EEG permits the recording of the collective electrical activity of the cerebral cortex as a summation of activity measured as a difference between two recording electrodes. Recording electrodes (leads) are placed on the scalp on at least 16 standard sites, and recordings of potential differences between key electrodes are obtained.
The principal wave forms recorded in the EEG are alpha (9 to 10 Hz, occipital location, predominant activity in adults, awake in resting state with eyes closed); beta (20 to 25 Hz, frontal and precentral locations, prominent in wakefulness, seen in light sleep); delta (2 to 2.5 Hz, frontal and central location, not prominent in wakefulness, generalized in deep sleep and coma or toxic states); and theta (5 to 6 Hz, central location, constant and not prominent when awake and active, sometimes generalized when drowsy). Electrode placement is shown in figure B. Examples are provided of a normal EEG taken when the patient is awake with eyes closed (C), and when sleeping normally (D). Abnormal patterns of activity can be seen in the presence of tumors (E) and in seizures (F); for example, the spike-and-wave appearance in a generalized tonic-clonic seizure (generalized fast repetitive spikes and generalized spikes and slow waves, respectively); and a 3 Hz spike-and-wave EEG in the case of an absence seizure.