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7 Electroencephalography and Event-Related Potentials
Interactions between neurons evoking current flow across cell membranes are the essence
of brain activity. The direction and magnitude of current flow in a neuron depends
on the neurons it communicates with. This activity may be recorded by an electrode that
will detect rapid change in voltage (or potential) caused by rapid changes in current
flow due to action potentials. Current flow can also observed in the vicinity of synapses.
The summed activity of many synapses on many neighbouring neurons (called field
potential) can be recorded by a pair of electrodes – one placed directly in neural tissue
and one some distance away.
Electroencephalography: Neuronal activity can also be recorded non-invasively from
electrodes placed on the scalp. Such record of fluctuating potential across time is considered
as an objective marker of neural activity that underlies cognitive processes,
called EEG. Its amplitude is much smaller than invasively recorded field potentials
because the skull and the scalp are strong electrical insulators.
The EEG is mainly caused by synchronously occurring post-synaptic potentials and
thought to consist of sources from numerous cerebral areas with opposite electrical
poles that constantly fluctuate. Its amplitude and polarity depends on the number and
the amplitude of the contributing synaptic potentials, on whether current is flowing into
or out of cells (i.e., movement of positive or negative ions, excitatory or inhibitory synaptic
potentials), and on the geometric relationship between the synapses and electrode
(i.e., current flow toward versus away from the electrode, or both toward and away,
which will lead to cancellation of the opposing signals; Nunez, 1981). Cortical pyramidal
cells (particularly in layer IV and V of the cortex) dominate the EEG signal, because
they are the largest and most numerous cell type, and their dendritic processes are spatially
parallel to their neighbours (Elul, 1971). Such an organization leads to summation
of the small electrical fields generated by each active synapse. Given the low electrical
conductivity of the skull, electrical potentials recorded from the scalp must reflect the
activity of large numbers of neurons, estimated 1000 to 10,000 for the smallest signals
recorded.
Usually, the electrodes used to measure EEG are placed at standardized positions at the
scalp. Historically, the system of locating electrodes is referred to as International 10-20
system (Jasper, 1958): In this system, the electrodes are placed at sites that are placed in
10% and 20% distances from four anatomical landmarks (nasion, inion, and the perauricular
points in front of the ears; see Figure 7-1). The American Electroencephalographic
Society (1994) added electrode placement nomenclature guidelines that designate specific
locations and identification of 75 electrode positions (see Figure 7-1, right part).