According to io9, The McGurk Effect illustrates how it is possible for the brain to hear the wrong sound if it is shown visual evidence that something else is being said. The effect is named after Harry McGurk from a paper he wrote entitled "Hearing Lips and Seeing Voices."
The McGurk Effect is an auditory illusion. The simplest way to demonstrate how the effect works is to have three people in a room. One person is the observer, the second person does the visual effect and the third person does the audio effect.
The observer faces the second person, the visual effect person. The third person, the audio effect person, stands behind the second person. The second person mouths the word "vase." Simultaneously, the third person says "base."
Even though "base" is what is vocalized, the McGurk Effect demonstrates that the observer hears "vase" instead, influenced by what they are observing visually.
The McGurk Effect does not work on everyone. It is most effective with people using a portion of their left brain known as the superior temporal sulcus, or STS. Io9 reports that researchers at the University of Texas discovered that the STS processes language and eye gazing together. Then they used magnetic stimulation to disrupt the functions of the STS, and the McGurk Effect no longer happened.