体性感覚の階層的処理と触知覚

Hierarchical somatosensory processing and tactile perception

　筆者らが無麻酔サル体性感覚中枢で単一ニューロン活動を記録，情報処理を階層的に見ることを提唱してから30年になる。階層的情報処理の帰結として，中心後回の最後部，頭頂間溝の前背壁に両手の統合を発見し，続いて上肢帯，下肢帯，足の両側統合の様子を明らかにした。一連の研究のなかで，同じ頭頂間溝の前背壁で，体性感覚と視覚の統合が起こることを発見した。最近盛んに行われているヒト中心後回の脳磁図や機能的脳イメージングによる体部位再現，触知覚情報処理についての研究は，同じ無麻酔の条件で行われたサルでの研究を支持する結果となっている。また階層構造の頂点に位置すべき触覚認識中枢部位として，第二体性感覚野を含む頭頂側頭連合皮質の重要性が指摘されている。

　I reviewed our 30-years-long research, by recording single neuronal activity in the somatosensory cortices of unanesthetized macaque monkeys. We found hierarchical processing in the first somatosensory cortex（SⅠ）, starting in area 3 and proceeding to areas 1 and 2, the increase in neuronal receptive field sizes and in the complexity of the most adequate stimuli for individual neurons. As the process proceeds, neurons representing the bilateral hands, shoulders, girdles and feet were found in the caudalmost part of the postcentral gyrus. Neuronal receptive field properties become far more complex in the second somatosensory cortex（SⅡ）when examined in unanesthetized monkeys. There, neurons with receptive fields combining bilateral sides were more common. They often were combination of hands and feet with or without face, trunk etc. Our data support the notion that SⅡis in higher position than SⅠ. I also reviewed current studies by other investigators in human subjects as well as monkeys. The review included the human studies seeking for the cortical region responsible for tactile object recognition. I argued that the hierarchical scheme we proposed is likely to be a leading principle in human somatosensory processing.