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.
Your brain on "da"
The McGurk effect reveals how the brain processes conflicting sensory inputs. When hearing speech, your brain integrates information from multiple sources. In this illusion, the eyes see a mouth making a velar consonant like /g/, where the tongue is at the back of the palate. The ears, however, hear a bilabial consonant like /b/, made with the lips.
The brain's solution is to compute a "best fit" perception. It resolves the conflict by creating a third, entirely new percept: the alveolar consonant /d/, which is formed with the tongue just behind the teeth. This location is intermediate between the lips (/b/) and the soft palate (/g/). Neuroimaging studies using fMRI show that the left superior temporal sulcus (STS) is a brain region for this process of integrating audio and visual speech cues. Temporary disruption of the STS can significantly reduce a person's susceptibility to the illusion. The effect is automatic; even when you are fully aware of the illusion, you continue to perceive "da".