Magnetoencephalography

Abstract

The fundamentals of magnetoencephalography (MEG) were developed and examined in the early 1970's, but it was very laborious to measure the magnetic field emitted from the brain by the MEG systems in those days, because they had fewer channels. A new phase of MEG study has begun with the commercial availability of multi-channel whole-cortex biomagnetometers from the mid 1990's. This new generation of MEGs has enabled us to record whole-cortex activities of a brain all at once (i.e., without repeating the measurements after changing the sensors' position), thus reducing measurement time and increasing measurement precision. This article explain the basic principles of MEG, which are the strength of the brain magnetic field, superconducting quantum interference devices (SQUIDs) and the Josephson Junction (JJ), pick-up coils, the measurement circuit and MEG system together with a recently developed helium circulation system. This article also explains the inverse problem of the measured MEG data, which is difficult, but is essentially important for the future widespread application of MEG. Dipole estimation methods have been commonly used for this, but have a persistent limitation in that they require a heuristic assumption of the dipole number and local minimization. Another approach to estimate dipole distribution by minimizing some performance functions has therefore been developed. This article explains its basic idea and limitations. The article goes on to present some application topics, such as finding the control center of eye accommodation which was identified in the occipito-temporal fissure, typical auditory response and an interesting oddball response which is an automatic auditory brain response to some irregular sounds. Finally, this article explains the time-special resolution of MEG compared with other brain measurement techniques which were already in use and will be explained in this lecture series to clarify the properties of MEG. This article also summarizes present problems which need to be fixed for the future widespread use of MEG.