Lexical meaning, grammatical meaning, and formal-syntactic rules elicit different types of late positive ERPs

In language processing, word meanings are retrieved from memory and converted into sentential expressions that have referential meaning at the level of utterances. Cognitive processes related to lexical retrieval versus the construction of referential meaning through grammar have not been properly disentangled yet. We addressed this with an experimental design aimed to link well-known N400 effects and late positive components (LPCs) to this distinction. We tested the hypothesis that semantic conflicts derived from a collision with stored (= ‘offline’) knowledge would elicit N400 effects, while semantic conflicts arising ‘online’ during the grammatical construction of referential meaning would exclusively trigger LPCs and also differ in their latency, amplitude or topography from formal-syntactic violations. Three anomalous conditions were compared against a baseline of correct sentences, across five word-positions within stimulus sentences: semantic anomalies that are lexically- but not grammatically-driven (I arrived in a button last week), semantic anomalies that are grammatically- but not lexically-driven (I arrived in a London last week), and formal-syntactic anomalies (I arrived in a cities last week). Results showed, in line with predictions, that (i) only the lexically-driven anomalies elicited N400 components, while (ii) all three anomalous conditions elicited LPCs, extending from the target word to subsequent word positions in the sentence; and (iii) the LPCs were all distinct in latency, amplitude and spatiotemporal topographies. These results demonstrate for the first time that grammar-level meaning, which is referential rather than lexical-conceptual in nature, has its own distinctive ERP, while confining the role of the N400 to effects of stored, or else contextually updated, memory.