Propagation of the N400 to subsequent words after lexical anomalies depends on the syntactic attachment point.

The N400 component is widely interpreted as an index of lexical processing, whereas its role in sentence-level meaning integration remains a subject of major debate. Moreover, there is growing evidence that Late Positive Component (LPC) is also elicited by the effort to integrate local meaning within the sentence structure. Here, we present an EEG study designed to test the sensitivity of both components to higher-order syntactic structures such as dependencies. The main question we addressed was whether either the N400 or LPC would propagate to upcoming words in a sentence after encountering a lexical anomaly, and whether such a cascading effect would be modulated by constituent structure. We created 200 Italian sentences in which the object noun could be either semantically correct or anomalous. Additionally, half of the last prepositional phrases (PPs) of the sentences depended on the object determiner phrase (DP) (ext-DP), while the other half functioned as verbal complements (VP-mod), resulting in four experimental conditions: Correct VP-mod (e.g., A customer books this table for tomorrow), Correct ext-DP (e.g., The postman collects many letters of best wishes), Semantic VP-mod (e.g., A customer books this enemy for tomorrow), and Semantic ext-DP (e.g., The postman collects many cultures of best wishes). Results revealed a significant N400 at the object noun and an LPC when comparing correct to semantically anomalous sentences. LPC extended to all subsequent words in anomalous sentences, continuing until sentence end in both ext-DP and VP-mod conditions –indicating a propagation independent of PP dependency. In contrast, N400 effects at the noun of the PP were observed only in the Semantic ext-DP condition. These findings suggest that, once a lexical anomaly is introduced, semantic integration costs cascade across the sentence; however, the N400 appears more sensitive than the LPC to constituent structure and syntactic dependency.