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

Inverse problem: practical problem of head models Displace the signalSmear the signal

Inverse problem: practical problem of head models

Working at the surface (for now)

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

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

1. Peaks absent in linked mastoids

Peaks absent in linked mastoids

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2. Peaks absent in average reference

Peaks absent in average reference

3. Translocated peaks

Translocated peaks

4. Spectral shift

Spectral shift

5. Relative amplitude

(relative) amplitude

6. Degenerated re-references

Degenerated re-references Bad average reference Bad mastoids reference

Conclusion 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. 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)

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. Initial positioning of the 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.Re-reference