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.

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Intro, Diagnostic, in practice, Clinical use & Limitations


EEG Procedure, Types, Methods & Normal vs Abnormal


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1. Electroencephalogram  

  • Electroencephalogram -EEG- : electro + encephalo + gram:
    • electro: Electrical related, using voltage and current units.
    • encephalo: Relating to the brain.
    • gram: Written record (record book) of brain signals.
  • Electroencephalography -EEG- : electro + encephalo + graphy:
    • electro: Electrical related, using voltage and current units.
    • encephalo: Relating to the brain.
    • graphy: The technique or the method used to record the EEG.

Electroencephalogram is a noninvasive test for measuring the electrical activity (patterns) of the brain, through measuring potential difference of placed electrodes (active and reference) on the scalp, to detect abnormalities. Electroencephalography is the technique for recording and interpreting it.
In simple words, electroencephalography is the result and the process of making an electroencephalogram.


Scientists first captured and recorded brain waves in dogs in 1912. German physiologist and psychiatrist Hans Berger (1873–1941) began his studies of the human EEG in 1920. He gave the device its name and is sometimes credited with inventing the EEG, though others had performed similar experiments. His work was later expanded by Edgar Douglas Adrian. In 1934, Fisher and Lowenback first demonstrated epileptiform spikes. In 1935 Gibbs, Davis and Lennox described interictal spike waves and the 3 cycles/s pattern of clinical absence seizures, which began the field of clinical electroencephalography. Subsequently, in 1936 Gibbs and Jasper reported the interictal spike as the focal signature of epilepsy. The same year, the first EEG laboratory opened at Massachusetts General Hospital. Franklin Offner (1911–1999), professor of biophysics at Northwestern University developed a prototype of the EEG that incorporated a piezoelectric inkwriter called a Crystograph (the whole device was typically known as the Offner Dynograph). In 1947, The American EEG Society was founded and the first International EEG congress was held. In 1953 Aserinsky and Kleitman described the REM sleep.
From that time and until the moment, EEG continues to develop to include the latest technologies that contribute to the diagnosis of patients with epilepsy and sleep disorder.

Hans Berger (1873–1941):

A German psychiatrist, best known as the inventor of EEG in 1924, which is a method used for recording the brainwaves. He is also the discoverer of the alpha brain wave rhythm (Have been eponymously referred to as the "Berger wave").

Edgar D. Adrian (1889–1977):

An English electrophysiologist and recipient of the 1932 Nobel Prize for Physiology, won jointly with Sir Charles Sherrington for work on the function of neurons. He provided experimental evidence for the all-or-none law of nerves.

Frederic A. Gibbs (1903–1992):

An American neurologist who was a pioneer in the use of electroencephalography (EEG) for the diagnosis and treatment of epilepsy.

Hallowell Davis (1896–1992):

An American physiologist, otolaryngologist and researcher. During the 1930s, Davis participated in the development of electroencephalography and was the first person in the United States to have his brain waves scanned by an EEG device.

William G. Lennox (1884–1960):

An American neurologist and epileptologist who was a pioneer in the use of electroencephalography (EEG) for the diagnosis and treatment of epilepsy.

Herbert H. Jasper (1906 –1999):

A Canadian psychologist, physiologist, neurologist, and epileptologist.

Eugene Aserinsky (1921–1998) is the scientist who co-discovered the rapid eye movement cycle of sleep. In 1952, he was a researcher at the University of Chicago when he and Nathaniel Kleitman (1895–1999) identified Rapid Eye Movement (commonly referred to as: REM).

When the brain cells send messages to each other, they produce tiny electrical signals. Your brain cells communicate via electrical impulses and are active all the time, even when you're asleep. This activity shows up as wavy lines on an EEG recording.

An EEG is one of the main diagnostic tests for epilepsy. An EEG may also play a role in diagnosing other brain disorders. In an EEG test, electrodes (flat metal discs) are placed onto your scalp using a sticky substance. These electrodes pick up the electrical signals from your brain and send them to an EEG machine, which will record and display the signals as wavy lines on a computer screen. The EEG machine records your brain's electrical activity as a series of traces, Each trace corresponds to a different region of the brain.

An electroencephalogram (EEG) is a painless procedure that takes 30 to 45 minutes with rarely causes of any side effects.

The main diagnostic application of EEG is in the case of epilepsy, as epileptic activity can create clear abnormalities on a standard EEG study.

A secondary clinical use of EEG is in the diagnosis of coma, encephalopathies and brain death. EEG can also help to identify causes of other problems such as sleep disorders and changes in behavior, as well it can be used to evaluate brain activity after a severe head injury or before heart or liver transplantation.

EEG used to be the first-line method for the diagnosis of tumors, stroke and other focal brain disorders, but this use has decreased with the advent of anatomical imaging techniques such as MRI and CT.

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The EEG shows patterns of normal or abnormal brain electrical activity. Some abnormal patterns may occur with a number of different conditions, not just seizures. For example, certain types of waves may be seen after head trauma, stroke, brain tumor, or seizures. A common example of this type is called "slowing," in which the rhythm of the brain waves is slower than would be expected for the patient's age and level of alertness. Certain other patterns indicate a tendency toward seizures. Your doctor may refer to these waves as "epileptiform abnormalities" or "epilepsy waves." These include spikes, sharp waves, and spike-and-wave discharges. Spikes and sharp waves in a specific area of the brain, such as the left temporal lobe, indicate that focal seizures might possibly come from that area. Generalized epilepsy, on the other hand, is suggested by spike-and-wave discharges that are widely spread over both hemispheres of the brain, especially, if they begin in both hemispheres at the same time.


In EEG, there are four brainwaves of interest, which usually appear in the recording. Based on them, their characteristics and appearance, the neurologist or epileptologist determines what the status is, and whether it is normal, abnormal, or insufficient, and requests additional tests like anatomical imaging, such as Magnetic resonance imaging (MRI), which can identify substrates underlying epilepsy, and guide clinicians in the determination of treatment and prognosis.
Each brain wave has different frequency, amplitude and meaning. Most of these rhythms may persist up to several minutes, note that, one rhythm is not present at all times, but an irregular, “arrhythmic” may prevail during long time intervals. The table below describes them in regard to EEG briefly.

Ten seconds of simulated EEG data in the five differently named frequency bands
of neural oscillations, or brainwaves: delta, theta, alpha, beta, and gamma.
Type Description
beta β Frequency range is from 13-30 Hz and it is the smallest and fastest with low amplitude among the four brainwaves. Typically, it is seen with arousal like attention to tasks, stimuli, outwardly focused concentration, under stress, experiencing psychological tension and mental, intellectual activity. Beta waves are usually maximal over the frontocentral areas of the scalp (frontal and central regions), but they may be widespread.
alpha α Frequency range is from 8-13 Hz and typically seen in healthy awake adults while they are quietly resting with their eyes closed. The Alpha waves disappear during sleep and fade away when there is concentration on a specific task. Alpha mostly is found over the posterior portions of the head during wakefulness with a background best known as dominant alpha rhythm (posterior dominant rhythm).
theta θ The frequency is usually between 4 and 8 Hz. Mostly, it is common among children, though, in normal adults may appear transiently during sleep ( drowsiness and in certain stages of sleep) or daydreaming. There are also other situations where theta waves can appear along with delta waves like in case of slow background, which indicate cerebral dysfunction.
delta δ Delta waves are predominate in the 1-4-Hz range and are commonly referred to as slow wave activity in EEG. In all ages of patients, it is typically seen during deep sleep and has a large amplitude. It is usually not found in the awake normal adult, but if present, it is indicating encephalopathy, such as cerebral damage or brain disease. Abnormal delta activity may occur with the person who has learning disabilities or difficulties maintaining conscious awareness.

Variables used in the EEG activity classification

  ⅰ. Periodicity: Referring to the distribution of patterns or elements in time (e.g., the appearance of a particular EEG activity at more or less regular intervals). The activity may be generalized, focal or lateralized. [Ref.]

  ⅱ. Synchrony: Referring to the simultaneous appearance of rhythmic or morphologically distinct patterns over different regions of the head, either on the same side (unilateral) or both sides (bilateral).

  ⅲ. Frequency: Referring to rhythmic repetitive activity (in Hz). The frequency of EEG activity can have different properties including: Rhythmic (EEG activity consisting in waves of approximately constant frequency); Arrhythmic activity (A sequence of EEG waves with an inconstant periodicity “no stable rhythms are present”); Dysrhythmic (Rhythms and/or patterns of EEG activity that characteristically appear in patient groups or rarely or seen in healthy subjects).

  ⅳ. Voltage: Referring to the average voltage or peak voltage of EEG activity.

  ⅴ. Morphology: Referring to the shape of the waveform. The shape of a wave or an EEG pattern is determined by the frequencies that combine to make up the waveform and by their phase and voltage relationships. Wave patterns can be described as being: Monomorphic (Distinct EEG activity appearing to be composed of one dominant activity); Polymorphic (distinct EEG activity composed of multiple frequencies that combine to form a complex waveform); Sinusoidal (Waves resembling sine waves. Monomorphic activity usually is sinusoidal); Transient (An isolated wave or pattern that is distinctly different from background activity).

Reference: The McGill Physiology Virtual Lab

Background EEG in Adults

alpha α: (8-13) Hz.
Y8-13 Hz, posterior predominant, symmetric, Amp 30-60 mV, R>L by 20-50%, ¯ by EO, drowsiness, Age 60-80, a=9.5 Hz; a < 8 in elderly suggests A.D. Slow (sub-harmonic, 4-5 Hz) and fast alpha (16-20 Hz)
theta θ (4-8) Hz.
Usually low amplitude at frontal central region (6-7Hz), Rhythmic temporal theta bursts of drowsiness, Midline theta rhythm (Cz max).
delta δ (0.5-4).
Diffuse in the deep sleep, metabolic encephalopathies, Focal in structural brain lesion.
beta β (13-30).
Frontal-central predominant, Amp < 20 mV and lower in elderly, by benzodiazepine, light sleep and skull defect.

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Clinical Use

A routine clinical EEG recording typically lasts 20–40 minutes (plus preparation time) and usually involves recording from scalp electrodes. Routine EEG is typically used in the following clinical circumstances:

  • To detect epilepsy.
  • To characterize seizures for the purposes of treatment.
  • To localize the region of brain from which a seizure originates for work-up of possible seizure surgery.
  • To distinguish epileptic seizures from other types of spells, such as psychogenic non-epileptic seizures, syncope (fainting), sub-cortical movement disorders and migraine variants.
  • To differentiate "organic" encephalopathy or delirium from primary psychiatric syndromes such as catatonia.
  • To serve as an adjunct test of brain death.
  • To prognosticate, in certain instances, in patients with coma.
  • To determine whether to wean anti-epileptic medications.

EEG can also be used in intensive care units for brain function monitoring:

  • To monitor for non-convulsive seizures/non-convulsive status epilepticus.
  • To monitor the effect of sedative/anesthesia in patients in medically induced coma (for treatment of refractory seizures or increased intracranial pressure).
  • To monitor for secondary brain damage in conditions such as subarachnoid hemorrhage (currently a research method).
  • In pediatric ICU (PICU), it is used for critically ill children to provide insight into brain function and to identify electrographic seizures.
  • When used in a pediatric intensive care unit (PICU), the most common clinical impact is identification of NCS or NCSE.
  • In pediatric ICU (PICU), Continuous electroencephalographic monitoring (cEEG) detects non-convulsive seizures (NCS) and non-convulsive status epilepticus (NCSE). Also it determines whether clinical events of concern are epileptic, and can identify meaningful background changes.

Additionally, EEG may be used to monitor certain procedures:

  • To monitor the depth of anesthesia.
  • As an indirect indicator of cerebral perfusion in carotid endarterectomy.
  • To monitor amobarbital effect during the Wada test.

At times, a routine EEG is not sufficient, particularly when it is necessary to record patients while they are having a seizure. In this case, the patient may be admitted to the hospital for days or even weeks, while EEG is constantly being recorded (along with time-synchronized video and audio recording). A recording of an actual seizure can give significantly better information about whether or not a spell is an epileptic seizure, and the focus in the brain from which the seizure activity emanates.

Irregular Waveforms

In EEG, certain waveforms are considered as indicatory signs of epileptic-seizure. These abnormalities are transient waveforms that stand out from the background EEG with an irregular, unpredictable activity. Their presence indicates a deviant neuronal behavior often found in patients suffering from epileptic seizures. The irregular EEG wave patterns or simply the abnormal one’s are called “Epileptiform EEG activity”. It is categorized based on the time it occurs as:

  • Ictal: Is defined as the period of a seizure (occurring during the seizure).
  • Preictal: Refers to the period before seizures.
  • Interictal: Refers to the period between seizures.
  • Postictal: Refers to and Seen in the period after the seizure.

Abnormal waveforms seen in an EEG recording include epileptiform and non-epileptiform abnormalities. The Epileptiform activity in EEG tracing may be focal or generalized. It includes spikes, sharp waves, or spike-and-wave complexes. They can be evident not only during a seizure (the ictal period) but also a short time before (the preictal period) as well as between seizures (the interictal period). Consequently. The focal discharges are arising from the temporal, frontal, occipital, centroparietal, centrotemporal, and midline regions of the brain. The Generalized epileptiform discharges are consisting of the 3-Hz spike-and-wave, slow spike-and-wave, atypical paroxysmal fast activity, and hypsarrhythmic patterns. Benign epileptiform variants unassociated with seizures can also be present in the EEG. Noting that, even normal EEG waveforms can be considered potentially abnormal, depending upon various factors

The abnormal EEG brainwaves activity can be categorized as:

  1. Triphasic waves:
Triphasic waves are abnormal electroencephalogram (EEG) waveforms seen in association with multiple clinical conditions, including encephalopathy and structural brain lesions, among others. They are nonspecific, high amplitude sharp/sharply contoured waves with three distinctive phases.

  2. Interictal epileptiform discharges (IED):
Between seizures, the epileptic patient brain generates pathological patterns of activity, designated as interictal epileptiform discharges (IEDs), that are clearly distinguished from the activity (ictal) observed during the seizure itself. It is an abnormal synchronous electrical discharge. IED have a low sensitivity in routine 30-minute EEG recording, and the yield increases with repeat EEG and prolonged EEG recordings. Though uncommon, they can occur in healthy persons without a history of seizures. IEDs are spikes, polyspikes, sharp waves, or spike and slow-wave complexes. "Spikes and sharp waves (SSWs) are transient waveforms that stand out from the background EEG with an irregular, unpredictable temporal pattern (paroxysmal activity). Their presence indicates a deviant neuronal behavior often found in patients suffering from epileptic seizures. Because of their relation to seizures, SSWs are often referred to as interictal since they occur between ictal events, i.e., epileptic seizures.

They can be subdivided into spike waves and sharp waves.

  Spike wave: Are very short in duration, with a sharp-pointed peak duration of 20 to 70 milliseconds. A spike is followed by a wave component, and this is generated by GABA-b mediated currents.

  Sharp wave: Are longer in duration than a spike and last 70 to 200 milliseconds.

The following patterns of interictal epileptiform discharges may be seen:3 Hz and spike-wave; Centro-temporal spikes/ Rolandic spikes; Epileptic encephalopathy with continuous spike-and-wave during sleep (CSWS); Slow spike and waves; Poly spike and waves; Generalized spike and waves; Lateralized periodic discharges (LPDs or PLEDs); Bilateral independent periodic discharges (BIPDs/ BiPLEDs); Generalized periodic discharges (GPDs); SREDA (Subclinical EEG discharges of adults); Brief (potentially ictal) rhythmic epileptiform discharges B(i)RDs/ BERDs.

Non-epileptiform abnormalities.

Slowing: It can be divided as.

  Diffuse slowing

  Focal slowing

Other diffuse or focal abnormal patterns in EEG:

  Electrocerebral inactivity (ECI)

  Burst suppression pattern

  Breach rhythm

Note: There are abnormal waveforms seen in EEG recording different than epileptiform discharges, but are not taken into consideration while the electroencephalographer interprets them, who in turn should has the proper knowledge to distinguish between the epileptiform and non-epileptiform abnormalities; know the normal EEG pattern in various physiological states in children and adults; has the significant skills to recognize artifacts, and also understanding of normal, benign variants.


Limitations of EEG

Among the limitations of EEG, the main is the spatial resolution, unlike temporal resolution which is considered perfect (counter to metabolic-based imaging techniques such as fMRI, PET, NIRS, which are considered as having a very good spatial resolution, but a rather poor temporal one).

Since measurements are taken at the scalp, the received signal is, essentially, the sum of the electric field (in the direction perpendicular to the scalp), the poor spatial resolution in EEG particularly at every spatial scalp position is due to resistive layers (including the skull) the cortical current has to pass through (Even if the distance is a few centimeters ) before being recorded by the electrodes placed on the scalp. The resistive layers induce a blurring effect at scalp level. As a result, at every spatial scalp position, the recorded activity is a mixture of the underlying brain sources, which causes the poor spatial resolution of scalp EEG. There are different methods which have been proposed to increase the spatial resolution of EEG.

Simply, One of the big disadvantages of EEG is that it's hard to figure out where in the brain the electrical activity is coming from.

Counter to the fact that spatial resolution is poor, the very good temporal resolution provides a direct measurement of cortical electrophysiology revealing, for example, the presence of interictal epileptiform discharges that identify regions of an epileptogenic brain.

Overall, with limitations and strengths of EEG, the clinician must interpret EEG findings within the overall clinical context.

Risks of EEG

The EEG procedure is very safe, painless and comfortable. Apart from possibly feeling a bit tired, you normally will not experience any side effects. However, the flashing lights or overbreathing (hyperventilation) required during the test may trigger seizures in those with seizure disorders, or make you feel lightheaded and notice a tingling in your lips and fingers for a few minutes during the hyperventilation. Some people develop a mild rash where the electrodes were attached.

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