What is an EEG?
All living brains produce electricity, which is detectable on the scalp or surface of the head. An EEG (electroencephalograph) is a computerized recording of the electrical activities of the brain. In order to record the electrical activity, the clinician or technician places electrodes on specified locations the client’s head.
System of Electrode Placement
The anatomical sites on the head are identified using the EEG 10-20 system of electrode placement, which is illustrated in Fig. 1 below.
Figure 1: The EEG 10-20 System of Electrode Placement
F= frontal (anterior) sites
C = Central sites
P = Parietal sites
T = Temporal sites
O = Occipital.
Z = Zero line, seperating the left and right hemispere. It overlies the cingulate gyrus.
Note: Even numbers are on the right hemisphere and odd numbers are on the left hemisphere.
On the computer screen, the EEG tracings look like this (Only 6 of the usual 19 EEG channels are shown here: FP1, FP2, F3, F4, C3, & C4….
Brainwaves are classified according to their frequencies and amplitudes into delta, theta, alpha, and beta as described in below
Unlike conventional EEG, quantitative EEG can generate head maps illustrating the distribution of components of the EEG wave, i.e. delta, theta, alpha, and beta, on the scalp. The head maps look like this.
On these head maps, a colour code is used to highlight level of deviation from normal. The score zero is represented by white colour. Green to blue indicates increasing levels of deficiency for the brainwave frequency band (- scores), whilst light blue through yellow to red indicates increasing excess of the frequency band.
Specialized quantitative EEG software such as LORETA (Low Resolution Brain Electro-Magnetic Tomography) generates brain scans which identify the cortical and subcortical sources (also called generators) of the abnormal brainwave patterns displayed on the head maps. These look like this.
From these scans, the clinician can identify the specific anatomical locations, the Brodmann areas, from which the abnormal brainwaves arise.