前頭葉と抑制機能

The Frontal Lobes and Inhibitory Function

はじめに

　前頭葉が関与すると考えられる認知機能はいくつか知られている^1,2)。その1つに，その個体にとって優位であるが状況的には不適切な反応や行動を抑制する機能，すなわち“抑制機能”がある。例えばGo/No-Go課題では，個体がもともと持っているGo反応の傾向を抑制することが求められる。この抑制機能は特に，“反応抑制”と呼ばれている。また，Wisconsinカード分類課題（Wisconsin card sorting test：WCST）では，色・形・数というカテゴリのうちの1つに基づいてカードを分類する行動を確立した後，新たに別のカテゴリに基づいてカードを分類するために，ひとたび確立したカード分類行動を抑制することが必要とされる。本稿では，このような状況で発現する抑制機能の機能分化と前頭葉内の機能局在について，神経心理学および神経イメージングからの知見を概説する。

Abstract

　Neuropsychological studies using traditional tasks of inhibitory functions,such as the Wisconsin card sorting test (WCST) and the Go/No-Go Task have revealed that the frontal lobe is responsible for several types of inhibitory functions. However,the detailed psychological nature of the inhibitory functions and the precise location of their critical foci within the frontal lobe remain to be investigated. Functional magnetic resonance imaging provides spatial and temporal resolution that allowed us to illuminate at least 4 frontal regions involved in inhibitory functions: the dorsolateral,ventrolateral,and rostral parts of the frontal lobe and the presupplementary motor area (preSMA). The ventrolateral part of the frontal lobe in the right hemisphere was activated during response inhibition. The preSMA in the left hemisphere was activated during inhibition of proactive interference immediately after the dimension changes of the WCST. The rostral part of the frontal lobe in the left hemisphere was activated during inhibition long after the dimension changes. The dorsolateral part of the frontal lobe in the left hemisphere was activated at the dimension changes in the first time,but not in the second time. These findings provide clues to our understanding of functional differentiation of inhibitory functions and their localization in the frontal lobe.