eeg & meg theory: neuronal basics

eeg & meg signal generators: The Neocortex shows laminar as well as horizontal structure in single columns. The pyramidal cells in Lamina II, III and V are the main generators of the EEG. Their apical dendrites reach into Lamina I and are the only ones with predominant vertical orientation. Moreover, 75% of all neocortical cells are pyramidal cells. The activation of an excitatory apical synapse results in sodium influx into and a proximal potassium efflux out of the cell.  The corresponding intracellular currents generate the magnetic fields (MEG),  the extracellular currents generate the electric potentials (EEG).

Inhibitory synapses are localized most often proximal and they are rare compared to excitatory ones (6%). The apical EPSP’s (excitatory postsynaptic potentials)  of pyramidal cells are the main generators of EEG and MEG. Magnetic fields and electric potentials are oriented perpendicular. The flow of the extracellular currents (EEG) heavily depend on the conductivity of the volume conductor (Rule of thumb: a negativity in the EEG reflects an activation).

spontaneous eeg:   In defined cognitive or active states the spontaneous EEG shows frequency intervals of amplified activation. Based on this the following frequency bands are defined:
Delta (0–4 Hz) in babies or during sleep. Theta (4–8 Hz) in early childhood or while falling asleep, drowsiness, deep relaxation like meditation. Alpha (8–12 Hz) during awake relaxation especially with closed eyes. Also with eyes open in an environment with very few stimuli or reduced visual attention of the subject. Beta (1220) and Gamma (>20 Hz) during mental activation. Because of the strong phase variability its also called desynchronized activity.