Electroencephalogram by Dr Khaled A Abulfadle

Electroencephalogram
(EEG)

Introduction

1. The electroencephalogram (EEG) is used for recording the electrical activity of the human brain.
2. Electrical impulses generated by nerve in the brain can be measured by electrodes placed on the scalp.
3. The EEG gives a view of neural activity and has been used to non-invasively study the physiology of the brain.
4. Unlike other imagining techniques (e.g. functional Magnetic Resonance Imaging [fMRI]), the EEG is continuous (i.e. in real time and ongoing). In its most simplistic form, when two electrodes are attached to the surface of the scalp and connected to an amplifier, the output will be variations in voltage over time.
5. EEG activity is quite small signal, measured in microvolt (µV) with the main frequencies of interest up to approximately 30 Hertz (Hz)

.
Source of EEG activity


• The brain's electrical charge is maintained by billions of neurons.
• Neurons are electrically charged (or "polarized") by membrane transport proteins that pump ions across their membranes.
• When a neuron receives a signal from its neighbour via an action potential, it responds by releasing ions into the space outside the cell.
• Ions of like charge repel each other, and when many ions are pushed out of many neurons at the same time, they can push their neighbors, who push their neighbors, and so on, in a wave.
• This process is known as volume conduction.
• When the wave of ions reaches the electrodes on the scalp, they can push or pull electrons on the metal on the electrodes.
• Since metal conducts the push and pull of electrons easily, the difference in push, or voltage, between any two electrodes can be measured by a voltmeter.
• Recording these voltages over time gives us the EEG.
• The electric potentials generated by single neurons are far too small to be picked by EEG.
• EEG activity therefore always reflects the summation of the synchronous activity of thousands or millions of neurons that have similar spatial orientation.
• If the cells do not have similar spatial orientation, their ions do not line up and create waves to be detected.
• Pyramidal neurons of the cortex are thought to produce most EEG signal because they are well-aligned and fire together.



The Nature of EEG Signal
*Signal range:
From 2µV (brain death) to several hundred µV.
*The frequency bands in EEG:
1-Delta (<4 Hz): deep sleep stages of normal adults.
2-Theta (4-8 Hz): normal infants and children as well as during sleep in adults.
3-Alpha (8-14 Hz): mostly below 50 µV, normal adults during relaxed and mentally inactive awakens.
4-Beta (14-30 Hz): mostly below 30 µV , It is enhanced by expectancy states and tension.
4-Gamma (>30 Hz): Usually, it is not of clinical and physiological interests and therefore often filtered out in EEG recordings

EEG machine consists of the following components:
1- Electrodes.
2- Amplifiers.
3- Filters.
4- Recording unit.



EEG electrode

Electrode: It is a metallic part which is used for transmitting the electrical activity of the brain to the input circuit of the amplifier in the EEG machine,
• Electrodes are placed on the scalp in special positions. These positions are identified by the recorders.
• There are two different types of EEG signals depending on where the signal is taken in the head: scalp or intracranial.
• For scalp EEG, electrodes, are placed on the scalp with contact.
• In intracranial EEG, the EEG signal is obtained by special electrodes implanted in the brain surface during a surgery.

 The brain (cerebrum) is formed of two cerebral hemispheres, right and left.
 Each cerebral hemisphere is
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