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 can be directly modulated by changes in speech intelligibility caused by spectral degradation. In the current study, we manipulate prior knowledge to increase the intelligibility of physically identical speech sentences and test the hypothesis that the tracking responses can be enhanced by this intelligibility improvement. Cortical magnetoencephalography (MEG) responses to intelligible speech followed by either the same (matched) or different (unmatched) unintelligible speech were measured in twenty-three normal hearing participants. Driven by prior knowledge, cortical coherence to "abstract" linguistic units with no accompanying acoustic cues (phrases and sentences) was enhanced relative to the unmatched condition, and was lateralized to the left hemisphere. In contrast, cortical responses coherent to word units, aligned with acoustic onsets, were bilateral and insensitive to contextual information changes. No such coherence changes were observed when prior experience was not available (unintelligible speech before intelligible speech). This dissociation suggests that cerebral responses to linguistic information are directly affected by intelligibility, which in turn are powerfully shaped by physical cues in speech. These results provide an objective and sensitive neural index of speech intelligibility, and explain why previous studies have reported no effect of prior knowledge on cortical entrainment.