Inverse problem, raw EEG and re-reference

1. Dr. E. Tognoli, 25 September 2008 HBBL meeting Inverse problem, raw EEG and re-reference From below the surface

2. Inverse problem: practical problem of head models Smear the signal Displace the signal

3. Inverse problem: practical problem of head models

4. Inverse problem: practical problem of head models

5. Working at the surface (for now)

7. Why reference: sense the noise • Principle of differential amplification: the CMR • (Signal + noise) – (noise) • Take a scalp electrode (say C3) and a fixed point (GND) • Measure one potential difference • Take a reference electrode (say MA1) and a fixed point • Measure a second potential difference • (Signal + noise) – (noise) = “a very clean” signal

8. COMPARISON OF SPECTRAL ANALYSIS ACROSS A RANGE OF REFERENCES In most circumstances, topography, frequency and amplitude are preserved across different references/re-references However…

9. 1. Peaks absent in linked mastoids

10. Peaks absent in linked mastoids

11. Peaks absent in linked mastoids

12. Peaks absent in linked mastoids

13. Peaks absent in linked mastoids

14. Peaks absent in linked mastoids *

15. 2. Peaks absent in average reference

16. Peaks absent in average reference

17. Peaks absent in average reference

18. 3. Translocated peaks

19. Translocated peaks

20. Translocated peaks

21. Translocated peaks

22. 4. Spectral shift

23. Spectral shift

24. 5. Relative amplitude

25. (relative) amplitude

26. 6. Degenerated re-references

27. Degenerated re-references Bad average reference Bad mastoids reference

28. Conclusion Mastoid (especially re-reference) Masking: alters temporal (peripheral) components. (Note however that EGI net does not support mastoid attenuation: electrodes are not exactly positioned over the mastoid bone) CZ or average reference Masking: alter vertex components. Splitting: tendency for competition of power: two peaks cannot dominate at a single frequency. Sparseness: The amplitude distribution of peaks is sparser on top, accordingly more difficult to separate from noise when amplitude is small. Of peaks and troughs (~): average reference is much better than other techniques to see power depression. Overall: However, in this study, less frequent missed peaks in average reference than in linked mastoid re-reference. Who is right? Some of the peaks from one montage may be spurious, always check supra- and sub-harmonics, organization, and consistency across conditions and temporal windows.

29. Initial positioning of the reference No single good recipe, depends on the study. Yet it is paramount to get the initial reference right. Anatomical wisdom: choose your reference electrode in an intelligent fashion with respect to your particular studies, and the anatomical structures it is expected to recruit. For instance, it does not make a lot of sense to study the motor cortex and record with a Cz ref or linked mastoids if you expect temporal (e.g. auditory) activity. Empiricism: In pilots, try two or three references. Symmetry: if you want to study hemispheric lateralization, you will need suitable bilateral or medial reference . Impedance matching: if you choose a pair of electrodes as reference, it is important to get them at a low and matched impedance. Ideally, the EEG amplifier should offer digitally linked reference leads.

30. Re-reference Most components will be robust to re-referencing, exhibiting minimal spatial shift and minimal amplitude, frequency… alteration. The examples I presented today were extreme cases that occur less frequently, but a serious investigator needs to keep an eye on those facetious spatial biases of the raw EEG. In the exploratory phase of your analysis, always try at least one complementary re-referencing to unmask hidden components from the vicinity of the reference electrode.