ABSTRACT
By the end of the twentieth century, there were several methods developed to measure human brain activities noninvasively. Magnetoencephalography (MEG) is one of the completely noninvasive techniques that records distribution of magnetic fields generated by the neuroelectrical activities of the human brain by using an array of ultrasensitive magnetic sensors located around the scalp (Hämäläinen et al., 1993). Since the first recording in 1929 (Berger, 1929), electroencephalography or EEG, which measures distributions and temporal changes in the electric potential generated by cerebral neural activities, was widely used both in clinical applications and in research on the working human brain. MEG is the magnetic counterpart of EEG that carries information complementary to that of EEG. More specifically, MEG mainly records neuroelectrical activity whose dipolar generator orients tangentially to the scalp, whereas EEG is sensitive to both tangentially and radially oriented sources. Also, MEG is thought to be useful to localize generators of the neural activities that lie in the cortical area. In EEG, accurate estimation of source location is often difficult because the distribution of electrical potentials on the scalp is severely affected by complex inhomogeneity of conductivity in the head and the poor electrical conductivity of the skull. On the other hand, spatial distribution of the magnetic field mainly reflects neural currents that flow in the macroscopically relatively homogeneous intracranial space (Hämäläinen et al., 1993). Under such favorable conditions where brain activity is modeled as a single current dipole, the location of the neural source in the brain can be determined within a few millimeters spatial accuracy.
