The Source of EEG

We have concluded our investigation into the sources of EEG, arriving at working hypothesis. We present our calculations and analysis in The Sources of EEG. The EEG signal is a potential in the extracellular fluid above the cortex. We can detect it well with a screw electrode 1 mm in diameter. Such an electrode cannot detect individual action potentials. The EEG signal results from tens of thousands of neurons acting coherently.

We believe there are two sources of the EEG signal. One is excitory post-synaptic currents flowing into the synapses of pyramidal neurons, down the apical dendrite, and out of the soma membrane capacitance. Current enters the extracellular fluid outside the soma and leaves it at the synapses, thus generating a current dipole in the extracellular fluid. When many, parallel pyramidal neurons act together, the effect of these circulating currents combines to produce a negative EEG voltage of order −1.5 mV for at least 10 ms.



The second source is a consequence of neuron activation. As activation starts in the soma and propagates down the apical dendrite of a pyramidal neuron, there is a fraction of a millisecond during which current is flowing along the apical dendrite prior to its activation. This current flows from the activated soma, along the apical dendrite, and out through the membrane capacitance of the dendrite endings. We call this the exctracellular activation current, because it flows from the dendrite endings, through the extracellular fluid, to the soma. We estimate it to be ten times the magnitude of the extracellular excitory current prior to activation, but its direction is opposite and its duration is only 1 ms.

With excitation of a layer of pyramidal neurons spread over 10 ms, we expect the activation to be spread out over a similar interval, so that the average activation current will be comparable to the excitory current, producing a +1.5 mV contribution to the EEG.

We believe that the baseline EEG signal, its usual frequency spectrum, hiss artifacts, spindle artifacts, and negative-going seizure spikes are all consistent with our hypothesis. But oscillations of greater than 150 Hz in the EEG are not.