An electroencephalogram, or EEG, records patterns of electrical activity through electrodes placed on the scalp. It listens to signals produced by groups of brain cells; the electrodes do not send electricity into the brain.
What the sensors collect
The recording unfolds over time, so a clinician can look for patterns that may help investigate seizures or other neurological questions. A single trace is affected by blinking, muscle movement, and the state of wakefulness. Technicians may ask someone to open and close their eyes or breathe in a particular way during a supervised test.
What EEG cannot reveal
EEG has excellent timing: it shows changes across fractions of a second. Its scalp sensors, however, mix activity from many sources, so a squiggle cannot be mapped to one thought or one tiny spot without further analysis. A normal recording also cannot rule out every condition, because an event may simply not occur during the test.
Related: how an ECG records the heart’s electrical activity.