How are the electrodes arranged?

The electrodes are placed on the scalp by an EEG technician according to internationally agreed-upon criteria. In order to locate the exact electrode position, this system uses four anatomical landmarks from which measurements can be made. The nasion is the indentation between the forehead and the nose, the inion is a ridge that can be felt at the midline of the back of the skull, over the occipital area, and the preauricular points are defined as the indentations just above the cartilage that covers the external ear openings. The electrode locations and distances between the electrodes are then defined as 10% or 20% of these anatomical distances. Occasionally, additional electrodes (sphenoidal and suboccipital, for instance) are placed to enhance the likelihood of detecting and determining the location of EEG waves that may be too small or too deep in the brain to be detected by routinely placed electrodes.

What do the letters and numbers mean?

The names of the electrode sites use alphabetical abbreviations that identify the lobe or area of the brain to which each electrode refers:
F = frontal
Fp = frontopolar
T = temporal
C = central
P = parietal
O = occipital
A = auricular (ear electrode).

The localization of the brain waves within the brain regions or lobes is further narrowed by adding electrodes, which are given numbers such as T3, T4, P3, P4. Even numbers identify electrode positions on the right side of the head, and odd numbers refer to the left side. The label "z" points to electrode sites in the midline of the head. For example, Cz refers to the midline central region of the head.

Often an EKG (electrocardiogram), a record of the heartbeat, also appears on the EEG printout.

How are the electrodes related to each other?

The EEG records brain waves from different amplifiers using various combinations of electrodes called montages. In bipolar montages, consecutive pairs of electrodes are linked by connecting the electrode input 2 of one channel to input 1 of the subsequent channel, so that adjacent channels have one electrode in common. The bipolar chains of electrodes may be connected going from front to back (longitudinal) or from left to right (transverse).

Another type of montage is the referential montage. In this type, various electrodes are connected to input 1 of each amplifier and a reference electrode is connected to input 2 of each amplifier. Ideally, inactive electrodes (ones that are uninvolved in the electrical field being studied) are chosen as references.

Being able to locate the origin of electrical activity ("localization") is critical to being able to interpret the EEG tracings meaningfully. Localization of normal or abnormal brain waves in bipolar montages is usually accomplished by identifying "phase reversal," a deflection of the two channels within a chain pointing to opposite directions. (See sample.) In a referential montage, all channels may show deflections in the same direction. If the electrical activity at the active electrodes is positive when compared to the activity at the reference electrode, the deflection will be downward. Electrodes where the electrical activity is the same as at the reference electrode will not show any deflection. In general, the electrode with the largest upward deflection represents the maximum negative activity in a referential montage.

Topic Editor: Steven C. Schachter, M.D.
Last Reviewed: 12/15/06