Electric field of EEG during anesthesia

Electroencephalogram (EEG) has been clinically used to estimate the level of consciousness during anesthesia, but its physiology and biophysics are poorly understood in anesthe-siological literature. The electrical sources of EEG are in cortical structures. EEG currents create closed-loops, which flow from the surface of the cortex and then return to the inside of the hemispheres. In the case of widespread synchronous activity like physiological sleep or anesthesia, the currents return through the base of brain and skull. Here we show with a typical EEG pattern of anesthesia, burst-suppression, that due to those currents EEG is recordable outside of scalp area. We also present the sensitivity field of electrodes located submentally, as well as the electrodes used for anesthesia monitoring, calculated from a realistic head model of the potential distribution and currents of EEG. Our results show that anesthesia EEG can be recorded with a pair of electrodes anywhere on the surface of head, as well as inside of head and brain, because the EEG current loops produce recordable voltage gradients in the whole head. A pair of electrodes submentally is most sensitive to basal parts of the brain. The typical electrodes used in anesthesia monitoring are most sensitive to basal surface of frontal lobes as well as frontal and mesial parts of temporal lobes.