A normal EEG cannot rule out epilepsy, and an abnormal one can mislead
Electrodes on the scalp can catch the brain's electrical chatter within milliseconds, faster than any MRI or CT scanner. Yet a standard half-hour recording misses the telltale signs in many people with epilepsy, and a small share of healthy people show odd spikes without ever having a seizure.
Electroencephalography records the brain's spontaneous electrical activity, largely the combined signals of pyramidal neurons in the cortex. Usually electrodes sit on the scalp in a standard arrangement called the 10-20 system; surgically placed electrodes give intracranial recordings. Because skull and tissue blur the signal like resistors and capacitors in a circuit, the EEG mainly reflects cortex close to the electrodes. Deep structures such as the hippocampus, thalamus and brain stem do not contribute directly.
Healthy recordings show rhythms tied to alertness, mostly between 1 and 30 Hz. Alpha waves, 8 to 13 Hz, dominate the back of the head during relaxed wakefulness and fade when the eyes open, while faster beta waves rise during concentration. Slow theta and delta waves in an awake adult suggest something is wrong. Averaging the EEG around a flash, sound or touch produces evoked potentials, and related event-related potentials are workhorses of cognitive psychology.
Clinically, EEG is the gold standard for confirming epilepsy, revealing spikes, sharp waves and spike-and-wave patterns. But routine recordings at epilepsy centres detect such discharges only 29 to 55 percent of the time, so doctors may repeat the test, record for longer, or use flashing lights, deliberate over-breathing or sleep deprivation to raise the yield. When discharges do appear they almost always confirm the diagnosis, though up to 3.5 percent of the general population show epileptiform patterns without seizures. To capture an actual seizure, patients may be filmed during days of monitoring, at home or in a hospital unit where medication is often withdrawn under supervision.
EEG also helps assess sleep disorders, anaesthesia depth, coma, brain injury after cardiac arrest and brain death. It was once a first-line tool for finding tumours and strokes, a role taken over by MRI and CT, but its portability and timing precision keep it central to research. Readings are still interpreted largely by eye, and different experts can disagree.
Source: Electroencephalography
