EEG Instrumentation, Montage, Polarity, and Localization

The source of the EEG potentials recorded from the scalp is the excitatory and inhibitory postsynaptic potentials of pyramidal neurons. Each pyramidal neuron has an apical dendrite and multiple basal dendrites (Fig. 2.1). Excitation of the postsynaptic membrane at the apical dendrite leads to depolarization with an intracellular shift of positive ions (Na). Subsequently the extracellular space nearby becomes relatively negatively charged. This is coupled by an inhibitory potential at the basal dendrites with a relatively positive charge nearby. At cortical layers III, V, and VI, neurons are aligned in a perpendicular fashion with the cortex. This allows for summation of the small potentials generated by each neuron when they fire in a synchronous fashion (Fig. 2.2). Cortical neuronal alignment effectively creates an electrical dipole. Whether a positive or negative potential is recorded on the scalp electrode depends on the location of the recording electrode with respect to these dipoles (Fig. 2.3). Epileptiform discharges (spikes or sharp waves) are commonly surface negative. Simultaneous intracranial and scalp recordings confirmed that at least 6 cm of synchronous cortical activation is indeed necessary to detect an individual epileptic spike on scalp electrodes [1].