Where agreement merges with disagreement: fMRI evidence of subject-verb integration.

Language comprehension is incremental, involving the integration of information from different words together with the need to resolve conflicting cues when unexpected information occurs. The present fMRI design seeks to segregate the neuro-anatomical substrates of these two processes by comparing well-formed and ill-formed sentences during subject-verb agreement computation. Our experiment takes advantage of a particular Spanish feature, the Unagreement phenomenon: a subject-verb agreement mismatch that results in a grammatical sentence ("Los pintores trajimos…" [The painters3.pl (we)brought1.pl…]). Comprehension of this construction implies a shift in the semantic interpretation of the subject from 3rd-person to 1st-person, enabling the phrase "The painters" to be re-interpreted as "We painters". Our results include firstly a functional dissociation between well-formed and ill-formed sentences with Person Mismatches: while Person Mismatches recruited a fronto-parietal network associated to monitoring operations, grammatical sentences (both Unagreement and Default Agreement) recruited a fronto-temporal network related to syntactic-semantic integration. Secondly, there was activation in the posterior part of the left middle frontal gyrus for both Person Mismatches and Unagreement, reflecting the evaluation of the morpho-syntactic match between agreeing constituents. Thirdly, the left angular gyrus showed increased activation only for Unagreement, highlighting its crucial role in the comprehension of semantically complex but non-anomalous constructions. These findings point to a central role of the classic fronto-temporal network, plus two additional nodes: the posterior part of the left middle frontal gyrus and the left angular gyrus; opening new windows to the study of agreement computation and language comprehension.