McGurk effect
What you see a mouth do changes what you hear it say.
What it means
The McGurk effect is a multisensory illusion in which seeing a speaker's mouth produce one sound while hearing a different sound yields the perception of a third, blended sound. It demonstrates that speech perception is not purely auditory but integrates visual information from the lips automatically and irresistibly, even when the observer knows the stimuli are mismatched. The fused percept arises because the brain combines concurrent auditory and visual cues into a single best estimate of what was said. The effect is robust across languages, though its strength varies with culture, attention, and individual differences. It is a foundational demonstration of audiovisual integration and of perception as the fusion of multiple senses.
Examples
Dubbing the sound 'ba' onto a face mouthing 'ga' makes most listeners hear 'da' — a percept present in neither channel.
A badly dubbed film sounds faintly wrong even with clean audio: the mouth says one thing, the soundtrack another, and your brain keeps trying to fuse the two into one voice.
Knowing the trick changes nothing. A lecturer can tell the audience exactly which sound is playing, and they still hear the blend — until they shut their eyes.
First described in Harry McGurk & John MacDonald (1976).