What is the source of the EEG?

Neurons in the human cortex generally process their information by means of electrical signals and thus enable the electrical recording of their activity, the electroencephalogram (EEG). Due to their unique orientation with their long apical dendrites perpendicular to the cortical surface, large cortical pyramidal neurons in deep cortical layers play a major role in the generation of the EEG. Specific and non-specific thalamic nuclei, as well as distant cortical areas, terminate on these apical dendrites and form myriads of excitatory and inhibitory afferents. The release of excitatory and inhibitory neurotransmitters by these fibers activates specific postsynaptic receptors and generates excitatory and inhibitory postsynaptic potentials, respectively. By electrotonic spread of postsynaptic potentials along the apical dendrites and equivalent capacitive currents, they become electrical dipoles. Positive or negative deflections are generated by both excitatory and inhibitory afferents, depending on the location of these synapses on the apical dendrites. Negative (upward) deflections are due to superficial excitatory or deep inhibitory inputs, whereas positive (downward) deflections represent deep excitatory or superficial inhibitory inputs.