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Source Localization for EEG/MEG

Source Localization for EEG/MEG. Stavroula Kousta Martin 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. Model Head. Model Data.

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Source Localization for EEG/MEG

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  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? EEG MEG

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

  4. Desired Model Solution Recorded Data ? ? ? ? ? -5 μV 5 μV -4 μV 4 μV -3 μV 3 μV -2 μV 2 μV -1 μV 1 μV 0 μV Brain Skull Scalp Inverse problem 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 (Vmodel) Calculate SSE Σ (Vmodel –Vdata)2 Accept dipole parameters ΔSSE<ε Adjust Parameters Inverse Solution Yes No Slotnick, 2005

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

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

  23. 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.

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

  25. 4) The Inverse Solution

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

  27. 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

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