Modulation of the N400 potential during auditory phonological/semantic interaction.

The processing of phonological and semantic word attributes has been commonly explored with electrophysiological methods using simple contexts where competition between features is eliminated. Conversely, the interaction between phonological and semantic attributes has not been systematically examined. We therefore recorded an event-related electrophysiological marker of word discordance, the N400, in response to sequences of auditory word pairs containing semantic incongruences, phonological discordances, or a mixture of the two. N400 enhancement to semantically unrelated words was systematically observed, whether the subjects heard the sequences passively (no instruction) or actively (semantic judgement task), and even in contexts where the task did not concern semantic attributes. In contrast, the N400 effect to phonologically unrelated (non-rhyming) words was exclusively obtained in the active situation (phonological judgment), while it disappeared in passive conditions and during semantic/phonological interference. This suggests that the detection of semantic incongruences is a more robust and automatized mechanism than that of phonological ones, and tends to occlude this latter when both features are in competition. Our data also provide new elements supporting the persistence of the semantic N400 during 'shallow' word processing tasks, i.e. tasks that discourage analysis of semantic aspects of the words.