Introduction to EEG Notes about EEG and Working Notes about EEG Electrodes ppt By, AJU REMESH (Ajumon R) ajuremesh007@gmail.com

1. Electro Encephalo Graph (EEG)
Ajumon . R

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Electroencephalography (EEG) is an
electrophysiological monitoring method to
record electrical activity of the brain.
• EEG describes general functions of brain activity.
• It sense the neuron signals operating in a non synchronized
manner in physical sense.

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• Monitoring the EEG has proven to be an effective method of diagnosing many
neurological illness and diseases.
such as epilepsy , tumour , cerebrovascular lesions , ischemia
and problems associated with trauma.
• It is also used to diagnose sleep disorders, coma, encephalopathies , and brain
death.

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An electroencephalogram (EEG) is a test that measures and records the electrical
activity of your brain. Special sensors are attached to your head and hooked by wires
to a computer. The computer records your brain's electrical activity on the screen or
on paper as wavy lines. Certain conditions, such as seizures, can be seen by the
changes in the normal pattern of the brain's electrical activity.
E E G
Several types of Electrodes may be used to record EEG.
1. Peel and Stick Electrodes.
2. Silver Plated Electrodes.
3. Needle Electrodes (rare case).

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• EEG Electrodes are smaller than ECG electrode.
• They may be applied separately to the scap or may be mounted in
special bands.
• In either case, electrode jelly or paste is used to improve the electrical
contact.
• If the electrodes are intended to be used under the skin or scalp,needle
electrodes are used (rare case).
• EEG Electrodes give high skin contact impedence as compared to ECG
electrodes.
• Good electrodes impedence should be generally below 5 kilohms.
• Impedence between pair of electrode must also balanced or the
difference between this should less than 2 kilohm.

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• Figure shows, most commonly used electrode for
EEG.
“Chlorided silver discs”
• Diameter approximately 6 – 8 mm .
• Contact with the scalp is made via an electrolytic
paste.
• They have AC resistance varying from 3–20 kilohm.
• Fig (A) is a “Pad Electrode”. Which made from silver rod
bulled out at the end and padded with sponge. It screwed
into an insulated mount and held in place on the head with
a rubber cap. To hold 3 such electrode, an adjustable tripod
mout is employed.
• Another type of electrode consist of multiple fine chloride
silver wires(fig. B) fixed in a rigid plastic cup. The plastic
cup is fixed with an adhesive. It is filled with jelly through a
hole in top

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Monopolar and Bipolar recording.
Monopolar: EEG may recorded by picking up the voltage difference between an
active electrode on the scalp with respect to a reference electrode
on the ear lobe or any other part of body.
Bipolar : In bipolar, voltage difference between the scalp electrode is
recorded. Bipolar recording is more popular than Monopolar.
Fig A: Bipolar
Fig B: Monopolar

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Block diagram of Electroencephalograph
The basic block diagram of EEG machine with both
analog and digital componets is shown in fig 5.11

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Placement of Electrodes
Electrodes are identified according to their postiton on the head:
Fp - Frontal-Polar P - Parietal
F - Frontal T - Temporal
C - Central O - Occipial

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EEG electrodes are arranged on the scalp according to a standard
known as the “10/20 system”. Adopted by the American EEG Society.
• There are 21 electrode locations in the 10/20 system.
• Placement of electrode at distance of 10% and 20% of measured
coronal,sagittal and circumferential arcs between the landmarks
on the cranium (fig 5.12)
A new montage convention has recently ben introduced in whch electrodes
are spaced at 5% distance along the cranium.
These are called “Closely spaced electrodes” (shown in fig 5.13)

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Electrode montage selector:
The montage selector panel containing switches that allow the user to
select which electrode pair will have signals subtracted from each
other to create an array of channels of output called montage.
Preamplifier:
Every channel has an individual, multistage, ac coupled, very sensitive
amplifier with different input and adjustable gain in a wide range.
Its frequency response can be selected by single stage passive filters.
Another parts of EEG machines are:
• Anolog to digital coverter.
• Oscilloscope and computer.
• Writer unit with Chart drive and Ink-writing Oscilloscope.

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Example of EEG Wave Form:

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EEG activity is dependent on the level of consciousness :

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