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

[Julie Onton]

I, for one, would definitely agree with this strategy, ie, have all electrodes referenced to the
same signal and include all channels in the ICA decomposition. I have never understood the idea of
a bipolar reference of electrodes above and below the eyes because most of the blink-related
activity will be cancelled out... At least from an ICA perspective, this is exactly the activity
you want to preserve in order to pull it out as a component and thus clean the rest of your data.
ICA can also be very sensitive to lateral eye movements if you include channels just lateral to
the eyes (referenced to the same signal as scalp electrodes).

Best, Julie

-- 
Julie Onton, PhD
Swartz Center for Computational Neuroscience
Institute for Neural Computation
University of California, San Diego
(858) 458-1927 ext 17
http://sccn.ucsd.edu/~julie

>
> 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> > Sorry for the late
> answer. Answer do come in time if you are patient.> > Best regards,> > Arno> >
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