Localizing the distributed language network responsible for the N400 measured by MEG during auditory sentence processing
We studied auditory sentence comprehension using magnetoencephalography while subjects listened to sentences whose correctness they had to judge subsequently. The localization and the time course of brain electrical activity during processing of correct and semantically incorrect sentences were estimated by computing a brain surface current density within a cortical layer for both conditions. Finally, a region of interest (ROI) analysis was conducted to determine the time course of specific locations. A magnetic N400 was present in six spatially different ROIs. Semantic anomalies caused an exclusive involvement of the ventral portion of the left inferior frontal gyrus (BA 47) and left pars triangularis (BA 45). The anterior parts of the superior (BA 22) and inferior (BA 20/21) temporal gyri bilaterally were activated by both conditions. The activation for the correct condition, however, peaked earlier in both left temporal regions (approximately 32 ms). In general, activation due to semantic violations was more pronounced, started later, and lasted longer as compared to correct sentences. The findings reveal a clear left-hemispheric dominance during language processing indicated firstly by the mere number of activated regions (four in the left vs. two in the right hemisphere) and secondly by the observed specificity of the left inferior frontal ROIs to semantic violations. The temporal advantage observed for the correct condition in the left temporal regions is supporting the notion that the established context eases the processing of the final word. Semantically incorrect words that do not fit into the context result in longer integration times.