Bioelectric Source Model and Brain Imaging

1. Bioelectric Source Model and Brain Imaging Dezhong Yao School of Life Sci & Tech,UESTC

2. Special Thanks to Prof Chen for giving the chance of the talk!

3. CONTENT • 1. Bioelectromagnetic Source models • 2. 2D Imaging of brain activities • 3. 3D Imaging of brain activities • 4. EEG reference problem

4. Bioelectric Source (1)For a live neuron, there are two currents (2) the Extracellular Current contributes directly to the scalp (EEG) (3) the Extracellular Current (EEG) is due to the intracellular current (source) Conclusion: the source of EEG is the intracellular current

5. Biomagnetic Source (1) MEG is generated by intracellular current (2) The source of both MEG and EEG is the intracellular current

6. 1.Bioelectromagnetic Source models (1) The bioelectromagnetic source is the intracellular current (2) The conventional source model is such as charge、dipole、quadruple... What is the bridge from current to charge or dipole model?

7. 1.Bioelectromagnetic Source models Bridge 1: the physics current is due to charge moving dipole is consisted of charges … This bridge is complex, we do not need to take care of it.

8. 1.Bioelectric Source models Bridge 2: performance and mathematics if a charge/dipole produces the same potential (EEG) of the actual current ---> charge/dipole is an equivalent source model of the current

9. 1.Bioelectric Source models • Equivalent charge model • (current source density)

10. 1.Bioelectric Source models • Equivalent dipole model • (Intracellular current) • By using Gauss Theorem

11. 1.Bioelectric Source models • Equivalent “potential” model

12. 1.Bioelectric Source models Neurophysiology of the equivalent model • (A) Equivalent charge model • extraqcellular current (EEG) • a negative current source density • (sink-negative charge ) • a positive current source density • (source-positive charge).

13. 1.Bioelectric Source models • (B) Equivalent dipole model • A paired • “negative charge- sink” • and • “positive charge- source” • ---> • a dipole model

14. 1.Bioelectric Source models • Equivalent source model in practice (1) Extracellular Current must flow in a regular way -- enough S/N -- -- to be recoded on the scalp surface (2) equivalent Source model - Macroscale collectively activities -not the microscale intracellular current

15. Summary of Source models • three kinds of Source models • each of them is an equivalent representation • of the actual neuron “assembly”

16. 2. 2D Imaging of brain activities • Two approaches • 1) Image processing • - Laplacian • (deblurring the skull smearing effect) • 2) Electric field analysis • Cortical potential reconstruction • Layer stripping(Equivalent dipole layer) • Layer replacing(Equivalent charge layer)

17. 2.2D Imaging of brain activities • Laplacian • --- try to find the current emerge or disappear in the scalp layer • current source density(CSD) • For a spherical head model (Yao, 2002) • h-radius of scalp, c-radius of the head

18. 2.2D Imaging of brain activities • （1）Laplacian

19. 2.2D Imaging of brain activities • （2) Electric field analysis • 1.Cortical potential reconstruction • (Sidman et al 1989;...) • 2.source potential in infinite medium • (Yao 2001) • 3.Layer stripping(Equivalent dipole layer) • (Freeman 1980, He Yao etal 2002) • 4.Layer replacing(Equivalent charge layer) • (Yao 2003)

20. 2.2D Imaging of brain activities • （2) Electric field analysis • The characteristics of the spatial spectra of the above four imaging approaches

21. 2.2D Imaging of brain activities • Forward • (Three dipoles) • Equivalent charge layer approach • -compared with Equivalent dipole layer (Yao 2003)

22. 2.2D Imaging of brain activities • Inverse

23. 2.2D Imaging of brain activities • Forward (four charges)

24. 2.2D Imaging of brain activities • Inverse

25. 2.2D Imaging of brain activities • Application

26. 3.3D Imaging of brain activities The source models may be: Dipole -- Potential -- charge

27. VEPs EC ED A ED X ED Y ED Z Real ERP result 3.3D Imaging of brain activities 1) Charge Loreta（He,Yao and Lian, IEEE TBME, 2002 ) Charge Vs Dipole model: lower computation complexity, and may image both charges and dipoles

28. 3.3D Imaging of brain activities 2) A Self-Coherence Enhancement Algorithm（ Yao et al 2001)

29. Step 1 Left: Actual source Right: LORETA 3.3D Imaging of brain activities 1) A Self-Coherence Enhancement Algorithm（ Yao et al 2001)

30. Step 2 3.3D Imaging of brain activities 1) A Self-Coherence Enhancement Algorithm（ Yao et al 2001) Two unknown parameters: K and alfa

31. Determine alfa 3.3D Imaging of brain activities 1) A Self-Coherence Enhancement Algorithm（ Yao et al 2001) Step 3 Determine K Actual neuronal source distribution is of neurophysiological smoothness. By defining a NBI (normalized blurring index ) Comparing the NBIs of the solution and the actual source to chose a proper K

32. 3.3D Imaging of brain activities 1) A Self-Coherence Enhancement Algorithm（ Yao et al 2001) Step 4

33. 4. EEG Reference problem EEG recordings • Reference is the oldest problem of EEG • There is not a point that its potential is zero all the time (Geselowitz, 1998） • A unitary reference is the best and ideal case

34. Method: Average ref:Va=GaX Inf ref V=GX Temporal waveform Real signal Average REST 4. EEG Reference problem （Yao, Physiol Meas, 2001）

35. Change of Spectra Real signal Average REST 4. EEG Reference problem The reference may have a large effect on the spectra

36. EEG/ ERP Lab at UESTC

37. Thanks