top of page




Electroencephalography (EEG) is the best test for analyzing brain function. While an MRI can take a picture of the brain to see what it looks like, the EEG will assess to see how the brain is functioning. This is most important for detection of seizures or forms of dementia, but we also rely upon EEG to help us in patients with coma.


EEG is the recording of electrical activity from the brain using electrodes placed on the scalp. Voltage changes in the brain result from ionic current flows within the nerve cells, called neurons, in the brain. We use EEG to measure the spectrum of signals within the brain that arise from the neurons' oscillations.


The most common condition that EEG is used for is epilepsy. There are obvious abnormalities seen in EEG readings should a seizure occur during the time EEG is recorded. However, EEG is also used to help diagnose sleep disorders, coma, conditions leading to confusion called encephalopathies, and helping to determine brain death. 

During an EEG, electrodes are stuck on the scalp in order to measure brain activity from as much of the brain as possible. Wires connect to a signal box and a computer to help determine the EEG signals and their meaning.

How does an EEG work?

EEG reads your brain's electricity. There are no shocks provided during EEG. Your brain can be active, meditating, in sleep, or having a seizure, and the electrodes will simply read the underlying signals coming from your brain over the time of 20–30 minutes generally. 

How does an EEG show us a signal?

The brain contains billions of neurons. These neurons are electrically charged (or "polarized") as ions are pumped across their cell membranes. These charged ions, when of similar charge, repel each other. When many ions are pushed out at the same time, a wave occurs, called volume conduction. This wave of ions will reach the electrodes at the scalp, where they will push or pull electrons within the metal on the electrodes (see picture to the left). This leads to a voltage - recording these voltages over time leads to the EEG signal.


We can never measure the electric potential of an individual neuron - this is simply too small to be picked seen on an EEG. Instead, EEG activity always reflects the summation of synchronous activity from thousands or even millions of neurons when they have similar spatial orientation. Those neurons that are well-aligned and fire together will show the greatest EEG signal. Also, it is easier to detect activity from the areas near the electrodes, like superficial cortex regions than from deep regions of the brain, which are more difficult to detect than currents near the skull.


EEG activity shows these oscillations at various frequencies in certain ranges. These ranges are associated with different states of brain functioning. For example, sleep has characteristic frequencies associated with it. Our baseline occipital rhythm at the back of the scalp also has a characteristic frequency.

What does an EEG determine?

A routine clinical EEG recording generally lasts 20–30 minutes. There are several minutes of preparation time before the EEG. Routine EEG is typically used for particular clinical circumstances:

  • to determine epileptic seizures as opposed to other types of spells, such as pseudoseizures, or psychogenic non-epileptic seizures (also called "stress seizures"), fainting spells (syncope), certain kinds of movement disorders and unusual types of migraine.

  • to help separate forms of brain disorder like encephalopathy or delirium from primary psychiatric syndromes like catatonia

  • to help in the determination of brain death

  • to permit prognosis to be assessed in patients with coma


For patients with epilepsy, EEG monitoring is typically performed when:

  • it is important to distinguish epileptic seizures from other types of spells which sometims occur in the same person

  • to determine the characteristics of seizures to help manage the best forms of treatment

  • to localize the brain region from which a seizure comes from to determine if surgery could be considered


At times, a routine EEG is not able to identify if patients are having a seizure. In this case, an admission to the hospital may be needed so that EEG can be constantly recordedin conjunction with video and audio recording to try to identify seizures. A recording of an actual seizure will help provide information about the spells and determine the type and location of a seizure, or determine if the spells are not truly epileptic seizures. 

What Happens During the EEG?


You will be lying down on the examining table or bed while about 20 electrodes are attached to your scalp (see above). You will be asked to relax. Your eyes will be open first and then later closed. Breathing deeply and rapidly is part of the assessment. Also, you will be requested to stare at a flashing light, as this can produce changes in the brain-wave patterns.


For patients with seizures, it is rare but possible that you may experience a seizure while being tested. Trained personnel will be nearby if this was to occur. 


Sometimes, the EEG is performed sleep deprived, after you have been kept awake all night. This helps to bring out seizures and related changes. There may be fasting, or going without eating, as well for the same reason. 


You may fall asleep during the EEG - this is alright and often helpful.


Once completed, the electrodes are removed along with the glue used to put them on and you can leave. If you are sleep deprived, please ensure that you do not drive to or from the EEG testing location.

EEG rhythmic activity frequencies 

Delta waves

  • Delta is the frequency range of 1-4 Hz

  • Often, these waves are the biggest and also the slowest

  • This is seen normally in slow wave sleep

  • This may be seen focally with subcortical lesions and generally with diffuse lesions, such as with encephalopathy or deep midline lesions

  • After a recent brain condition, it can be seen most prominent frontally in adults as FIRDA (Frontal Intermittent Rhythmic Delta) or posteriorly in children (e.g. OIRDA - Occipital Intermittent Rhythmic Delta).


Theta waves.

  • Theta is the frequency range from 4-7 Hz

  • This is normally seen in young children

  • Theta occurs in drowsiness or arousal in older children and adults

  • During times of meditation, theta is evident

  • If awake and theta is excessive, this means abnormal activity is present. This can be seen with focal subcortical lesions. It is generalized in a diffuse condition of brain such as with metabolic encephalopathy


Alpha waves.

  • Alpha is in a frequency range from 7-14 Hz

  • This is the "posterior basic rhythm" when we are awake

  • Alpha rhythm emerges with eye closing and during times of relaxation

  • Alpha diminishes with eye opening or at times of mental exertion

  • In some cases, alpha can be abnormal, such as with "alpha coma".


Beta waves.

  • Beta is a frequency range between 15-30 Hz

  • This is most evident frontally

  • Beta is seen during times of motor behavior and is less visible during active movements

  • Low amplitude beta can be associated with active, busy or anxious thinking and during times of concentration when it is arhythmic

  • Rhythmic beta, on the other hand, can be seen with various pathologies and drug effects, especially with benzodiazepines

  • Sometimes, beta can be absent or reduced at areas of brain damage


Gamma waves.

  • Gamma is the frequency range of 30–100 Hz

  • Gamma occurs when different populations of neurons join together into a network for performance of a certain cognitive or motor function


EEG transient activities


There are several changes on EEG that come and go, called transients.

  • Spikes and sharp waves are associated with epilepsy. In some cases, these occur in people without epilepsy but who are predisposed towards epilepsy.

  • Other transients are normal, such as vertex waves and sleep spindles in sleep. Mu rhythm is something that can be seen when you are thinking about moving your arm, for example.


bottom of page