Spatio-temporal distribution of brain activity associated with audio-visually congruent and incongruent speech and the McGurk Effect.

INTRODUCTION Spatio-temporal distributions of cortical activity to audio-visual presentations of meaningless vowel-consonant-vowels and the effects of audio-visual congruence/incongruence, with emphasis on the McGurk effect, were studied. The McGurk effect occurs when a clearly audible syllable with one consonant, is presented simultaneously with a visual presentation of a face articulating a syllable with a different consonant and the resulting percept is a syllable with a consonant other than the auditorily presented one. METHODS Twenty subjects listened to pairs of audio-visually congruent or incongruent utterances and indicated whether pair members were the same or not. Source current densities of event-related potentials to the first utterance in the pair were estimated and effects of stimulus-response combinations, brain area, hemisphere, and clarity of visual articulation were assessed. RESULTS Auditory cortex, superior parietal cortex, and middle temporal cortex were the most consistently involved areas across experimental conditions. Early (<200 msec) processing of the consonant was overall prominent in the left hemisphere, except right hemisphere prominence in superior parietal cortex and secondary visual cortex. Clarity of visual articulation impacted activity in secondary visual cortex and Wernicke's area. McGurk perception was associated with decreased activity in primary and secondary auditory cortices and Wernicke's area before 100 msec, increased activity around 100 msec which decreased again around 180 msec. Activity in Broca's area was unaffected by McGurk perception and was only increased to congruent audio-visual stimuli 30-70 msec following consonant onset. CONCLUSIONS The results suggest left hemisphere prominence in the effects of stimulus and response conditions on eight brain areas involved in dynamically distributed parallel processing of audio-visual integration. Initially (30-70 msec) subcortical contributions to auditory cortex, superior parietal cortex, and middle temporal cortex occur. During 100-140 msec, peristriate visual influences and Wernicke's area join in the processing. Resolution of incongruent audio-visual inputs is then attempted, and if successful, McGurk perception occurs and cortical activity in left hemisphere further increases between 170 and 260 msec.