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 the then British prime minister

Margaret Thatcher Margaret Hilda Thatcher, Baroness Thatcher (; 13 October 19258 April 2013), was a British stateswoman who served as Prime Minister of the United Kingdom from 1979 to 1990 and Leader of the Conservative Party (UK), Leader of th ...

, on whose photograph the effect was first demonstrated. The effect was originally created in 1980 by Peter Thompson, professor of psychology at the

University of York The University of York (abbreviated as or ''York'' for Post-nominal letters, post-nominals) is a public Collegiate university, collegiate research university in York, England. Established in 1963, the university has expanded to more than thir ...

Overview

The effect is illustrated by two originally identical photos, which are inverted. 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. The Thatcher effect is thought to be due to specific psychological cognitive modules involved in

face perception Facial perception is an individual's understanding and interpretation of the face. Here, perception implies the presence of consciousness and hence excludes automated facial recognition systems. Although facial recognition is found in other spe ...

which are tuned especially to upright faces. Faces seem unique despite the fact that they are very similar. It has been hypothesized 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

eye An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...

nose A nose is a sensory organ and respiratory structure in vertebrates. It consists of a nasal cavity inside the head, and an external nose on the face. The external nose houses the nostrils, or nares, a pair of tubes providing airflow through the ...

and

mouth A mouth also referred to as the oral is the body orifice through which many animals ingest food and animal communication#Auditory, vocalize. The body cavity immediately behind the mouth opening, known as the oral cavity (or in Latin), is also t ...

. There is evidence that rhesus monkeys as well as chimpanzees 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 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.

Further investigations

The Thatcher illusion has also been useful in revealing the psychology of face recognition. Typically, experiments using the Thatcher illusion look at the time required to see the inconsistent features either upright or inverted. Such measures have been used to determine the nature of the processing of holistic facial images. By looking at the intermediate angles between upright and inverted, studies have explored the gradual or sudden appearance of the illusions. Sensitivity to the Thatcher illusion has been found in children, including children with

autism Autism, also known as autism spectrum disorder (ASD), is a neurodevelopmental disorder characterized by differences or difficulties in social communication and interaction, a preference for predictability and routine, sensory processing d ...

. However, people with congenital prosopagnosia have been shown to exhibit an overall much weaker response to the illusion compared to people without such condition. Their response time was shown to be weakly and linearly affected by the grotesque face's orientation.Carbon, C. C., Grüter, T., Weber, J. E., & Lueschow, A. (2007). Faces as objects of non-expertise: Processing of Thatcherised faces in congenital prosopagnosia. ''

Perception Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous syste ...

'', 36(11), 1635-1645. This last observation suggests that the part of the brain responsible for "configural processing" in face recognition is the

fusiform face area The fusiform face area (FFA, meaning spindle-shaped face area) is a part of the human visual system (while also activated in people blind from birth) that is specialized for facial recognition. It is located in the inferior temporal cortex (I ...

of the cortex, which is affected in both the congenital and the acquired condition.

References

External links

Thatcher illusion demonstration

Open University YouTube Thatcher illusion demonstration

BBC article on the Thatcher illusion

{{Optical illusions Psychological effects Vision Optical illusions Margaret Thatcher Face perception 1980 neologisms Eponyms