Magnetoencephalography (MEG): physiological background

Magnetoencephalography (MEG) is a non-invasive technique able to spatially identify the synchronous firing neurons in restricted cortical areas, either for spontaneous cerebral activity or in response to an external stimulus. MEG is unaffected by scalp and skull, and preferentially reflects the tangential component of dipoles in the depth of gyri and sulci.

The neuromagnetic technique consists of measurement of the magnetic field over the scalp, as generated by the bioelectrical currents in the brain. In order to achieve the sensitivity needed to measure these very weak magnetic fields (about 10 as compared to the earth magnetic field), the use of new superconducting magnetometers (superconducting quantum interferences devices or SQUID) and of devoted shielding is mandatory. Under the symmetry conditions, well approximated in the case of the head, it can be shown that the component of the magnetic field perpendicular to the skull is mostly sensitive to the tangential component of the primary current source, and negligibly to the field generated by the volume currents and to the distortions, smearing effects and filtering of frequency components, mainly in the faster rhythms caused by the intervening tissues (Romani & Rossini, 1988; Okada et al., 1999). This represents an advantage in respect to the purely electric measurement of neural activity, especially for the postsynaptic component at the level of the cortical mantle. The magnetic field is simultaneously measured over many scalp sites, with a rapid sampling in the time domain, and from these data the isofield contour maps are calculated and studied.