Theory of the EEG Inverse Problem - Quantitative EEG Analysis Methods and Clinical Applications - page 123

PSPs occurring in large clusters of neurons. A typical cluster size must cover at least
40 to 200 mm 2 of cortical surface in order to produce a measurable scalp signal.
Summarizing, there are two essential properties:

1. The EEG sources are confined to the cortical surface, which is populated
mainly by pyramidal neurons (constituting approximately 80% of the
cortex), oriented perpendicular to the surface.
2. Highly synchronized PSPs occur frequently in spatial clusters of cortical
pyramidal neurons.

This information can be used to narrow significantly the nonuniqueness of the
inverse solution, as explained later in this chapter.
The reader should keep in mind that there is a very strict limitation on the use of
the equivalent terms EEG generators and electric neuronal generators. This is best
illustrated with an example, such as the alpha rhythm. Cortical pyramidal neurons
located mainly in occipital cortical areas are partly driven by thalamic neurons that
make them beat synchronously at about 11 Hz (a thalamocortical loop). But the
EEG does not “see” all parts of this electrophysiological mechanism. The EEG only
sees the final electric consequence of this process, namely, that the alpha rhythm is
electrically generated in occipital cortical areas. This raises the following question:
Are scalp electric potentials only due to electrically active cortical pyramidal neu-
rons? The answer is no. All active neurons contribute to the EEG. However, the
contribution from the cortex is overwhelmingly large compared to all other
structures, due to two factors:

1. The number of cortical neurons is much larger than that of subcortical
neurons.
2. The distance from subcortical structures to the scalp electrodes is larger than
from cortical structures to the electrodes.

This is why EEG recordings are mainly generated by electrically active cortical
pyramidal neurons.
It is possible to manipulate the measurements in order to enhance noncortical
generators. This can be achieved by averaging EEG measurements appropriately, as
is traditionally done in average ERPs. Such an averaging manipulation usually
reduces the amplitude of the background EEG activity, enhancing the brain
response that is phase locked to the stimulus. When the number of stimuli is very
high, the average scalp potentials might be mostly due to noncortical structures, as
in a brain stem auditory evoked potential [14].

5.2.2 The Equivalent Current Dipole
From the physics point of view, a cortical pyramidal neuron undergoing a PSP will
behave as a current dipole, which consists of a current source and a current sink sep-
arated by a distance in the range of 100 to 500

m. This means that both poles (the
source and the sink) are always paired, and extremely close to each other, as seen
from the macroscopic scalp electrodes. For this reason, the sources of the EEG can
be modeled as a distribution of dipoles along the cortical surface.

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