Download - Source Localization for EEG/MEG Stavroula Kousta Martin Chadwick Methods for Dummies 2007
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Source Localization for EEG/MEG
Stavroula KoustaMartin Chadwick
Methods for Dummies 2007
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Source Localization: The Question
Is it possible to find the locus of electrical activity in the brain based upon the map of recorded electrical activity?
EEG MEG
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Forward Solution
Model Head
Brain
Skull
Scalp
-4 μV
-5 μV
Model Data
0 μV
-1 μV-2 μV
-3 μV
1 μV2 μV
3 μV
4 μV
5 μV
+
Location OrientationAmplitude
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Inverse problem
-5 μV
-4 μV
Recorded Data
0 μV
-1 μV-2 μV
-3 μV
1 μV2 μV
3 μV
4 μV
5 μV
Desired Model Solution
Brain
Skull
Scalp
?
?
? ?
?
ILL-DEFINED!!!
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+
-
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+ -
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+-
+ -
Any field potential vector could be consistent with an infinite number of possible dipoles
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Specify initial dipole parameters in head model
Generate scalp potentials/fields using forward solution (Vmodel)
Calculate SSE
Σ (Vmodel –Vdata)2 ΔSSE<εAccept dipole parameters
Adjust Parameters
Yes
No
Inverse Solution
Slotnick, 2005
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Source Model Initial Parameters
‘Imaging’‘Imaging’‘Equivalent Current Dipoles’ (ECD)
‘Equivalent Current Dipoles’ (ECD)
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Head Model
Spherical Head Model
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Head Model
3-spheres Head Model*
* Rush & Driscoll, IEEE-TBME, 1969
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Head Model
Realistic Geometry Head Model*
* He et al., IEEE-TBME, 1987
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Head Model
3-shell BEM RG Head Model*
* Hamalainen & Sarvas, IEEE-TBME, 1989
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Head Model
FEM RG Head Model*
* Yan et al. Med. & Biol. Eng. & Compt. 1991
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Forward Solution
Source Model (e.g. ECD) + Head Model (e.g. boundary-element):
V = C V + G
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Inverse Solution
• Model fitting– Minimize the sum of squares error between
the data and the model by adjusting model parameters (dipole location, orientation, magnitude, time-course).
• Parameter fitting routines– Levenberg_Marquardt method (Marquardt, 1963)– Simplex method (Nelder and Mead, 1965)– Simulated annealing (Kirkpatrick, 1984)– Genetic algorithms (McNay et al., 1996)
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EEG & MEG Source Localisation:The Buttons
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Components of the source reconstruction process
Source/Head Model
Source/Head Model Forward modelForward model Inverse solutionInverse solutionRegistrationRegistration
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1) Source/Head Model
• Select data file
• Select title for the analysis
• Select “MRI”
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‘Imaging’‘Imaging’‘Equivalent Current Dipoles’ (ECD)
‘Equivalent Current Dipoles’ (ECD)
• Select the size of your head model
• Select the relevant source model
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Head Model
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2) Registration
•This stage co-registers the EEG sensory positions with the head model
•Approximately matches anatomical markers between the two spaces
•Followed by a more accurate surface-matching routine.
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3) The Forward Model
• Select “Forward Model”
• This will now create a forward model based on the specifications of your source model and your head model.
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Important!
Source model
Source model
Forward model
Forward model
Inverse solution
Inverse solution
Registration
Registration
The same for all conditions.
Therefore, only done ONCE for each subject
Repeated for each condition
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4) The Inverse Solution
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The 3D reconstruction is now complete, and you are ready to perform you statistical analyses.
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Sources
• Cuffin (1998) http://ieeexplore.ieee.org/iel4/51/15468/00715495.pdf
• Slotnick, D. (2005). In T.C. Handy (Ed.), Event-related potentials: A methods handbook. MIT Press.
• Mattout, Phillips & Friston (2005) SPM course
http://www.fil.ion.ucl.ac.uk/spm/course/slides05/ppt/MEEG_inv.ppt
• Bahador Bahrami (2006)
http://www.fil.ion.ucl.ac.uk/~mgray/Presentations/Source%20localization%20for%20EEG%20and%20MEG.ppt