Adaptation of neuromagnetic N1 responses to phonetic stimuli by visual speech in humans

The technique of 306-channel magnetoencephalogaphy (MEG) was used in eight healthy volunteers to test whether silent lip-reading modulates auditory-cortex processing of phonetic sounds. Auditory test stimuli (either Finnish vowel /ae/ or /ø/) were preceded by a 500 ms lag by either another auditory stimulus (/ae/, /ø/ or the second-formant midpoint between /ae/ and /ø/), or silent movie of a person articulating /ae/ or /ø/. Compared with N1 responses to auditory /ae/ and /ø/ when presented without a preceding stimulus, the amplitudes of left-hemisphere N1 responses to the test stimuli were significantly suppressed both when preceded by auditory and visual stimuli, this effect being significantly stronger with preceding auditory stimuli. This suggests that seeing articulatory gestures of a speaker influences auditory speech perception by modulating the responsiveness of auditory-cortex neurons.