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Continuous Theta-Burst Stimulation (cTBS) on the Left Posterior Inferior Frontal Gyrus Revealed a Trend of Selective Inhibition on Complex Syntactic Processing in Mandarin Chinese
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Poster D27 in Poster Session D, Wednesday, October 25, 4:45 - 6:30 pm CEST, Espace Vieux-Port
Luyao Chen1,2, Yao Cheng1, Xingfang Qu1, Gesa Hartwigsen1, Junjie Wu3; 1Beijing Normal University, 2Max Planck Institute for Human Cognitive and Brain Sciences, 3Tianjin Normal University
Objective: Human language is hierarchical in nature. The hierarchical structures of the sentences are assumed to be built up via Merge, a binary syntactic operation that combines two elements to form a new constituent. The left posterior inferior frontal gyrus (LpIFG) is proposed to support Merge, a shared syntactic region across languages. However, whether LpIFG plays a causal role in syntactic processing in Mandarin Chinese, and whether the causality would be modulated by the syntactic complexity rather than the working memory effects were unspecified yet. Therefore, we set out to answer these questions by administering the transcranial magnetic stimulation (TMS) with the inhibitory continuous theta-burst stimulation (cTBS) protocol over LpIFG to assess the behavioral changes of Mandarin Chinese syntactic processing. Methods: 32 healthy adult Chinese native speakers underwent this within-subject experiment, composed of a stimulation session and a sham session with the order counter-balanced across participants (the interval between the sessions was 7 days). In the cTBS session (i.e., the stimulation condition), the TMS was administered to LpIFG with the cTBS protocol that triplets of TMS pulses at 50 Hz were delivered at 5 Hz, giving a 40 s train of 600 pulses in total. The sham stimulation (i.e., the sham condition) was performed by flipping the coil over with the settings of cTBS. In each session, participants were required to process syntactically-complex sentences with relative clauses embedded [i.e., the complex syntactic processing condition, including subject relative clause (SR) and object relative clause (OR) processing sub-conditions], semantically-matched coordinate sentences (i.e., the simple syntactic processing condition), and word lists (i.e., the word-list condition). As for the syntactic processing conditions, participants should judge whether the content of the probing sentence was correct according to the previously-presented sentence, and in the word-list condition, participants were asked to recall and to judge whether the word and its given position were matched, that is, a working memory task. Results & Discussion: The behavioral changes (“real cTBS - sham”, denoted as “Δ”) were compared in the conditions. The Δkurtosis of reaction time (RT) was significantly reduced and the Δcoefficient of variation was increased under the complex syntactic processing condition, especially when the subject relative clauses were embedded in the sentences (i.e., the SR processing sub-condition). No Δd-prime differences were identified. These results suggested that (1) inhibition of the left pIFG activity might cause the change in the stability of syntactic processing, the more complex, the more unstable (thus with a larger variation of the RT distribution), (2) the processing differences were not qualitative as implied by the null results of Δd-prime, possibly since cTBS on the healthy participants would not cause a real impairment on the behavioral performances, and that (3) the causal role of the left pIFP should be specific to syntactic processing (as shared by Mandarin Chinese), but not to the working memory capacity. To sum up, this study demonstrated the causal role of LpIFG as a core syntactic region universally backing up complex syntactic processes across languages.
Topic Areas: Syntax and Combinatorial Semantics, Reading