Electroencephalogram (EEG)

The brain generates rhythmically potentials which originate in the individual neurons of the brain. These potentials get summated as millions of cell discharge synchronously and appear as a surface waveform, the recording of which is known as the electroencephalogram.

The neurons like other cells of the body are electrically polarized at rest. The interior of the neuron is at a potential of about -70 mV relative to the exterior. When a neuron is exposed to a stimulus above a certain threshold, a nerve impulse, seen as a charge in membrane potential, is generated which spreads in the cell resulting in the depolarization of the cell. Shortly afterwards, repolarization occurs.

The EEG signal can be picked up with electrodes either from the scalp or directly from cerebral cortex. The peak to peak amplitude of the waves that can be picked up from the scalp is normally 100 μV or less while that on the exposed brain is about 1 mV. The frequency varies greatly with different behavioural states. The normal EEG frequency content ranges from 0.5 to 50 Hz. The nature of the wave varies over the different parts of the scalp.

Electroencephalography is the study and analysis of these signals due to the electrical activity of the brain.

The instrument which records this activity is called Electroencephalograph.

The variations in EEG signals both in terms of amplitude and frequency are of diagnostic value. Since 1950, EEG has become a method of clinical examination and can give information on specific functional disorders like Epilepsy, Brain tumours, Brain haemorrhage and its location, etc. however it cannot be used for psychiatric diseases.

It was in 1924 that the Germany psychiatrist Hans Berger succeeded in obtaining the first EEG tracings from the intact human scalp.

Frequency information is particularly significant since the basic frequency of the EEG range is classified into the following bands for purposes of EEG analysis:

Delta waves (δ) → 0.5 Hz to 4 Hz

Theta (ϴ) → 4 Hz to 8 Hz

Alpha (α) → 8 Hz to 13 Hz

Beta (β) → 13 Hz to 22 Hz

Gamma (γ) → 22 Hz -30 Hz

The above waves are from different parts of the brain. The amplitude of waves varies from 10 μV to 200 μV.

The Alpha waves mostly come from the occipital and parietal regions and they have amplitude of 50 μV.  The amplitude of the waves from the right lobe is higher than those from the left lobe for right handed person. The alpha rhythm is one of the principal components of the EEG and is an indicator of the state of alertness of the brain.

Pathological disorder exists if the amplitude of the waves varies  ±30 % from the normal and also if the frequency difference is 2 Hz over a sufficiently long period.

Beta waves are predominant in the central region. These waves become blocked when the fist is clenched but are less inhibited upon opening the eyes. Muscle potentials should not be confused with beta waves. An increased beta activity is regarded as an expression of cortical tonus.

Theta waves of frequencies 6 Hz to 7 Hz are found in the parietal region and those of 4 Hz to 6 Hz in the basal, temporal and frontal regions. Their amplitude does not exceed 40 μV. They are pronounced when a person is in state of fatigue.

Delta waveforms occur during the sleep in normal adults and are seen in the early part of the night with superimposed spindles. This is also a normal finding in infants and young children. Disturbances in delta waves indicate disturbances of cerebral function. Using the 10-20 electrode placement method, one set of waveforms is plotted with the eyes closed and one set of waveforms with the eyes open.

Besides the importance of frequency content of the EEG pattern, phase relationships between similar EEG patterns from different parts of the brain are also studied with great interest in order to obtain additional knowledge regarding the functioning of the brain.

Another important measurement is the recording of, ‘’Evoked response”, which indicate the disturbance in the EEG pattern resulting from the external stimuli. The stimuli could be a flash of light or a click of sound. Since the responses to the stimuli are repeatable, the evoked response can be distinguished from the rest of the EEG activity by averaging techniques to obtain useful information about the functioning of particular parts of the brain.

Types of Electrodes used for Electroencephalography measurements

Two types of electrodes are used:

• Surface electrodes
• Inserted electrodes

Electrodes (surface type) placed on the scalp are the most commonly used. In some instances those which are inserted inside are used too.

1. Scalp electrodes of platinum alloy or stainless steel and they are placed on the scalp after applying conducting jelly.
2. Silver chloride wires fixed in a rigid plastic cup that gives a large area.
3. Adhesive electrodes, these are commonly used.
4. Sphenoidal electrodes are hypodermic insulated needles, which are inserted through the muscular tissue in the nostrils. They are used to take recordings of waves from the parts of the brain which are inaccessible.
5. Electrocortigraphic electrodes are inserted on the underside of the brain. During a seizure, they are free from artifacts as there is no movement from these electrodes.

Electrode Placement

Placement of electrodes is shown below:

The system shown in the above figure is known as the 10-20 system. The placing of electrodes is equal along any anteroposterior or transverse line and they are symmetrical. This system gives a precise and comprehensive coverage of the convexity of two hemispheres.