EEG: electro + encephalo + gram:

Electro: Represented in electrical units like Voltagr or Current.

Encephalo- Related to brain.

Gram- The record book of brain signals.


Electroencephalogram is a noninvasive test for measuring the electrical activity (patterns) of the brain, where Electroencephalography is the technique for recording and interpreting it.

EEG Procedure Information Page


EEG Procedure, Types, Protocols, Normal vs Abnormal & EEG FAQ's

2. Procedure, Types, Protocols & Normal vs Abnormal  

EEG Procedure

EEG is a non-invasive test which depends on amplifying the tiny electrical signal generated and used by brain neurons communication. This signal practically shows how the human brain handles the tasks. If there is any interruption or miss-behavior of the nature of the signal, it will directly reflect to how the human body behaves. In an EEG, this faint electrical activity (in microvolts -µV-) is measured by putting electrodes directly on the scalp with a sticky material (paste) to guarantee proper conduction, recording and to get a high-quality reading (electrodes are conductor through which an electric current enters or leaves). The test is performed by an EEG technician in health care provider's office either it is a clinic, laboratory or a hospital. The procedure itself is not complicated, and it requires some preparation before starting the recording. You will be asked to lie on your back on a bed or in a reclining chair, and then, the technician will apply between 16 and 25 flat metal disks electrodes (for routine EEG) in different positions on the scalp according to 10 20 international electrode placement system (Cap can be used too). The electrodes themselves are with wires connecting them, where one side is on the scalp and the other is hooked to an electronic component called “amplifier”, which picks up the signal, amplifies, filters it, and then send it to a sophisticated software on a computer, which processes it and then it is displayed as a wavy line on a computer screen. The recording machine converts the electrical impulses into patterns that can be seen on a computer screen, which is automatically saved and archived on a computer hard disk. Historically, before the advancement in technology, the activity was printed on paper. In the procedure, you will have to do certain activities when asked, like eyes closed and opened or hyperventilation (breathe deeply and rapidly for several minutes) or look at a Photic flash light, but generally during the test you should lie still most of the time, because any movement can alter the results.

Image Sources: Slide 1: NHS / Slide 2: MedlinePlus / Slide 3: Al-basheer, CC BY-SA 3.0, via Wikimedia Commons / Sebastian Nagel via ResearchGate / Nachtelijke EEG by maartenzam via flickr , 2.0 Generic (CC BY-NC-SA 2.0) /

Before the Test

Before doing the EEG test, you may consider the following which will help you to prepare for the test, in addition to your healthcare provider instructions.

  • Preparatory steps.
    • Make sure your hair is washed the night before the test, and don’t use spray or gel in your hair till the test is done.
    • Once your test is scheduled, check with your doctor if you should take or avoid taking any medications in your EEG test day.
    • Avoid taking any food or drinks containing caffeine for at least eight hours prior to the test, such as coffee, tea, or cola, as it can affect the test. However, it's important that you do not fast the night before or day of your test. Low blood sugar can interfere with your results.
    • According to the type of EEG, your doctor or the technician might instruct you to sleep fewer hours than usual the night before the procedure, especially, if you’re required to sleep during the EEG.

- Depending on the type of EEG test, the time varies, but usually for a routine outpatient one (doctor's office, hospital, lab, or clinic.), it is mostly from 20 to 40 minutes plus the preparation time. Overall, from the start to the end, it is about an hour, and then, the rest of the day is yours.

- If the procedure is a long-time monitoring “LTM”, it might be up to 8 hours ∓ or if in-Hospital, then it requires you to be admitted to the hospital for 24 hours or a few days depending on what your clinician want to diagnose (including sleeping, usual day activity, etc.).

- If the procedure is a routine EEG, you'll be sitting or lying down for some time, wherefore, it is better to wear something which is comfortable and easy to handle. If you will be admitted to the hospital, you'll change into a hospital gown.

- If you take medication for epilepsy, follow your clinician instructions. You might be asked to lower your dose or not take your prescription at all prior to the test in order to bring on abnormal brain activity, which is useful for your case diagnosis.

During the Test

As there are different types of EEG Tests (routine, LTM, Ambulatory), how each test is carried out depends on the type being performed, but generally, they are almost the same with little differences in term of preparation, and varies according to the time taken to perform the procedure, wither it is outpatient or in-house with hospital admission.

The test will be performed in a room prepared to be with a quite environment and dimly lit, so you will be relaxed as you are sitting on a reclining chair or lying down on a bed.

The technician will then start measuring and marking spots on your head for the electrodes to be attached (also cap with attached electrodes can be used); may gently scrub each area where an electrode will be placed with a cream that's mildly abrasive to let the disc stick better and improve the quality of the recording, and then places around 16 to 25 electrodes in total with a sticky material (paste) on your scalp.

Before starting the recording, the technician will brief you with what and not to do during the procedure, and instruction to follow once the recording starts, like: Open your eyes; close your eyes; breathe deeply and fast (hyperventilate); exposure to flashing lights.

In most of the time, you will have to stay still and quiet, as each and every activity or movement (blink, swallow, cough or smile) can alter the reading and distort it.

If the EEG to be performed is ambulatory (24 to 72 hours), then there are few differences compared to doing the test at the clinic. Since you are going home, the technician will make sure the electrodes are attached and will not move out of place. This can be done by using stronger glue (no worries, this can be cleaned later easily using acetone or a similar solution). You will have to wear a small device where the other side of electrode’s wire will be attached to. At home, you will be asked to act normal and be as you are in your usual daily routine with a few notable exceptions which your doctor or technician will mention to avoid altering the recording or the device.

How the electrodes are placed on patients head?

Electrodes are not placed randomly or roughly on the scalp, but rather with an accurate precise spot according to an internationally classified protocol called “International 10/20 electrode placement system, or in some cases International 10/10 extended electrode placement system). This relies on taking measurements between certain fixed points on the head, and then, placed at points that are 10% and 20% of these distances.

Each electrode site is labelled with a letter and a number. The letter refers to the area of brain underlying the electrode. e.g.: “P” refers to the parietal lobe and “O” to the occipital lobe.
Even numbers denote the right side of the head and odd numbers the left side of the head. The label "z" points to electrode sites in the midline of the head. For example, Cz refers to the midline central region of the head.

Watch our video to know how electrodes are placed in EEG and to learn about 10 20 electrodes international placement system.

What do the letters and numbers mean?

The names of the electrode sites use alphabetical abbreviations that identify the lobe or area of the brain to which each electrode refers:

  • F: Frontal.
  • Fp: Frontopolar.
  • T: Temporal.
  • C: Central.
  • P: Parietal.
  • O: Occipital.
  • A: Auricular (ear electrode).
  • Z: Refers to the midline central region of the head.

EEG Montage

Since an EEG signal represents two electrodes potential difference, the display of the recorded signal by the EEG machine may be set up in one of several ways. The representation of the EEG channels is referred to as a montage.

The montage can be set up in various different methods and ways, depending on the clinician requirements, how to view, where, and what are the one’s of interest. Below are examples (not limited to):

1. Bipolar montage: Each channel (waveform) represents the difference between two adjacent electrodes. The entire montage consists of a series of these channels. For example, the channel "Fp1-F3" represents the difference in voltage between the Fp1 electrode and the F3 electrode. The next channel in the montage, "F3-C3," represents the potential difference between F3 and C3, and so on through the entire array of electrodes.

2. Referential montage: Each channel represents the difference between a certain electrode and a designated reference electrode. There is no standard position for this reference; it is, however, at a different position than the "recording" electrodes. Midline positions are often used because they do not amplify the signal in one hemisphere vs. the other. Another popular reference is "linked ears," which is a physical or mathematical average of electrodes attached to both earlobes or mastoids.

3. Average reference montage: The outputs of all of the amplifiers are summed and averaged, and this averaged signal is used as the common reference for each channel.

4. Laplacian montage: Each channel represents the difference between an electrode and a weighted average of the surrounding electrodes. With digital EEG, all signals are typically digitized and stored in a particular (usually referential) montage; since any montage can be constructed mathematically from any other, the EEG can be viewed by the EEG machine in any display montage that is desired.

After the Test

Once the EEG procedure is over, the technician will remove the electrodes from your scalp, and you might have to wash your head at home to clean it from the remaining’s of the sticky paste used to attach the electrodes. No steps further to be taken, and you can resume your regular routine for the rest of the day. However, in certain cases you might be given a sedative, which will be worn off after sometime and not immediately, therefore you should not drive home by yourself. instead, it is advisable that someone should take you home after the test.

Interpreting the Results

The EEG is read by a neurologist, optimally the one who has specific training in the interpretation of EEGs. This is done by visual inspection of the waveforms, called graphoelements.
The result of your test will be either normal or abnormal, showing healthy normal brainwave patterns or abnormalities, but might not be exclusive, as there are occasions where the person has seizure or epilepsy but his/her EEG shows normal patterns. In this case, your doctor might ask for further investigative tests like anatomical imaging (MRI, fMRI, PET, etc)

EEG Types

A routine or a standard EEG is usually the first type your doctor asks you to have (may be done with or without video). It is performed as an outpatient and lasts from 20 – 45 minutes plus preparation. You can go home as soon as the test is done. During the procedure, you will be asked to lie on your back on a bed or in a reclining chair. The technician will attach electrodes to your scalp using paste and then hook their end to an amplifier connected to a computer. The acquired signal will be displayed as wavy lines on a screen. You might be asked to do certain things like: open your eyes, close it, do hyperventilation, and be exposed to an intermittent flashing light (photic stimulation). All of these activities can change the electrical activity in your brain. This can help the doctor to make a diagnosis. However, most of the time you will have to be relaxed and stay still, because any movement can change the electrical activity of your brain and can affect the results.

A sleep EEG is carried out while you're asleep. It may be used if a routine EEG does not give enough information when you were awake, and didn’t show any unusual electrical activity. This could be because your seizures happen when you’re asleep or when you are tired. When you’re asleep, your brainwave patterns change and may show more unusual electrical activity. In some cases, you may be asked to stay awake the night before the test or just to wake up much earlier than you usually do, to ensure you can sleep while it's carried out. This is called a sleep-deprived EEG.
A sleep EEG test is usually done in hospital using a routine EEG machine, lasting from few hours to eight hours, and then you may go home once you’ve woken up (unless otherwise instructed by your doctor).

You would usually have a video-telemetry test if you have already been diagnosed with epilepsy, and your doctor wants a thorough investigation to determine: what type of seizures you have and during what activity; or if your AED (anti-epileptic drug) is not working well; or if your seizures are not caused by epilepsy, but something else; or you are considered or subject to epilepsy surgery.

In a video-telemetry test, you need to stay in the hospital over a few days wearing an ambulatory EEG amplifier and filmed while recording is taken. During that time, you are kept under regular surveillance by trained staff. When the test is completed, your doctor or the epileptologist will review as well watch any occurred seizures, and see if there were any changes to your brainwave patterns at the time of the seizure(s). Note that, sometimes your anti-epileptic drugs may be reduced or withdrawn. This is to increase the chances of recording your seizures.

Ambulatory means the amplifier is not fixed with the EEG machine, but rather a mobile wearable one performing continuous recording of your normal daily activities throughout the day and night over a period of one or more days (You don’t usually stay in hospital while the test is being done.). You may be asked to have an ambulatory EEG test if your doctor wants to record your brain activity over a few hours, days or weeks. This allows more time for the test to pick up any unusual electrical activity than during the routine EEG. An ambulatory EEG uses electrodes similar to those used for a routine procedure but with a stronger glue for attaching the electrodes to make sure they don’t move out from their position. Your doctor may ask you to keep a brief diary while you’re wearing the ambulatory EEG. This can show if there is anything you do or any particular situations that cause your brainwave patterns to change.

Sample EEG recordings

Protocols “activation procedures”

What we mean here by the protocols are the additional activation procedures that are done during the EEG test, specifically, photic stimulation and hyperventilation, noting that the term “protocol” itself may be used by some to enumerate the types of EEG or methods, etc.

In EEG and during the test, the most common used procedures are:

1. Intermittent Photic Stimulation “IPS”

    Consists of:
  • Arm-mounted light assembly
  • Light source
  • Intensity control and a Trigger-in
photic An example of Photic Stimulator - Image source: Lifelines Neuro


The main use of Intermittent photic stimulation (IPS) or simply Photic Stimulation “PS” in EEG is to detect abnormal epileptogenic sensitivity to flickering light in photosensitive patients. IPS can be very helpful with patients with a clear history of visually induced seizures or complaints; in those known to be photosensitive and to assess whether there is a susceptibility to visual stimuli in daily life.

As per International League Against Epilepsy “ILAE”, new IPS procedure standardization algorithm Has been published by Epilepsia (official journal of ILAE) which can be viewed on Wiley Online Library (Article: Methodology of photic stimulation revisited: Updated European algorithm for visual stimulation in the EEG laboratory).

IPS procedure can be of two levels as of the following (Cited from the above article):

Ⅰ.   IPS procedure on the basic level.

# Steps
A Stop the visual stimulus immediately as soon as generalized epileptiform discharges occur during any flash frequency, regardless of whether the discharges stop at the end of the stimulus or continue after that (i.e., they are self‐sustaining).
B Determine IPS sensitivity in three eye conditions with separate trains of flashes of 5 s duration each during eye closure, eyes closed, and eyes open.
If there is not enough time, choose the eye closure condition (closure of the eyes on command at the start of a flash train) and stimulate for 7 s per flash frequency
C Use the following flash frequencies separately and in this order: 1 – 2 – 8 – 10 – 15 – 18 – 20 – 25 – 40 – 50 – 60 Hz. If there is a generalized response at a certain frequency (lower threshold), skip the remainder of the series and start again with 60 Hz and go down in frequencies (60 – 50 – 40 – 25 Hz‐ …) until again a generalized PPR occurs (upper threshold). When in doubt if a particular frequency has provoked a generalized PPR, repeat the frequency after a rest of 10 s or give a frequency of 1 Hz difference.
D Observe clinical signs during the PPRs and ask the patient about any complaints he/she might have felt.

Ⅱ.   IPS procedure on a higher level of sophistication.

# Steps
A Use electrodes for recording eye movements and surface axial electromyography (EMG) recording to detect subtle myoclonus.
B Test IPS sensitivity in three separate eye conditions (eye closure, eyes closed, and eyes open).
C Use the following flash frequencies in this order: 1 – 2 – 6 – 8 – 9 – 10 – 13 – 15 – 18 – 20 – 23 – 25 – 30 – 40 – 50 –60 Hz. If there is a generalized response at a certain frequency, skip the remainder of the series and continue then with 60 Hz and go down in frequencies (60 – 50 – 40 – 30 – 25 Hz‐ …) until again a PPR occurs.
D Stimulation with colored flashes in patients with a history of TV and videogame epilepsy.
E Stimulation with black‐and‐white evenly striped patterns (gratings that are circular in outline and centrally fixated) with spatial frequency between 2 and 4 cycles per degree, Michelson contrast >0.8, and a mean luminance of at least 300 cd/m in a well‐lit room or on an LCD monitor with a steady backlight. Patterns of increasing size are presented in succession having radii of 3, 6, 12, and 24 degrees, and the series is terminated if a PPR occurs. In one series the stripes are black and white and in a second series they are red and blue.
F Video games and cartoons on TV and computer screens.
G Observe clinical signs with precision during the PPRs and ask the patient about any complaints he/she might have felt (Note: Recording of eye movement/myoclonus or of axial myoclonus is fundamental).

2. Hyperventilation (Overbreathing)

  • Rapid and deep breath.
  • Breathe deeply in and out through mouth.
  • For a minimum of three minutes.


Hyperventilation also called overbreathing is a technique used in EEG where you are asked to breath faster and/or deeper than necessary (breathing deeply in and out through mouth ensuring as much air is expelled during expiration as possible). This activation method may provoke physiological slowing of brain rhythms, interictal discharges, epileptiform abnormalities and in some cases seizures.

It is considered reasonably safe but carries a small risk of inducing potentially ‘adverse events’, including seizures, cardiac, respiratory and cerebrovascular.

The desired respiratory rate in HV is approximately 1 breath every three seconds for a minimum of three minutes, and extend to five minutes if absence seizures are suspected. Continue the post hyperventilation EEG recording for at least two minutes after cessation of hyperventilation. Allow a minimum period of three minutes separating the performance of hyperventilation and intermittent photic stimulation. During this period, monitor the background EEG for the persistence of any changes produced by the hyperventilation response.

American Clinical Neurophysiology Society advices that Hyperventilation should be used routinely unless medical or other justifiable reasons (e.g., a recent intracranial hemorrhage, significant cardiopulmonary disease, sickle cell disease or trait, or patient inability or unwillingness to cooperate) contraindicate it.

Results: Normal vs Abnormal

Normal Results

A normal EEG waveform patterns mean you don’t have a brain disorder, but that does not mean a seizure did not occur. This is because, The EEG shows only the brainwave activity during the test, so if you didn’t have abnormal brain wave patterns or seizures during the test “though you are epileptic”, your EEG might be normal, and therefore your doctor may request further EEG test or brain imaging. Approximately, one-half of all EEGs done for patients with seizures are interpreted as normal.
Brainwave patterns have a certain number of waves per second (frequencies) that are normal for different levels of alertness. For example, brain waves are faster when you are awake and slower in certain stages of sleep.

Abnormal Results

Abnormal results on an EEG test may be due to:

  • Epilepsy or another seizure disorder.
  • Sleep disorder.
  • Head injury.
  • An abnormal structure in the brain (such as a brain tumor).
  • Encephalitis (swelling of the brain, edema).
  • Dementia.
  • Abnormal bleeding (hemorrhage).
  • Dead tissue due to a blockage of blood flow.
  • Substance abuse (Drug or alcohol abuse).
  • Migraines (in some cases).

Factors affecting EEG interpretation

  • Age: maturation of EEG.
  • Arousal: refers to different sleep stages.
  • Medications e.g. benzodiazepines.
  • Pathological brain condition (e.g.craniotomy).
  • Environment e.g. a.c. interference, ICU setting.
  • Quality of recording – Artifacts.

Common factors affecting EEG recording ( artifacts)

It is very important while monitoring and reviewing EEG to recognize and eliminate artifacts, which can be either patient related artifacts (e.g., movement, sweating, ECG, etc.), or technical artifacts (50/60 Hz interference, cable movements, electrode paste-related, radio frequency, external noise, magnetic waves, etc.). These artifacts have to be handled differently. Generally, Common factors are:

  • Electrical mains.
  • Electrical mains with no proper grounding.
  • 60/50 cycle interference.
  • Radio frequency (RF) waves.
  • Electromagnetic interference.
  • placement of electrodes in exact positions..
  • Muscle and movement artifacts
  • Other physiological signals such as ECG.
  • Sweating.
  • Poor conductive electrodes, gels, paste.
  • No proper patient preparation.
  • EEG machine related artifacts.


Can an EEG test show what type of seizures?
Any abnormal activity of brainwaves during the EEG test reflects the brain area (scientifically defined as lobe) where it was originated. Each electrode picks up the activity in the part of the brain directly underneath it. Therefore, depending on the type of seizure, either focal or generalized, the attached electrodes will indicate the place of the unusual electrical activity in regard to the location where they are placed in. This can help the doctor to know the type of seizure, but with consideration to the fact that spatial resolution which is poor in EEG, and regarded as a mixture of the underlying brain sources does not disclose where in the brain the electrical activity is coming from.
Will an EEG test causes a seizure?
The EEG procedure itself does not induce seizures, it just a non-invasive test where electrodes are place on the scalp to record electrical activity, although, “activation procedures” used in the test in particular flashing light or Hyper ventilation (HV) in a small percentage would cause to have a seizure. The risk may increase if your doctor asks you to reduce your anti-epileptic drugs or have less sleep than usual before you have an EEG test.
I’m epileptic or had seizures before, but my EEG shows normal result, why?
In almost one half of performed routine EEG’s, the results are normal though some patients had seizures before the test. This is because a normal EEG doesn’t mean you are seizure-free because the EEG only shows brain activity during the time of the test. If you aren't having a seizure at that time, there may not be any unusual brain waves for the test to record. If a seizure occurs during the EEG test (the likelihood of recording a seizure during a routine EEG is small), the brain electrical activity becomes abnormal. Once the seizure is over, the brain rapidly returns to normal in most individuals. You may note that some people with seizure history may have abnormal brain waves and EEG even if they don’t have seizures during the procedure.
Is EEG test the only way and source of information to diagnose seizures?
No. In fact, before performing the EEG test, your doctor will note down your past medical history to see if it includes injuries or illnesses that would make you more likely to have seizures, and will try to know your seizure history, in addition to the description of the event from you or anyone who witnesses it, which might help to specify if it is focal, generalized, motor or absence, as well as will perform some physical examinations to evaluate your relevant brain abnormalities. If all of these are not enough for the diagnosis, imaging tests may be requested such as MRI, fMRI, MEG, etc.
Based on all the previous, your neurologist may diagnose your seizures.
How many EEG Tests do I have to have?
If you have a routine EEG test that doesn’t show any unusual electrical activity in your brain, your doctor may ask you to have a video long term monitoring EEG (8 – 24 - 72 hours) with hospital admission; or in-house ambulatory EEG; or other types such as sleep deprived EEG. This will depend on factors like the times when you’re more likely to have a seizure, for example early in the morning or, for some women, around the time of your period, etc.
Distinctive abnormal pattern?
Regular repetition of spikes, sharp waves, slow waves or any of the combination, e.g. PLED, burst suppression, triphasic waves, pseudoperiodic generalized sharp waves in CJD, pseudoperiodic slow complexes in HSV encephalitis.
What are the Epileptiform discharges?
Spikes, polyspikes, sharp and slow waves.
Abnormal slow activity?
A sensitive indicator of encephalopathy if diffuse; correlate with regional cerebral dysfunction if localized; may appear as intermittent rhythmic delta (FIRDA in adult or OIRDA in children).
Abnormal changes in normal rhythm?
If asymmetrical, the side with lower amplitude is usually pathological.

Biomedresearches (bmr’S) EEG Educational Materials.

Understanding EEG Video Series:

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