The waitress that the customer served: Disentangling two accounts of verb processing when nouns have unexpected thematic roles

The N400 brain potential is usually large at unexpected words in a sentence. The N400 semantic illusion arises in sentences such as “the waitress that the customer served”, where the N400 at “served” is small despite it being unexpected that a customer would serve a waitress. The surprisingly small N400 has been interpreted as a transient illusion of plausibility due to the lexico-semantic association of waitress, customer, and served (Kuperberg et al., 2003; Kim & Osterhout, 2005; Rabovsky et al., 2018; and more). Recent evidence, including our own, has shown that delaying the presentation of the verb prevents the illusion (Chow et al., 2018; Nakamura et al., 2024; Stone & Rabovsky, 2024). Liao et al. (2022) propose that this demonstrates the different stages of generating a verb prediction, where predictions are initially based on lexical association and only later constrained by thematic roles. If the verb appears immediately after its arguments, only the first of these steps is complete and so “served” is incorrectly predicted. Since “served” matches the input, only a small N400 is elicited. In contrast, the SG model (Rabovsky et al., 2018) proposes that initial understanding of the sentence is uncertain due to the mismatch between the literal sentence and a lifetime of experience with the usual roles of waitresses and customers. When the verb appears immediately after its arguments, the more probable, experience-based interpretation is still stronger and “served” is considered plausible, resulting in a small N400. These two accounts make similar predictions about N400 amplitude at the verb, making them difficult to disentangle, but doing so is important as they propose different mechanisms about how argument-verb relationships are computed. I will present evidence from a speeded lexical decision task in an attempt to disentangle the accounts by probing the effect that very early processing has on parsing of the verb.