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McGurk Effect

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McGurk accidentally discovered the phenomenon of the McGurk effect that is also known as audio-visual speech perception. McGurk discovered that visual information had a strong influence on the process of speech perception. Visual information in this instance is concerned with the movements of the lips. This is generally recognised as lip reading

Speech perception is what is heard. He made his discovery whilst setting up an experiment to investigate "ability of infants to integrate auditory and visual information". McGurk was checking his stimuli and incorrectly discovered the replaced words were incorrect yet when he closed his eyes they were correct.

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In the demonstration of the McGurk effect used in the study, the participant is asked to keep his or her eyes closed while listening to a video that shows a person making the sounds "ba ba ba." Then that individual is asked to open their eyes and watch the mouth of the person in the video closely, but with the sound off. Now, the visuals look like the person is saying "ga ga ga." In the final step of the experiment, the exact same video is replayed, but this time the sound is on, and the participant is asked to keep his or her eyes open

People who are sensitive to the McGurk effect will report hearing "da da da" — a sound that doesn't match up with either the auditory or visual cues previously seen. That's because the brain is attempting to resolve what it thinks it's hearing with a sound closer to what it visually sees. If the person closes their eyes again, and the video's sound is replayed, he or she will once again hear the original sound of "ba ba ba." The effect was first described in an experiment done in 1976 by psychologists Harry McGurk and John MacDonald, which showed that visual information provided by mouth movements can influence and override what a person thinks he or she is hearing.

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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. This definition includes all variants of the illusion, and it has been used by MacDonald and McGurk (1978) themselves, as well as by several others (e.g., Rosenblum and Saldaña, 1996; Brancazio et al., 2003). The different variants of the McGurk effect represent the outcome of audiovisual integration. When integration takes place, it results in a unified percept, without access to the individual components that contributed to the percept. Thus, when the McGurk effect occurs, the observer has the subjective experience of hearing a certain utterance, even though another utterance is presented acoustically.

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In conclusion, the McGurk effect is an excellent tool to investigate multisensory integration in speech perception

The main messages of this opinion paper are, first, that the McGurk effect should be defined as a change in auditory perception due to incongruent visual speech, so that observers hear another speech sound than what the voice uttered, and second, that the perceptual properties of the acoustic and visual stimulus components should be taken into account when interpreting the McGurk effect as reflecting integration.

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McGurk H., MacDonald J. (1976). Hearing lips and seeing voices. Nature 264, 746–748

Rosenblum L. D., Saldaña H. M. (1996). An audiovisual test of kinematic primitives for visual speech perception. J. Exp. Psychol. Hum. Percept. Perform. 22, 318–331

Sams M., Aulanko R., Hämäläinen M., Hari R., Lounasmaa O. V., Lu S.-T., et al. (1991). Seeing speech: visual information from lip movements modifies activity in the human auditory cortex. Neurosci. Lett. 127, 141–145

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