MEGによる高次脳機能研究

Studies of higher brain functions by MEG

　はじめに

　脳磁場計測法（magnetoencephalography：MEG）による頭蓋外からの微弱脳磁場の計測が行われたのは，脳波の出現後40年を経た1960年代後半のことであった。最初の装置はワイヤを200万回巻いた電磁誘導コイルであった。その後1970年代になり，超伝導技術の進歩により超伝導量子干渉素子装置（Superconducting Quantum Interference Device：SQUID）が開発され，それによるMEGの導入で，脳磁場計測がより現実的なものとなった。その結果，電気的な基準を必要とせずに，ヒト脳の随意脳活動や誘発磁場信号の記録が可能となった。さらに近年の工業技術の急速な進歩により，記録センサーの増加と解析法が進化し，脳機能解明研究と臨床応用が進められている。本稿では，MEGの基本原理を簡潔に述べ，MEGによる高次脳機能研究として，運動関連脳磁場，感覚誘発脳磁場，注意，そして感覚運動連関機構について紹介する。

　Postsynaptic neuronal currents arising in synchronously activated cortical pyramidal cells produce extremely small magnetic fields, only about 10^-9times the steady magnetic field of the earth. These fields are mainly generated by currents in fissural cortex and can be measured with sensitive superconducting quantum interference devices（SQUIDs）entirely non-invasively from outside the head. To locate the activated areas in the brain, it is often useful to consider the local neuronal activations as a current dipole. The purpose of MEG recordings is to identify and accurately locate the dipole in the brain, and to follow changes in its strength as a function of time. Skull and scalp distort the electric potential distributions but are transparent for magnetic signals so that the MEG pa-ttern is restricted to a more local area than the corresponding EEG distribution. The further advantages of MEG in human brain research include the non-invasiveness, the excellent temporal resolution, and the possibility to obtain quantitative information on activation strengths（net intracellular currents）of local neuronal populations. The selectivity of MEG to activation of fissural cortex can be considered an advantage as well because these areas are difficult to reach with other means, including intracranial recordings. In this chapter, brain activities related to the higher brain functions, for example movement-related magnetic fields, pain perception and selective attention, and sensori-motor integration mechanisms are presented. Based on these advantages of MEG and development of MEG instrumentation, recently brain research by MEG has rapidly progressed. Further development of efficient MEG analysis methods and the combined use of MEG and other methods could allow us to significantly facilitate our understanding of human brain functions, in particular, the temporal dynamic aspects of higher brain functions.