反応選択と前頭前野

Response selection and prefrontal cortex

前頭前野は複雑な状況での反応決定に重要な役割を果たしていると言われてきたが，その神経機構は明らかになっていない。筆者らは，サルにコンフリクトがある状況とない状況で判断を行わせ，そのときの前頭前野のニューロン活動を記録・解析した。その結果，前頭前野ニューロン，特に反応選択にかかわる外側部のニューロンは，コンフリクトがある場合にのみ行動決定に関与していることがわかった。このことは，ムシモル局所注入による可逆的破壊実験によっても支持された。

The prefrontal cortex（PFC）is believed to be a brain region for decision-making, particularly in complex situations. Although some recording studies have shown that prefrontal neurons support decision-making even in simple situations,s ome lesion studies disagree with this view. To understand the role of the PFC, particularly the lateral prefrontal cortex（LPFC）, in decision-making, we recorded the neuronal activity from this region while the monkey performed a go/no-go task with a multidimensional stimulus. Each stimulus had two visual dimensions：color and direction of motion；in turn, each dimension had a go feature（e. g., green in the color dimension and upward movement in the motion dimension）and a no-go feature（e. g., red and downward movement, respectively）. The monkey was required to attend to one of the visual dimensions in this two-dimensional stimulus. For congruent trials, the monkey was presented with go features both in the relevant and irrelevant dimensions. For the incongruent trials, the monkey was presented with a go feature in the relevant dimension but a no-go feature in the irrelevant dimension. Analyses focused on the activity of neurons that discriminated the go stimulus from the no-go stimulus in the color dimension. For congruent stimuli, we observed no differences in go/no-go differential activity for correct and incorrect trials. For incongruent stimuli, on the other hand, the discrimination of go/no-go differential neurons was correlated with behavioral performance-the lower the discrimination, the lower the performance. Moreover, inactivation study using muscimol micro-injection revealed that the LPFC dysfunction led to decreased performance only in incongruent trials. These results suggest that the brain has several independent pathways to calculate an appropriate behavior for a given situation and that the LPFC output is critical for decision-making only in complex situations. This theory can explain why prefrontal deficits lead to stereotyped behaviors such as imitation and utilization.