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In everyday communication, natural spoken sentences are expressed in a multisensory way through auditory signals and speakers' visible articulatory gestures. An important issue is to know whether audiovisual speech plays a main role in the linguistic encoding of an utterance until access to meaning. To this end, we conducted an event-related potential experiment during which participants listened passively to spoken sentences and a lexical recognition task. The results revealed that N200 and N400 waves had a greater amplitude after semantically incongruous words than after expected words. This effect of semantic congruency was increased over N200 in the audiovisual trials. Words presented audiovisually also elicited a reduced amplitude of the N400 wave and a facilitated recovery in memory. Our findings shed light on the influence of audiovisual speech on the understanding of natural spoken sentences by acting on the early stages of word recognition in order to access a lexical-semantic network.

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Speakers use prosodic emphasis to express the content of their message in order to help listeners to infer meaning. By measuring event-related potentials (ERPs) to semantically congruent and incongruent final words embedded in a sentential context that was emphasized or de-emphasized, we investigated whether prosodic emphasis conveyed by a sentential context leads listeners to a finer semantic analysis. The negative shift (N400) triggered by the difficulty to combine the incongruent word with the sentence representation was increased by prosodic emphasis at an early stage. Over the later stages, the amplitude of the N400 wave was increased by prosodic emphasis of the sentential context, whatever the semantic congruency of final words. As shown by the N400 wave, emphasizing a sentential context affected the lexical-semantic processing of the following word. This study provides clear evidence that prosodic emphasis plays a role in the semantic analysis of sentences by inducing a deeper analysis.

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