言語流暢性課題中の前頭前皮質活動に及ぼす加齢の影響—近赤外分光法を用いた検討

Effects of aging on prefrontal activity during verbal fluency tasks:A near-infrared spectroscopy study

　加齢による認知機能の低下をモニタリングし，認知症を早期にスクリーニングする検査として言語流暢性課題の有用性が示唆されている．本研究では，日常生活を支障なく過ごしている高齢者15名と若年者14名を対象として，カテゴリー流暢性課題（CFT）および文字流暢性課題（LFT）遂行中の前頭前皮質活動を近赤外分光法（24チャンネルNIRS）によって計測し，遂行成績と脳活動に及ぼす加齢の影響を調査した．両課題の生成語数は高齢群で有意に低下した．酸素化ヘモグロビン量の群間比較では有意な年齢の効果は検出されなかった．群ごとに実施した3要因（課題，チャンネル，課題期間）の分散分析の結果，高齢群では背外側前頭前野に位置する一部のチャンネルでoxy-Hb量の有意な増加が認められなかった．若年群に比し，高齢群では，課題遂行時に背外側前頭前野で酸素化ヘモグロビン増加量の低下が示唆され，加齢の影響と推察された．支障なく社会生活を維持している高齢群においても，課題遂行中に前頭前皮質の微細な活動低下を認めた．

　Verbal fluency tasks have been suggested as a useful test for monitoring cognitive decline due to aging and screening for early dementia. In this study, we used near-infrared spectroscopy (24-channel NIRS) to measure prefrontal cortex activity during category fluency tasks (CFT) and letter fluency tasks (LFT) in 15 elderly and 14 young subjects who were able to carry out their daily lives, and investigated the effects of aging on performance and brain activity. The number of words produced in both tasks was significantly lower in the elderly group. No significant age effect was detected in the between-group comparison of oxygenated hemoglobin (oxy-Hb). In a three-factor analysis of variance (task, channel, and task duration) performed for each group, significant increase in oxy-Hb in some channels located in the dorsolateral prefrontal cortex was not observed in the elderly group. Compared to the younger group, the elderly group showed a decreased amount of increase in oxy-Hb in the dorsolateral prefrontal cortex during task performance, which was thought to be due to the effect of aging. Even in the elderly group who were able to maintain normal social lives, subtle decreases in prefrontal cortex activity were observed during task performance.