Hierarchical Model Comparisons on Effective Connectivity in Semantic Judgments of Chinese Characters

In functional magnetic resonance imaging (fMRI), dynamic causal modelling (DCM) and hierarchical model comparisons were used to investigate the effective connectivity during semantic judgments to visual Chinese characters. Thirty-five participants were asked to indicate if character pairs were related in meaning. Experimental stimuli were character pairs that included semantically-related and semantically-unrelated pairs. In this study, DCM was used to examine the directional influences among brain regions, and hierarchical model comparisons were used to seek the optimal model with parsimonious connections in the semantic network. In conventional fMRI analysis, participants showed activations in left ventral inferior frontal gyrus (vIFG, BA 47), left dorsal inferior frontal gyrus (dIFG, BA 9), left posterior middle temporal gyrus (pMTG, BA 21) and left fusiform gyrus (FG, BA 37). Based on a fully connected model among these four regions, we built sixteen types of models that differed in the modulation of the ”related” or ”unrelated” conditions on specific connections. From hierarchical model comparisons, the results of the optimal model showed significant modulatory effects from vIFG to pMTG, suggesting top-down influences of the frontal cortex on retrieval of semantic representations. Significant modulatory effects was also found from FG to pMTG, suggesting bottom-up influences of orthographic representations on semantic representations.