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EEGBy: Dionne Boc
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ELELTROENCEPHALOGRAM
(EEG) a recording of the electrical activity of the brain from the
scalp.
The first recordings were made by Hans Berger in 1929.
Gives a coarse of neural activity and has been used tonon-invasively study the physiology of the brain.
EEG activity is quite small signal, measured in microvolt
with the main frequencies of interest up to
approximately 30Hz.
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Origin of EEG waves
Electrical impulses
generated by nerve
firings in the brain
can be measured byelectrodes placed
on the scalp.
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EEG Waves
Signal range: 2 V (brain death)several hundred V.
The frequency bands in EEG:
Alpha wave -- 813 Hz.
Beta wave -- >13 Hz. (1430 Hz.)
Theta wave -- 47.5 Hz.
Delta waves13.5 Hz.
D T A B
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Different types of brain waves
in normal EEG
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Alpha wave
rhythmic, 8-13 Hz
mostly on occipital lobe
20-200 V
normal,
relaxed awake rhythm with eyes closed
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Beta wave
irregular, 14-30 Hz
mostly on temporal and frontal lobe
mental activity
excitement
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Theta wave
rhythmic, 4-7 Hz
Drowsy, sleep
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Delta wave
slow, < 3.5 Hz
in adults
normal sleep rhythm
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Rhythm Amplitude
(uV)
Recording
& Location
Alpha() 50 100 Adults, rest, eyes closed.Occipital region
Beta() 20 Adult, mental activityFrontal region
Theta() Above 50 Children, drowsy adult,emotional distress
Occipital
Delta() Above 50 Children in sleep
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Components
Electrode
Amplifiers
Filters
Recording unit
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EEG Electrode
Each electrode site is labeled with a letter and a number.
The letter refers to the area of brain underlying the
electrode
e.g. F - Frontal lobe and T - Temporal lobe.
Even numbers denote the right side of the head and
Odd numbers the left side of the head.
Electrolyte: may be a conducting solution (gel or paste)
or may be fluid of living tissue as when electrodeinserted below skin. It is a salt solution, sodium chloride.
This means that current flow within brain becomes
electron flow in the electrodes and electrode wires.
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EEG Electrode Placement
Two types of EEG signals:
Scalpelectrodes are placed on the scalp with good mechanical
and electrical contact.
IntracranialEEG signal is obtained by special electrodes
implanted in the brain surface during surgery. EEG system cover all parts of brain by placing electrodes on all
parts of the head.
Two systems of electrode placement:
10-20 international system: includes 21 electrodes.
10-10 international system: includes 64 electrodes.
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Circuit Design
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Block Diagram
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Block Diagram
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Differential Amplifier
Measure the voltage difference between the two signals
at each of its inputs.
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Electrode Montages
Bipolar: The potential difference is measured between
two active electrodes.
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Filters
Notch filtera band pass filter
around the power
lines frequency (50 or 60 Hz).
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Use of EEG
Epilepsy Generalized (grandmal) seizures.
Absence (petitmal) seizures.
Localize brain tumors.
Sleep disorders (Polysomnography) Narcolepsy
Sleep apnea syndrome
Insomnia and parasomnia
Helpful in knowing the cortical activity, toxicity,
hypoxia and encephalopathy &
Determination of brain death.
Flat EEG(absence of electrical activity) on two records run24 hrs apart.
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Advantages and Disadvantages
EEG has two clear advantages:
The first is characteristic of any electrical recording
systemhigh precision time measurements. Changes
in the brains electrical activity occur very quickly, and
extremely high time resolution is required to
determine the precise moments at which these
electrical events take place.
Unlike other electrical recording devices that require
inserting electrodes into the brain, EEG electrodes aresimply stuck onto the scalp. It is therefore a non-
invasive procedure.
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Advantages and Disadvantages
The main disadvantage of EEG recording is poor spatial
resolution. The EEG signal is not useful for pinpointing the
exact source of the activity.
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EEG Artifacts
Biological artifacts
Eye artifacts (including eyeball, ocular muscles and
eyelid)
ECG artifacts
EMG artifacts
Glossokinetic artifacts (minor tongue movements)
External artifacts
Movement by the patient
settling of the electrodes
Poor grounding of the EEG electrodes
the presence of an IV drip
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