Basics of electroencephalography

Basics of electroencephalography

1. Basics of EEG Dr. Vaishal shah Neurology sr Govt. medical college, kota

2. Introduction The EEG records electrical activity from the cerebral cortex. Must be amplified by a factor of 10,00,000 in order to be displayed on a computer. Number of possible sources Action potentials Post-synaptic potentials (PSPs)

3. Introduction The resting membrane potential (electrochemical equilibrium) is typically –70 mv on the inside. At the post-synaptic membrane the neurotransmitter produces a change in membrane conductance and transmembrane potential. If the signal has an excitatory effect on the neuron it leads to a local reduction of the transmembrane potential (depolarization) and EPSP.

4. Introduction IPSPs result in local hyperpolarization typically located on the cell body of the neuron. The combination of EPSPs and IPSPs induces currents that flow within and around the neuron with a potential field sufficient to be recorded on the scalp. The EEG is essentially measuring these voltage changes in the extracellular matrix.

5. Introduction The mechanisms of EEG rhythmicity, although not completely understood, are mediated through two main processes. Interaction between cortex and thalamus. Functional properties of large neuronal networks in the cortex that have an intrinsic capacity for rhythmicity.

6. Technical considerations

7. Electrodes Small, non-reactive metal discs or cups applied to the scalp with a conductive paste. Gold, silver/silver chloride, tin, and platinum. Electrode contact must be firm in order to ensure low impedance (resistance to current flow) minimizing both electrode and environmental artifacts .

8. Electrode placement Standardized in the US and indeed in most other nations. Allows EEGs performed in one laboratory to be interpreted in another. 10-20 international system 10-10 international system

9. Electrode placement Skull is taken in three planes – sagittal, coronal, and horizontal. The summation of all the electrodes in any given plane will equal 100%. Electrodes designated with odd numbers are on the left; those with even numbers are on the right.

10. 10 – 20 system Sagittal plane

11. Coronal plane

12. Horizontal plane

13. F3 and F4 are defined by the halfway points between F7 and Fz on the left and F8 and Fz on the right. Similarly, P3 and P4 are defined by the halfway points between P7 and Pz on the left and P8 and Pz on the right.

14. 10 – 20 system

15. 10 – 10 system

16. Potential fields The summation of IPSPs and EPSPs in a neuronal net creates electrical currents. The flow of current creates a field that spreads out from the origin of an electrical event such as same as the concentric rings created on a glassy pond when one tosses a pebble onto its surface. Field’s effect diminishes as the distance from the source increases.

17. Amplification Each amplifier has two inputs, I and II.

18. Bipolar recording The voltage at one electrode is compared with the voltage affecting adjacent electrodes (potential difference).

19. Consider a spike with a very limited potential field involving only T8. Channel 1 : F8–T8 = –20 μ v– (–100 μ v) = 80 μ v (downward deflection) Channel 2 : T8–P8 = (–100 μ v) – (–20 μ v) = –80 μ v (upward deflection) This is phase reversal – the localization Principle of bipolar recording.

20. Channel 1: Fp2–F8 = (–50 μ V) – (–100 μ V) = 50 μ V (downward deflection) Channel 2: F8–T8 = (–100 μ V) – (–50 μ V) = –50 μ V (upward deflection) Channel 3: T8–P8 = (–50 μ V) – (–20 μ V) = –30 μ V (a smaller upward deflection) Channel 4: P8–O2 = (–20 μV) – (–20 μV) = 0 μV (no deflection)

21. Referential recording Signals from each of the scalp electrodes are conducted to input I of the associated amplifier, while signals from the reference are conducted to input II. Recording the potential difference between a particular scalp electrode and a referential electrode. Opposite ears are used as a referential electrode.

22. Channel 1: Fp2–A1 = (–50 μV ) – (–20 μV ) = –30 μV (small upward deflection) Channel 2: F8–A1 = (–100 μV ) – (–20 μV )= –80 μV (big upward deflection) Channel 3: T8–A1 = (–50 μV ) – (–20 μV ) = –30 μV (small upward deflection) Channel 4: P8–A1 = (–20 μ V) – (–20 μ V) = 0 μ V (no deflection)

23. Referential recording In referential recording, the localization principle is amplitude. That is, the electrode recording the greatest amplitude of the wave defines the focus.

24. Montage selection Montage refers to the pattern of systematic linkage of the scalp electrodes designed to obtain a logical display of the electrical activity. In bipolar recording the longitudinal arrangement is perhaps the most popular (known in the trade as the “ double banana ,” and by some as the queen square montage).

25. A typical longitudinal bipolar montage.

26. A transverse bipolar montage

27. Referential montage With respect to referential montage, the recording is usually displayed in both A-P and transverse arrangements, reprising commonly used bipolar montages. A low-amplitude temporal spike during bipolar recording can rapidly be inspected on a referential montage.

28. The paradox of bipolar recording

29. Calibration Standard display timebase is 30 mm/sec with 10 seconds of EEG per display. The sensitivity of each channel refers to the amplitude of the display produced by the received signal. Standard sensitivity is 7 μv /mm. Impedances should not exceed 5 kohms .

30. Calibration High-frequency filters - attenuates undesirable high frequencies (e.g., Muscle action potentials) and passes low frequencies. The standard HF setting is 70 hz . Low-frequency filters - marked attenuation of slow potentials below the cutoff frequency (such as those caused by sweat artifact , respirations, and tongue movement), with little effect on rapid potentials such as spikes or muscle artifacts . The LFF is typically set at 1 hz . Notch filter - selectively reducing environmental interference .

31. Thank you