2.1 The EEG and Electrode Placement

The EEG is a powerful tool used in the study of both normal and abnormal brain function.  Because the electrical signals are about one-tenth the magnitude of electrocardiogram (EKG) signals (i.e. on the order of micro-volts), low-noise electrodes coupled with high precision amplifiers and filters are used to improve the signal characteristics for recording.

The International 10-20 System of electrode placement has become the standard instrument in research and clinical neurophysiology.  Use of the 10-20 System of electrode placement assures accurate placement of electrodes for repeated EEGs in any laboratory and allows the comparison of EEG signals between subjects.

The detailed head measurements and electrode application techniques will not be described in this report, but can be found in almost any handbook on EEG technology.  The various positions on the scalp are described using a letter/number combination.  Odd numbers refer to locations on the left hemisphere of the head, even numbers are located on the right side, and using the letter 'z' in place of a number is used to denote a location that is on the centerline between the left and right hemispheres.

Electrodes on the front hemisphere are called frontal (F) locations, the back hemisphere contains the parietal (P) and occipital (O) locations, and the centerline between the front and back are the central (C) and temporal (T) locations.  Locations A1, A2, or the linked combination of A1+A2 are typically used in conjunction with one or more electrodes located on the scalp to create the common bipolar montage and are called the anterior (A) locations.  These "A" electrodes can be located either on the mastoid bone behind the ear lobe or attached directly to the ear lobe (usually on the underside).

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