Re-referencing and MMN/N400

[N.K.]

Dear Coen!

I've read a as much as I found on CMS and DRL and re-referencing, but I'm still a little confused. Maybe you'd have a concrete advice for my two problems.

I recorded one data-set with a MMN-paradigma and wanted to use the nose (EXG5) as the reference. I realized that the left-side options are for online-view only and are not recorded that way.
For data analysis, I converted the .bdf-data over eeg-lab to .dat-files to import them into VisionAnalyzer.
In VisionAnalyzer, I could now simply put "new reference - EXG5". Would my data then be as I need it to be, or is there something i overlooked? / how do I have to involve that CMS/DRL-loop into my analysis?

Previously, I recorded another data-set with a typical N400-paradigma. there we wanted to use the average as the reference.
After data-analysis, we couldn't find the expected N400 and are currently rather clueless. I was wandering, if it had anything to do with the re-referencing I might not have fully understood by now...?

Thanks a lot for your help!

[Coen]

In general, you can check your referencing step because the reference trace should be flat after referencing (everything zero, no noise). In your example, the EXG5 trace should be flat after referencing all channels to EXG5. The CMS/DRL loop cancels out after referencing

In general, the reference choice affects the waveforms. So it is certainly possible to decrease expected responses by choosing the wrong reference.

Best regards, Coen (BioSemi)

[N.K.]

Thanks a lot for your answer!

As I struggled doing the ICA in VisionAnalyzer, i finished the analysis in eeglab. the problem there is that eeglab doesn't recognise my EXG5-coordinates.

Are there official coordinates for the EXG5-Electrode when it's used as nose-reference?

I tried to reference to joined Mastoids instead and Common Average. but i would prefere a nose-reference as i'm doing an MMN paradigma.

thanks a lot for your help!

[Coen]

I don't know an "official" coordinate. However, the coordinates of the nose can be quite easily extrapolated from the existing ones. The azimuth is surely 90 degrees (same as Fpz, Fz, etc), the inclination should be something around 138 degrees (Fpz is on 92, arc length from nose to Fpz is approx equal to the arc length from Fpz to Fz, 92+(92-46) = 138).

Best regards, Coen (BioSemi)