Prior exposure to speech rapidly modulates cortical processing of high-level linguistic structure.

Neural activity has been shown to track hierarchical linguistic units in connected speech, and these responses are modulated by changes in speech intelligibility resulting from spectral degradation. In this study, we manipulated prior knowledge to enhance the intelligibility of physically identical speech sentences and tested whether this improvement would strengthen neural tracking responses. Cortical magnetoencephalography responses were recorded from 23 normal-hearing participants while they listened to intelligible speech followed by either the same (matched) or different (unmatched) unintelligible speech. When prior knowledge was available, cortical coherence at higher-order linguistic rates, particularly phrase and sentence rates, was enhanced relative to the unmatched condition and was predominantly lateralized to the left hemisphere. In contrast, cortical coherence to word-level units, which aligned with acoustic onsets, was bilateral and did not show a significant modulation by contextual information. No such coherence enhancement was observed when unintelligible speech preceded intelligible speech. This dissociation suggests that cerebral tracking of linguistic information is directly influenced by intelligibility, which itself is strongly shaped by physical speech cues. These findings provide an objective and sensitive neural index of speech intelligibility and help explain why previous studies have reported no effect of prior knowledge on cortical entrainment.