Audiovisual Speech Perception

Persons with hearing loss use visual information from articulation to improve their speech perception. Even persons with normal hearing utilise visual information, especially when the stimulus-to-noise ratio is poor. A dramatic demonstration of the role of vision in speech perception is the audiovisual fusion called the ‘McGurk effect’. When the auditory syllable /pa/ is presented in synchrony with the face articulating the syllable /ka/, the subject usually perceives /ta/ or /ka/. The illusory perception is clearly auditory in nature. We recently studied the audiovisual fusion (acoustical /p/, visual /k/) for Finnish (1) syllables, and (2) words. Only 3% of the subjects perceived the syllables according to the acoustical input, ie in 97% of the subjects the perception was influenced by the visual information. For words the percentage of acoustical identifications was 10%. The results demonstrate a very strong influence of visual information of articulation in face-to-face speech perception. Word meaning and sentence context have a negligible influence on the fusion. We have also recorded neuromagnetic responses of the human cortex when the subjects both heard and saw speech. Some subjects showed a distinct response to a ‘McGurk’ stimulus. The response was rather late, emerging about 200 ms from the onset of the auditory stimulus. We suggest that the perisylvian cortex, close to the source area for the auditory 100 ms response (M100), may be activated by the discordant stimuli. The behavioural and neuromagnetic results suggest a precognitive audiovisual speech integration occurring at a relatively early processing level.