The McGurk Effect

Most people think of listening as something done with the ears alone. A discovery made in the 1970s showed that the eyes take part as well.

The psychologist Harry McGurk and his research assistant, John MacDonald, were studying how infants perceive speech. While preparing materials, they asked a technician to combine a video of a woman saying the syllable "ga" with a recording of her voice saying "ba." When they played the result, both researchers were surprised. What they heard was neither syllable. They heard a third one, "da."

The explanation lies in how the brain handles speech. It does not let the ears finish their work and only then check the eyes. Instead, it combines both sources of information at once. In the video, the shape of the lips did not match "ba," so the brain settled on the sound that best fit both the picture and the voice. This process is so automatic that knowing about the illusion does not stop it. A viewer who closes their eyes hears "ba" clearly, yet when they open them again, the "da" often returns.

McGurk and MacDonald described their findings in the journal Nature in 1976, and the illusion is now known as the McGurk effect. It shows that seeing a speaker's face is part of hearing what that speaker says.