Watch the video below. You’ll hear either “ga-ga” or, more likely, “da-da.” Now, close your eyes and listen again. You’ll hear it as “ba-ba.”
This is known as the McGurk effect, and works by offering inconsistent auditory and visual information. The audio of “ba” is placed over video of someone mouthing “ga.” Faced with this conflict, the listener will hear the intermediate phoneme “da.” The illusion was identified by Harry McGurk and John MacDonald in a 1976 Nature paper. Before then, speech perception was viewed as always and everywhere an auditory phenomenon.