An illusion by mistake
The McGurk effect was an accidental discovery. In 1976, at the University of Surrey's psychology department, cognitive psychologist Harry McGurk and his research assistant John MacDonald were studying how infants perceive speech at different stages of development. Their work involved playing audio and video recordings of mothers speaking simple syllables. A technician preparing the experimental tapes made an error, dubbing the sound of a person saying "ba-ba" onto a video of the same person articulating "ga-ga".
When McGurk and MacDonald reviewed the mismatched tape, they did not perceive the auditory "ba" or the visual "ga". Instead, they both clearly perceived the syllable "da-da". This surprising result was not a fault of the equipment, but a product of the brain's attempt to make sense of conflicting sensory information. The discovery demonstrated a fusion of auditory and visual information in speech perception. Their subsequent paper, "Hearing lips and seeing voices," published in the journal Nature in December 1976, detailed this newly found illusion.