The present study investigates the status of rhythmic irregularities occurring in natural speech and the importance of rhythmic alternations in cognitive processing. Previous studies showed the relevance of rhythm for language processing, but there has been only little research using the method of event-related potentials to investigate this phenomenon in a natural metrical context. To this end, an experiment was conducted in which the so-called Rhythm Rule (alternation of stressed and unstressed syllables) was either met or violated by stress clashes or stress lapses which are known to occur in German. The comparison of rhythmic well-formed conditions with the conditions including rhythmic irregularities revealed biphasic EEG-patterns for rhythmically marked structures, i.e., stress clashes and lapses. The present results show that irregular but possible rhythmic variants are costly in language processing, reflected by an early negativity and an N400 in contrast to the well-formed control conditions. Supposedly, the early negativity reflects error detection in rhythmical structure and supports the view that the brain is sensitive to subtle violations of rhythmical structure. A late positive component reflects the evaluation process related to the task requirements. The study shows that subtle rhythmical deviations from the Rhythm Rule are perceived and treated differently from well-formed structures during processing, even if the deviation in question is permitted and can therefore occur in language production.
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