Workshops (WS) Workshop 1: Electroencephalogram (EEG) WS1.1. The origin of EEG, recording techniques and quality

EEG records the electrical activity of the cerebral cortex, mainly from large pyramidal neurons close to the surface, and this activity is influenced by subcortical structures. At least 6 cm2 of synchronised cortical activity is required to be recorded by scalp EEG. Surface electrodes are placed equidistantly over the scalp according to an international convention (10–20 System), ensuring good electrical contact, with electrode impedances Digital EEG amplifiers then use analogue-to-digital converters to translate the recorded voltage differences into numerical values, requiring an amplitude resolution of at least 12 bits (usually ≥16 bits), and with the number of measurements per second (sampling rate) at least three times the highest frequency being recorded (≥256 Hz). Filtering highlights activity that is biologically relevant, using a bandpass of 0.5–70 Hz for routine diagnostic EEG, but filtering distorts raw data and is no substitute for good technique. Digital EEG recording uses a system reference electrode separate from the 10–20 recording electrodes and stores the voltage measurements for each recording electrode relative to this system reference electrode. This allows reformatting of EEG channels and montages after the recording simply by combining any electrode pair to create the channel. At least longitudinal bipolar, transverse bipolar and referential montages should be used to read EEG. Unfortunately, EEG is commonly misinterpreted, leading to misdiagnosis. Age and state dependency dramatically influence the normal EEG, and there are many variations of normal, especially during drowsiness and sleep. ECG, careful clinical observation and video recording (routine if possible) are required throughout the recording to recognise artefacts and other important changes. For EEG to be useful, comprehensive knowledge, training and experience are required under the supervision of a skilled mentor. When in doubt, it is best to assume that background slowing is related to drowsiness/sleep, and every sharply contoured wave is an artefact or normal variant until proven otherwise.