構文処理の脳内メカニズム

Brain mechanisms of syntactic processing

　本論文では，構文処理の脳内メカニズムに関する様々な臨床的ならびに実験的研究を紹介した。まず，自然言語の主要な特徴についてまとめた後，ブローカ失語の特徴に関して述べた。次に自然言語およびアメリカ手話言語の意味理解における言語中枢（特にブロードマン44野，45野，40野，22野）の役割に関して，ニューロイメージング研究および言語領野の電気刺激を用いた研究を概観した。これらの研究から，ブローカ領野が構文処理に関わっていることが明らかになった。最後に「運動系列予測学習仮説」（乾，1998）を紹介した。

　In this paper, we have reviewed clinical and experimental research on the mechanisms of syntactic processing in the brain. Firstly, the main features of natural language are summarized. Secondly, symptoms of Broca's aphasia are reviewed. Thirdly, neuroimaging and electrical stimulation studies of language areas；especially the role of Brodmann's areas 44, 45, 40, and 22, in understanding the meaning of sentences and the American Sign Language are reviewed in detail：Evidence from which indicate that the Broca's area is involved in syntactic processing. Finally, we have introduced the'motor sequence prediction learning hypothesis'（Inui, 1998）,w hich can explain several types of aphasias on the basis of experimental knowledge regarding the functions of the brain. In this model, the region that generates articulation and hand movement is the posterior part of the Broca's area, namely BA 44, also known as the pars opercularis of the inferior frontal gyrus. What then is the role of BA 45―the pars triangularis―within Broca's area?We believe that visual and auditory information are transformed into articulatory and hand activities in this region, and that BA 45 works to predict motor sequences. In other words, the pars triangularis of the Broca's area transforms visual and auditory information into information for the individual's articulatory and/or motor behavior；and by means of interactions with the working memory, permit the prediction of subsequent motor sequences. Furthermore, the interaction between BA 40 and 44 transforms between motor information and auditory and visual information. I have argued that BA 40 performs these functions through interactions with the Wernicke's area within Brodman's area 22. The motor programs for the production of silent, or inner speech are generated and activated in BA 44. Via the articulatory-to-auditory transformation occurring at the supramarginal gyrus, we can“hear”our own inner speech. This process is known to be fundamental to various thinking behaviors of human beings.