Processing of changes in visual speech in the human auditory cortex

Seeing a talker's articulatory gestures may affect the observer's auditory speech percept. Observing congruent articulatory gestures may enhance the recognition of speech sounds [J. Acoust. Soc. Am. 26 (1954) 212], whereas observing incongruent gestures may change the auditory percept phonetically as occurs in the McGurk effect [Nature 264 (1976) 746]. For example, simultaneous acoustic /ba/ and visual /ga/ are usually heard as /da/. We studied cortical processing of occasional changes in audiovisual and visual speech stimuli with magnetoencephalography. In the audiovisual experiment congruent (acoustic /iti/, visual /iti/) and incongruent (acoustic /ipi/, visual /iti/) audiovisual stimuli, which were both perceived as /iti/, were presented among congruent /ipi/ (acoustic /ipi/, visual /ipi/) stimuli. In the visual experiment only the visual components of these stimuli were presented. A visual change both in audiovisual and visual experiments activated supratemporal auditory cortices bilaterally. The auditory cortex activation to a visual change occurred later in the visual than in the audiovisual experiment, suggesting that interaction between modalities accelerates the detection of visual change in speech. (C) 2002 Elsevier Science B.V. All rights reserved.