[Eeglablist] reference during import of EMG data acquired with Biosemi

[Andrew Hill]

Hi Sebastian,

Biosemi isn't exactly reference-free, right?  e.g the combination of CMS/DRL channel creates a reference, which is why electrodes close to that location have lower amplitude than ones more distant when viewed "raw" on montage setting in the acquisition software.

But the answer to your question - it doesn't really matter, I don't think.  The way I understand it, a reference channel is needed on import to avoid losing the huge DC-offset potential in the signal, if it's there (I tend to use the BDF->EDF Converter program first, to drop file sizes and avoid using an aggressive 1hz filter against the BDF offset, which might smear ERPs).  

So any channel will do.  I tend to pick Pz because it's rather near the midpoint of the CMS/DRLs, and thus looks pretty close to the "raw" as I recorded it.

Best,
Andrew


On Sep 19, 2011, at 7:57 AM, seba wrote:

> Dear eeglab specialists,
> 
> I hope you can help me on this one:
> 
> I have recorded facial electromyography (EMG) with a Biosemi system, and want to analyze it in EEGLAB/matlab.
> I am not sure about the question of the reference during importing.
> 
> I know that when first importing EEG data into EEGLAB, which was recorded reference-free with with a Biosemi system (www.biosemi.com), one should indicate whatever channel as a reference, in order to improve the signal-to-noise ratio.
> 
> However, it is unclear to me what I should do when importing into EEGLAB EMG data recorded with the same system.
> Should I indicate one electrode as a reference? And after having done so, should I rereference the data? to what?
> Or is the EMG signal so strong that one can afford to keep some noise in the data and therefore does not need to indicate a reference during importing?
> Would this also be true if I am interested in tiny EMG differences between conditions, as usual in facial mimicry paradigms?
> 
> Thank you very much for your help.
> 
> Best,
> Sebastian
> 
> 
> 
> ----------------------------------------------------------------------
> Sebastian Korb, Ph.D.
> Swiss Center for Affective Sciences
> University of Geneva, Switzerland
> Tel.: +41223799812
> Email: Sebastian.Korb at unige.ch
> Webpage: http://www.affective-sciences.org/user/34
> 
> 
> 
> 
> 
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