In 1976 psychologist Harry McGurk discovered that seeing a person speak affects our impression of the sound we hear. Faced with conflicting information, the brain seems to make its “best guess” as to what it’s perceiving. In some cases a third sound is produced: When the syllables /ba-ba/ are spoken over the lip movements /ga-ga/, the perception is /da-da/.

This casts doubt on the assumption that the senses operate separately and can be studied in isolation. Psychologists and philosophers are still considering the implications.

(Harry McGurk and John MacDonald, “Hearing Lips and Seeing Voices,” Nature 264:5588 [1976], 746.)