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
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
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
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!!!
+
-
+ -
+-
+ -
Any field potential vector could be consistent with an infinite number of possible dipoles
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
Source Model Initial Parameters
‘Imaging’‘Imaging’‘Equivalent Current Dipoles’ (ECD)
‘Equivalent Current Dipoles’ (ECD)
Head Model
Spherical Head Model
Head Model
3-spheres Head Model*
* Rush & Driscoll, IEEE-TBME, 1969
Head Model
Realistic Geometry Head Model*
* He et al., IEEE-TBME, 1987
Head Model
3-shell BEM RG Head Model*
* Hamalainen & Sarvas, IEEE-TBME, 1989
Head Model
FEM RG Head Model*
* Yan et al. Med. & Biol. Eng. & Compt. 1991
Forward Solution
Source Model (e.g. ECD) + Head Model (e.g. boundary-element):
V = C V + G
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)
EEG & MEG Source Localisation:The Buttons
Components of the source reconstruction process
Source/Head Model
Source/Head Model Forward modelForward model Inverse solutionInverse solutionRegistrationRegistration
1) Source/Head Model
• Select data file
• Select title for the analysis
• Select “MRI”
‘Imaging’‘Imaging’‘Equivalent Current Dipoles’ (ECD)
‘Equivalent Current Dipoles’ (ECD)
• Select the size of your head model
• Select the relevant source model
Head Model
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.
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.
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
4) The Inverse Solution
The 3D reconstruction is now complete, and you are ready to perform you statistical analyses.
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