EEG

Electroencephalography (EEG) is a noninvasive method to record the brain's electrical activity. It measures voltage fluctuations resulting from ionic currents within neurons, captured by electrodes placed on the scalp or, in some cases, on the surface of the brain. EEG traces reflect the summed activity of large neuronal populations and are characterized by waveforms with various amplitudes and frequencies.

Electrodes are commonly arranged using the international 10-20 system, which places sensors at standardized positions on the scalp. Recordings are typically made in a quiet, sleep or wake state, with references and montages applied in analysis. EEG data are analyzed in terms of frequency bands: delta (0.5–4 Hz), theta (4–8 Hz), alpha (8–12 Hz), beta (12–30 Hz), and, in some cases, gamma (>30 Hz). Event-related potentials (ERPs) are time-locked responses to sensory, cognitive, or motor events, obtained by averaging many trials to boost signal-to-noise ratio.

EEG has broad clinical use. It is essential in diagnosing epilepsy and characterizing seizure types, evaluating syncope versus epileptic activity, and monitoring coma or encephalopathy. It is also used in sleep studies to stage sleep and diagnose disorders such as sleep apnea or narcolepsy. In research, EEG supports studies of perception, attention, memory, and brain-computer interfaces, and is often complemented by imaging methods.

Limitations include relatively low spatial resolution and susceptibility to artifacts from eye movements, muscle activity, and electrical noise. Safety concerns are minimal, with rare skin irritation at electrode sites. The technique has evolved from Hans Berger's first recording in 1929 to modern high-density EEG and ambulatory systems.