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What do we measure with EEG and MEG?. What do we measure with EEG? Xavier de Tiege Isabell Zlobinski 03/05/06. Characteristics of the EEG. Localize neural electric activity using non-invasive measurements

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what do we measure with eeg and meg

What do we measure with EEG and MEG?

What do we measure with EEG?

Xavier de Tiege

Isabell Zlobinski


characteristics of the eeg
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)
recording eeg
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
the eeg machine
The EEG machine
  • 8 – 64 identical channels recording simultaneously from as many different pairs of electrodes
  • Electrodes & electrode board
  • Amplifiers
  • Filters
  • Pen & chartdrive (screen)
when do we use eeg
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)
Physiologic artifacts

Eye movement

Muscle activity

ECG artifacts

Skin artifacts

Extraphysiologic artifacts


Alternating current (60 Hz) artifact

Movements in the enviroment

event related potentials
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

erps failures limits
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
evoked potentials
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
  • 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 14-30.
  • www.dkgn.de
  • http://edoc.hu-berlin.de/dissertationen/eichholz-stephan-2004-10-22/HTML/chapter2.html
  • http://www.emedicine.com/neuro/topic678.htm