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.
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