[Eeglablist] imported BrainVision data: finding the reference electrode

Wed Sep 26 06:34:46 PDT 2012

Hi Ricardo,

the reference of the recording is implicit in all EEG systems I know. The potential difference of the reference electrode site to itself is always zero, thus, it does not make to much sense to store this as a separate signal channel.

As the implicit reference is zero re-referencing to linked/averaged mastoids is trivial on the command line:
EEG.data = EEG.data - repmat(EEG.data(<rightmastoid_channum>, :) * 0.5, [EEG.nbchan 1]);
or 
EEG.data = EEG.data - repmat(EEG.data(<rightmastoid_channum>, :, :) * 0.5, [EEG.nbchan 1 1]);
for epoched data. Do not forget to store back into ALLEEG.

Using the GUI it is somewhat more complicated:
(1) Add an empty reference channel LM using the channel location tool with "Append chan" button. Set the reference of the other channels to the new channel LM with the "Set reference" button.
(2) Re-reference to any channel ticking "Retain old reference channels in data" and selecting your new appended channel LM in "Add current reference channel back to the data". This will result in a dataset with the channel you re-referenced to being zero and your new channel LM showing the inverted signal of this channel.
(3) Now you can select both mastoids in the re-reference tool.

However, I would like to note that re-referencing to linked channels is not necessarily a recommended procedure (cf. e.g. Dien, J., 1998, BRMIC)

Hope this helps. Best,
Andreas

Am 25.09.2012 um 05:13 schrieb Stephen Politzer-Ahles <politzerahless at gmail.com>:

> Hi Ricardo,
> 
> That's strange, it does indeed sound like it referenced to just that channel. I'll have to go back and double-check how I did my data at the time.
> 
> It's definitely possible to reference the data in Analyzer and then import it. After referencing the data in Analyzer, you can use the Generic Data Export tool there to export the header, marker, and eeg files in .dat format, and then use pop_loadbv() in EEGLAB to import them. 
> 
> Best,
> Steve
> 
> On Mon, Sep 24, 2012 at 10:09 PM, Ricardo Moura <ricardoojm at gmail.com> wrote:
> Hi Steve,
> 
> I have just done what you said, and when I plot the data, the signal in the right mastoid electrode is turned to a flat line. It seems that all of the data was re-referenced only to this channel. If I try "add current reference channel to the data", I get the error message again (there are no reference channels)...
> 
> Do you know if there is a way to export the data already re-referenced from the BrainVision analyzer? Now I am trying the analysis using the raw data.
> 
> Thank you very much once again.
> 
> Best,
> Ricardo
> 
> On 24 September 2012 23:59, Stephen Politzer-Ahles <politzerahless at gmail.com> wrote:
> Hi Ricardo,
> 
> If I'm understanding correctly, you should be able to get the average mastoid by choosing right mastoid as your reference and selecting "retain old reference channels". This should average together the right mastoid with the current reference (the left mastoid) and compute a new reference based on that average. 
> 
> Best,
> Steve
> 
> 
> On Mon, Sep 24, 2012 at 9:39 PM, Ricardo Moura <ricardoojm at gmail.com> wrote:
> Hi,
> Thanks for your answer.
> The reference channels were recorded at the left (recording reference) and right (recorded as a normal eeg channel) mastoids. The right reference is available when I open the data in EEG lab, but not the recording reference. The reference channel always appear as "unknown" at the main eeglab window, and when I go to the "channel locations", it only shows me 64 electrodes (the 65th electrode, which would be the recording reference, is missing). This is a problem because I need this channel in order to re-reference the data to linked-ears (don't I?). But I don't need to use it as an active electrode. 
> 
> So when I try the  "Add current reference channel back into the data" , I allways have the following message: "there are no reference channel defined, add it using the channel locations".
> 
> 
> Any idea about how to recover this channel (in case I really need it to perform the linked-ears reference)?
> 
> Best,
> Ricardo
> 
> 
> On 24 September 2012 14:50, Stephen Politzer-Ahles <politzerahless at gmail.com> wrote:
> Hi Ricardo,
> 
> May I ask what your reference was? In my experience using Brain Vision data, the recording reference (in my case it was on the nose) was just not included in the data that I imported to EEGLAB. This wasn't a problem for me because I wasn't interested in analyzing that electrode anyway after I re-referenced; but it might matter for you if your reference is something that you care about as an active electrode, like Cz. You may be able to get that channel back by using the "Add current reference channel back into the data" command and specifying its location (but I haven't tried that myself with BV).
> 
> Best,
> Steve
> 
> On Mon, Sep 24, 2012 at 8:55 AM, Ricardo Moura <ricardoojm at gmail.com> wrote:
> Dear all,
> 
> I am new to the EEGLab. Now I am trying to analyze data that were recorded using the BrainVision recorder, but I never find the reference electrode. Going to "Channel Locations" only shows me the EEG electrodes. It is a problem for me because I want to re-reference the eeg signal to the linked-ears. The second reference electrode is always there (once it was recorded as a regular eeg electrode), but not the actual reference.
> 
> I would appreciate any help,
> Best,
> Ricardo Moura
> 
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> 
> -- 
> Stephen Politzer-Ahles
> University of Kansas
> Linguistics Department
> http://people.ku.edu/~sjpa/
> 
> 
> 
> 
> -- 
> Stephen Politzer-Ahles
> University of Kansas
> Linguistics Department
> http://people.ku.edu/~sjpa/
> 
> 
> 
> 
> -- 
> Stephen Politzer-Ahles
> University of Kansas
> Linguistics Department
> http://people.ku.edu/~sjpa/
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