A phenomenon that occurs when a speech sound does not match the shape of the lips producing it, as when the sound corresponding to the usual pronunciation of the word gay is dubbed on to a video image of a person uttering the word bay, causing the listener to hear a word intermediate between the two (day).
The effect shows that the visual channel conveys important information not just to deaf people but also to listeners with normal hearing. For those with minor hearing loss, speech reading can be a very valuable way to maximize the hearing they still do have. Also, this reveals more about why watching the mouth is so important in intense language learning.
The phenomenon is named after the Scottish psychologist Harry McGurk (1936-98) who co-authored the first article on it, entitled 'Hearing Lips and Seeing Voices' in the journal Nature in 1976.