Orchestration of language and motor systems in language understanding : neuroimaging investigations

Over the past few decades, neuroimaging studies have provided evidence for the involvement of the motor system in semantic processing. However, the way that the motor system contributes to semantic processing is still understudied. In this dissertation, neuroimaging techniques (fMRI and MEG) were applied to explore the interaction between language and motor systems in reading verb phrases with varying abstractness in native (L1) and second language (L2). By using fMRI, Study I investigated the effect of abstractness (literal/metaphorical/abstract) and language proficiency (L1/L2) on the involvement of the motor system. Results showed attenuated motor activation from literal to abstract via metaphorical phrases in both L1 and L2. In addition, overall greater motor activation was found for L2 than L1. The graded motor involvement modulated by abstractness and the greater motor activation in L2 suggested a dual-functional role of the motor system in semantic processing. By employing MEG, Study II further investigated temporal dynamics of the involvement of language and motor regions in L1 and L2 processing. Results showed greater activation of language areas for L1 than L2 during 300-500 ms and greater activation of motor areas for L2 than L1 at 600-800 ms. No effect of abstractness was found. The underactivation in language areas in the early stage and overactivation in motor areas in the late stage suggested a compensatory role of the motor system in L2 semantic processing, i.e., over-recruitment of the motor system to compensate for the inadequate engagement of the language network. Study III investigated spatiotemporal dynamics of abstract concept processing in L1. Results showed delayed activation in the posterior superior temporal sulcus at 200-300 ms for abstract phrases relative to concrete and metaphorical ones. In addition, greater activation in the anterior temporal pole at 300-400 ms was found for abstract phrases. Findings shed light on the spatiotemporal dynamics of abstract concept processing. In summary, by exploring the time-varying interplay between the motor and language systems, the above studies deepen our understanding of how the motor cortex contributes to language understanding and bring alternative interpretations of the functional role of the motor cortex in language processing.