Event-related Brain Potentials Suggest a Late Interaction of Meter and Syntax in the P600

Many studies refer to the relevance of metric cues in speech segmentation during language acquisition and adult language processing. However, the on-line use (i.e., time-locking the unfolding of a sentence to EEG) of metric stress patterns that are manifested by the succession of stressed and unstressed syllables during auditory syntactic processing has not been investigated. This is surprising as both processes rely on abstract rules that allow the building up of expectancies of which element will occur next and at which point in time. Participants listened to metrically regular sentences that could either be correct, syntactically incorrect, metrically incorrect, or doubly incorrect. They either judged syntactic correctness or metric homogeneity in two different sessions. We provide first event-related potential evidence that the metric structure of a given language is processed in two stages as evidenced in a biphasic pattern of an early frontal negativity and a late posterior positivity. This pattern is comparable to the biphasic pattern reported in syntactic processing. However, metric cues are processed earlier than syntactic cues during the first stage (LAN), whereas both processes seem to interact at a later integrational stage (P600). The present results substantiate the important impact of metric cues during auditory syntactic language processing.