[FieldTrip] EEG reference

[Stephen Politzer-Ahles]

Eduardo,

Steve Luck's (2005) book *An Introduction to the Event-Related Potentials
Method* has a chapter discussing reference methods. If you search the
archives of the EEGLAB mailing list there are also many recent discussions
(including references) about this issue.

I am not personally experienced with spectral analyses, but in general, the
online reference you use during the recording does not matter very much,
since you can re-calculate the data to any other reference offline. For
example, in both labs where I have done experiments, we use various online
references but always re-reference to the average of both mastoids during
the offline analysis; this is a common procedure, if you want to do that
all you need to do is make sure to buy a cap that includes mastoid
electrodes. Another common procedure is to use the average reference (see
Luck (2005) or the EEGLAB online wiki for detailed discussion about average
reference); for that, the most important thing is for the cap to have
sufficient electrodes, evenly covering enough of the head. Of course, there
are also many other referencing procedures available.

Best,
Steve Politzer-Ahles


>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Mon, 25 Jun 2012 12:09:35 -0300
> From: Eduardo Schenberg <eduardoschenberg at gmail.com>
> To: fieldtrip at science.ru.nl
> Subject: [FieldTrip] EEG reference
> Message-ID: <7D53A322-3BF0-46F4-8D4B-B72E5B973EEC at gmail.com>
> Content-Type: text/plain; charset=us-ascii
>
> Hello all,
>
> I am new to this list, as I just started ding EEG experiments. I did some
> pilot tests with a BrainProducts/Brain Vision with an Acticap and I am
> doing some preliminary analysis using EEGLab/fieldtrip. I am now planning
> to buy my own EEG form Brain Products, but I am bothered by the fact that
> in most cap configs they sell, the reference is near Cz, usually FCz. As
> far as my knowledge goes, if I want to study power spectrum alterations and
> coherence in different frequencies between different electrodes, a
> reference in the head can be a problem, specially if some of the regions of
> interest are synchronous with the reference itself in the given frequency
>
> Can anyone point me to some good review or chapter about these issues so I
> can study them better and buy the best config available?
>
> Many many thanks for all
>
> eduardo schenberg
>
>
>
>
>
> ------------------------------
>
> Message: 2
> Date: Tue, 26 Jun 2012 11:31:32 +0200
> From: Marcel Heers <marcel.heers at googlemail.com>
> To: fieldtrip at donders.ru.nl
> Subject: [FieldTrip] Source localization using DICS for EEG data from
>        combined MEG/EEG recordings
> Message-ID:
>        <CAMq9WnPig4WAhsiM5ogwnXrPcPWj34TyWv8JuTAus5f=xNecrA at mail.gmail.com
> >
> Content-Type: text/plain; charset=ISO-8859-1
>
> Dear all,
>
> I am trying to perform source localization in the frequency domain
> from EEG data using DICS. The data were recorded combined with MEG
> (Neuromag data format).  The volume conductor is a 3 shell realistic
> head model created with bemcp.
> When running DICS with the following settings
>
> cfg                   = [];
> cfg.method       = 'dics';
> cfg.grad            = elec;
> cfg.frequency    = 14;
> cfg.vol              = vol_eeg;
> cfg.dics.projectnoise = 'yes';
> cfg.dics.lambda       = 5;
> source = ft_sourceanalysis(cfg, freq);
>
>
> I am am getting the following error message:
>
> ??? Error using ==> svd
> Input to SVD must not contain NaN or Inf.
>
> Error in ==> beamformer_dics>pinv at 568
>  [U,S,V] = svd(A,0);
>
> Error in ==> beamformer_dics at 314
>        filt = pinv(lf' * invCf * lf) * lf' * invCf;              %
> Gross eqn. 3, use
>        PINV/SVD to cover rank deficient leadfield
>
> Error in ==> ft_sourceanalysis at 584
>      dip(i) = beamformer_dics(grid, sens, vol, [],
> squeeze(Cf(i,:,:)), optarg{:});
>
>
> and I am not sure what might be the cause.
>
> Maybe anyone can help me! Thank you in advance!
>
> Marcel
>
>
> ------------------------------
>
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> End of fieldtrip Digest, Vol 19, Issue 42
> *****************************************
>



-- 
Stephen Politzer-Ahles
University of Kansas
Linguistics Department
http://www.linguistics.ku.edu/
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