|The processing of relative clauses (RCs) has been an important topic owing to its grammatical complexity as well as its rich typology. Extensive research has found that there is a processing asymmetry between the two most common types of RCs, subject-extracted relative clauses (SRCs) and object-extracted relative clauses (ORCs). Comparison of SRCs and ORCs, which differ minimally in word order, provides us valuable materials to investigate how the human brain interprets sentences with different levels of processing difficulty. A great deal of research has indicated that SRCs are consistently easier to comprehend than ORCs in Indo- European languages, which seems to raise a very attractive proposition of a universal preference for SRCs; however, it has been a long-standing debate about whether Chinese SRCs are also easier to process than ORCs. Moreover, the neural correlates underlying the processing of Chinese RCs have been poorly understood before. Therefore, one aim of this dissertation is to investigate the processing preference pattern between Chinese SRCs and ORCs by conducting a series of self-paced reading experiments (Chapter 2) and also explore the neural mechanisms underlying the comprehension of Chinese RCs by running fMRI experiments (Chapter 3 and 4). Another aim is to elaborate on how the human brain dynamically adjusts itself for comprehending Chinese RCs with different levels of processing difficulty by applying a newly-developed single-trial analysis (Chapter 5).
The present findings from a series of self-paced reading experiments (Chapter 2) demonstrated that there was a processing asymmetry between Chinese SRCs and ORCs and Chinese ORCs were easier to comprehend than Chinese SRCs. The ORC preference in Chinese thus poses a challenge to the theoretical approaches that predict a universal SRC advantage and also highlight the importance of investigating language-specific processing mechanisms in sentence comprehension. Besides, these behavioral findings set a reference for interpretations of the neural mechanisms underlying the processing of Chinese SRCs and ORCs.
The neuroimaging studies (Chapter 3 and 4) then presented clear neuroimaging evidence, for the first time in the literature, to support the ORC preference in Chinese, as reflected in enhanced activation of the LIFG and the LSTG during the comprehension of Chinese SRCs than ORCs. In addition to crucial roles of the LIFG and the LSTG, these neuroimaging results also indicate that enhanced effective connectivity from the LIFG to the LSTG may prominently contribute to the comprehension of Chinese SRCs that are more difficult than ORCs.
Further, through a single-trial analysis by combining fMRI and eye-tracking data (Chapter 5), the strength of the connectivity from the LIFG to the LSTG was found to be significantly correlated with the processing difficulty of Chinese SRCs. The findings further confirm our assumption that the effective connectivity from the LIFG to the LSTG plays an important role during the processing of RC structures. Enhanced strength of this connectivity is claimed to reflect increased integration demands and restructuring attempts for sentence comprehension.
In sum, the findings of this dissertation contribute to the understanding of the subject-object asymmetry in Chinese and the neural mechanisms underlying the comprehension of Chinese RCs. Moreover, the additional sources of evidence from a single-trial analysis reveal that brain activation in between the LIFG and the LSTG can dynamically attune to different levels of processing difficulty. Besides, the collected behavioral and neuroimaging evidence from the same set of stimuli in a series of studies demonstrate that grammatically different sentences but with subtle differences in semantic content can be detected in brain activation patterns. Therefore, in addition to traditional measures (i.e., reading time, eye fixation time), the dynamic networking pattern change might be another useful index to differentiate the difficulty level of sentence comprehension, as what we found in the current neuroimaging studies.