Brain areas associated with sentence processing: A functional MRI study and a lesion study

The aim of this study was to identify the location associated with primitive sentence processing. Processing related to generation and comprehension of sentences (“sentence processing”) is postulated to be largely divided into syntactic processing (processing related to the formation of sentences and to verb reflection and particles) and verb information (argument structure and thematic role). Numerous lesion studies and functional brain imaging studies on unimpaired individuals have suggested that the left inferior frontal gyrus (IFG) is involved in syntactic processing. In addition, some studies have reported that the area from the left parietal lobe to the posterior superior temporal gyrus is involved in processing information such as argument structure and thematic role. However, studies on sentence processing using functional brain imaging have used complex sentences as tasks, raising the possibility that the results show the demands on not only language processing, but also on working memory. To clarify the brain areas involved in basic sentence processing in human language, there is a need to examine tasks involved in sentence processing that assume more primitive processing with minimal demands on working memory. The present study used sentence-completion tasks in the Japanese language that include basic sentence processing. The results showed activation in the left IFG and left parietal lobe, suggesting that these areas are involved in sentence processing. We then investigated the proportion of patients showing impaired sentence processing from among patients with aphasia and a lesion in the left IFG or from the parietal lobe to the posterior superior temporal gyrus. Four of 5 patients (80%) with lesions mostly in the left IFG showed impaired sentence processing, suggesting that this site plays a critical role in sentence processing. Of the 4 patients with lesions mostly in the area from the left parietal lobe to the posterior superior temporal gyrus, 1 patient (25%) showed impaired sentence processing. Unlike the other 3 subjects, this subject exhibited impaired recalling of verbs. This area is mainly involved in lexical-semantics and the present results suggest that verb information within that field became impaired, in turn causing impaired sentence processing.

► The study identifies the location associated with primitive sentence processing.
► 14 healthy subjects performed sentence-completion tasks in Japanese and 9 patients with aphasia participated in the lesion study.
► Activation was observed in the bilateral IFG and inferior parietal lobule (IPL).
► 80% left-IFG-lesion-patients and 25% left-IPL-lesion-patients showed impaired sentence processing.
► The left IFG and the left IPL involved in primitive sentence processing.