ElectroencephalogrElectroencephalographyaphy ( EEG ) ( EEG )

Nidhin Thomas Kartik Jain

“Electroencephalography (EEG) is the recording of electrical activity along the scalp produced by the firing of neurons within the brain.”

HistoryHistory1929: Hans Berger developed the electroencephalography (=graphic representation of the difference in voltage between two different cerebral locations plotted over time) following the studies of Richard Caton in non-human animal species.

He described the human alpha and beta rhythms

Gray Walter – Brain Gray Walter – Brain ImagingImaging

In 1957, Gray WalterIn 1957, Gray WalterMakes recordings Makes recordings with large numbers with large numbers of electrodesof electrodesVisualizes brain Visualizes brain activity with the activity with the toposcope toposcope Shows that brain Shows that brain rhythms change rhythms change according to the according to the mental task mental task demandeddemanded

The toposcope by Gray Walter

Functions of EEGFunctions of EEGThe EEG uses highly conductive silver electrodes coated The EEG uses highly conductive silver electrodes coated with silver-chloride and gold cup electrodes to obtain with silver-chloride and gold cup electrodes to obtain accurate measures… use impedance device to measure accurate measures… use impedance device to measure effectiveness, resistance caused by cerebrospinal fluid, effectiveness, resistance caused by cerebrospinal fluid, and skull boneand skull bone

Monopolar Technique : Monopolar Technique : the use of one active recording the use of one active recording electrode placed on area of interest, a reference electrode placed on area of interest, a reference electrode in an inactive area, and a ground electrode in an inactive area, and a ground

Bipolar Technique : Bipolar Technique : the use of two active electrodes on the use of two active electrodes on areas of interestareas of interest

Measures brain waves (graphs voltage over time) Measures brain waves (graphs voltage over time) through electrodes by using the summation of many through electrodes by using the summation of many action potentials sent by neurons in brain. Measured action potentials sent by neurons in brain. Measured amplitudes are lessened with electrodes on surface of amplitudes are lessened with electrodes on surface of skin compared to electrocorticogram skin compared to electrocorticogram

EEG AcquisitionEEG AcquisitionElectrodes: Ag/AgCl, tin…

Active electrodes: Attached to the scalp

Reference electrode: Mastoid, nose, ear lobe...The EEG records differences in voltage – difference in electrical potential from one electrode to another!!

EEG in clinical diagnostics EEG in scientific research

Electrode Positioning Electrode Positioning systemsystem

EEG ElectrodesEEG Electrodes

Sliver Electrodes Electrodes Cap

Procedure of EEG Procedure of EEG recordingrecording

A standard EEG makes use of 21 A standard EEG makes use of 21 electrodes linked in various ways electrodes linked in various ways (Montage).(Montage).Ask the subject to lie down in bed.Ask the subject to lie down in bed.Apply electrode according to Apply electrode according to 10/20% system.10/20% system.Check the impedance of the Check the impedance of the electrodes.electrodes.

10 /20 % system of EEG 10 /20 % system of EEG electrode placementelectrode placement

Major Brain RegionsMajor Brain Regions

What does the EEG What does the EEG record?record?Mainly NOISE!!Mainly NOISE!!

The electrical activity flows The electrical activity flows through the tissue through the tissue between the electrical between the electrical generator and the generator and the recording electrode.recording electrode.

Thus, the EEG is a 2-D Thus, the EEG is a 2-D representation of a 3-D representation of a 3-D reality, which poses a reality, which poses a problem in localizing the problem in localizing the sources of the electrical sources of the electrical activityactivity

Sodium-Potassium PumpSodium-Potassium Pump

The mechanism within neurons that creates The mechanism within neurons that creates action potentials through the exchange action potentials through the exchange between sodium and potassium ions in and out between sodium and potassium ions in and out of the cellof the cellAdenosine Triphosphate (ATP) provides energy Adenosine Triphosphate (ATP) provides energy for proteins to pump 300 sodium ions per for proteins to pump 300 sodium ions per second out of the cell while simultaneously second out of the cell while simultaneously pumping 200 potassium ions per second into pumping 200 potassium ions per second into the cell (concentration gradient)the cell (concentration gradient)Thus making the outside of the cell more Thus making the outside of the cell more positively charged and the neuron negatively positively charged and the neuron negatively chargedchargedThis rapid ionic movement causes the release of This rapid ionic movement causes the release of action potentials action potentials

• ERPs– “Electrical Potentials associated with specific

sensory, perceptual, cognitive, or motor events”

• From EEG to ERP…– Time-locked average of EEG from many trials

involving same ‘event’– Signal/Noise Ratio reduction; what is left is

‘related to the event’• EEG = 20-50v / ERP = 1-10 v

What are Event-Related Potentials?What are Event-Related Potentials?

Filter & AmplifyAverage

across Trials & Individuals

Collapsed to form a “Grand Average” Or

mean of means

Electrical activity at the

onset of a stimulus recorded

Different types of brain Different types of brain waves in normal EEGwaves in normal EEGRhythm Frequenc

y(Hz)

Amplitude(uV)

Recording& Location

Alpha(α) 8 – 13 50 – 100 Adults, rest, eyes closed.Occipital region

Beta(β) 14 - 30 20 Adult, mental activityFrontal region

Theta(θ) 5 – 7 Above 50 Children, drowsy adult, emotional distressOccipital

Delta(δ) 2 – 4 Above 50 Children in sleep

Factors influencing EEGFactors influencing EEGAge

Infancy – theta, delta waveChild – alpha formation.Adult – all four waves.

Level of consciousness (sleep) Hypocapnia(hyperventilation) slow & high amplitude waves.HypoglycemiaHypothermiaLow glucocorticoids

Desynchronization or Alpha Desynchronization or Alpha blockblock

Cause:Cause:Eyes opening (after closure)Eyes opening (after closure)Thinking by the subject Thinking by the subject (mathematical calculation)(mathematical calculation)

Sound (clapping)Sound (clapping)

Eye openingEye opening

Alpha rhythm changes to beta on Alpha rhythm changes to beta on eye opening (desynchronization / eye opening (desynchronization / αα- - block) block)

ThinkingThinking

Beta waves are observedBeta waves are observed

Provocation testProvocation test

Intermittent photic stimulationIntermittent photic stimulationIncrease rate & decrease amplitudeIncrease rate & decrease amplitude

HyperventilationHyperventilationDecrease rate & increase in amplitudeDecrease rate & increase in amplitude

Strengths and Advantages Strengths and Advantages of EEGof EEG Is a measure of Is a measure of

brain function; brain function; supplement supplement neuroimaging neuroimaging studiesstudies

Provides direct Provides direct rather than rather than indirect evidence indirect evidence of epileptic of epileptic abnormalityabnormality

May be the only May be the only test that shows test that shows abnormalities in abnormalities in epileptic patientsepileptic patients

Provides some Provides some spatial or spatial or localization localization informationinformation

Low costLow cost

Low morbidityLow morbidity

Readily Readily repeatablerepeatable

Portable / Portable / ambulatory ambulatory

Limitations and Limitations and Disadvantages Of EEGDisadvantages Of EEG

Detects cortical dysfunction but rarely discloses its Detects cortical dysfunction but rarely discloses its etiology etiology Relatively low sensitivity and specificityRelatively low sensitivity and specificitySubject to both electrical and physiologic artifactsSubject to both electrical and physiologic artifactsInfluenced by state of alertness, hypoglycemia, Influenced by state of alertness, hypoglycemia, drugsdrugsSmall or deep lesions might not produce an EEG Small or deep lesions might not produce an EEG abnormalityabnormalityLimited time sampling (for routine EEG) and Limited time sampling (for routine EEG) and spatial samplingspatial samplingMay falsely localize epileptogenic zoneMay falsely localize epileptogenic zone

EEG Artifacts EEG Artifacts

Biological artifacts Biological artifacts Eye artifacts (including eyeball, ocular Eye artifacts (including eyeball, ocular muscles and eyelid) muscles and eyelid) ECG artifacts ECG artifacts EMG artifacts EMG artifacts Glossokinetic artifacts (minor tongue Glossokinetic artifacts (minor tongue movements)movements)

External artifacts External artifacts Movement by the patient Movement by the patient settling of the electrodes settling of the electrodes Poor grounding of the EEG electrodes Poor grounding of the EEG electrodes the presence of an IV drip the presence of an IV drip

Computerized EEG Computerized EEG MachineMachine

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