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What do we measure with EEG and MEG?

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Presentation on theme: "What do we measure with EEG and MEG?"— Presentation transcript:

1 What do we measure with EEG and MEG?
Xavier de Tiege Isabell Zlobinski 03/05/06

2 Characteristics of the EEG
Localize neural electric activity using non-invasive measurements Measures electric potential differences between pairs of scalp electrodes Temporal precision Rhythmic fluctuations in voltage Standard time interval: cycles per second = hertz (Hz) Amplitude in microvolts (µV)

3 The EEG Frequency Spectrum
Waves Frequencies per second Amplitude in µV Characteristics Beta-waves 5 - 50, mostly below 30 Sharp spike-waves over 35 Hz, Frontocentral, precentral & posterior Criteria of light sleep stages Alpha-waves 8 - 13 , mostly below 50 Posterior-dominant, awake, eyes closed, mental inactivity, physical relaxation Theta-waves 4 - 7 Strictly rhythmic or highly irregular Awake & drowsiness or light sleep stages Delta-waves 0,5 - 3 Abnormality in waking adults, Accompaniment of deep sleep Gamma-waves -10 Legality of appearance and site not well established

4 The EEG Frequency Spectrum

5 Recording EEG EPSP at apical dentritic trees of pyramid cells
Dentritic membrane  depolarized Potential difference cause a current  flow through volume conductor from the nonexited membrane of the soma to the apical dentritic tree Extracellular currents = secondary / volume currents

6 Recording EEG

7 The EEG machine 8 – 64 identical channels recording simultaneously from as many different pairs of electrodes Electrodes & electrode board Amplifiers Filters Pen & chartdrive (screen)

8 When do we use EEG? Sleep research Clinical diagnosis
Epileptic patients Sleep disorders Encephalopathies Biophysiologic research (e.g. evoked potentials) Cognitive research (e.g. ERPs)

9 Artifacts Physiologic artifacts Extraphysiologic artifacts
Eye movement Muscle activity ECG artifacts Skin artifacts Extraphysiologic artifacts Electrodes Alternating current (60 Hz) artifact Movements in the enviroment

10 Event Related Potentials
Voltage fluctuations in cortex because of cognitive procedures or stimuli responses Designed by summation & averaging of event related EEG parts Waves described after polarity and latency (e.g. P300), method of release (mismatch negativity) and psychophysiological correlatives

11 ERPs – failures & limits
Artifacts possible Difficult to analyse high complex cognitive procedures when stimuli need more time than 100 ms  short ERP duration between individuals very variable depends on age combined from several spacial and temporal overlapping components  low specificity

12 Evoked Potentials Record of low amplitude potentials evoked by different types of sensory stimulus Voltage fluctuation is slow with a very small amplitude of the response (about 1/100 of spontaneous EEG activity) high amplification is essential Special computer averaging technic is required clinical diagnostic, neurophysiologic & cognitive research Visual evoked Potential Brainstem auditory evoked Potential Somatosensory evoked Potential

13 Sources Duffy, Iyer, Surwillo (1989). Clinical Electroencephalography and Topographic Brain Mapping. Springer Verlag S. Baillet, J.C. Mosher, R.M. Leahy. (2001). Electromagnetic Brain Mapping. IEEE Signal Processing Magazine. P

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