[Eeglablist] [Fwd: Eye Movement Artifacts and Filtering]

[Scott Makeig]

ICA can decompose common reference and bipolar channel pairs simultaneously,
since the sources sum linearly at all these channels proportional to the
difference in source projection strengths to the 'active' and 'reference'
channels. However, plotting the scalp maps of ICA components including both
types of channels is not apt to be wholly accurate, since the baselines for
the two channel types are different.However, iIt should be possible to
reconstruct the full component scalp maps (at least for 'dipolar'
components) using a forward (BEM) head model... In the absence of such a
procedure, one can always remove the bipolar channels *after* running ICA
decomposition, using the EEGLAB 'remove channels' function. This will allow
plotting the  component maps based on the (common reference) montage.

Scott Makeig

On Sun, Apr 20, 2008 at 3:45 PM, Nicolas Robitaille <
nicolas.robitaille at umontreal.ca> wrote:

>  Dear Arno,
>
> Just a related idea:
>
> Would it be a better practice to simply include the EOG channels as if
> they were just others EEG electrodes (if they were all recorded with say a
> left-earlobe reference, as the others EEG channels, or reference-free using
> a Biosemi system) instead of bipolar channels, when possible? In my mind,
> this would help ICA to isolate these signals (which is not always trivial
> for horizontal eye movements, which are smaller and can be less frequent
> than eye blinks), according that at least some channels would have clear
> eye-related signals. This would also avoid the difference in referencing
> methods across channels you mentionned.
>
> Nicolas
>
> > Date: Sun, 20 Apr 2008 23:13:52 +0200
> > From: arno at ucsd.edu
> > To: modestino at ccs.fau.edu
> > CC: eeglablist at sccn.ucsd.edu
> > Subject: Re: [Eeglablist] [Fwd: Eye Movement Artifacts and Filtering]
>
> >
> > Dear Justin,
> >
> > > I am planning to use ICA in EEGLAB to remove eye movement artifacts. I
> > > know that ICA does not use EOG channels locate the component
> associated
> > > with eye movements. However, my advisor will want me to show ERP
> averages
> > > of the EOG channels as evidence that the eye movements have been
> removed
> > > using ICA. So, I will be including the EOG channels (bipolar, but
> > > represented as individual difference waves as one channel, for HEO and
> > > VEO).
> > >
> >
> > It is OK to include bipolar channels for that purpose. ICA should be
> > able to detect the difference in reference (although you might have to
> > search for the component that accounts for that). Then you may compute
> > ERP for your EOG channels. A better idea though would be to do that for
> > frontal channels such as FPz.
> >
> > > Finally, I need to run an acausal FIR high pass filter at 1 Hz. [which
> > > will not change the temporal components of the EEG] on the data before
> > > running ICA, as there is an extreme trend in the data that made
> automated
> > > and even manual eye movement removal inadequate using the raw data
> outside
> > > of EEGLAB. Also, the trend needs to be removed anyway. However,
> removing
> > > the linear trend will not adequately alter the tend in the continuous
> data
> > > as the FIR filter will. So, I wish to use such a filter. [I may remove
> > > the linear trend once the data is epoched per condition, but not on
> the
> > > continuous data.] What sort of filters does EEGLAB have built in? Is
> this
> > > filter already available?
> > >
> >
> > Yes, there are 2 types of filters available in EEGLAB by default. I
> > would advise to use the non-linear IIR filter which has better design
> > than the linear FIR filter (and does not introduce phase distortion
> > because it is applied in both directions). There is also an additional
> > plugin that allows more advanced filtering.
> >
> >
> http://www.uni-leipzig.de/~biocog/content/widmann/eeglab-plugins/#firfilt<http://www.uni-leipzig.de/%7Ebiocog/content/widmann/eeglab-plugins/#firfilt>
> >
> > Sorry for the late answer. Answer do come in time if you are patient.
> >
> > Best regards,
> >
> > Arno
> >
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-- 
Scott Makeig, Research Scientist and Director, Swartz Center for
Computational Neuroscience, Institute for Neural Computation, University of
California San Diego, La Jolla CA 92093-0961, http://sccn.ucsd.edu/~scott
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