Editing Thatcher effect

{{short description|Optical illusion}}
{{redirect-distinguish|Thatcherization|Thatcherism}}
[[File:Thatcher_effect.jpg|thumb|upright=1.3|The Thatcher effect, shown here on a photograph of [[Margaret Thatcher]]. The two upside-down images both appear superficially correct as faces. When these images are rotated, however, it becomes clear that the face on the right had its eyes and mouth inverted.]]
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]], 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]].<ref>{{cite journal |author=Thompson, P. |year=1980 |title=Margaret Thatcher: a new illusion |journal=Perception |doi=10.1068/p090483 |pmid=6999452 |volume=9 |pages=483–484 |number=4|s2cid=32492890 |url=http://eprints.whiterose.ac.uk/115530/1/thatcher1980.pdf }}</ref>

== Overview ==
The effect is illustrated by two originally identical photos,<ref>{{cite web |url=http://www.faculty.ucr.edu/~rosenblu/VSinvertedspeech.html |title=Reading Upside-down Lips |publisher=faculty.ucr.edu |year= |access-date=11 April 2013}}</ref> 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]] 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 [[Human eye|eye]]s, [[human nose|nose]] and [[Human mouth|mouth]].

There is evidence that [[rhesus monkeys]]<ref>Adachi Ikuma, Chou Dina P., Hampton Robert R. Thatcher Effect in Monkeys Demonstrates Conservation of Face Perception across Primates, Current Biology 2009, 19, 1270–1273. {{doi|10.1016/j.cub.2009.05.067}}.</ref><ref>Dahl Christoph D, Logothetis Nikos K, Bülthoff Heinrich H, Wallraven Christian 'The Thatcher illusion in humans and monkeys', Proceedings of the Royal Society B: Biological Sciences, 2010, 277 (1696)</ref> as well as [[Common chimpanzee|chimpanzees]]<ref>Weldon, K. B., Taubert, J., Smith, C. L., & Parr, L. A. (2013). 'How the Thatcher illusion reveals evolutionary differences in the face processing of primates'. Animal cognition, 16(5), 691-700.</ref> 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.<ref>Mirenzi A, Hiris E, 2011, "The Thatcher effect in biological motion" Perception 40(10) 1257 – 1260</ref>

==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.<ref>Sjoberg, W., & Windes, J. D. (1992)</ref> Such measures have been used to determine the nature of the processing of holistic facial images.<ref>Lewis, M.B. & Johnston, R.A. (1997). The Thatcher Illusion as a test of configural disruption. Perception, 26, 225-227.</ref>

By looking at the intermediate angles between upright and inverted, studies have explored the gradual or sudden appearance of the illusions.<ref>Stuerzel, F., & Spillmann, L. (2000). Thatcher illusion: dependence on angle of rotation. ''[[Perception (journal)|Perception]]'', 29(8), 937-942.</ref><ref>Lewis, M. B. (2001). The lady's not for turning: Rotation of the Thatcher illusion. ''[[Perception (journal)|Perception]]'', 30(6), 769-774.</ref> Sensitivity to the Thatcher illusion has been found in children,<ref>Lewis, M.B. (2003). Thatcher’s children: Development and the Thatcher illusion. Perception, 32, 1415-1421.</ref> including children with [[autism]].<ref>Rouse, H., Donnelly, N., Hadwin, J. A., & Brown, T. (2004). Do children with autism perceive second-order relational features? The case of the Thatcher illusion. Journal of Child Psychology and Psychiatry, 45(7), 1246-1257.</ref>

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 (biology)|response time]] was shown to be weakly and linearly affected by the grotesque face's orientation.<ref>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 (journal)|Perception]]'', 36(11), 1635-1645.</ref> This last observation suggests that the part of the brain responsible for "configural processing" in face recognition is the [[fusiform face area]] of the cortex, which is affected in both the congenital and the acquired condition.

==References==
{{Reflist}}

== External links ==
{{Commons category|Thatcher effect}}
* [http://www.faculty.ucr.edu/~rosenblu/VSinvertedspeech.html Thatcher illusion demonstration]
* [https://www.youtube.com/watch?v=jdADSx8JpfI Open University YouTube Thatcher illusion demonstration]
* [https://www.bbc.co.uk/bang/article_thatcher.shtml BBC article on the Thatcher illusion]
* [http://www.milbo.users.sonic.net/stasm/download.html Windows utility for "thatcherizing" the face in the webcam (open source)]

{{Psychology}}
{{Optical illusions}}

[[Category:Psychological effects]]
[[Category:Vision]]
[[Category:Optical illusions]]
[[Category:Margaret Thatcher]]
[[Category:Face perception]]
[[Category:1980 neologisms]]
[[Category:Eponyms]]