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.
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.
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.
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.
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