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.