Electrophysiological evidence for speech-specific audiovisual integration

Lip-read speech is integrated with heard speech at various neural levels. Here, we investigated the extent to which lip-read induced modulations of the auditory N1 and P2 (measured with EEG) are indicative of speech-specific audiovisual integration; and we explored to what extent the ERPs were modulated by phonetic audiovisual congruency. In order to disentangle speech-specific (phonetic) integration from non-speech integration, we used Sine-Wave Speech (SWS) that was perceived as speech by half of the participants (they were in speech-mode), while the other half was in non-speech mode. Results showed that the N1 obtained with audiovisual stimuli peaked earlier than the N1 evoked by auditory-only stimuli. This lip-read induced speeding up of the N1 occurred for listeners in speech and non-speech mode. In contrast, if listeners were in speech-mode, lip-read speech also modulated the auditory P2, but not if listeners were in non-speech mode, thus revealing speech-specific audiovisual binding. Comparing ERPs for phonetically congruent audiovisual stimuli with ERPs for incongruent stimuli revealed an effect of phonetic stimulus congruency that started at similar to 200 ms after (in)congruence became apparent. Critically, akin to the P2 suppression, congruency effects were only observed if listeners were in speech mode, and not if they were in non-speech mode. Using identical stimuli, we thus confirm that audiovisual binding involves (partially) different neural mechanisms for sound processing in speech and non-speech mode. (C) 2013 Elsevier Ltd. All rights reserved.