Effects of fissures in the brain on electroencephalograms and magnetoencephalograms Available to Purchase

The electroencephalogram and magnetoencephalogram maps measured on the head can be used to estimate the location, orientation, and amplitude of electrical sources in the brain. It is known that differences in the electrical conductivities of the various tissues in the head can affect these maps and estimates, e.g., the low conductivity of the skull ‘‘smears’’ electroencephalogram maps and makes sources appear deeper in the head than they actually are. Since fissures in the brain are filled with cerebrospinal fluid, which is several times more conductive than the brain tissue in which the sources are located, and since fissures are close to many of the sources, they may have significant effects on the maps and estimates of source parameters. However, little or no information is available about these effects. This study uses a spherically‐shaped computer model of the head which contains fissures to determine these effects. It is found that even a fissure as large as the interhemispheric fissure has a maximum effect on location estimates of 0.75 cm and a maximum effect on orientation estimates of 15°. The maximum effect of this fissure on amplitude estimates is 28% with the estimates being larger than the actual source for sources perpendicular to a fissure and smaller for sources parallel. In general, there are no significant differences in the effects of the fissures on estimates using electroencephalogram and magnetoencephalogram maps. The fissures cause a radial source in the spherically‐shaped head model to produce only a small magnetic field with a map like that of a source perpendicular to the fissure; a radial source in a sphere with no fissures would produce no magnetic field. Since most fissures in the brain are smaller than the ones in this head model, it is concluded that actual fissures in the brain have little effect on electroencephalogram and magnetoencephalogram maps and source estimates using these maps.