Sight is a very powerful sense and easily dominates our lives, and since 1976, researchers have known that it can actually override at least our sense of hearing. The McGurk effect is when someone hears one sound, but sees something associated with another sound, and believes the other sound is what they actually heard. Researchers at the University of Utah have recently found why the brain does this, which could have impacts on understanding how we learn language and how dyslexia develops.
To capture the McGurk effect's source, the researchers placed electrodes on the brains of four subjects, who were undergoing brain surgery for severe epilepsy. With the electrodes monitoring brain signals, videos were played for the subjects of a mouth saying the syllables 'ba,' 'va,' 'ga,', and 'tha,' but sometimes the audio was different than what the lips actually said. When the difference was obvious, like in a badly dubbed movie, the subjects would report hearing the correct sound. When the difference was not obvious though, such as the audio being for 'va' but the video saying 'ba,' the subjects reported the sound they saw spoken, instead of what they heard.
The data from the electrodes showed that whenever the sound matched the video or was obviously different, the brain used the audio to determine what it heard. When the McGurk effect occurred though, the brain activity changed to use the video to determine what it heard. This could have implications in understanding language processing in humans as the infant brain develops to learn a language, or leads to dyslexia.
Source: University of Utah