[Mne_analysis] EEG reference

[Christopher Bailey]

Hi Mengting,

So as I understand, reference is only a required step for source localization,

The _average_ reference is required for source localisation

but not any other steps like measuring the connectivity between brain regions.

I’m not sure I understand what you mean by “brain regions” here. If you’re going to measure connectivity in source space, your need an (inverse) operator of some sort to project your electrode-data “into the brain”.

If you want to do sensor-level connectivity calculations on EEG data, the effect of the reference might depend on the connectivity metric you use. For those time/frequency-domain measures I’m aware of (but have little first-hand experience with), it doesn’t matter which reference your data is in.

Sounds reference removed the DC componets but remains the oscillation. If so, does it influence the power in DC components?

Yes and no. Relative differences within and between channels remain the same in any (proper) reference. Note though that the time-domain is not involved in (re-)referencing, so possible ‘DC components’ still remain in the data.

 Also, does it matter the conventional source localization methods such as dipole fitting?

All localisation methods require the average reference, otherwise the recorded and predicted (forward model) data cannot be compared.

Best,

Chris

On 27 Sep 2017, at 20.54, Liu Mengting <bigting84 at gmail.com<mailto:bigting84 at gmail.com>> wrote:

Hi Chris,

Thanks for the info, this really helps in understanding the whole procedures. So as I understand, reference is only a required step for source localization, but not any other steps like measuring the connectivity between brain regions. Sounds reference removed the DC componets but remains the oscillation. If so, does it influence the power in DC components? Also, does it matter the conventional source localization methods such as dipole fitting?

Really appreciate for help,

Mengting

2017-09-25 4:55 GMT-04:00 Christopher Bailey <cjb at cfin.au.dk<mailto:cjb at cfin.au.dk>>:
Hi Mengting,

The average reference is the only valid scheme when performing ‘source localisation’, no matter what you do afterwards. To first order, the localisation procedure is

- have measured (EEG) data
- build forward model that computes sensor-level readings for know sources
- compare measured and predicted (forward-projected) data to each other
- minimise prediction error under chosen prior/model

The predicted data are calculated relative to a hypothetical absolute reference potential of zero at infinity, whereas a real dataset could be referenced to a number of points on the head. To be able to compare the two datasets, both are re-referenced to their respective average: the average reference does not depend on the location of the on-line reference electrode (as long as it was functioning properly). After this re-scaling, the values can be compared directly.

Note that like any (proper) re-referencing procedure, taking the common average only shifts the zero-point; relative differences between electrode readings remain unaltered.

/Chris

On 24 Sep 2017, at 01.28, Liu Mengting <bigting84 at gmail.com<mailto:bigting84 at gmail.com>> wrote:

Hello MNE users,

I noticed that in MNE inverse operations, all EEG data were forced by an average referencing.
Does anyone has insight about how does the average reference influence the inverse operations in MNE (I mean use dSPM or sLoreta)? Especially would this average referencing influence functional connectivity measure in source space (e.g. using phase locking)?

Thanks,
Mengting
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