1 The Electroencephalogram
Introduction to EEG
Karri Haen Whitmer
The electroencephalogram, or EEG, is a noninvasive method for recording the electrical activity of millions of neurons. The neurons, mostly pyramidal cells, are stratified in 3-5 layers, composing the outer-most section of the human cerebral cortex. EEG is a surface electrophysiology method, which employs recording electrodes placed upon the scalp, in a variety of configurations called montages.
Unlike other biopotentials we have investigated in this text, EEG waves are described in several ways. Wave parameters include the standard measurements of frequency recorded in cycles per second (cps) or Hertz (Hz) and wave amplitude (in mV). Additionally, frequency may be described as a range of values, or EEG bands, including delta, theta, alpha, beta, mu, and gamma waves. Each EEG wave band has several characteristics associated with it (see Table 1).
One major difference between the EEG and other surface electrophysiology methods is the amplitude of the recorded EEG waves. The fluctuations in voltage measured across the skull by the EEG are very small compared to the wave amplitudes recorded with ECG, EMG and even EOG (Table 2), making the EEG particularly prone to corruption by artifacts that must be filtered from the raw EEG data. Often, the more powerful EMG, ECG and EOG bursts are the source of these artifacts.
Another major difference between the EEG and other standard surface-based electrophysiological measurements is the source of the electrical activity being measured. Whereas other techniques like the EMG measure the action potentials of excitable cells, the EEG mainly records extracellular currents that arise from synaptic activity in the dendrites of neurons. These extracellular potentials are derived from excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs). For example, excitation of a postsynaptic neuron creates an extracellular voltage near the dendrite that is more negative than elsewhere along the neuron. This creates a dipole, that is, a region of positive charge separated from a region of negative charge. The positively charged region is referred to as a source, and the negatively charged region is referred to as a sink. If the source and sink are at different distances from the recording electrode, then the voltage difference can be measured. This means the neurons that elicit EEG recordings must be in a particular orientation with respect to the surface of the skull, where the recording electrode sits. Thus, EEG recordings typically emerge from pyramidal neurons oriented with cell bodies and axons perpendicular to the surface of the skull (Figure 1).
Electroencephalography technique
Scalp electrodes are placed at standard positions on the skull to assure the repeatability of EEG results. One standard EEG electrode array or montage is the “international 10–20 system.” EEG arrays place electrodes in reference to anatomical landmarks on the skull. For e