In addition to providing exquisite timing information, ERP measures provide a means of assessing the qualitative similarity of two (or more) effects -- e.g., at different processing levels. N400 studies offered critical evidence for both temporal and qualitative similarity between the effects of a word prime and those of a sentence context on word processing. Studies showed that the morphology, timing, and topography of the visual N400 semantic priming effect for target words following semantically related versus unrelated primes were indistinguishable from those for the visual N400 effect to sentence final words of congruent versus anomalous sentences. This was an especially important finding because, on most accounts, word level priming was thought to be mediated by automatic spreading activation or at least by some process internal to the mental lexicon, impervious to any external context. Sentence context and other top-down effects, by contrast, were assumed to exert their influence via qualitatively different controlled processes, acting upon the representation of already “recognized” words – that is, post-lexically. Direct, within-subject comparisons of lexical and sentential N400 context effects, however, strongly argued against the possibility that they arose from different stages of language processing (Kutas 1993).
