Electroencephalography (EEG) is the noninvasive recording and measuring of the electrical activity of the cerebral cortex, with surface EEG performed via electrodes placed on the scalp. The recorded voltage activity is the net difference between ionic current flows within the brain, controlled by N-methyl-D-aspartate (NMDA) glutamate receptors becoming permeable to calcium ions. An EEG recording is a summation of numerous different frequencies. The depolarization and synchronous activation of many neurons generates epileptiform activity. The spike and wave activity seen in epilepsy is likely caused by cyclic depolarization and repolarization. The inhibiting feedback of the neurons results in the ultimate cessation of the epileptiform activity. The thalamus is considered to be the main site for the origin of cortical excitability. A minimum of 6 cm² of cortical synchronous activity is needed to create a recordable scalp potential on surface EEG.

This short review of the basic principles of EEG recording and interpretation, while by no means exhaustive, should assist the reader in becoming competent in requesting and interpreting an EEG report.

Biological signals consist of various different sequences. Filters are used to exclude frequencies that are less useful. EEG machines have a 60-Hz filter, which removes activity in the 60-Hz range and helps in eliminating artifacts from the line voltage that affect physiological recordings. The use of different montages, that is, different organizations of scalp electrodes, allows for different methodologies to be used in reading the EEG recording. With these montages, the same event will vary because of the different electrode pairings.

The following are the frequencies of different waves seen in a routine EEG recording: