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McGurk Effect
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Overview
The McGurk effect describes a phenomena that shows how speech perception is not dependent solely on auditory information. Visual information in the form of reading the lips is also taken into account and combined with the auditory information that is heard to produce the final stimuli perceived. This can get particularly interesting when the auditory information of one sound, paired with the spoken lips of another sound, ultimately combine to form the perception of a third different sound.

Explanation
When humans perceive speech, they not only take in auditory information but also visual information as well in the form of reading lips, facial expression, and other body bodily cues. Usually, these two sources of information are consistent with each other so the brain simply combines them to form one unified perceived stimuli. When the McGurk effect is tested and the incoming information from the ears and eyes differ, the brian tries to make sense of the contradictory stimuli which ultimately results in a fusion of both. In humans, information received from the eyes dominates other sensory modalities, including audition, so for instance when 'ba' is heard and 'ga' is seen, the resulting stimuli is heard is 'da'. . The resulting stimuli is what happens when the brain tries to make sense of the two different sets of information.

History
Before the McGurk Effect was discovered, the motor theory of speech perception was initially conceived in the 1950s by Alvin Liberman and Franklin S. Cooper. This theory suggests that people perceive speech not only from auditory information but also by facial movements and vocal tract gestures. The theory was later expanded by Donald Shankweiler, Michael Studdert-Kennedy, Ignatius Mattingly, Carol Fowler, and Douglas Whalen.

Harry McGurk
Initially, Harry McGurk's (February 23 1936 – April 17 1998) field of psychological research was human development, focusing especially on infant behavior. In the early 1970's, interested in how infants perceived the world, he ran numerous infant perception studies. Specifically, his early work dealt with unimodal visual perception by infants. He found that infants sensory information was "combined cross-modally as the result of operations on the environment or more generally, experience." At the same time, two researchers by the name of Aronson and Rosenbloom published a paper in Science stating that infants' spatial perception occurs within a common auditory-visual space. This view by Aronson and Rosenbloom was for the most part consistent with the currently accepted view at the time which was that all the senses were essentially unified as one. This view though, contradicted what McGurk had found which was that "the unity of the senses, especially in speech, is a product of experience." In response to Aronson and Rosenbloom's difference in findings, McGurk and Macdonald decided to run a similar test in 1974 which tested 1-, 4-, and 7-month old infants in a similar fashion. Once again, the results from McGurk and Macdonalds' experiment did not confirm Aronson and Rosenbloom's findings.

What McGurk and Macdonald decided to do next though set the stage for the accidental discovery of what is now known as the McGurk effect. After rejecting Aronson and Rosenbloom's ideas about intermodal spatial dislocation, they decided to test the unity of the senses hypothesis by pitting the senses against each other.

''"As documented in the accompanying address, McGurk took videotapes of productions of /ba/ and /ga/ and had them dubbed to produce not just the matching ba-voice/ba-lips, and the ga-voice/ga-lips sequences, but also conflicting ba-voice/ga-lips and ga-voice/ba-lips sequences. What is not documented in McGurk’s address is his displeasure when the videotapes returned from the auditory-visual centre at Surrey. Harry said he thought the technician had made a mistake, and that he told the technician so in no uncertain terms. He soon realized however that there had been no mistake and that he was experiencing something quite remarkable, a “da” percept when presented with an auditory [ba] and visual [ga], and a “bga” percept from an auditory [ga] and visual [ba]." ''

After this monumental realization in 1976, McGurk and Macdonald decided to scrap their intermodal conflict studies with infants and instead publish their findings of visual speech perception in a paper titled "Hearing Lips and Seeing Voices" in the journal Nature.

Similar Findings by Others
Interestingly enough, around the same time that Harry McGurk and John Macdonald discovered what is now known as the McGurk effect, another British researcher also stumbled upon this phenomena. Barbara Dodd discovered a similar effect with audio-visual speech interpretation but instead it was with the visual cue of 'hole' and the audio cue of 'tough' which ultimately generated the audio perception of 'towel'. In 1987 Barbara Dodd along with Ruth Campbell then published these findings along with other contributions from researchers also studying audio-visual speech perception in an edited book titled Hearing by Eye: The Psychology of Lip-Reading. These discoveries in audio-visual speech perception ultimately changed the way scientists and researchers view the interaction of different senses in the brain.

Infants
Interestingly enough, infants also show signs similar to the McGurk effect. Obviously you cannot ask the infant what they hear since they cannot verbally communicate yet but by measuring certain variables such as their attention to audiovisual stimuli, effects similar to the McGurk effect can be seen. Very soon after infants are born, sometimes even within minutes of birth, they are able to imitate adult facial movements; an important first step in audiovisual speech perception. Next comes the ability to recognize lip movements and speech sounds a couple of weeks after birth. Evidence of the McGurk effect is not visible though until about 4 months of age, with a much stronger presence at around 5 months after birth. To test this effect on infants, infants are first habituated to a stimuli. Once the stimuli gets changed, the infant exhibits an effect similar to the McGurk effect. As infants grow older and continue to develop, the McGurk effect also becomes more prominent as visual cues start to override purely auditory information in audiovisual speech perception.

Effect in Other Languages and Cultures
Although the McGurk effect has been primarily studied in English because of it's origins in English speaking countries, research has now spread to others countries with different languages as well. In particular, the comparison between English and Japanese has been prominent. Research has shown that the McGurk effect is much more prominent in English listeners compared to Japanese listeners. One strong hypothesis for this is the difference between cultures and how each culture behaves and interacts. Japanese culture is notable for being politeness and avoiding direct eye or face contact when interacting.

This phenomena has also been studied in French Canadian children and adults with similar findings. When compared to adults though, children tend to show less susceptibility to the McGurk effect since their primary sense of speech perception is dominated by auditory information. This is evident in children scoring lower on lip reading tasks when compared to adults. Nevertheless, the McGurk effect was present in certain contexts but the effects were much more variable than when the tests were run on adults.

Broader Impact on Society
Although the McGurk effect's importance may seem isolated to just psychological researchers and scientists, this phenomena has been expanded to everyday audio and visual speech perception as well. Two researchers by the name of Wareham and Wright conducted a study in 2005 that may suggest that the McGurk effect can influence how everyday speech is perceived. This is especially important in witness testimony where the observations and accounts of the witness are usually expected to be accurate and correct. With this information, witness testimonies now must be interpreted with the notion that the witness may be unaware of their own inaccurate perception.

Video Examples
http://www.youtube.com/watch?v=G-lN8vWm3m0

http://www.youtube.com/watch?v=jtsfidRq2tw