What is the McGurk effect?

McGurk and MacDonald (1976) reported a powerful multisensory illusion occurring with audiovisual speech. They recorded a voice articulating a consonant and dubbed it with a face articulating another consonant. Even though the acoustic speech signal was well recognized alone, it was heard as another consonant after dubbing with incongruent visual speech. The illusion has been termed the McGurk effect. It has been replicated many times, and it has sparked an abundance of research. The reason for the great impact is that this is a striking demonstration of multisensory integration. It shows that auditory and visual information is merged into a unified, integrated percept. It is a very useful research tool since the strength of the McGurk effect can be taken to reflect the strength of audiovisual integration. Here I shall make two main claims regarding the definition and interpretation of the McGurk effect since they bear relevance to its use as a measure of multisensory integration. First, the McGurk effect should be defined as a categorical change in auditory perception induced by incongruent visual speech, resulting in a single percept of hearing something other than what the voice is saying. Second, when interpreting the McGurk effect, it is crucial to take into account the perception of the unisensory acoustic and visual stimulus components. There are many variants of the McGurk effect (McGurk and MacDonald, 1976; MacDonald and McGurk, 1978)1. The best-known case is when dubbing a voice saying [b] onto a face articulating [g]