“The McGurk effect is a perceptual phenomenon which demonstrates an interaction between hearing and vision in speech perception. It suggests that speech perception is multimodal, that is, that it involves information from more than one sensory modality.”
This effect may be experienced when a video of one phoneme‘s production is dubbed with a sound-recording of a different phoneme being spoken. Often, the perceived phoneme is a third, intermediate phoneme. For example, a visual /ga/ combined with an audio /ba/ is often heard as /da/. Further research has shown that it can exist throughout whole sentences. The effect is very robust; that is, knowledge about it seems to have little effect on one’s perception of it. This is different from certain optical illusions, which break down once one ‘sees through’ them.
Study into the McGurk effect is being used to produce more accurate speech recognition programs by making use of a video camera and lip reading software. It has also been examined in relation to witness testimony; Wareham & Wright’s 2005 study showed that inconsistent visual information can change the perception of spoken utterances, suggesting that the McGurk effect may have many influences in everyday perception.”