Where and When are the Heard and Seen Speech Integrated: Magnetoencephalographical (MEG) Studies

We have recently recorded neuromagnetic responses of the human brain when the subjects both hears and sees speech. When we presented infrequent deviant discordant pairings (auditory /pa/ together with visual /ka/) among frequent concordant stimuli (auditory and visual /pa/), the deviants elicited a specific response, which was argued to be generated in the auditory cortex. In another experiment, the subjects were experienced lip readers and the concordant and discordant stimuli were also delivered equiprobably. The measured MEG signals were complex and showed a lot of individual variability. However, some subjects showed a distinct response to a discordant stimulus both when presented equiprobably with concordant stimuli and when presented as an infrequent deviant. 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 100-ms response (M 100), may be activated by the discordant stimuli.