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Electroencephalography (EEG)

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Introduction to Electroencephalography

Electroencephalography is a technique that was first developed in 1929 by a German scientist named Hans Berge. Electroencephalography can be defined as a technique to record and intercept the activity of the brain with the help of brain waves. The nerve cells of the brain generate electrical waves which when recorded and interpreted give the idea of the brain’s activity. The graph generated by electrical waves of the brain is known as an electroencephalograph, whereas the device that is used to study and record the electrical impulse of nerve present in the brain is known as the electroencephalogram. The data interpreted as electroencephalograph is also known as EEG.

Electroencephalography is used as the diagnostic tool for conditions such as epilepsy, brain tumor, head injury, sleeping disorder, cerebral infection, and brain death. The most widespread use of encephalography has been recorded in the case of epilepsy. It is also a crucial diagnosis in case of brain death because it allows healthcare professionals to maintain the vitality of the organs in case of brain death of a donor.


The nerves of the brain communicate through electrical signals EEG or electroencephalograph is a graphical representation of the recorded electrical signal of the rain. EEG helps understand brain activity. EEG is used to diagnose the following conditions

  1. Brain tumor

  2. Seizures

  3. Epilepsy

  4. Encephalitis (inflammation of the brain)

  5. Encephalopathy (dysfunction of the brain as an effect of various diseases)

  6.  Neurodegenerative disease like Alzheimer.

EEG Test Procedure

It is a simple procedure that includes the following steps: the first patient is laid down then two steps of electrodes are attached to either side of the brain. These electrodes are the part of electroencephalograms, they are used to detect neural electrical impulses. Electrodes are connected through a wire to the amplifier, where it sends the message, and waves get amplified enough to be sent to the computer, and the computer deciphers the electrical signals and the graph is plotted on the computer screen. This graph is known as an electroencephalograph. Now, this complete setup is called an electroencephalogram, and the procedure is known as electroencephalography. 

Risk Associated

Only a small part of the population has the risk of seizure due to EEG. The signal-to-noise ratio in EEG tests is poor, hence there are relatively large numbers of subjects required to gain a piece of correct pathological information by EEG.

Mechanism of Electroencephalography

Electroencephalography works on the principle of volume conduction. Volume conduction refers to the mechanism of measuring various electrical potentials generated from a distant source. The human brain consists of several millions of neurons, these neurons maintain common resting potential and when needed generates an action potential. These potentials are generated and maintained by manipulating the concentration of ions inside and outside of the neuron. During any activity a large number of ions are expelled from the neurons, ions with similar charges repel each other. This repulsion causes a push action, that is ions push each other causing the formation of waves. Thus this wave represents the activity of the human brain. When the wave generated by the virtue of repulsion of ions reaches the electrodes, it pushes the electrons of the electrode. The electrode is made of metal that can easily push or pull electrons (ions). The difference between the voltages of push or pull between electrodes can be measured by a voltmeter. When these voltages are recorded over time this gives us the EEG wave.

Types of Wave in EEG

EEG can be divided into 2 two main components, rhythmic activity, and transients. The rhythmic activity of the brain can be divided into bands according to frequency. The frequency ranges from less than 4 Hz to 32 Hz. The frequency bands can be divided as follows-

  1. Delta

  2. Theta

  3. Alpha

  4. Beta 

  5. Gamma 

  6. Mu 

Delta Waves- These waves range from 1 to less than 4 Hz. These waves are pathologically important for the detection of subcortical lesions, diffuse lesions, deep midline lesions, and metabolic encephalopathy hydrocephalus. It has the highest amplitude and slowest wave propagation. It is generally generated during discontinuous attention tasks. 

Theta Wave- These range from 4 to 7 Hz. it is more common in young children. These waves are pathologically important for the detection of focal subcortical lesions, deep midline disorders, metabolic encephalopathy, and minor cases of hydrocephalus. Theta waves are also associated with the repression of an elicited response. 

Alpha Waves-  These waves range from 8 to 15 Hz. they are abundantly found in the posterior region of the brain. Hence these waves are also named the posterior basic rhythm. It is also associated with the inhibition of the elicited response. They are pathological determinants of the coma. 

Beta Waves- These range from 16 to 31 Hz, they are predominantly found in the frontal region of the brain. They have symmetrical distribution, these waves have a low amplitude. These waves are associated with high thinking and anxious behaviour. These waves are pathologically important for the detection of Dup15q syndrome.

Gamma Waves- These waves have frequencies up to 30 Hz max. Gamma waves decrease are associated with a decline in cognitive ability, but it still needs evidence research to prove the hypothesis to be used as a diagnostic measurement.

Mu Waves- This has a frequency range of 8 to 12 Hz. These are predominantly found in the sensorimotor cortex. They are associated with the diagnosis of autism.

EEG Scan Waves

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EEG Scan Artifact

Artifacts are the signals that are of non-cerebral origin. These serve as noise (unwanted waves) in an EEG scan. These are generally large as compared to the signal of interest. There are the following artifacts-

  1. Eye induced (blinking)

  2. EMG induced (muscle contraction induced)

  3. ECG induced (cardiac induced)

  4. Glossokinetic artifact.

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FAQs on Electroencephalography (EEG)

1. State Electroencephalogram Definition.

Ans - It refers to the device that is being used to calculate the electrical measure of the electrical impulse of the brain thus providing information about brain activity. 

2. What are the Disorders Diagnosed By the EEG Test?

Ans- EEG test is used to diagnose epilepsy, seizures, brain tumor, and encephalitis.

3. Electroencephalography is Based on Which Principle?

Ans- It is based on the principle of volume conduction where ions repel each other thus creating a geta push action that generates waves, which then reaches to electrodes. The varying voltage is recorded over a period generating an EEG scan.