Electroencephalogram Notes

Electroencephalogram Introduction

• Electroencephalography is the study of the electrical activities of the brain. Electroencephalogram (EEG) is the graphical recording of the electrical activities of the brain. The 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 a 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 to as Berger’s waves.

Read And Learn More: Medical Physiology Notes

Significance Of EEG

EEG is useful in the diagnosis of neurological disorders and sleep disorders. Following are the common neurological disorders in which the change in the EEG pattern occurs:

1. Epilepsy, which occurs due to excessive discharge of impulses from the cerebral cortex
2. The disorders of the midbrain affecting the ascending reticular activating system
3. Subdural hematoma, during which there is a collection of blood in the subdural space over the cerebral cortex.

Method Of Recording EEG

• Electroencephalograph is the instrument used to record EEG. The electrodes are called scalp electrodes from the instrument placed over the unopened skull or over the brain after opening the skull, or by piercing into the brain. The electrodes are of two types, unipolar and bipolar electrodes.
• While using bipolar electrodes, both terminals are placed in different parts of the brain. When unipolar electrodes are used, the active electrode is placed over the cortex and the indifferent electrode is kept on some part of the body away from the cortex.

Waves Of EEG

The electrical activity recorded by EEG may have synchronized or desynchronized waves. Synchronized waves are 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.

1. Alpha Rhythm:

• Alpha rhythm consists of rhythmical waves, which appear at a frequency of 8-12 waves/second with an amplitude of 50 pV. The alpha waves are synchronized waves.
• Alpha rhythm is obtained in the 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 the eyes are opened.
• The waves of alpha rhythm are most marked in the parietooccipital area. Sometimes these waves appear in other areas also.

Alpha Block:

• The 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 a 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 the replacement of regular alpha waves with irregular low-voltage waves. It is due to the loss of synchronized activity in the neural elements that are responsible for regular wave patterns.

2. Beta Rhythm:

• It includes high frequency waves of 15-60/second but, the amplitude is low, i.e. 5-10 pV. 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, states like conscious activity, problem-solving, and fear, very high-frequency waves of 30-100/sec appear. Some controversy exists in naming such waves. Often the very high-frequency waves are called gamma rhythm. However, many scientists consider these waves as beta rhythms.

3. Delta Rhythm:

• Delta rhythm includes waves with low frequency and high amplitude. These waves have a frequency of 1-5/second with an amplitude of 20-200 pV. It is common in early childhood during waking hours. In adults, it appears mostly during deep sleep.
• The presence of delta waves in adults during conditions other than sleep indicates the pathological process in the brain like tumors, 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. The frequency of theta waves is 4-8/second and the amplitude is about 10 pV.