Abstract
Although the connection between the left inferior frontal gyrus (LIFG) and the left superior temporal gyrus (LSTG) has been found to be essential for the comprehension of relative clause (RC) sentences, it remains unclear how the LIFG and the LSTG interact with each other, especially during the processing of Chinese RC sentences with different processing difficulty. This study thus conducted a 2 × 2 (modifying position × extraction position) factorial analyses to examine how these two factors influences regional brain activation. The results showed that, regardless of the modifying position, greater activation in the LIFG was consistently elicited in Chinese subject-extracted relative clauses (SRCs) with non-canonical word order than object-extracted relative clauses (ORCs) with canonical word order, implying that the LIFG subserving the ordering process primarily contributes to the processing of information with increased integration demands due to the non-canonical sequence. Moreover, the directional connection between the LIFG and the LSTG appeared to be modulated by different modifying positions. When the RC was at the subject-modifying position, the effective connectivity from the LIFG to the LSTG was dominantly activated for sentence comprehension; whereas when the RC was at the object-modifying position thus being more difficult, it might be the feedback mechanism from the LSTG back to the LIFG that took place in sentence processing. These findings reveal that brain activation in between the LIFG and the LSTG may be dynamically modulated by different processing difficulty and suggest the relative specialization but extensive collaboration involved in the LIFG and the LSTG for sentence comprehension.
Highlights
One of the challenging questions in the neuroscience of language has been how the human brain processes syntactically complex sentences
As for reaction time (RT), the contrast between the OM-subject-extracted relative clauses (SRCs) (1308.66 ± 241.77 ms) and OM-object-extracted relative clauses (ORCs) (1292.2 ± 200.48 ms) conditions did not result in any significant difference (t (18) = 0.262, p = 0.797)
Regardless of the modifying position, Chinese SRCs with non-canonical word order were consistently more difficult to process than the ORCs, as reflected in increased brain activation in the left inferior frontal gyrus (LIFG) and the left superior temporal gyrus (LSTG) and the directional connectivity between them
Summary
One of the challenging questions in the neuroscience of language has been how the human brain processes syntactically complex sentences. Because in Chinese SRCs (e.g., [renshi zhangsan de siji] weifanle guiding), the relative clause (RC) did not follow canonical word order (i.e. verb + object + subject), greater processing costs were required during the storage and ordering processing to construct argument-verb relations For this reason, increased activation in and between the LIFG and the LSTG was inferred to facilitate the comprehension of the SRCs. On the other hand, for the ORCs with canonical word order (e.g., [zhangsan renshi de siji] weifanle guiding), incremental integration of all the upcoming words was sufficient for the reading; less activation in between the LIFG and the LSTG was involved in the processing of the ORCs compared to the SRCs. Based on accumulating evidence, it appeared that complex sentence processing was probably driven by word-order analysis, supported by the LIFG [27, 28]. 規定 guīdıng rule sentence complexity and brain connectivity information flow between the LIFG and the LSTG and attempted to answer which of the directional connections between the LIFG and the LSTG could be modulated by different processing difficulty involved in different types of RC sentences
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