Syntactic complexity representation in sentence production reveals a fronto-temporal syntactic network

A milestone of human language faculty is to communicate with syntactically complex sentence structures. Although various syntactic complexity representations have been studied in language comprehension, a clear framework that systematically organizes syntactic complexity into different levels and the corresponding neural substrates during sentence production are largely unknown. Therefore, this functional magnetic resonance imaging (fMRI) study investigated the neural substrates of syntactic processing during production responsive to a well-organized hierarchy of syntactic complexity: Sentence versus Word list, Complex sentence versus Simple sentence, and Subject relative clause versus Object relative clause. Thirty healthy adult Cantonese native speakers underwent the picture-description task during scanning. Behavioral results showed that producing sentences with higher syntactic complexity was associated with greater processing difficulty (i.e., lower accuracy and longer reaction time). Results of the brain activation analysis, peak intensity analysis, and of the effective connectivity modeling converged on a critical fronto-temporal syntactic network, including the left inferior frontal gyrus (IFG), middle frontal gyrus (MFG), and posterior temporal lobe (PostTemp), in managing the increased demands of syntactic structure building. Moreover, the role of the left MFG in adapting to the increased difficulty of syntactic structure building during sentence production was identified. Taking together, this study specified a fronto-temporal syntactic network in sentence production by establishing a well-controlled syntactic complexity hierarchy, and thus sheds further lights on the neural underpinnings of the remarkable complex human language faculty.