Decoding the Neural Dynamics of Headed Syntactic Structure Building

The brain builds hierarchical phrases during language comprehension; however, the representational details and dynamics of the phrase-building process remain underspecified. This study directly probes whether the neural code of verb phrases involves reactivating the syntactic property of a key subcomponent (the "head" verb). To this end, we train a part-of-speech sliding-window neural decoder (verb vs. adverb) on EEG signals recorded while 30 participants (17 females) read sentences in a controlled experiment. The decoder reaches above-chance performance that is spatiotemporally consistent and generalizes to unseen data across sentence positions. Appling the decoder to held-out data yields predicted activation levels for the verbal "head" of a verb phrase at a distant non-head word (adverb); the critical adverb appeared either at the end of a verb phrase or at a sequentially and lexically matched position with no verb phrase boundary. There is stronger verb activation beginning at ∼600 milliseconds at the critical adverb when it appears at a verb phrase boundary; this effect is not modulated by the strength of conceptual association between the two subcomponents in the verb phrase nor does it reflect word predictability. Time-locked analyses additionally reveal a negativity waveform component and increased beta-delta inter-trial phase coherence, both previously linked to linguistic composition, in a similar time window. With a novel application of neural decoding, our findings delineate the dynamics by which the brain encodes phrasal representations by, in part, reactivating the representation of key subcomponents. We thus establish a link between cognitive accounts of phrasal representations and electrophysiological dynamics.Significance Statement Neuroimaging studies suggest that the brain constructs hierarchical linguistic representations. However, current evidence does not specify the details of minimal hierarchical units, namely phrases. On the other hand, theoretical consensus postulates phrases represented with properties derived from a key subcomponent, so-called the "head". Here, we explore the neural code of headed phrases. Leveraging advances in neural decoding, this study introduces a training-prediction pipeline to probe the activation dynamics of the phrasal head in electrophysiological recordings. Our analysis provides novel evidence regarding the neural representation of phrases that, at phrasal boundaries, the head of a phrase is reactivated and integrated into the higher-level representation. This is a fundamental step to understanding the neural bases of language comprehension at the sentence level.