Margret Thatcher illusion or Thatcher effect is the name given to a cognitive process in the perception of human faces. See the picture. In the panel (1) the face is beautiful. In the panel (2) also is the same face. What is the reason for this difference? The answer is: Thatcher illusion or Thatcher effect. Let us examine the cognitive processes behind the Thatcher illusion.
For the face perception there is a specialized module in the brain which is formed by specific circuits in the cerebral cortex. In short the face perception is special form of pattern perception. The term pattern perception refers to the step between perception of the stimulus from an object in the environment and the conversion of this stimulus into a meaningful mental representation of the object in the mind. To make the stimuli from an object into a meaningful percept of the object in the mind it is necessary to categorize the object on the basis of its perceived features. See the pictures of the Sheltie dog. First a pattern is perceived and then the dog is recognized.
Inability of recognition of object is called agnosia. This is seen in patients suffering from lesions in certain regions of the brain. They can see the object but cannot recognize the object. The patient suffering from this lesion may not recognize the Sheltie dog in the picture in spite of the clear perception.
Module for face recognition
Specialized pattern recognition is adopted for perception of human faces. This specialized process for the face recognition is called modularity in face recognition. A module refers to a set of processes that are automatic, fast, separated and independent of other cognitive processes. The module acts through the activation of specific nerve circuits in the brain. There may be several modules, each dedicated to the perception of an important class of stimuli such as face or speech.
Social interactions are crucial to our survival and reproduction. These depend on the ability to recognize faces and speech. So the cognitive system included specialized modules for processing these categories of stimuli. Facial expressions provide a key means for communicating emotional states.
Perception of parts and whole
In the process of perception, stimulus or light from parts of the object reach the brain and in the brain these parts are assembled and the individual sees only the meaningful whole object. This is the holistic perception. But in the process the parts are perceived first. For example in the perception of human face various parts like nose, eyes, lips, ears etc. are perceived separately and they are synthesized together and ultimately the individual sees the whole face only. The first part of the perception of face is called analytical process. The individual is unaware of the analytical process or the perception of the parts.
When one perceived human face in the upright position the holistic process is dominant and face is recognized as a whole. On the other hand when the human face is perceived in the upside down position the analytic process becomes dominant and each part, the nose, the eyes, the eyebrows, the lips and the ears are perceived separately.
|Peter Thompson's demonstration - Thatcher illusion|
Professor Peter Thompson in 1980 proved this phenomenon through a strange demonstration. The eyes and mouth from a picture of British former Prime Minister Margaret Thatcher were cut and pasted upside down. This change caused grotesqueness to the picture which, surprisingly, disappeared when the cut-pasted picture is viewed upside down. This demonstration is called Thatcher illusion. This happened because when the picture is turned upside down each part of the face is perceived separately and appeared normal.
The psychological processes involved in face perception are tuned 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 eyes, nose and mouth. When a face is upside down, the configuring processing cannot take place, and so minor differences are more difficult to detect.
This effect is not present in people who have some forms of prosopagnosia, a disorder where face processing is impaired, usually acquired after brain injury or illness. This suggests that their specific brain injury may damage the process that analyses facial structures.
Japanese neuroscientist Takashi Adachi-Yamada and associates in 2009 proved that rhesus monkeys also show the Thatcher effect raising the possibility that the brain mechanisms involved in processing faces may have evolved in a common ancestor 30 million years ago.
The human face's proportions and expressions are important to identify origin, emotional tendencies, health qualities, and some social information. From birth, faces are important in the individual's social interaction. Face perceptions are very complex as the recognition of facial expressions involves extensive and diverse areas in the brain. Sometimes, damaged parts of the brain can cause specific impairments in understanding faces.
|Medial (inner) surface of brain. Yellow arrow indicates Fusiform gyrus|
Neuro-anatomy of face perception
Face perception has well identified neuro-anatomical correlates in the brain. Most scientists agree that during the perception of faces, major activations occur in the fusiform gyri. This part of the brain is often called Fusiform Face Area (FFA).
Sex-related differences in face perception
Studies using electrophysiological techniques have demonstrated gender-related differences during a face recognition memory (FRM) task and a facial affect identification task (FAIT). The male subjects used a right, while the female subjects used a left, hemisphere neural activation system in the processing of faces and facial expressions. Gender-related differences may suggest a role for sex hormones. In females there may be variability for psychological functions related to differences in hormonal levels during different phases of the menstrual cycle.
Ethnicity in face perception
Humans tend to perceive people of other races than their own to all look alike. Other things being equal, individuals of a given race are distinguishable from each other in proportion to our familiarity, to our contact with the race as whole. Thus, to the uninitiated American all Asiatic look alike, while to the Asiatic, all White men look alike. This phenomenon is known as the cross-race effect, own-race effect, other-race effect, own-race bias or interracial-face-recognition-deficit.
Artificial face perception
A great deal of effort has been put into developing computer software that can recognize human faces. Much of the work has been done by a branch of artificial intelligence known as computer vision which uses findings from the psychology of face perception to inform software design.
Another interesting application is the estimation of human age from face images. As an important hint for human communication, facial images contain lots of useful information including gender, expression, age, etc. Compared with other cognition problems, age estimation from face images is still very challenging. This is mainly because the aging progress is influenced by not only personal gene but also many external factors. Physical condition, living style etc. may accelerate or slow aging process. Besides, since aging process is slow and with long duration, collecting sufficient data for training is a fairly strenuous work.
In Greek "prosopon" means face and "agnosia" means not knowing. Prosopagnosia is a disorder of face perception where the ability to recognize faces is impaired, while the ability to recognize other objects may be relatively intact. The specific brain area usually associated with prosopagnosia is the fusiform gyrus. No successful treatment has so far been developed for affected people. Affected individuals often learn to use 'piecemeal' or 'feature by feature' recognition strategies. This may involve secondary clues such as clothing, gait, hair color, body shape, and voice. Because the face seems to function as an important identifying feature in memory, it can also be difficult for people with this condition to keep track of information about people, and socialize normally with others.
The study of prosopagnosia has been crucial in the development of theories of face perception. Because prosopagnosia is not a unitary disorder (i.e., different people may show different types and levels of impairment), it has been argued that face perception involves a number of stages, each of which can be separately damaged. This is reflected not just in the amount of impairment displayed, but also in the qualitative differences in impairment that a person with prosopagnosia may exhibit. Prosopagnosia is usually acquired through extensive neurological damage.