EEG electrodes are placed on the scalp at 10 and 20 percent of certain measured distances. If a measurement was made around your skull, (i.e - a hat size measurement) a circumference distance would be determined. This measurement would be around 55 cm for an average-sized person. Therefore, 10% of this measurement or 5.5 cm would be used to determine precise locations around the skull. An anatomic landmark is needed to know where to start with the measurements. The landmarks are the ear canals, the bridge of the nose (nasion) and the little bump at the very back of your skull (inion).
The 10-20 system is based generally on the relationship between the location of an electrode and the underlying area of cerebral cortex, although exact relationships can only be determined with MRI confirmation of exactly where the various parts of brain are. There are slight variations amongst individuals in brain shape and relationship to the skull landmarks. Each point (e.g., Cz, C3, Fp1, F7, etc.) represents a standard place for a recording electrode. Each site has a letter (to identify the lobe) and a number or another letter to identify the hemisphere location. The letters F, T, C, P, and O stand for Frontal, Temporal, Central, Parietal and Occipital. (Note that there is no "central lobe", but this is just used for identification purposes.) Even numbers (2,4,6,8) refer to the right hemisphere and odd numbers (1,3,5,7) refer to the left hemisphere. The z refers to an electrode placed on the midline. Also note that the smaller the number, the closer the position is to the midline. Many more placements can be defined for research purposes. Some researchers use 64, 128, or even 256 electrode placements. For clinical purposes 16 to 19 channel recording is most common.
Nasion - point between the forehead
Inion - Bump at back of skull
Why are percentages used?
The skull may be different from patient to patient. A pediatric
adolescent may be smaller than an adult, and different adults have
different size head (and brains). The percentage relationship remains
same for the location of the internal brain lobes.
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