Differential integration efforts of mandatory and optional sentence constituents.

We investigated the online sensitivity of the semantic integration system to the different roles played by sentence constituents that are necessary (verbs and nouns) or optional (adjectives) for argument completion. We compared the effect of semantic incongruities introduced in both types of words on the N400 ERP component. Participants read sentences for meaning, half of which were rendered anomalous by an incongruent verb, noun, or an early/late adjective. Incongruent adjectives led to smaller N400 effects than did incongruent nouns and verbs, and the congruity effect for sentence-final adjectives was not significant. All incongruities are therefore not created equal: Incongruent optional sentence constituents create less of an integrative burden than incongruent mandatory sentence constituents, suggesting that online sentence integration processes are sensitive to the distinct roles played by different words in shaping sentence meaning.