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Role of Broca’s Subregions in Syntactic Processing: A Comparative Study of Japanese Patients with Lesions in the Pars Triangularis and Opercularis

Background/Aim: To clarify whether Broca’s subregions, comprising the pars triangularis and pars opercularis, play different roles in syntactic processing, we investigated Japanese infarct patients with selective damage in each subregion. Methods: Four patients participated in this study: 3 showing lesions mostly restricted to the pars opercularis (Op group) and a rare case with a lesion mostly restricted to the pars triangularis (Tr). Using the Object Manipulation Task, scores and error types were compared between the Tr and the Op patients. Results: No marked differences in total score were evident between the Tr and Op patients. However, the Tr patient showed greater difficulty with dative sentences than the Op patients. Conversely, the Tr patient showed rather high scores with relative sentences, whereas the Op patients were rarely able to manipulate items correctly for relative sentences. In addition, the Tr patient tended to posit items in canonical word order incorrectly, whereas the Op group tended to make simplification errors. Conclusion: Broca’s subregions play different roles in syntactic processing. Considering Japanese sentence structure, the results indicate that the pars triangularis is involved in comprehension of semantic relationships among syntactically equivalent nominal phrases assigned thematic roles, while the pars opercularis is involved in thematic role transformation and/or reinterpretation.