What do we What do we measure with measure with

EEG and MEG?EEG and MEG?What do we measure with EEG?What do we measure with EEG?

Xavier de TiegeXavier de TiegeIsabell Zlobinski Isabell Zlobinski 03/05/0603/05/06

Characteristics of the Characteristics of the EEGEEG

Localize neural electric activity using non-Localize neural electric activity using non-invasive measurementsinvasive measurements

Measures electric potential differences between Measures electric potential differences between pairs of scalp electrodespairs of scalp electrodes

Temporal precision Temporal precision

Rhythmic fluctuations in voltageRhythmic fluctuations in voltage Standard time interval: cycles per second = Standard time interval: cycles per second =

hertz (Hz)hertz (Hz) Amplitude in microvolts (Amplitude in microvolts (µV)µV)

The EEG Frequency The EEG Frequency SpectrumSpectrum

WavesWaves FrequenFrequen

cies per cies per secondsecond

AmplitudAmplitude in e in µVµV

CharacteristicsCharacteristics

Beta-Beta-waveswaves

14 - 30 5 - 50, mostly below 30

Sharp spike-waves over 35 Hz,Frontocentral, precentral & posterior Criteria of light sleep stages

Alpha-Alpha-waveswaves

8 - 13 5 - 120, mostly below 50

Posterior-dominant, awake, eyes closed, mental inactivity, physical relaxation

Theta-Theta-waveswaves

4 - 7 20 - 100 Strictly rhythmic or highly irregular Awake & drowsiness or light sleep stages

Delta-Delta-waveswaves

0,5 - 3 5 - 250 Abnormality in waking adults,Accompaniment of deep sleep

Gamma-Gamma-waveswaves

31 - 60 -10 Legality of appearance and site not well established

The EEG Frequency The EEG Frequency SpectrumSpectrum

Recording EEGRecording EEG EPSP at apical dentritic EPSP at apical dentritic

trees of pyramid cellstrees of pyramid cells Dentritic membrane Dentritic membrane

depolarizeddepolarized Potential difference Potential difference

cause a current cause a current flow flow through volume through volume conductor from the conductor from the nonexited membrane of nonexited membrane of the soma to the apical the soma to the apical dentritic treedentritic tree

Extracellular currents = Extracellular currents = secondary / volume secondary / volume currentscurrents

Recording EEGRecording EEG

The EEG machineThe EEG machine 8 – 64 identical 8 – 64 identical

channels recording channels recording simultaneously from as simultaneously from as many different pairs of many different pairs of electrodeselectrodes

Electrodes & electrode Electrodes & electrode boardboard

AmplifiersAmplifiers FiltersFilters Pen & chartdrive Pen & chartdrive

(screen)(screen)

When do we use EEG?When do we use EEG? Sleep researchSleep research Clinical diagnosis Clinical diagnosis

Epileptic patientsEpileptic patients Sleep disordersSleep disorders EncephalopathiesEncephalopathies

Biophysiologic research (e.g. evoked Biophysiologic research (e.g. evoked potentials)potentials)

Cognitive research (e.g. ERPs)Cognitive research (e.g. ERPs) ……

ArtifactsArtifacts

Physiologic artifactsPhysiologic artifacts Eye movement Eye movement Muscle activityMuscle activity ECG artifactsECG artifacts Skin artifactsSkin artifacts

Extraphysiologic Extraphysiologic artifactsartifacts

ElectrodesElectrodes Alternating current Alternating current

(60 Hz) artifact(60 Hz) artifact Movements in the Movements in the

enviromentenviroment

Event Related PotentialsEvent Related Potentials Voltage fluctuations in cortex because of cognitive Voltage fluctuations in cortex because of cognitive

procedures or stimuli responsesprocedures or stimuli responses Designed by summation & averaging of event Designed by summation & averaging of event

related EEG parts related EEG parts

Waves described after Waves described after

polarity and latency (e.g. P300), polarity and latency (e.g. P300),

method of release (mismatch negativity) method of release (mismatch negativity)

and psychophysiological correlativesand psychophysiological correlatives

ERPs – failures & limitsERPs – failures & limits

Artifacts possibleArtifacts possible Difficult to analyse high complex cognitive Difficult to analyse high complex cognitive

procedures when stimuli need more time than procedures when stimuli need more time than 100 ms 100 ms short ERP duration short ERP duration

between individuals very variablebetween individuals very variable depends on agedepends on age combined from several spacial and temporal combined from several spacial and temporal

overlapping components overlapping components low specificity low specificity

Evoked PotentialsEvoked Potentials Record of low amplitude potentials evoked by different Record of low amplitude potentials evoked by different

types of sensory stimulustypes of sensory stimulus

Voltage fluctuation is slow with a very small amplitude of Voltage fluctuation is slow with a very small amplitude of the response (about 1/100 of spontaneous EEG activity)the response (about 1/100 of spontaneous EEG activity)

high amplification is essentialhigh amplification is essential Special computer averaging technic is required Special computer averaging technic is required

clinical diagnostic, neurophysiologic & cognitive clinical diagnostic, neurophysiologic & cognitive research research

Visual evoked PotentialVisual evoked Potential Brainstem auditory evoked PotentialBrainstem auditory evoked Potential Somatosensory evoked PotentialSomatosensory evoked Potential

SourcesSources Duffy, Iyer, Surwillo (1989). Clinical Duffy, Iyer, Surwillo (1989). Clinical

Electroencephalography and Topographic Brain Electroencephalography and Topographic Brain Mapping. Springer VerlagMapping. Springer Verlag

S. Baillet, J.C. Mosher, R.M. Leahy. (2001). S. Baillet, J.C. Mosher, R.M. Leahy. (2001). Electromagnetic Brain Mapping. Electromagnetic Brain Mapping. IEEE Signal IEEE Signal Processing MagazineProcessing Magazine. P 14-30.. P 14-30.

www.dkgn.dewww.dkgn.de http://edoc.hu-berlin.de/dissertationen/eichholz-http://edoc.hu-berlin.de/dissertationen/eichholz-

stephan-2004-10-22/HTML/chapter2.htmlstephan-2004-10-22/HTML/chapter2.html http://www.emedicine.com/neuro/topic678.htmhttp://www.emedicine.com/neuro/topic678.htm