記憶における前頭葉の役割

Memory and frontal lobe.

はじめに

　前頭葉に限局した損傷では，健忘が生じないといわれている。それでは，前頭葉は記憶の機能とまったく無関係であろうか。答えは「否」である。最近の研究によれば，前頭葉に限局した損傷で，時間的順序の判断や記憶が選択的に阻害されることが明らかになってきたからである。

　本論文では，記憶における前頭葉の役割を明らかにする目的で，まず，ヒトの前頭葉損傷が時間的文脈にもとづく情報の記憶に及ぼす影響を調べた研究について述べる。次に，健忘患者で前頭葉症状を示す患者では，とりわけ，時間的・空間的文脈に関する情報の処理や再生が悪いことを示唆している研究について論述する。つづいて，サルを用いた動物実験の知見をreviewし，最後に記憶における前頭葉の役割に関して考察する。

It has been acknowledged that lesions restricted to the frontal lobe do not produce an ordinary amnesic syndrome in humans since pure memory test rarely show problems in frontal patients. Several investigators, however, have provided data concerning the role of the frontal lobes in human memory. This article begins with a brief review of data concerning the restricted frontal lesions on memory functions in humans. Next, the data from recent research on amnesic patients with frontal lobe pathology are considered. Thesedata indicate that substantial impairments can be detected when frontal patients are given tasks requiring memory for temporal order or demand-ing temporal discrimination. Thus, it becomes apparent that frontal lobes play a specific role in certain memory functions. In the next section I described the evidence concerning the effects of frontal ablation on certain memory tasks in mon-keys. These studies concerning nonhuman primates indicate (1) that the dorsolateral region of the prefrontal cortex is related specifically to memory for spatio-temporal information, (2) that the ventromedial region is related to visual recogni-tion memory. Additional information concerning the involvement of the frontal lobe in memory comes from the chronic unit recording studies. Neurons in the sulcus principalis area of the dorsolateral prefrontal cortex responded differ-entially depending on the spatial location of the cue. Thus, the sulcus principalis area is special-ized for a particular type of memory function. In the final section a possible role of frontal cortex in memory functions was discussed. It was concluded that the frontal lobes are involved in organizing and establishing the specific spatio-temporal contexts.