1. Source Localization for EEG/MEG Stavroula Kousta Martin Chadwick Methods for Dummies 2007

2. 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? EEGMEG

3. Forward Solution Model Head Brain Skull Scalp -4 μV -5 μV Model Data 0 μV -1 μV -2 μV -3 μV 1 μV 2 μV 3 μV 4 μV 5 μV + Location Orientation Amplitude

4. Inverse problem -5 μV -4 μV Recorded Data 0 μV -1 μV -2 μV -3 μV 1 μV 2 μV 3 μV 4 μV 5 μV Desired Model Solution Brain Skull Scalp ? ? ?? ? ILL-DEFINED!!!

5. + -

6. + -

7. + - + - Any field potential vector could be consistent with an infinite number of possible dipoles

8. Specify initial dipole parameters in head model Generate scalp potentials/fields using forward solution (V model ) Calculate SSE Σ (V model –V data ) 2 Δ SSE<ε Accept dipole parameters Adjust Parameters YesYes No Inverse Solution Slotnick, 2005

9. Source Model Initial Parameters ‘Imaging’ ‘Equivalent Current Dipoles’ (ECD)

10. Head Model Spherical Head Model

11. Head Model 3-spheres Head Model* * Rush & Driscoll, IEEE-TBME, 1969

12. Head Model Realistic Geometry Head Model* * He et al., IEEE-TBME, 1987

13. Head Model 3-shell BEM RG Head Model* * Hamalainen & Sarvas, IEEE-TBME, 1989

14. Head Model FEM RG Head Model* * Yan et al. Med. & Biol. Eng. & Compt. 1991

15. Forward Solution Source Model (e.g. ECD) + Head Model (e.g. boundary-element): V = C V + G

16. 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)

17. EEG & MEG Source Localisation: The Buttons

18. Components of the source reconstruction process Source/Head Model Forward model Inverse solution Registration

19. 1) Source/Head Model Select data file Select title for the analysis Select “MRI”

20. ‘Imaging’ ‘Equivalent Current Dipoles’ (ECD) Select the size of your head model Select the relevant source model

21. Head Model

22. 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.

24. 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.

25. Important! Source model Forward model Inverse solution Registra tion The same for all conditions. Therefore, only done ONCE for each subject Repeated for each condition

26. 4) The Inverse Solution

27. The 3D reconstruction is now complete, and you are ready to perform you statistical analyses.

28. Sources Cuffin (1998) http://ieeexplore.ieee.org/iel4/51/15468/00715495.pdf 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/MEE G_inv.ppt Bahador Bahrami (2006) http://www.fil.ion.ucl.ac.uk/~mgray/Presentations/Source %20localization%20for%20EEG%20and%20MEG.ppt