Theta Oscillations Underpin Syntactic Encoding in First and Second Language Sentence Production

Successful language communication relies on efficient syntactic encoding to produce well-structured sentences, yet the real-time neural dynamics of this process, particularly in second language (L2) learners, are poorly understood. Here, we used scalp EEG to investigate the neural dynamics of syntactic encoding in first language (L1) and L2 production of Mandarin Chinese. Participants prepared sentences of varying syntactic complexity during a cued-delay period, allowing us to isolate syntactic encoding from articulation. We found qualitatively distinct neural dynamics between groups. Time-resolved multivariate decoding revealed that L1 speakers formed robust and sustained complex syntactic representations from the onset of sentence planning, whereas L2 representations were delayed and transient. This divergence was underpinned by theta-band oscillations, which flexibly tracked the subjective syntactic computational load for each group. Furthermore, L1 speakers engaged a specialized left-lateralized frontotemporal network, while L2 speakers recruited a compensatory bilateral frontal-posterior network. These results initially provide a multi-level account of syntactic encoding, demonstrating that L1 and L2 sentence production rely on fundamentally different neural computations and establishing that theta-band phase coherence is a core mechanism for building hierarchical syntactic structure.