Formant transition-specific adaptation by lipreading of left auditory cortex N1m.

To test for the feature specificity of adaptation of auditory-cortex magnetoencephalographic N1m responses to phonemes during lipreading, we presented eight healthy volunteers with a simplified sine-wave first-formant (F1) transition shared by /ba/, /ga/, and /da/, and a continuum of second-formant (F2) transitions contained in /ba/ (ascending), /da/ (level), and /ga/ (descending), during lipreading of /ba/ vs. /ga/ vs. a still-face baseline. N1m responses to the F1 transition were suppressed during lipreading, further, visual /ga/ (vs. /ba/) significantly suppressed left-hemisphere N1m responses to the F2 transition contained in /ga/. This suggests that visual speech activates and adapts auditory cortex neural populations tuned to formant transitions, the basic sound-sweep constituents of phonemes, potentially explaining enhanced speech perception during lipreading.