An EEG (electroencephalogram) is a diagnostic test that measures and records the electrical activity of the brain. During an EEG test, a technician attaches 20–40 small cup-shaped electrodes to precise locations on the scalp using a conductive gel or paste. These electrodes detect the electrical impulses produced by the brain's neurons and transmit the signals to an EEG machine, which amplifies and records them as wave patterns on a graph called an electroencephalogram.
Key facts about the EEG test:
- Non-invasive — no needles, no cuts, no radiation
- Painless — the electrodes measure electrical activity; they do not produce electrical current
- Duration: 30–90 minutes for standard EEG; up to 72 hours for ambulatory or video-EEG monitoring
- Performed by an EEG technologist; results read and interpreted by a neurologist
- Available at hospitals, neurology clinics, and outpatient diagnostic centers
The brain produces four primary types of electrical waves that appear on an EEG test:
- Delta waves (0.5–4 Hz): Slowest waves, normal during deep sleep; abnormal when present in waking adults — may indicate brain damage or deep sleep disorders
- Theta waves (4–8 Hz): Common in children and during light sleep; may indicate problems in adults if excessive during waking state
- Alpha waves (8–13 Hz): Produced when relaxed with eyes closed; should reduce when eyes open or during concentration
- Beta waves (13–30 Hz): Associated with alertness, concentration, and problem-solving; most common waking rhythm
Abnormal EEG patterns — such as spike-and-wave complexes — indicate abnormal electrical discharges associated with seizures and epilepsy. The eeg test field is a core component of clinical neurology, with EEG tests performed millions of times annually for diagnostic and monitoring purposes.
Why Is an EEG Test Ordered?
Physicians order EEG tests to investigate a wide range of neurological conditions:
Primary indications:
- Epilepsy and seizure disorders: The most common reason for an EEG test. EEG can identify abnormal electrical patterns that confirm epilepsy, help classify seizure type, and guide treatment decisions. A normal EEG does not rule out epilepsy — the test may need to be repeated or the patient monitored for longer periods.
- First seizure evaluation: After a patient's first seizure, an EEG helps determine the risk of recurrence and whether to begin antiepileptic medication
- Unexplained loss of consciousness: EEG distinguishes between seizure-related and non-seizure causes of loss of consciousness
- Encephalopathy: EEG assesses diffuse brain dysfunction caused by metabolic disorders, organ failure, infections, or toxins
- Brain death determination: EEG is used (alongside other tests) to confirm brain death in critically ill patients — an isoelectric (flat) EEG is one criterion
- Sleep disorders: EEG is a component of polysomnography (sleep studies) used to diagnose sleep apnea, narcolepsy, and REM sleep behavior disorder
- Monitoring in the ICU: Continuous EEG monitoring detects non-convulsive seizures in comatose patients and monitors depth of sedation
- Brain tumors and strokes: EEG may reveal focal slowing over affected brain regions, helping localize lesions
Proper preparation is important for a successful EEG test. Follow your doctor's specific instructions, which may differ from general guidelines. Common preparation steps:
Hair and scalp:
- Wash your hair the night before or morning of the test with shampoo only — no conditioner, oils, hair spray, gel, or other styling products
- Clean, dry hair ensures good electrode contact with the scalp for a clear, artifact-free recording
- Avoid braids, weaves, extensions, or dreadlocks that cannot be removed — discuss with your provider if applicable
Medications:
- Continue taking your regular medications unless your doctor specifically tells you to stop
- Do NOT stop antiepileptic medications without physician guidance — stopping seizure medications can trigger breakthrough seizures
- Inform the EEG technologist of all medications, including over-the-counter drugs and supplements
Food and caffeine:
- Eat a normal meal before the test — low blood sugar can affect brain activity patterns
- Avoid caffeine (coffee, tea, energy drinks, soda) for 8–12 hours before the test
Sleep preparation:
- If ordered as a standard EEG: get a normal night's sleep
- If ordered as a sleep-deprived EEG: stay awake all night or follow your doctor's specific sleep restriction instructions
- Patients who are sleep-deprived will often fall asleep during the recording — this is expected and helps capture sleep-related patterns
What to bring:
- A list of all current medications with dosages
- Your insurance card and photo ID
- Comfortable clothing — you will be asked to lie or sit still for 30–90 minutes
- A driver if you are sleep-deprived, as driving after sleep deprivation is unsafe
What Happens During an EEG Test?
The EEG procedure typically follows these steps:
- Setup (20–30 minutes): An EEG technologist measures and marks your scalp to identify electrode placement locations using the standardized 10-20 system. Electrode positions are determined by anatomical landmarks (nasion, inion, ear lobes). The technologist cleans each spot with a mild abrasive gel, then attaches electrodes using conductive paste. A cap or net system may be used for faster application. You will be asked to sit or lie comfortably in a reclining chair or on a bed.
- Baseline recording (5–10 minutes): Brain activity is recorded while you lie still with your eyes open, then eyes closed. You must remain as still as possible to avoid movement artifacts that can obscure the recording.
- Activation procedures: To bring out abnormal patterns, the technologist will ask you to:
- Hyperventilate: Breathe deeply and fast for about 3 minutes — this can provoke absence seizures
- Photic stimulation: A strobe light flashes at varying frequencies — can provoke photosensitive epilepsy responses
- Sleep recording (if applicable): You may be asked to sleep during part of the recording. If sleep-deprived, you will likely fall asleep naturally.
- Electrode removal: Electrodes are removed and the gel is cleaned from your hair. You may wash your hair at home to remove residue.
The EEG practice questions on our site cover the clinical interpretation concepts that EEG technologists and neurology students study to understand waveform patterns and electrode placement systems.
Understanding EEG Test Results
EEG results are interpreted by a neurologist who reviews the wave patterns recorded during your test. Results are typically available within 1–7 days and communicated by your referring physician.
Normal EEG: A normal EEG shows expected wave patterns for your age and state of consciousness — alpha waves when relaxed with eyes closed, beta waves when alert, and appropriate changes with sleep. Important: A normal EEG does not rule out epilepsy. Seizure activity is often intermittent and may not occur during the 30–90 minute recording window.
Abnormal EEG patterns and their significance:
- Spike-and-wave complexes: The hallmark of epilepsy — sharp spikes followed by slow waves, often in 3-per-second (3 Hz) complexes typical of absence epilepsy, or irregular patterns in generalized or focal epilepsy
- Focal slowing: Abnormally slow waves over one brain region, suggesting focal dysfunction (tumor, stroke, abscess, or focal cortical injury)
- Generalized slowing: Diffuse slow wave activity across the entire brain, seen in encephalopathy, metabolic disorders, or diffuse injury
- Burst-suppression pattern: Alternating bursts of activity and suppression (flat periods) — seen in severe brain injury, deep anesthesia, or hypothermia treatment
- Isoelectric (flat) EEG: Absence of electrical brain activity — one criterion used in brain death evaluation
Your neurologist will correlate the EEG test findings with your symptoms, medical history, neurological exam, and other diagnostic tests (MRI, CT scan) before drawing conclusions. Explore our eeg practice test for visual walkthroughs of common EEG waveforms and what they mean clinically.