An EEG is the record of brain activity of an individual obtained through the use of scalp or
intracranial electrodes. Scalp electrodes are placed on the scalp with a gooey conducting substance. There are a number of methods of placement of electrodes, the most common being the international 10-20 system, in which their placement is constant for all individuals according to head size. These scalp electrodes are referenced to relatively inactive sites such as the tip of the nose or the ears, and all are referenced to a ground, most commonly placed on the
forearm.
Once in place, the
electrodes are connected to an
oscilloscope that records electrical activity at each site. The activity from each electrode is displayed as a function of time.
An electroencephalogram records activity from
bipolar fields only. Thus, only the activity of certain cells is reflected in the EEG.