The so-called Thatcher effect illustrates the phenomenon that certain features of a face are not recognised when the face is viewed upside down, despite obvious changes. The effect was named after former Prime Minister Margaret Thatcher, as a photograph of her face was used to make this phenomenon most famous.

 

The effect was originally created by Peter Thompson in 1980. You see two identical photos, one of which is reversed. The second image is strikingly altered; the eyes and mouth are the wrong way round. The phenomenon is interesting because the strong changes in the reversed photograph are not immediately noticed.
This is due to the psychological perceptual abilities of a face, because these are set to recognise certain facial features in faces that are viewed the right way round. This enables us to distinguish the uniqueness of faces, even though all human faces are very similar.
Humans develop certain processes in the brain to distinguish faces based on the configuration of individual features, i.e. details of the appearance of the mouth, eyes and nose etc. are perceived.
If you turn a face upside down, the learned recognition process cannot be applied, so that even apparent feature differences are difficult to detect.