EEG Test: How Brain Wave Monitoring Works

What Is an EEG Test and Why Is It Done?

An EEG (electroencephalography) is a diagnostic test that measures and records electrical activity in the brain using small electrodes placed on the scalp. It is primarily used to diagnose epilepsy and seizure disorders, but also helps evaluate sleep disorders, encephalopathy, head injuries, and other neurological conditions.

The human brain contains approximately 86 billion neurons that communicate through electrical impulses. These electrical signals create patterns called brain waves, which can be detected and recorded by an EEG. Different brain states – such as being awake, drowsy, or in various stages of sleep – produce characteristic wave patterns that trained neurologists can interpret.

The technology behind EEG was first developed in the 1920s by German psychiatrist Hans Berger, who recorded the first human electroencephalogram in 1924. Since then, EEG has become one of the most important tools in neurology, providing real-time information about brain function that other imaging techniques like MRI or CT scans cannot capture.

Unlike structural imaging tests that show the physical appearance of the brain, EEG reveals how the brain is functioning electrically. This makes it uniquely valuable for detecting abnormal electrical activity that occurs during seizures, sleep disorders, or conditions affecting brain function. The test is also used to monitor brain activity during surgery, assess patients in coma, and confirm brain death in critical care settings.

How Brain Waves Work

Brain waves are categorized into different types based on their frequency (measured in Hertz, or cycles per second). Each type of brain wave is associated with different states of consciousness and mental activity. Understanding these patterns helps doctors identify abnormalities that may indicate neurological conditions.

Alpha waves (8-13 Hz) appear when you are awake but relaxed with your eyes closed. They are most prominent in the occipital region (back of the head) and decrease when you open your eyes or begin mental activity. The presence of normal alpha activity is an important indicator of healthy brain function.

Beta waves (14-30 Hz) dominate when you are alert, focused, or actively thinking. They are associated with problem-solving, decision-making, and active concentration. Higher beta activity may be seen during anxiety or stress.

Theta waves (4-7 Hz) occur during light sleep, drowsiness, and deep relaxation. In adults, excessive theta activity during wakefulness may indicate drowsiness, medication effects, or certain brain disorders.

Delta waves (0.5-3 Hz) are the slowest brain waves and appear during deep, dreamless sleep. In awake adults, significant delta activity may indicate brain injury, encephalopathy, or other serious conditions.

Conditions Diagnosed with EEG

EEG is an essential diagnostic tool for numerous neurological conditions. The most common reason for ordering an EEG is to evaluate suspected epilepsy or seizure disorders. During a seizure, neurons fire abnormally in synchronized patterns that create distinctive EEG signatures. Even between seizures, many people with epilepsy show characteristic abnormalities called interictal epileptiform discharges.

Beyond epilepsy, EEG helps diagnose sleep disorders such as narcolepsy and REM sleep behavior disorder. It is used to assess encephalopathy (diffuse brain dysfunction) caused by infections, metabolic disorders, toxins, or organ failure. EEG can help evaluate the effects of head injuries, monitor brain function during surgery, and assess patients who are unconscious or in coma.

What Are the Different Types of EEG Tests?

There are several types of EEG tests: routine (awake) EEG takes about 60 minutes, sleep EEG records brain activity during sleep, ambulatory (long-term) EEG monitors for 24 hours to several weeks, video-EEG combines brain monitoring with video recording, and stereo-EEG uses surgically implanted electrodes for precise localization.

The type of EEG your doctor orders depends on what information is needed and the suspected condition. Each type has specific advantages and is designed to capture different aspects of brain electrical activity. Understanding the differences helps you prepare appropriately and know what to expect.

Routine (Awake) EEG

A routine EEG is the most common type and is typically the first EEG test ordered when evaluating possible epilepsy or other neurological conditions. During this test, you recline comfortably while approximately 21-25 electrodes are attached to your scalp using a conductive gel or paste. Thin wires connect these electrodes to an EEG machine that displays your brain wave patterns on a monitor.

The test usually takes about 60 minutes total, including preparation time for placing the electrodes. During the recording portion (typically 20-40 minutes), you will be asked to remain still and relaxed. The technician may ask you to open and close your eyes at various intervals, as this produces characteristic changes in brain wave patterns that help assess normal brain function.

Toward the end of the test, two activation procedures are commonly performed to increase the likelihood of detecting abnormalities. Hyperventilation involves breathing deeply and rapidly for about 3 minutes, which can trigger abnormal activity in people with certain types of epilepsy. Photic stimulation involves looking at a flashing strobe light at various frequencies, which may provoke photosensitive epileptic activity in susceptible individuals.

Sleep EEG

A sleep EEG records brain activity while you sleep, which is valuable because many types of abnormal brain activity are more likely to appear during sleep. The electrode placement is similar to a routine EEG, and the test takes one to two hours. After the recording, you are awakened, the electrodes are removed, and you can go home.

Sleep deprivation is often required before a sleep EEG because it helps you fall asleep during the test and also increases the likelihood of detecting certain abnormalities. Your doctor will provide specific instructions about how little sleep you should get the night before. For adults, this often means staying awake all night or sleeping only a few hours.

Children do not need to stay awake all night before a sleep EEG. The amount of sleep restriction depends on the child's age – younger children need less restriction, and infants who still nap during the day may not need any sleep deprivation at all. In some cases, children may receive melatonin to help them fall asleep during the test.

Ambulatory (Long-Term) EEG

Ambulatory EEG monitoring allows continuous recording over an extended period, typically 24 hours to several weeks. This type of monitoring significantly increases the chance of capturing abnormal activity that may occur infrequently. You wear a portable EEG device while going about your daily activities at home.

Electrodes are attached to your scalp and held in place with a thin cap or adhesive. Thin wires connect to a small, lightweight recording device worn in a waist pouch or small backpack. The device continuously records your brain activity, which is later analyzed by a neurologist.

During ambulatory monitoring, you typically keep a diary noting the times of your activities, any symptoms you experience, and when you take medications. This information helps the neurologist correlate brain wave patterns with your symptoms. You can eat and drink normally during the monitoring, but you should avoid chewing gum (which creates electrical artifacts) and bathing may be restricted.

You will return to the clinic periodically – usually once daily – so the technician can download the recordings, check the equipment, and ensure electrode connections remain good. At the end of the monitoring period, the device and electrodes are removed.

Video-EEG Monitoring

Video-EEG combines continuous EEG recording with simultaneous video monitoring, allowing doctors to see exactly what happens during episodes or seizures. This is typically done in a hospital epilepsy monitoring unit over several days. The primary goal is to capture and characterize seizures or other events to determine their origin and nature.

Video-EEG monitoring is particularly valuable for distinguishing epileptic seizures from non-epileptic events (such as psychogenic seizures or fainting episodes), localizing where seizures begin in the brain for surgical planning, characterizing seizure types to guide treatment decisions, and quantifying how often seizures occur.

During video-EEG monitoring, antiepileptic medications may be reduced to increase the likelihood of capturing seizures. Other activation procedures like sleep deprivation may also be used. Medical staff continuously monitors patients for safety, as reducing medications can increase seizure risk.

Stereo-EEG (Invasive EEG)

Stereo-EEG is a specialized, invasive procedure used in the evaluation of patients with epilepsy who may be candidates for surgery. Under general anesthesia, thin electrode probes are surgically inserted through small holes in the skull directly into the brain tissue. This provides much more precise information about where seizures originate than scalp EEG.

Stereo-EEG is only used when non-invasive tests have not provided sufficient information to plan surgery, and when surgery is being seriously considered as a treatment option. The procedure requires hospitalization for several days to weeks while monitoring captures seizure activity. Extensive discussions with the medical team occur before this procedure is undertaken.

How Should I Prepare for an EEG Test?

To prepare for an EEG: wash your hair the night before or morning of the test (avoid conditioner, gel, or hairspray), tell your doctor about all medications, eat normally unless instructed otherwise, and follow specific instructions if having a sleep EEG. Clean hair helps electrodes attach properly to your scalp.

Proper preparation for an EEG helps ensure accurate results and a smooth testing experience. The most important preparation step is hair care – electrodes must make good contact with your scalp to detect brain signals clearly, and hair products or excessive oils can interfere with this connection.

Wash your hair thoroughly the night before or morning of your test using regular shampoo. After washing, do not apply any styling products such as conditioner (unless you rinse it out completely), hair gel, mousse, hairspray, or leave-in treatments. These products create a barrier between the electrodes and your scalp, potentially affecting the quality of the recording.

Medications and Your EEG

Inform your doctor about all medications you are taking, including prescription drugs, over-the-counter medications, and supplements. Certain medications can affect brain wave patterns and may need to be considered when interpreting your results. However, you should never stop taking any medication without your doctor's explicit instruction.

For some EEG tests, particularly those evaluating epilepsy before possible surgery, your doctor may specifically instruct you to reduce or temporarily stop antiepileptic medications. This is only done under careful medical supervision, usually in a hospital setting where you can be monitored for safety. Never adjust your medications on your own.

Eating and Drinking

Unless your doctor gives you different instructions, eat your normal meals before the EEG. Low blood sugar can affect brain wave patterns, potentially making results harder to interpret. A comfortable, well-fed patient is also more likely to relax during the test, which helps capture accurate recordings.

If you are having a sleep EEG, avoid caffeine on the day of your test. Coffee, tea, energy drinks, chocolate, and some sodas contain caffeine that can make it difficult to fall asleep during the test. Check ingredient labels if you're unsure about a particular food or drink.

What to Wear

Wear comfortable clothing to your EEG appointment. If you are having long-term ambulatory monitoring, wear button-up or zip-up shirts rather than pullover tops. This makes it easier to dress and undress without disturbing the electrodes on your scalp.

Leave valuable jewelry at home, although most jewelry does not need to be removed for the test. Your technician will let you know if anything needs to be removed.

Sleep EEG Specific Preparation

If you are scheduled for a sleep EEG, follow your doctor's specific instructions about sleep deprivation. Adults are often asked to stay awake all night before the test or to sleep only a few hours. This serves two purposes: it makes it easier for you to fall asleep during the test, and sleep deprivation itself can increase the likelihood of detecting certain types of abnormal brain activity.

Because you will be sleep-deprived, arrange for someone else to drive you to and from your appointment. Driving while severely tired is dangerous and comparable to driving under the influence of alcohol. Many facilities will not perform the test if you have driven yourself after sleep deprivation.

What Happens During an EEG Test?

During an EEG, you lie or recline comfortably while a technician attaches 21-25 electrodes to your scalp using conductive gel. The test records your brain waves as you rest quietly, and may include opening/closing your eyes, deep breathing, and looking at flashing lights. The entire process is painless.

When you arrive for your EEG, a trained technician (often called an EEG technologist) will explain the procedure and answer any questions you have. Understanding what will happen can help you feel more relaxed, which is important for obtaining a good quality recording.

Electrode Placement

The technician will measure your head and mark specific locations on your scalp where electrodes will be placed. These locations follow an international standard called the 10-20 system, which ensures electrodes are positioned consistently and cover all areas of the brain.

A mild abrasive gel may be applied first to lightly clean each electrode site – this removes dead skin cells and oils that could interfere with the recording. Then, each electrode is attached using a conductive paste or gel that helps transmit electrical signals from your scalp to the electrode. The process is not painful, though the paste may feel slightly cold or sticky.

Thin wires connect each electrode to the EEG machine, which amplifies and records the tiny electrical signals produced by your brain. Modern EEG machines display this information digitally on a computer screen, allowing the technician to monitor the quality of the recording throughout the test.

The Recording Process

Once all electrodes are in place and the equipment is calibrated, the actual recording begins. You will be asked to relax and remain as still as possible. Movement creates electrical signals that can interfere with the brain wave recording – these artifacts make the EEG harder to interpret.

During the recording, the technician will give you specific instructions at various times. You may be asked to open and close your eyes – this produces characteristic changes in brain waves (particularly alpha waves) that help assess normal brain function. You should keep your eyes focused on a single point when they are open.

The hyperventilation portion involves breathing deeply and rapidly for about 3 minutes. This can feel tiring and may cause lightheadedness, tingling in the fingers and around the mouth, or mild dizziness. These sensations are temporary and resolve quickly once normal breathing resumes. Hyperventilation can provoke certain types of abnormal brain activity, making it a valuable part of the test.

Photic stimulation involves looking at a strobe light that flashes at various frequencies. You will be asked to keep your eyes open, then closed, at different points during the flashing. This test can trigger abnormal activity in people with photosensitive epilepsy. If you are known to be photosensitive or if abnormal activity is detected, the stimulation is immediately stopped.

Staying Comfortable

The key to a successful EEG is remaining relaxed and still. Try to avoid clenching your jaw, furrowing your brow, or tensing facial muscles, as these actions create electrical signals that appear on the recording. If you feel the need to move, blink, or swallow, try to do so gently and minimally.

If you become uncomfortable, need to use the restroom, or have any concerns during the test, inform the technician. Brief pauses can be accommodated if needed. The technician monitors you throughout the procedure and can make adjustments to ensure your comfort.

How Will I Feel After an EEG Test?

After a routine or sleep EEG, you will feel completely normal. The electrode paste washes out easily with regular shampoo. If you did hyperventilation, temporary dizziness passes within minutes. After a sleep EEG with sleep deprivation, do not drive until you have caught up on sleep.

One of the advantages of EEG testing is that there are no lasting effects – most people feel completely normal immediately after the test. Since the electrodes only record brain activity (they don't send any signals into your brain), there are no side effects from the test itself.

Immediate Aftercare

After the recording is complete, the technician will remove all the electrodes from your scalp. The conductive paste or gel leaves some residue in your hair, which washes out easily with regular shampoo and water. Some people prefer to wash their hair at the clinic before leaving, while others wait until they get home.

If hyperventilation was performed during the test, you may have experienced lightheadedness or tingling that resolves quickly once you resume normal breathing. These sensations typically pass within a few minutes and should not persist after you leave the clinic.

After a Sleep EEG

Following a sleep EEG with sleep deprivation, your most important consideration is safety. You will likely feel tired and should not drive, operate machinery, or engage in activities requiring alertness until you have had adequate sleep. Most people take the rest of the day to sleep and recover.

The quality of your recovery sleep may actually be better than usual due to the sleep deprivation – your body will be ready for deep, restorative sleep. Plan to have a quiet day at home where you can nap or sleep as needed.

After Long-Term Monitoring

How you feel after ambulatory or video-EEG monitoring depends on several factors, including how long the monitoring lasted, whether you had any seizures during monitoring, and whether your medications were reduced. If medication reduction resulted in seizures, you may feel fatigued and need time to recover.

After any EEG involving medication changes, follow your doctor's instructions carefully about resuming your regular medication schedule. Do not make any changes to your medications without medical guidance.

When Will I Get My EEG Results?

EEG results are typically available within 1-2 weeks after the test. A neurologist reviews the recordings to identify normal and abnormal brain wave patterns. Your referring doctor receives the interpretation and will discuss the results with you and explain their implications for your diagnosis and treatment.

Understanding how EEG results are generated and interpreted can help you know what to expect. The raw EEG recording is just the beginning – skilled interpretation by a trained specialist is essential to extract meaningful diagnostic information.

The Interpretation Process

After your EEG is completed, the digital recording is sent to a neurologist or clinical neurophysiologist who specializes in EEG interpretation. This specialist reviews the entire recording, analyzing the patterns of brain waves across all electrode locations and throughout all stages of the test.

The interpreter looks for both normal and abnormal features. Normal findings include age-appropriate background rhythms, expected responses to eye opening/closing, and normal patterns during hyperventilation and photic stimulation. Abnormal findings may include epileptiform discharges (spikes, sharp waves), slowing of background rhythms, asymmetries between the two sides of the brain, or unusual responses to activation procedures.

The interpreter creates a written report that describes the technical quality of the recording, the background brain activity, any abnormalities detected, and a clinical impression or conclusion. This report is sent to your referring doctor.

Understanding Your Results

Your doctor will discuss the EEG results with you and explain what they mean in the context of your symptoms and overall medical situation. EEG results are one piece of the diagnostic puzzle – they are interpreted alongside your medical history, physical examination, and other tests.

It's important to understand that a normal EEG does not necessarily mean you don't have epilepsy or another neurological condition. Abnormal brain activity may not occur during every EEG session. If your symptoms strongly suggest a condition but your EEG is normal, your doctor may recommend repeat testing, longer monitoring, or additional evaluation.

Conversely, some people may have EEG findings that require careful interpretation. Certain patterns that look abnormal may actually be normal variants in some individuals. Your neurologist considers many factors when making diagnostic conclusions.

Frequently Asked Questions About EEG