Left occipital and right frontal involvement in syntactic category prediction: MEG evidence from Standard Arabic

Though recent years have seen a growth in research on predictive processes in language comprehension, their scope and mechanisms remain partially elusive. While mechanisms involved in predicting specific words are relatively well understood, those underlying syntactic prediction are still unclear. In part, this is because of the difficulty in designing experiments that manipulate syntactic predictability while controlling other variables. In this MEG study, we achieved this with a manipulation of syntactic category predictability within fully well-formed expressions of Standard Arabic. Participants read sentences beginning with a subject-adjective context, in which the presence of at least one of two possible cues (gender-incongruity and/or an intervening relative pronoun) was sufficient for predicting a target word's syntactic category. Absence of both cues (i.e., congruent subject-adjective context with no relative pronoun) increased uncertainty about the target's syntactic category. Using source analysis, we compared activity evoked by targets with predictable and unpredictable categories in the occipital lobe. We found an interaction effect consistent with previous findings: in the primary visual cortex, an early evoked component (visual M100) is enhanced only when the syntactic category was unpredictable. We also compared responses to pre-target predictive and unpredictive contexts across five bilateral frontal and temporal regions. In the right-hemispheric frontal region, we found a temporal cluster (~230 ms after adjective onset), where unpredictive contexts elicited more activation than predictive contexts. By hypothesis elimination, we conclude that the most likely variable driving this effect is syntactic entropy. Our results show that predictive mechanisms recruited during reading also involve predicting upcoming syntactic categories, implicating at least two cortical regions: the left visual cortex and the right frontal cortex.