On the biphasic nature of the N400-P600 complex underlying language comprehension

The ERP literature on language comprehension reveals variability in observing monophasic N400 versus biphasic N400-P600 effects in response to incongruent input, with the reasons for this inconsistency remaining unclear. Two interrelated factors may contribute: spatiotemporal overlap between the N400 and P600, where a strong N400-effect can obscure the P600, and the P600's sensitivity to depth of processing, as determined by the experimental setting. Building on previous findings reporting monophasic N400-effects with plausibility judgments, we investigated whether comprehension questions, encouraging more natural reading and deeper processing of the full content, would elicit a biphasic effect, suggesting reduced component overlap in such settings. Using a design fully crossing lexical association and plausibility, we found that the N400 is modulated by association and the P600 by plausibility. Crucially, a biphasic pattern emerged for implausible and unrelated words, suggesting a mitigation of component overlap compared to previous studies employing plausibility judgments. We interpret the results in light of current accounts of the N400 and P600, arguing that the empirical evidence strongly supports single-stream over multi-stream models. Importantly, our findings highlight the critical role of both component overlap and task demands in shaping the data that inform the development and evaluation of theoretical models.