"Virtual N400" and slow wave topography to auditory sentence incongruence.
In a simple auditory sentence paradigm, two ERP components were found to be sensitive to violations of sentence congruence: a "virtual N400" and the Slow Wave (SW). Unlike previous studies, the N400 was found to occur very early (peak latency of 250 msec) and have a short duration (196-287 msec), while maintaining the previously described slightly asymmetric right central-parietal negative topography. Because of the early latency, this component was referred to as a "virtual N400." The SW (418-530 msec) was also enhanced and topographically shifted to incongruent sentence terminal words. Its topography consisted of a symmetric frontal minimum and posterior maximum. Although the difference map was also symmetric, a t value map of the congruence effect was maximal over the left posterior temporal area. The very early latency of the "virtual N400" and its decreased duration were produced by a paradigm that allowed for very early recognition of a semantic violation, even before the spoken word was complete. It is postulated that the "virtual N400" indexes word recognition. Semantic processing in indexed by the "virtual N400" as well, but the left posterior temporal asymmetry in the t maps of the Slow Wave may be of even greater import.