An EEG, or electroencephalogram, records the electrical activity of the brain through electrodes placed on the scalp. It provides a continuous, real-time trace of the summed activity of large populations of neurons.

Different frequency bands correspond roughly to different states. Alpha waves appear during relaxed wakefulness with eyes closed; beta waves accompany active thinking and attention; theta and delta waves dominate in drowsiness and deep sleep. Spikes and sharp waves can indicate seizure activity. The EEG is therefore a primary tool in the diagnosis of epilepsy and in the staging of sleep.

The method has excellent temporal resolution—it can track changes on the order of milliseconds—but limited spatial resolution, because the signal is filtered through the skull and represents the activity of many neurons at once. Event-related potentials extracted from the ongoing EEG reveal the brain’s response to specific stimuli with precise timing.

When a clinician needs to know what the brain is doing moment by moment, the EEG remains one of the most direct and accessible windows available.