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