[2] Many people are affected differently by the McGurk effect based on many factors, including brain damage and other disorders.
It was first described in 1976 in a paper by Harry McGurk and John MacDonald, titled "Hearing Lips and Seeing Voices" in Nature (23 December 1976).
The McGurk effect arises during phonetic processing because the integration of audio and visual information happens early in speech perception.
Some people, including those that have been researching the phenomenon for more than twenty years, experience the effect even when they are aware that it is taking place.
[13] In people with lesions to the left hemisphere of the brain, visual features often play a critical role in speech and language therapy.
[12] People with lesions in the left hemisphere of the brain show a greater McGurk effect than normal controls.
[12] There is a lack of susceptibility to the McGurk illusion if left hemisphere damage resulted in a deficit to visual segmental speech perception.
[14] Integration only appears if visual stimuli is used to improve performance when the auditory signal is impoverished but audible.
[15] The smaller McGurk effect may be due to the difficulties dyslexics have in perceiving and producing consonant clusters.
[15] Children with specific language impairment show a significantly lower McGurk effect than the average child.
[16] They use less visual information in speech perception, or have a reduced attention to articulatory gestures, but have no trouble perceiving auditory-only cues.
A reason for the smaller effect in this population is that there may be uncoupled activity between anterior and posterior regions of the brain, or left and right hemispheres.
[22] People with aphasia show impaired perception of speech in all conditions (visual-only, auditory-only, and audio-visual), and therefore exhibited a small McGurk effect.
[23] The greatest difficulty for aphasics is in the visual-only condition showing that they use more auditory stimuli in speech perception.
[23] A small study (N=22 per group) showed no apparent difference between individuals with bipolar disorder and those without, with respect to the McGurk effect.
[24] Discrepancy in vowel category significantly reduced the magnitude of the McGurk effect for fusion responses.
[25] This could be because /a/ has a wide range of articulatory configurations whereas /i/ is more limited,[25] which makes it much easier for subjects to detect discrepancies in the stimuli.
[31] If a male face is dubbed with a female voice, or vice versa, there is still no difference in strength of the McGurk effect.
[32] When a person was expecting a certain visual or auditory appearance based on the semantic information leading up to it, the McGurk effect was greatly increased.
[33] While looking at oneself in the mirror and articulating visual stimuli while listening to another auditory stimulus, a strong McGurk effect can be observed.
[33] In the other condition, where the listener speaks auditory stimuli softly while watching another person articulate the conflicting visual gestures, a McGurk effect can still be seen, although it is weaker.
[2] In order for the McGurk effect to become insignificant, the listener's gaze must deviate from the speaker's mouth by at least 60 degrees.
[37][40][41][42][43][44] The cultural practice of face avoidance in Japanese people may diminish the McGurk effect, as well as tone and syllabic structures of the language.
[37][42] Regardless, listeners of all languages resort to visual stimuli when speech is unintelligible; the McGurk effect then applies to them equally.
The McGurk effect is still present in individuals with impaired hearing or using cochlear implants, although it is quite different in some aspects.
By measuring an infant's attention to certain audiovisual stimuli, a response that is consistent with the McGurk effect can be recorded.
[10][48] The strength of the McGurk effect displays a developmental pattern that increases throughout childhood and extends into adulthood.