Neural harmonics reflect grammaticality

Can neural activity reveal syntactic structure building processes and their violations? To verify this, we recorded electroencephalographic and behavioral data as participants discriminated concatenated isochronous sentence chains containing only grammatical sentences (regular trials) from those containing ungrammatical sentences (irregular trials). We found that the repetition of abstract syntactic categories generates a harmonic structure of their period independently of stimulus rate, thereby separating endogenous from exogenous neural rhythms. Behavioral analyses confirmed this dissociation. Internal neural harmonics extracted from regular trials predicted participants9 grammatical sensitivity better than harmonics extracted from irregular trials, suggesting a direct reflection of grammatical sensitivity. Instead, entraining to external stimulus rate scaled with task sensitivity only when extracted from irregular trials, reflecting attention-capture processing. Neural harmonics to repeated syntactic categories constitute the first behaviorally relevant, purely internal index of syntactic competence.