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

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

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

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

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

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

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. 1.Bioelectromagnetic Source models

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

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

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

1.Bioelectric Source models Equivalent “ potential ” model

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

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

1.Bioelectric Source models (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 Equivalent source model in practice

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

2. 2D Imaging of brain activities 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) Two approaches

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

2.2D Imaging of brain activities （ 1 ） Laplacian

（ 2) Electric field analysis 2.2D Imaging of brain activities 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)

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

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

2.2D Imaging of brain activities Inverse

2.2D Imaging of brain activities Forward (four charges)

2.2D Imaging of brain activities Inverse

2.2D Imaging of brain activities Application

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

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

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

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

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

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

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

 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 4. EEG Reference problem EEG recordings

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

4. EEG Reference problem Change of Spectra Real signalAverageREST The reference may have a large effect on the spectra

EEG/ ERP Lab at UESTC

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