If your ears register /ba/ and your eyes a mouth saying /ga/, you’ll “hear” /da/. It’s called the McGurk effect:
The most striking demonstration of the combined (bimodal) nature of speech understanding appeared by accident. Harry McGurk, a senior developmental psychologist at the University of Surrey in England, and his research assistant John MacDonald were studying how infants perceive speech during different periods of development. For example, they placed a videotape of a mother talking in one location while the sound of her voice played in another. For some reason, they asked their recording technician to create a videotape with the audio syllable “ba” dubbed onto a visual “ga.” When they played the tape, McGurk and McDonald perceived “da.” Confusion reigned until they realized that “da” resulted from a quirk in human perception, not an error on the technician’s part. After testing children and adults with the dubbed tape, the psychologists reported this phenomenon in a 1976 paper humorously titled “Hearing Lips and Seeing Voices,” a landmark in the field of human sensory integration. This audio-visual illusion has become known as the McGurk effect or McGurk illusion.
Amazing. (Via Sally Thomason at Language Log.)