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Electroencephalography

(E.E.G)

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METHOD

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In conventional scalp EEG, the

recording is obtained byplacing electrodes on the scalpwith a conductive gel or paste.

Many systems typically useelectrodes, each of which isattached to an individual wire.Some systems use caps or netsinto which electrodes areembedded.

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BASIS OF EEG

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Electric activity of neurons is manifested

by generation of action potentials whichis caused due to rapid increase ofpermeability for Na+ ions.

Their influx in the cell causes a rapidincrease of the potential inside the celland the change of polarity of the inside ofthe neuron from negative to positive(about +30 mV).

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A subsequent increase of membrane

permeability to K+ ions (leading to theiroutflow from the cell), and a decrease ofpermeability for Na+ ions makes the insideof the cell negative again with respect to

the surrounding medium.

In this way, action potential of

characteristic spike-like shape (durationabout 1 ms) is created.

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A typical EEG spike

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LEAD POSITIONS

Electrode locations and names are specified

by the International 1020 system.

In most applications 19 recording electrodes

(plus ground and system reference) are used.

Additional electrodes can be used for betterspatial resolution for an area of the brain

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10-20 system

Internationally recognizedmethod to describe and

apply the location of scalpelectrodes in the contextof an EEG test orexperiment

Developed to ensurestandardization innomenclature of electrodes

This system is based on the

relationship between thelocation of an electrodeand the underlying area ofcerebral cortex

The "10" and "20" refer to the

fact that the actual distancesbetween adjacentelectrodes are either 10% or20% of the total front-back orright-left distance of the skull

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F- Frontal Lobe

T-Temporal LobeC-Central Lobe

P-Parietal Lobe

O-Occipital Lobe

Z refers to an

electrode placed

on the central line

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Side View and Top View of electrodes

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WORKING

Each electrode is connected toone input of a differentialelectrode (one amplifier per pairof electrodes), referenceelectrode is connected as the

other input.

The amplifiers amplify

the voltage betweenactive electrode andreference (1000 to100,000 times).

In analog EEG signal isfiltered and the EEG signal isoutput as the deflection ofpens as paper passes

underneath

In digital EEG the amplifiedsignal is digitized viaan analog-to-digitalconverter, after being

passed through a filter

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Block diagram of recording of a single EEG channel

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The digital EEG signal is stored electronically and can befiltered for display.

High Pass filter 0.5-1Hz frequency range Filters out slow artifact such

as electrogalvanic signalsand movement artifact

Low Pass filter 35-70 Hz frequency range Filters out high-frequency

artefacts, such aselectromyographic signals.

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EEG MACHINE

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EEG machine essentially consists of two

parts:

1. The electrodes

2. Computer to analyze and print the data

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MODERN EEG MACHINE

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A normal EEG plot

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Abnormal EEG plot

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EEG Montage

An EEG voltage signal represents adifference between the voltages at two

electrodes. The display of EEG may be set up in

different ways.

The representation of EEG channels is

called montage.

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BIPOLAR MONTAGE

Each channel represents thedifference between twoadjacent electrodes. Theentire montage consists of aseries of these channels.

REFERENTIAL MONTAGE

Each channel representsthe difference between a

certain electrode and adesignated referenceelectrode.

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AVERAGE REFERENCE MONTAGE

The outputs of all of theamplifiers are summed andaveraged, and this averagedsignal is used as the commonreference for each channel.

LAPLACIAN MONTAGE

Each channel representsthe difference betweenan electrode and aweighted average of the

surrounding electrodes.

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EEG ACTIVITY

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The EEG is typically described in terms of(1) rhythmic activity and (2) transients. The

rhythmic activity is divided into bands byfrequency.

Most of the cerebral signal observed in

the scalp EEG falls in the range of 120 Hz

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A. Delta wave

FREQUENCY RANGE : up to 4Hz

LOCATION Frontally in adults, posteriorly

in children high amplitude waves

OCCURENCE adults (slow wave sleep) in babies

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DELTA WAVE PATTERN

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B. Theta waveFREQUENCY RANGE : 4-8 Hz

OCCURENCE young children Drowsiness

Associated with inhibition ofelicited responses (has beenfound to spike in situationswhere a person is actively tryingto repress a response or action)

LOCATION

Found in locationsnot related to taskat hand

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THETA WAVE PATTERN

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c. Alpha waveFREQUENCY RANGE: 8-13Hz

LOCATION posterior regions of

head, both sides,higher in amplitude ondominant side. Centralsites (c3-c4) at rest

OCCURRENCE Relaxed position

closing the eyes Also associated with inhibition control,

seemingly with the purpose of timinginhibitory activity in different locationsacross the brain

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ALPHA WAVE PATTERN

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D. Beta wave

FREQUENCY RANGE: 13-30 Hz

LOCATION Both sides, symmetrical

distribution, most evidentfrontally; low amplitude waves

OCCURRENCE

Alert/working active, busy or anxious thinking,active concentration

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BETA WAVE PATTERN

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SLEEP EEG

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Sleep period can be divided into 4 stages

STAGE 1: (Drowsiness)

STAGE 2: (Light Sleep)

STAGE 3: (Deep Sleep) STAGE 4: (Very deep sleep)

REM stage : (Dream state)

Different EEG pattern is recorded in eachstage

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STAGE 1 : Drowsiness

Decrease of alpha

rhythm in 2-7 Hzfrequency band andlow amplitude rhythm of15-25 Hz band

Medium amplitude mixedfrequency (mainly ),sometimes with vertex

sharp waves

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STAGE 2 : Light sleep

Frequency ranging from

0.75 Hz to 4 Hz prominent.Frequencies b/w 15-30 Hzmay be present too

Less than 20% portion

contains Delta Waves

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STAGE 3 : Deep sleep

Slow rhythm in Delta

frequency range (0.75-3Hz) Activity of lower

amplitude in 5-9 Hz is alsoquite common

Stage 3 is scored when

2050% of the epochcontains delta waves of0.52.5 Hz frequencyand of 75 mV or greaterpeak-to-peakamplitude.

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STAGE 4 : Very deep sleep

Dominated by slow wave

activity if high amplitude

Stage 4 is scored whenmore than 50% of the

epoch contains deltaactivity conforming to thecriteria defined above.

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STAGE 5 : Dream state

Decrease of EEGamplitude, occurrence

of fast rhythms, rapideye movements, andloss of muscular activity

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Conditions influencing EEG

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Pathological conditions EEG is affected by the CNS disorders (e.g.,

cerebral anoxia, cerebral inflammatory

processes, cerebral palsy, and metabolic anddegenerative nervous system disorders).

It is influenced by brain tumours and cranio-

cerebral traumas;

EEG is also an important test in psychiatric

diseases, sleep disorders, and developmental

disorders

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Epileptic seizures In epileptic discharges, the membrane

potential of cortical and deeper located

neurons changes in a dramatic way,which leads to massive bursts of actionpotentials and large fluctuations of intra-and extracellular fields.

As a result fluctuation in EEG pattern isobserved.

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Drugs EEG is very sensitive to the action of a

wide range of pharmacological

substances, especially psychotropicdrugs, anaesthetics, and anticonvulsants.

Influence of drugs on EEG primarily

include changes in its spectral contentand topographic characteristics.

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APPLICATIONS

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Diagnose epilepsy and see what type of

seizures are occurring.

Check for problems with loss ofconsciousness or dementia.

Help find out a person's chance ofrecovery after a change in consciousness.

Find out if a person who is in a coma isbrain-dead.

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Study sleep disorders, such as narcolepsy.

Watch brain activity while a person isreceiving general anaesthesia duringbrain surgery.

Help find out if a person has a physicalproblem (problems in the brain, spinalcord, or nervous system) or a mentalhealth problem.

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Research Uses EEG, is used extensively

in neuroscience, cognitive science, cognitive

psychology,and psychophysiological research.

EEG can detect covert processing (i.e.,

processing that does not require a response)

EEG can be used in subjects who are

incapable of making a motor response

Cheaper as compared to MRI

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FUTURE OF EEG

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At present techniques like MRI are used

for diagnosis of pathologic neurologicalstates and in brain research.

However, these methods give informationabout the absorption of certain

substances in specific structures or aboutthe metabolism rate or glucoseconsumption, not directly about the brainelectrical activity.

Although their spatial localizationproperties are good, their time resolutionis much lower than EEG.

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Moreover, in the information processing

by brain, EEG rhythms have a differentspecific role, which cannot bedistinguished by imaging techniques.

Therefore, these techniques are not likelyto replace EEG, which is a totally non-invasive and low-cost technique capableof providing information aboutrelationships between cortical sites andthe time evolution of brain processes.

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QUESTIONS???

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THANK YOU