Separating the visual sentence N400 effect from the P400 sequential expectancy effect: cognitive and neuroanatomical implications.

The N400 is an event-related potential (ERP) component that is elicited by semantically meaningful stimuli; one of its defining characteristics is that it is amplified for sentence completions that are semantically unexpected or incongruous with the preceding context. Some prior reports using visual sentence reading paradigms have suggested that there may also be a Cz-centered P400 (a P400cz) that is also responding to semantic congruity manipulations, distinct from the classic Pz-centered N400 (the N400pz). In the present experiment, sentences were presented visually one word at a time, and half of the sentences ended with a semantically incongruent ending. High-density 129-channel event-related potential data were recorded from 26 participants. A combination of temporo-spatial principal components analysis (PCA) and item averaging was applied to decompose the waveforms. The presence of the P400cz was confirmed. The P400cz was much more sensitive to congruity and somewhat more sensitive to cloze probability than the N400pz. The separation of the N400 semantic effect into these two portions is consistent with both MEG studies and intracranial studies. The data suggest that the N400pz has its major source in the bilateral anterior medial temporal lobe (AMTL) whereas it is suggested that the P400cz has its major source in the medial parietal region. It is further suggested that whereas the N400pz process appears to be semantic in nature, some prior reports suggest that the P400cz reflects a general sequential expectancy system.