Electrical Principles

Electrical Principles

Surface electrodes are applied to the scalp to record the underlying cortical activity. Each electrode records the synchronized electrical activity arising from a population of neurons in an area of cortex in an angle subtended by the surface electrode of approximately 6 cm², slightly more than the surface area of a quarter.

Since most of the neurons in the cerebral cortex are radially oriented, the synaptic events in the area just under the surface of the scalp tend to be negative in polarity, reflecting the electrical activity in the extracellular space around apical dendrites as positive ions rush into the cell during excitation. Electrical activity surrounding the soma is generally positive in polarity, but due to its depth, is not typically recorded by surface electrodes.

Electrode Placement - International 10-20 Electrode Placement System

The surface electrodes are placed on the scalp in a systematic manner that standardizes their position in relation to underlying brain regions and ensures a reproducible study between patients or for an individual patient across multiple recordings.

EEG electrodes are placed according to an internationally agreed-upon system called the International 10-20 System. This system is based on measurements of the head that are divided into 10% and 20% fractions of the total circumference of the head, the ear to ear measurement and the anterior-posterior measurement from the nasion to the inion.

The electrodes are named based on convention, with letters that represent particular cerebral regions (Frontal, Temporal, Central, Parietal, Occipital), and numbers that represent sidedness. Odd numbered electrodes are on the left, even numbered on the right, and electrodes with a “z” designation at the midline.

Additional electrodes placed in between at the 10% locations to improve the ability to localize. This system is called the 10-10 electrode placement system. Electrodes F9/10, T9/10 and P9/10 are examples on this head diagram.