The electroencephalogram (EEG) is a non-invasive neuroimaging test that can detect and record minute changes in electrical activity within the brain. This is recorded using microelectrodes (large, flat electrodes stuck to the skin or scalp).
EEG recordings show brainwaves as wavy lines with rising and falling patterns.
Key Takeaways
- An EEG (electroencephalogram) records your brain’s electrical activity using painless electrodes placed on the scalp.
- It measures brainwaves like delta, theta, alpha, beta, and gamma, helping to assess brain states such as sleep, alertness, and relaxation.
- EEGs are used to diagnose conditions like epilepsy, sleep disorders, and brain injuries, and also have wide applications in psychological and developmental research.
- The test is safe, non-invasive, and provides real-time data about your brain’s function without using radiation.
- While highly sensitive to changes in time, EEGs cannot pinpoint exact brain locations and may occasionally be affected by external noise or movement.
What Does an EEG Do?
An EEG measures your brain’s electrical activity using small sensors called electrodes. These electrodes are placed on your scalp, where they detect tiny voltage fluctuations caused by the firing of neurons.
When neurons communicate, they do so by generating small electrical impulses known as action potentials.
While a single neuron’s signal is too weak to detect at the scalp, large groups of neurons firing in synchrony create electrical fields strong enough to be picked up by surface electrodes.
Combined neuron signals form wave-like patterns called brainwaves, which the EEG machine amplifies and records.
The results appear as brainwaves—rhythmic patterns of activity that reflect different mental and physiological states.
Doctors and psychologists analyze these patterns to understand how your brain is functioning in real time.
Key Concepts:
- Electroencephalography: The technique of recording electrical activity from the brain.
- Electrode: A small metal disc placed on the scalp to detect brain activity.
- Action potential: An electrical signal used by neurons to transmit messages.
- Brainwave: A rhythmic pattern of neural activity shown on EEG.
What Does an EEG Measure?
EEG machines measure brainwaves, which vary based on your alertness, mood, and brain state. Each type of wave has a unique frequency, measured in hertz (Hz).
Types of Brainwaves:
| Wave Type | Frequency Range | State Associated | Typical Location | Notes |
|---|---|---|---|---|
| Delta | 0.5–4 Hz | Deep sleep | Diffuse | Highest amplitude, slowest frequency |
| Theta | 4–8 Hz | Light sleep, drowsiness | Temporal | Seen in early sleep and meditation |
| Alpha | 8–12 Hz | Calm wakefulness | Occipital | Linked to relaxed, eyes-closed states |
| Beta | 13–30 Hz | Alert, focused | Frontal | Often seen during active thinking |
| Gamma | >30 Hz | Cognitive processing | Various | Less understood; linked to perception and memory |
Understanding these wave patterns helps clinicians assess everything from seizure activity to attention and sleep quality.
What Is an EEG Used For?
EEGs help diagnose and monitor many neurological and psychiatric conditions. They’re also used in research to understand brain function during different tasks and mental states.
Clinical Uses:
- Epilepsy: Detects abnormal spiking waves during or between seizures.
- Sleep disorders: Measures brain activity during different sleep stages.
- Brain injuries: Assesses functioning after trauma, stroke, or surgery.
- Dementia: Slowed waves can indicate neurodegeneration.
- Encephalitis: Can show diffuse brain inflammation.
- Tumors or stroke: May present as localized slowing or abnormal patterns.
Research Uses:
- Cognitive psychology: Tracks brain response to stimuli or tasks.
- Developmental studies: Used in infants to assess early brain function.
- Mental health: Some studies explore EEG patterns in conditions like ADHD or depression.
Example Studies:
- A study by Coutin-Churchman et al. (2003) found abnormal EEG activity in 83% of participants with mental health conditions. Decreases in delta and theta waves, often alongside increased beta activity, were common indicators of brain dysfunction.
- Bell (2012) used EEG to study working memory in 8-month-old infants. Increased frontal-parietal EEG coherence was associated with better task performance, showing how EEG can track developmental differences in cognition.
A Brief History of EEG
The EEG was first developed by German psychiatrist Hans Berger in 1924. He was the first to record electrical activity from a human brain and coined the term “electroencephalogram.”
Berger’s early work laid the foundation for decades of research in brain science, leading to the clinical and research tool we know today
Step-by-Step EEG Procedure
The procedure is painless, safe, and usually takes 20–60 minutes (longer for sleep studies).
Before the Test:
- Wash your hair to remove oils and product.
- Avoid caffeine and alcohol.
- Follow sleep instructions if doing a sleep EEG.
During the Test:
- A technician measures your head and marks electrode positions.
- Electrodes are placed using paste or adhesive.
- You sit or lie still while the machine records brainwaves.
- You may be asked to close your eyes, breathe deeply, or respond to flashing lights.
- For some tests, you might be asked to sleep.
After the Test:
- Electrodes are removed, and your scalp is cleaned.
- If sedatives were used, you may need someone to drive you home.
Are There Any Risks to EEG?
EEGs are considered extremely safe. The electrodes do not emit electricity—they simply record signals. You won’t feel pain or discomfort during the test.
However, there are a few minor considerations:
- Seizure risk: In rare cases, a seizure may be triggered during testing in people with epilepsy—especially if flashing lights or hyperventilation are used as stimuli. This occurs under controlled, supervised conditions.
- Skin irritation: Some people experience mild irritation from the adhesive or paste used to attach the electrodes.
- Fatigue or drowsiness: Particularly if you were asked to sleep less the night before.
Compared to other neuroimaging techniques, EEG has fewer risks and is suitable even for young children and medically vulnerable individuals.
How Are EEG Results Interpreted?
The raw EEG data appears as wave patterns—like rolling hills—that reflect your brain’s ongoing activity. A neurologist interprets these patterns to look for abnormalities.
Normal Results:
- Clear alpha waves during relaxed wakefulness
- Appropriate delta and theta waves during sleep
Abnormal Results:
- Spikes or sharp waves: May indicate seizure activity
- Slowing: May suggest brain damage, stroke, or encephalopathy
- Burst suppression: A severe pattern often seen in coma
Keep in mind that some people show unusual EEG patterns even without any health condition. Results always need to be interpreted alongside symptoms and medical history.
What Are the Benefits and Limitations of an EEG?
Benefits:
- Non-invasive and safe: No radiation or electric current
- Real-time feedback: Excellent temporal resolution (millisecond-level)
- Cost-effective: Less expensive than MRI or PET
- Good for all ages: Especially useful with infants or people who can’t undergo scans
Limitations:
- Poor spatial resolution: Can’t precisely locate brain activity
- Susceptible to noise: Movement or muscle tension can interfere
- Surface activity only: Doesn’t measure deep brain regions
Is an EEG Right for You?
If your doctor suspects a seizure disorder, sleep problem, or cognitive change, an EEG may be recommended as part of your evaluation. It’s often one step in a broader neurological or psychological workup.
While it doesn’t give you a diagnosis on its own, it provides valuable information that helps doctors understand what’s happening in your brain.
References
Bell, M. A. (2012). A psychobiological perspective on working memory performance at 8 months of age. Child Development, 83 (1), 251-265.
Coutin-Churchman, P., Anez, Y., Uzcategui, M., Alvarez, L., Vergara, F., Mendez, L., & Fleitas, R. (2003). Quantitative spectral analysis of EEG in psychiatry revisited: drawing signs out of numbers in a clinical setting. Clinical Neurophysiology, 114 (12), 2294-2306.
Mayo Clinic. (April 15, 2020). EEG (Electroencephalogram) . https://www.mayoclinic.org/tests-procedures/eeg/about/pac-20393875
Further Reading
- Electroencephalography (EEG)
- Casson, A. J., Abdulaal, M., Dulabh, M., Kohli, S., Krachunov, S., & Trimble, E. (2018). Electroencephalogram. In Seamless Healthcare Monitoring (pp. 45-81). Springer, Cham.
