[Eeglablist] Average Reference after measuring 128 Ch using one of the 128 channels as reference (Cz)?

Ernesto Gonzalez Trejo gonzalez at snn-unit.de
Wed Dec 2 03:11:03 PST 2015


Dear Makoto,

thank you for your answer. 

There is no problem when importing the data and setting Cz (ch 30) as reference
when i set the channel locations. But i don't know if it is valid for EEGLAB if
i assign one of my existing 128 channels as reference (by edit->channel
locations->set reference-> Channel Indices 1:128, Reference Cz).  There is no
exclusive reference channel appended at the end. Is this refrence assignment
only a label? does it matter when computing the average reference?

I can "synthesize" my question as follows: Should I remove the reference channel
from my data (select data->remove) before processing the average reference, even
if it is part of my data?

>From the 128 acquired electrodes, 63 and 64 are earlobes (so i remove them) and
30 (cz)  is used as a reference, while channel 30 itself is measured against the
ground. Unfortunately our setup has no option to use a dedicated reference
electrode, so either EEG has to be measured against ground (no reference) or one
of the 128 channels.

So in order to get an accurate average-referenced EEG, should i remove the
reference before calculating it? 

Thanks again!

Ernesto


> On December 1, 2015 at 9:17 PM Makoto Miyakoshi <mmiyakoshi at ucsd.edu> wrote:
> 
> Dear Ernesto,
> > i have a brief question
> 
> I thought it was quite long, which is why I did not understand the
> question.Let me start from here: What's the problem of importing the data and
> apply average reference?
> REST is a good solution but I've never been used it myself. Does REST has
> Matlab code? If it's available I'd love to try it out.
> Another approach is Nima Bigdely-Shamlo's PREP. It'll do conscientious channel
> cleaning before average
> referencing.http://journal.frontiersin.org/article/10.3389/fninf.2015.00016/abstract
> Makoto 
> On Thu, Nov 26, 2015 at 7:14 AM, Ernesto Gonzalez Trejo <gonzalez at snn-unit.de>
> wrote:
> > Hi everyone, 
> > 
> > i have a brief question regarding a correct average referencing in EEGLAB,
> > that until now i've been unable to solve. I've read the EEGLAB tutorial and
> > also searched for some answers in the EEGLAB list but until now i didn't
> > find one that helps me solve the problem.
> > 
> > I am doing EEG measurements with 128 Channels, using the channel in the
> > position Cz as reference(ch. 30 in my cap). During the acquisition, the
> > amplifier properties have Cz set as reference for all electrodes, and  the
> > Cz electrode is then measured then against the ground. Additionally, two
> > more channels (63-64) are on the earlobes and are usually discarded since
> > i'm not using them as reference or for data analysis.
> > 
> > Given the nature of the measurement I consider the average reference the way
> > to go in order to represent better the brain activity. 
> > 
> > The acquisition is done through SIMULINK and saved into a MAT file, for
> > post-processing in MATLAB. (The output is a 128xL matrix, where L is the
> > length of the measurement in samples).
> > 
> > The problem i have is that i can't seem to indicate correctly to EEGLAB that
> > the reference used in my measurement is also part of the channels. When i
> > import the mat file, i cannot give explicitely the channel (as stated on the
> > import window's help:
> > 
> > "Ref. channel indices or mode" - [edit box] current reference. This edit box
> >  cannot be edited. To change data reference, use menu 
> >  Tools > Re-reference calling function pop_reref(). The reference 
> >  can be a string, 'common' indicating an unknow common reference, 
> >  'averef' indicating average reference, or an array of integer 
> >  containing the indices of the reference channels."
> > 
> > Then, after I import the data without a reference, I select the channel
> > locations (using a self-made loc file for the cap i use). Here, each one of
> > the 128 channels has a label (10-10 system names for electrodes 1-64 and
> > just the electrode number for 65-128). Once loaded, i click on "Set
> > Reference" and write 1:128 as channel indices and Cz as "Reference".
> > (Channel 30 has the label Cz and was the reference in the measurement).
> > After the locations are set, i remove channels 63 and 64, since i don't want
> > to use them for AR.
> > 
> > So far, so good. After this i can just go and compute the average reference
> > using the tools from EEGLAB. I wanted to validate the results so i kept
> > doing some tests, and i realized that if i clicked on tools-rereference and
> > then to "add current reference channel back to the data", the error :
> > 
> > "There are no Reference channel defined, add it using the channel location
> > editor"
> > 
> > appeared, although in the main EEGLAB window the Reference is stated to be
> > Cz and Channel locations as "Yes".
> > 
> > Question 1:
> > 
> > Could it be that i am giving the Reference wrong in the channel locations?
> > After reading the tutorials i only got that the reference is usually given
> > as an extra channel, which is appended to the locations. In this case, it is
> > not an extra channel, but channel 30 of 128. Is it wrong then, to input this
> > channel as i did in the channel locations?
> > 
> > Question 2:
> > 
> > Since i wanted to see how exactly the average referencing was working, I
> > checked also the reref.m function from EEGLAB and tried to run it standalone
> > as follows:
> > 
> > output=reref(eeg,30,'exclude',[63 64],'keepref','on');,
> > 
> > where i am only interesed on the average-referenced EEG (output) and i give
> > a 128xL matrix (my EEG measurement), I indicate my reference was the channel
> > (row) 30, and i exclude the earlobes (63 64). 
> > 
> > The output is however very different to the one i obtain with the method
> > explained above in EEGLAB.
> > 
> > In order to obtain the (mostly) same result as in EEGLAB i have to input the
> > following:
> > 
> > output=reref(eeg,30,'exclude',[30 63 64],'keepref','on');
> > 
> > So, what exactly am i doing wrong here? I am probably specifying the
> > reference in a wrong way. How should one manage such a reference system
> > within EEGLAB or in the reref function? As i understand the calculation of
> > average referencing, i should NOT use the values of the reference to
> > calculate the mean of all electrodes, which would be afterwards subtracted
> > from each one. And if i wanted to see the signal of Cz post-AR, i would have
> > it by inverting the mean of all electrodes. If so, then the right way to go
> > here would be to exclude also the channel 30 in the reref function. Is that
> > right?
> > 
> > I am using currently EEGLAB v13.4.4b on MATLAB R2013a
> > 
> > Thanks in advance!
> > 
> > EG
> > 
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> -- 
> Makoto Miyakoshi
> Swartz Center for Computational Neuroscience
> Institute for Neural Computation, University of California San Diego


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