Left parietal alpha enhancement during working memory-intensive sentence processing

Both functional magnetic resonance imaging (fMRI) and event-related brain potential (ERP) studies have shown that verbal working memory plays an important role during sentence processing. There is growing evidence from outside of sentence processing that human alpha oscillations (7-13 Hz) play a critical role in working memory. This study aims to link this to the sentence processing domain. Time-frequency analyses and source localization were performed on electroencephalography (EEG) data that were recorded during the processing of auditorily presented sentences involving either a short or a long distance between an argument (subject or object) and the respective sentence-final verb. We reasoned that oscillatory activity in the alpha band should increase during sentences with longer argument-verb distances, since decreased temporal proximity should result in increased memory demands. When verbal working memory-intensive long-dependency sentences were compared to short-dependency sentences, a sustained oscillatory enhancement at 10 Hz was found during storage prior to the sentence-final verb, turning into a transient power increase in the beta band (13-20 Hz) at the sentence-final verb. The sources of the alpha oscillations were localized to bilaterally occipital and left parietal cortices. Only the source activity in the left parietal cortex was negatively correlated with verbal working memory abilities. These findings indicate that the parsimonious role of alpha oscillations in domain-general working memory can be extended to language, that is, sentence processing. We suggest that the function of left parietal cortex underlying verbal working memory storage during sentence processing is to inhibit the premature release of verbal information that will subsequently be integrated.