: