Task-dependent and automatic tracking of hierarchical linguistic structure

Linguistic phrases are tracked in sentences even though there is no clear acoustic phrasal marker in the physical signal. This phenomenon suggests an automatic tracking of abstract linguistic structure that is endogenously generated by the brain. However, all studies investigating linguistic tracking compare conditions where either relevant information at linguistic timescales is available, or where this information is absent altogether (e.g., sentences versus word lists during passive listening). It is therefore unclear whether tracking at these phrasal timescales is related to the content of language, or rather, is a consequence of attending to the timescales that happen to match behaviourally-relevant information. To investigate this question, we presented participants with sentences and word lists while recording their brain activity with MEG. Participants performed passive, syllable, word, and word-combination tasks corresponding to attending to rates they would naturally attend to, syllable-rates, word-rates, and phrasal-rates, respectively. We replicated overall findings of stronger phrasal-rate tracking measured with mutual information (MI) for sentences compared to word lists across the classical language network. However, in the inferior frontal gyrus (IFG) we found a task-effect suggesting stronger phrasal-rate tracking during the word-combination task independent of the presence of linguistic structure, as well as stronger delta-band connectivity during this task. These results suggest that extracting linguistic information at phrasal-rates occurs automatically with or without the presence of an additional task, but also that that IFG might be important for temporal integration across various perceptual domains.