One of the first major contributions of the N400 to psycholinguistic research was therefore to show that effects of semantic manipulations could be seen almost immediately: the N400 congruity effect began ~200 ms (and peaked before 400 ms) into the processing of a critical word – written, spoken or signed. Furthermore, because ERPs can be examined in response to every stimulus, not just selected targets, they provide an instantaneous and continuous look at language processing. The ERP to every word read one at a time in the center of the screen (RSVP) contains N400 activity, which is affected by context, thereby revealing the inherently incremental nature of language processes. Comparing normal English sentences with those that were syntactically structured but semantically anomalous revealed a linear decline in N400 amplitudes of open class words across the course of a congruent sentence (i.e., word position effect), which thus seemed to reflect the incremental build-up of semantic (and not syntactic) constraints (reviewed in Van Petten 1993).
