Understanding EEG

download Understanding EEG

of 31

  • date post

  • Category


  • view

  • download


Embed Size (px)

Transcript of Understanding EEG

Electroencephalography ( 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.

History1929: 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 nonhuman animal species. He described the human alpha and beta rhythms

Gray Walter Brain ImagingIn 1957, Gray Walter Makes recordings with large numbers of electrodes Visualizes brain activity with the toposcope Shows that brain rhythms change according to the mental task demandedThe toposcope by Gray Walter

Functions of EEGThe EEG uses highly conductive silver electrodes coated with silversilverchloride and gold cup electrodes to obtain accurate measures use impedance device to measure effectiveness, resistance caused by cerebrospinal fluid, and skull bone Monopolar Technique : the use of one active recording electrode placed on area of interest, a reference electrode in an inactive area, and a ground Bipolar Technique : the use of two active electrodes on areas of interest Measures brain waves (graphs voltage over time) through electrodes by using the summation of many action potentials sent by neurons in brain. Measured amplitudes are lessened with electrodes on surface of skin compared to electrocorticogram

EEG 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 system

EEG Electrodes

Sliver Electrodes

Electrodes Cap

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

10 /20 % system of EEG electrode placement

Major Brain Regions

What does the EEG record?Mainly NOISE!! The electrical activity flows through the tissue between the electrical generator and the recording electrode. electrode. Thus, the EEG is a 2-D representation of a 3-D reality, which poses a problem in localizing the sources of the electrical activity

Sodium-Potassium Pump Sodium-

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

What are Event-Related Potentials? Event 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 = 20-50Qv / ERP = 1-10 Qv

Electrical activity at the onset of a stimulus recorded Filter & Amplify Average across Trials & Individuals Collapsed to form a Grand Average Or mean of means

Different types of brain waves in normal EEGRhythm Frequency Amplitude (Hz) (uV) 8 13 50 100 Recording & Location Adults, rest, eyes closed. Occipital region Adult, mental activity Frontal region Children, drowsy adult, emotional distress Occipital Children in sleep

Alpha( ) Beta( ) Theta( ) Delta( )

14 - 30



Above 50


Above 50

Factors influencing EEGAgeInfancy theta, delta wave Child alpha formation. Adult all four waves.

Level of consciousness (sleep) Hypocapnia(hyperventilation) slow & high amplitude waves. Hypoglycemia Hypothermia Low glucocorticoids

Desynchronization or Alpha block

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

Eye openingAlpha rhythm changes to beta on eye opening (desynchronization / - block)

ThinkingBeta waves are observed

Provocation testIntermittent photic stimulationIncrease rate & decrease amplitude

HyperventilationDecrease rate & increase in amplitude

Strengths and Advantages of EEGIs a measure of brain function; supplement neuroimaging studies Provides direct rather than indirect evidence of epileptic abnormality May be the only test that shows abnormalities in epileptic patients Provides some spatial or localization information Low cost Low morbidity Readily repeatable Portable / ambulatory

Limitations and Disadvantages Of EEGDetects cortical dysfunction but rarely discloses its etiology Relatively low sensitivity and specificity Subject to both electrical and physiologic artifacts Influenced by state of alertness, hypoglycemia, drugs Small or deep lesions might not produce an EEG abnormality Limited time sampling (for routine EEG) and spatial sampling May falsely localize epileptogenic zone

EEG ArtifactsBiological artifactsEye artifacts (including eyeball, ocular muscles and eyelid) ECG artifacts EMG artifacts Glossokinetic artifacts (minor tongue movements)

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

Computerized EEG Machine