Electrophysiological evidence for differences between fusion and combination illusions in audiovisual speech perception

Incongruent audiovisual speech stimuli can lead to perceptual illusions such as fusions or combinations. Here, we investigated the underlying audiovisual integration process by measuring ERPs. We observed that visual speech‐induced suppression of P2 amplitude (which is generally taken as a measure of audiovisual integration) for fusions was similar to suppression obtained with fully congruent stimuli, whereas P2 suppression for combinations was larger. We argue that these effects arise because the phonetic incongruency is solved differently for both types of stimuli. Integration of audiovisual (AV) speech is characterized by lip‐read‐induced suppression of the electrophysiological auditory P2. We show equal P2 suppression for incongruent McGurk fusions (AbVg perceived as ‘d’) and congruent speech, whereas suppression for McGurk combinations (AgVb perceived as ‘bg’ or ‘gb’) is larger. This difference is likely related to a difference in processing of phonetic AV congruency for fusions and combinations.