言語機能の局在論をめぐって

Localization of Language Function in the Brain

はじめに

　自然言語は，文法・意味・音韻などを中心としたいくつかの独立した「モジュール（機能単位）」により構成されていると考えられている。中でも文法（syntax）は，「再帰的計算（recursive computation）」という特異的なプロセスで人間の言語を特徴づけている。再帰的計算とは，新たな文や句を再び元の文や句の中に埋め込むことである。この言語の本質を看破し「生成文法」に始まる文法理論を主導してきたChomsky¹⁾は，新たな文を無限に生成できる言語能力に着目して「言語の創造的側面」と述べている。

　近年の脳機能イメージング技術の進展と実験パラダイムの精緻化によって，単語に関する音韻や意味のモジュールだけでなく，文法機能のモジュールの局在も明らかになりつつある。これにより，古典的な発話・理解モデルに基づく失語症の病因についても再検討する必要が出てきた。実際，文法処理に選択的な脳活動の解析と，損傷研究との融合によって，これまで議論の多かった文法障害の存在がはっきりしている。本稿では言語の脳科学における新しい展開について紹介しながら，人間の言語について理解を深めたい。

Abstract

　Since the first report of an aphasic patient by Paul Broca, the localization of brain function has been disputed for 150 years. In lesion studies, double dissociation has been a key concept to show the localization of particular cognitive functions. The advancement of non-invasive brain imaging methods enables us to investigate the brain activities under well-controlled conditions, further promoting the studies on the localization of the cognitive functions, including language function. Brain imaging studies, together with subtraction and correlation analyses, have accumulated evidence that syntax, phonology, and sentence comprehension are separately processed by modules in different cortical regions. More specifically, it has been clarified that the module for syntax localizes in the left lateral premotor cortex and the opercular/triangular parts of the left inferior frontal gyrus. This modular structure further suggests that aphasia is interpreted as deficits in either syntactic or phonological processing. Therefore, the classical model of contrasting speech production and comprehension should be updated.

　According to theoretical linguistics,on the other hand,the recursive computation of syntactic structures is an essential feature of human language faculty. One direction of research would be to contrast human beings and animals for the abilities of processing symbolic sequences. Another direction is to clarify that the human brain is indeed specialized in language processing,which can be revealed by well-controlled language tasks and functional imaging techniques. Here we will review recent studies that demonstrate the existence of grammar center in the left frontal cortex. The future studies in the neuroscience of language will eventually elucidate the cortical localization of language function in a more precise way,i.e.,what is really computed in the human brain.