Read my lips: asymmetries in the visual expression and perception of speech revealed through the McGurk effect.

It is well known that the right side of the mouth moves more than the left during speech, but little is known about how this asymmetry affects lipreading. We investigated asymmetries in the visual expression and perception of speech using the McGurk effect-an illusion in which incongruent lip movements cause listeners to misreport sounds. Thirty right-handed participants watched film clips in which the left, the right, or neither side of the mouth was covered. The McGurk effect was attenuated when the right side of the mouth was covered, demonstrating that this side is more important to lipreading than is the left side of the mouth. Mirror-reversed images tested whether the asymmetry was the result of an observer bias toward the left hemispace. The McGurk effect was stronger in the normal than in the mirror orientation when the mouth was fully visible. Thus, observers attend more to what they think is the right side of the speaker's mouth. Asymmetries in mouth movements may reflect the gestural origins of language, which are also right lateralized.