What Is EEG?

If you have a brain injury or illness, you may need an EEG. An EGG (electroencephalogram) is a convenient way of letting doctors see how the brain is behaving.

An electroencephalogram (EEG) is a recording of brain electrical activity. Nerve cells transmit messages in the form of electrical impulses in the brain. An EEG records these electrical signals, which are then read by a doctor or clinical physiologist.

Below, we look at the different types of EEG, the situations in which they are used, how an EGG is conducted, as well as what to expect afterwards.


An electroencephalogram (EEG) is a recording of electrical activity in the brain. Nerve cells or neurons transmit messages by firing electrical impulses, and recording these electrical signals can provide valuable information about the health and activity of the brain.1 

During an EEG test, electrodes (electrical conductors) are placed on the scalp. There are three types of electrodes: gel, dry and water electrodes, though gel electrodes are the most widely used. Electrodes conduct electrical activity from the scalp and allow it to be recorded and displayed on a screen.1

Advantages of EEG1

Ability to record fast processes: EEG is able to record neuronal activity in the sub-second range, which means it captures brain activity as it occurs. In other words, it has a high temporal resolution.

Portable: Compared to other methods of recording brain activity such as fMRI (functional magnetic resonance imaging), EEG is more convenient because of its small size and portability.

Affordable: EEG is relatively inexpensive compared to other neuroimaging tests.

Versatile: It can be used for a wide variety of purposes, as described below in ‘Why is an EEG performed’.

Disadvantages of EEG1

Poor spatial detail: Spatial resolution is the amount of detail that a sensor can capture relating to space. EEG is said to have poor spatial resolution. High temporal resolution and low spatial resolution means that an EEG can accurately record when an electrical signal occurs, but cannot as accurately measure where the signal occurs.

Different types of EEG

There are different types of EEG which can be used depending on the nature of the problem and symptoms.

Routine EEG

A routine EEG takes approximately 20 to 40 minutes. You may be asked to look at flashing lights or to breathe quickly to check the effect on your brain activity.

Sleep EEG

A sleep EEG is performed over several hours while you sleep. It is usually used when the doctor needs more information than a routine EEG provides. A sleep EEG is also used to check for symptoms of a sleep disorder.

Ambulatory EEG

An ambulatory EEG is recorded over 1 to 3 days as you go about your daily life. A small EEG recorder is attached to your clothes and connected to electrodes on your scalp.

Video EEG

This type of EEG is also known as video telemetry. A video recording is taken along with the EEG to give more information about the disorder and help in confirming a diagnosis.

Invasive EEG with video

This is also known as intracranial EEG. Brain surgery is performed to place electrodes on the surface of the brain. It is usually followed by video monitoring to see how the condition develops.

Who performs EEG?

Clinical neurophysiology is a branch of science that is concerned with diagnosing and managing neurological disorders. The EEG is typically performed by a clinical neurophysiologist - a person who specialises in this area. In some cases, they may be assisted by an EEG technologist.

Why is an EEG performed?

An EEG is performed to allow a doctor or medical specialist to understand how the brain is behaving. It is used to examine patients who may have:


People with epilepsy experience seizures; sudden, uncontrollable bursts of electrical activity in the brain. This can cause jerking of the limbs or “fits”, stiffness, staring into space or loss of consciousness. EEG can be used to diagnose epilepsy in patients, as well as to investigate it in research studies.2

Sleep disorders

EEG is used to diagnose and monitor various sleep disorders. During normal sleep, the pattern of brain activity (brain waves) goes through several phases. In sleep disorders such as apnoea, these brain wave patterns are disrupted and this can be seen in an EEG recording.3


EEG can be used to diagnose dementia.4 Dementia is a general term which describes a progressive decline in the functioning of the brain, such as memory, decision-making and language comprehension. Alzheimer’s disease, Parkinson’s disease and Huntington’s disease are all types of dementia.

Head injury

A traumatic brain injury is an injury to the head that affects brain functioning, such as one caused by a fall or car accident.  It can cause loss of consciousness, difficulty learning information and seizures.5 EEG can be used to diagnose and monitor the progression of traumatic brain injury.5


For some people, infection with COVID-19 causes changes in the brain that lead to cognitive difficulties like problems with processing information, planning or thinking flexibly. These symptoms usually get better over several months, though they can persist for longer in some people. This discovery was made by researchers using EEG and other brain imaging methods.6

What should I expect during and after EEG?

The clinical neurophysiologist first explains the purpose of the EEG and what you will need to do. During the EEG test, you will be asked to sit or lie down quietly. The test can take a few minutes if it is a routine EEG or longer if it is a sleep or ambulatory EEG. You may be asked to breathe quickly or look at flashing lights depending on the purpose of the EEG. 

An EEG test is painless and does not cause discomfort. Once the EEG recording is done, you should be able to return to your daily activities. 

Are there any side effects of EEG?

An EEG test generally does not have any side effects. If gel electrodes are used, some gel will be applied on your scalp and you will need to wash it out afterwards. This should not cause any pain or discomfort. In the case of a sleep EEG, you may be asked not to sleep the night before to make sure you fall asleep during the EEG test. This means you will be sleep-deprived before the test and may need to avoid activities like driving.


An electroencephalogram (EEG) is a recording of electrical activity produced by nerve cells in the brain. It is used to understand and monitor brain activity, which is important to diagnose conditions like epilepsy, sleep disorders, dementia, and traumatic brain injury. It is also used in research to study how COVID-19 infection affects brain function in the long term. There are different types of EEG which are suited for different purposes, and are generally painless and non-invasive, except for invasive EEG, which involves surgery.


  1. Michel CM, Brunet D. Eeg source imaging: a practical review of the analysis steps. Frontiers in Neurology [Internet]. 2019 [cited 2023 Feb 3];10. Available from: https://www.frontiersin.org/articles/10.3389/fneur.2019.00325
  2. Sharmila A, Geethanjali P. A review on the pattern detection methods for epilepsy seizure detection from EEG signals. Biomedical Engineering / Biomedizinische Technik [Internet]. 2019 Oct 1 [cited 2023 Feb 3];64(5):507–17. Available from: https://www.degruyter.com/document/doi/10.1515/bmt-2017-0233/html
  3. Raut AD, Dixit B. Sleep apnoea disorder: a review. European Journal of Engineering and Technology Research [Internet]. 2020 Mar 20 [cited 2023 Feb 3];5(3):339–42. Available from: https://ej-eng.org/index.php/ejeng/article/view/1822
  4. Sánchez-Reyes LM, Rodríguez-Reséndiz J, Avecilla-Ramírez GN, García-Gomar ML, Robles-Ocampo JB. Impact of eeg parameters detecting dementia diseases: a systematic review. IEEE Access. 2021;9:78060–74.
  5. Ianof JN, Anghinah R. Traumatic brain injury: An EEG point of view. Dement Neuropsychol [Internet]. 2017 [cited 2023 Feb 3];11(1):3–5. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5619208/
  6. Cecchetti G, Agosta F, Canu E, Basaia S, Barbieri A, Cardamone R, et al. Cognitive, EEG, and MRI features of COVID-19 survivors: a 10-month study. J Neurol [Internet]. 2022 Jul 1 [cited 2023 Feb 4];269(7):3400–12. Available from: https://doi.org/10.1007/s00415-022-11047-5
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Karabo Sibasa

Doctor of Philosophy - PhD, Psychology,The University of Manchester

Karabo has a BSc in Biochemistry and is currently doing a PhD in Psychology at the University of Manchester. She has a background in teaching and research, and is interested in health, science communication and education.

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