Demonstration, Part 1

As noted in the text, the McGurk effect, named after its discoverer, Harry McGurk (McGurk and MacDonald, 1976), has to be seen and/or heard to be believed. The stimulus for the effect is quite simple: a video of human face, or just a human mouth, saying a speech sound, combined with an audio track of a human voice saying a speech sound.

The movie at left includes just such a combined video and audio track. To start the demonstration, play the movie with the sound on your computer turned on, so that you both see and hear the movie.

The movie is only a few seconds long, and will loop over and over. Once you’ve viewed and heard it for a while, stop the movie and click a link at left to choose which speech sound you heard.

bah | dah | gah | lah

Demonstration, Part 2

Now play the movie again, but this time close your eyes or hide the movie so that you can hear but can’t see what the mouth is saying.

Once you’ve listened to it a few times, stop the movie and click a link below to choose which speech sound you heard.

bah | dah | gah | lah

Demonstration, Part 3

For the final part of the demonstration the video has been muted. Play the movie again, and click a link below to choose which speech sound you think you are seeing the mouth say.

bah | dah | gah | lah

Conclusion

Here’s what is actually going on in this movie:

1. The audio track is playing the sound “bah”
2. The mouth is articulating the sound “gah”

When most people listen to the audio alone (with their eyes closed) and view the video alone (with the sound muted), these are exactly the sounds they see and hear. What most people perceive when viewing the video and listening to the audio, however, is a third speech sound, either “dah” or “lah.”

When this effect was first reported in the mid-1970s, it was taken as evidence for the “motor theory” of speech perception, which postulates that we perceive speech by somehow running the processes that produce speech in reverse. While the motor theory is not generally accepted by speech perception researchers today, the McGurk effect is still interesting as a great example of cross-modal perception. Clearly, information from your visual system is altering what your auditory system is perceiving (and/or vice versa).