Electroencephalogram (EEG) uses and function

Electroencephalogram (EEG) is the graphical recording of electrical activities of brain. Electrical activity of the brain is complicated when compared to that of a single nerve fiber or neuron. It is due to the involvement of large number of neurons and synapses. German psychiatrist Hans Berger was the first one to analyze the EEG waves systematically and hence the EEG waves are referred as Berger waves.

SIGNIFICANCE OF EEG

Electroencephalogram is useful in the diagnosis of neurological disorders and sleep disorders. EEG pattern is altered in the following neurological disorders:

1. Epilepsy, which occurs due to excessive discharge of impulses from cerebral cortex

2. Disorders of midbrain affecting ascending reticular activating system

3. Subdural hematoma during which there is collection of blood in subdural space over the cerebral cortex.

METHOD OF RECORDING EEG

Electroencephalograph is the instrument used to record EEG. The electrodes called scalp electrodes

from the instrument are placed over unopened skull or over the brain after opening the skull or by piercing into brain. Electrodes are of two types, unipolar and bipolar electrodes. While using bipolar electrodes, both the terminals are placed in different parts of brain. When unipolar electrodes are used, the active

electrode is placed over cortex and the indifferent electrode is kept on some part of the body away from

cortex.

WAVES OF EEG

Electrical activity recorded by EEG may have synchronized or desynchronized waves. Synchronized waves are the regular and invariant waves, whereas desynchronized waves are irregular and variant. In normal persons, EEG has three frequency bands :

1. Alpha rhythm

2. Beta rhythm

3. Delta rhythm.

In addition to these three types of waves, EEG in children shows theta waves.

ALPHA RHYTHM

Alpha rhythm consists of rhythmical waves, which appear at a frequency of 8 to 12 waves/second with

the amplitude of 50 μV. Alpha waves are synchronized waves. Alpha rhythm is obtained in inattentive brain or mind as in drowsiness, light sleep or narcosis with closed eyes. It is abolished by visual stimuli or any other type of stimuli or by mental effort. So, it is diminished when eyes are opened. parietooccipital

area. Sometimes these waves appear in other areas also.

Alpha Block

Alpha block is the replacement of synchronized alpha waves in EEG by desynchronized and low voltage

waves when the eyes are opened. The desynchronized waves do not have specific frequency. It occurs due to any form of sensory stimulation or mental concentration, such as solving arithmetic problems. Desynchronization is the common term used for replacement of regular alpha waves with irregular low

voltage waves. It is due to the loss of synchronized activity in neural elements that are responsible for

regular wave pattern.

BETA RHYTHM

Beta rhythm includes high frequency waves of 15 to 60 per second but, the amplitude is low, i.e. 5 to 10 μV. Beta waves are desynchronized waves. Beta rhythm is recorded during mental activity or mental tension or arousal state. It is not affected by opening the eyes. During higher mental activity or peak performance state like conscious activity, problem solving and fear, very high frequency waves of 30 to 100 per second appear. Some controversy exists in naming such waves. Often very high frequency waves are called gamma rhythm. However, many scientists consider these waves as beta rhythm.

DELTA RHYTHM

Delta rhythm includes waves with low frequency and high amplitude. These waves have the frequency of 1 to 5 per second with the amplitude of 20 to 200 μV. It is common in early childhood during waking hours. In adults, it appears mostly during deep sleep. Presence of delta waves in adults during conditions

other than sleep indicates the pathological process in brain like tumor, epilepsy, increased intracranial

pressure and mental deficiency or depression. These waves are not affected by opening the eyes.

THETA WAVES

Theta waves are obtained generally in children below 5 years of age. These waves are of low frequency and low voltage waves. Frequency of theta waves is 4 to 8 per second and the amplitude is about 10 μV.