A new method for magnetoencephalography

Magnetoencephalogram (MEG) techniques passively measure the magnetic fields emanating from neuronal sources in the brain. The MEG is the magnetic analogue of the electroencephalogram (EEG), which is a measure of potential differences about the brain caused by the electric fields emanating from neuronal sources within the brain. MEG and EEG signals originate from sources that can be modeled by current dipoles. Figure 1 shows a hypothetical dendrite of dipole moment Q immersed in the electrically conducting brain. The current dipole causes volume currents to flow in the brain and surrounding tissue, resulting in potential differences at the scalp. Those potentials are the signals measured by the EEG. A magnetic field, B, associated with the current dipole also is generated. This neuromagnetic field, which is measured by the MEG, has a field-spatial pattern giving contours of constant field (plus contours for B exiting, minus contours for B entering). The magnitude of the field diminishes as the contour radius increases. Figure 2 illustrates the magnetic field contours from a source in the temporal lobe of the brain. The MEG has theoretical advantages over the EEG. ) The vector B gives directional information about the source orientation. The neuromagnetic field B is not distorted by the brain, because the brain has the same permeability as air. The MEG is an absolute measure of source strength, not measured with respect to a reference, as is the EEG. The MEG is not affected by bad electrode contact or tissue artifacts, as is the EEG. The MEG does not need to touch the head, as does the EEG. Originally discovered in 1968,2 the MEG has led to several investigations for locating the neuronal sources of magnetic signals emanating from the brain under normal or pathological conditions. Neuromagnetic signals seen under normal conditions might be evoked responses from visual, auditory, somatic, or other stimuli. 3 Normal brain rhythms (e.g., alpha and beta) are also seen. Neuromagnetic signals observed under pathological conditions might be associated with epilepsy4 or some other disease. The MEG shows potential for noninvasively localizing some sources of epilepsy located deep in the brain. At present, such sources are often localized by using EEG electrodes penetrating the brain. The MEG also shows potential for straightforward functional imaging of brain activity in mind-brain investigations. For