1 / 25

Source localization

Source localization. Sensor space. Signal at sensors. Surface potentials. Positive & negative potentials. Always with respect to the reference. Current source density (CSD). CSD = -1 * scalp conductivity * Laplacian of scalp potential. Current source density (CSD).

maille
Télécharger la présentation

Source localization

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Source localization

  2. Sensor space

  3. Signal at sensors

  4. Surface potentials Positive & negative potentials Always with respect to the reference

  5. Current source density (CSD) CSD = -1 * scalp conductivity * Laplacian of scalp potential

  6. Current source density (CSD) 'scalp current density‘ Divergence of the current density in the scalp. The rate of change of current flowing into and through the scalp.

  7. Current source density (CSD) CSDs are three dimensional vectors (direction and amplitude) They are a function of spatial voltage differences across electrodes

  8. Dipoles Neural activity (inside the brain) is modeled by electric dipole (created by the movement of electrically charged ions).

  9. + - + - Source localization Which neurons (dipoles) are creating the signal at the scalp? Locate the dipoles creating the potential at the scalp…

  10. + - Inverse solution CSD measurement at time x Dipole location and strength at time x

  11. + - + - How do we know which one is correct? We can’t. There is no correct answer. Source localization is an ILL-DEFINED PROBLEM We can only see which one is better Can we find the best answer? Only among the alternatives that you have considered.

  12. + - Forward solution EEG measurement at time x Dipole location and strength at time x

  13. Build a forward model Individual anatomy (gyri and sulci) Conduction/Resistance boundaries (electricity travels differently across different mediums: skull, CSF) Electrode locations and reference

  14. Examples of expected potentials

  15. If then If then If then If Simulate many dipole sources then And on and on and on and …

  16. Find the simulation with the best fit Forward Model Experimental DATA Model with multiple dipoles, not just two…

  17. error iteration HUNTING for best possible solution Forward DATA Inverse Solution Iterative Process Until solution stops getting better (error stabilises)

  18. Mathematical definition This problem is ill posed because there are many more dipoles than electrodes (P >> N). Remember that each dipole is an x,y,z vector…

  19. Mathematical definition Different models use different numbers of dipoles (from one to many) and put different constraints on the solution.

  20. Different solutions - BESA Assumes sparse predetermined (fixed) dipoles

  21. Different solutions - LORETA Estimates the direction and amplitude of many (thousands) dipoles throughout the brain using a smoothness constraint.

  22. Different solutions - Beamforming Estimates the behavior of multiple dipoles using a set of “spatial filters”. y(t) = WTm(t) Where WT is a spatial filter applied to the electrode measures m(t) to compute the dipole activity y(t). Many hundreds of simultaneous dipoles can be estimated.

  23. Different approach Computing ICA components Independent spatio-temporal components This approach keeps the data in sensory space, but re-structures (transforms) it into parts with “maximal statistical independence”

  24. Open Matlab!

  25. Before and after CSD analysis Using the EEGLAB plugin for computing CSD Note that the top row contains potentials µV and the bottom row contains CSD units µV/m² CSD measures are more focal than potential measures.

More Related