Aims: 

EEG is usually recorded with scalp electrodes which are located above the cerebral cortex. Recently we have shown that cortical burst suppression EEG can also readily be recorded with a pair of electrodes located in subthalamic nucleus for stimulation in Parkinson patients. This is because the electrodes are relatively large in size and the patient is in deep anaesthesia so that local activity in nuclei is minimally recorded. We now demonstrate that a pair of electrodes with a distance of 3 cm records EEG everywhere in the head in deep anaesthesia at burst suppression level. We then show with a realistic 3D model of head, how the electric currents of cortex spread everywhere inside the head and on the surface and calculate the sensitivity of electrode pairs to activity of different parts of cortex.

Methods: 

A patient undergoing gynaecological surgery was anaesthetized to EEG burst suppression level with desflurane and EEG was recorded with electrodes on tip of nose, tip of chin, posterior angle of mandible and zygomatic arch. A realistic 3 D model was used to analyze the sensitivity of these electrodes to EEG from different parts of cortex.

Results: 

High amplitude EEG was recorded from nose tip-chin, chin –angle of mandible and angle of mandible – zygomatic arch derivations. Relative amplitudes of these signals were roughly 5:1:3. Thus, a relatively strong EEG current reaches the mandible, although it is far from the cerebral cortex. Calculation with our model shows that the mandibular EEG is likely to be generated mostly in basal forebrain and temporal lobes.

Conclusion: 

EEG currents produce closed loops and, in the case of activity which is synchronous in the whole cortex, are readily recordable with relatively high amplitude with any electrode pair inside the head or on the surface of the head, including lower face.