[Eeglablist] Conducting ICA on correct or all epochs

Baris Demiral demiral.007 at googlemail.com
Wed Dec 1 13:23:08 PST 2010

Dear David,

Thank you for your e-mail. I should mention that I am enthusiastically
following your work on the reliability of the ICA and other EEG related
issues. I also plan to use Mass Univariate ERP toolbox sometime in future.

It took me a while to test and play with the data. Well, here is what I have
done, and my evaluation:

I had max 40 trials from each condition. Participants made mean of 6
incorrect decisions per condition, so the odds of answering a question
correctly was 34/6. I analyzed the data in this order:

1-Filtered, re-referenced (bi-mastoid), epoched data goes into ICA (epochs
with very gross artifacts are removed as suggested by ADJUST algorithm
leading to around 1 or at most 2 epochs to be rejected)
2- Baseline to -200-0ms
3-If correct trials should be used, then select the correct epochs and go to
5, else go to 4
4-Use all trials
5- Use ADJUST algorithm to detect and remove problematic ICs automatically
6-Take out the incorrect trials if there are any
7-Export data for statistics

I also ran the classical method of rejecting the epochs with over 100
microvolts observed on the EOG electrodes (H1, H2 and VA2) before
baselining, leading to the elimination of mean of 6-7 epochs per participant
most probably due to the eye blinks. But, note that even though this method
is quiet common, it somehow ignores the individual EOG potential difference
strengths (some subjects might have eye blinks less than 100 microvolts

When I used all the epochs for feeding ICA, the statistical output (ran
ANOVA) was almost mirroring the statistical output I obtained from the
classical method, and the components which I was suspecting of being
artifacts disappeared. When I used the correct trials only, some earlier and
later components (mainly centro-temporal components which I believe not
artifacts) disappear/attenuated and in one case one new components appeared
mainly frontally.

My overall experience suggest that using 40 epochs per condition including
the incorrect epochs might be better IF you are only concerned about
'artifact correction' via a toolbox like ADJUST which is mainly depending on
the ICAs. Ignoring ICA reliability issues and assuming ADJUST treats the
data similarly every time, I think we need to use more trials.


On Thu, Nov 11, 2010 at 3:33 PM, David Groppe <dgroppe at cogsci.ucsd.edu>wrote:

> Hi Baris,
>   ICA's performance will generally degrade as the number of
> electrical sources increases (see
> http://www.cogsci.ucsd.edu/~dgroppe/PUBLICATIONS/GroppeCSO2008.pdf).
> The incorrect trials probably have some EEG activity not present (or
> at least less present) in the correct trials.  So if you have
> sufficient data to run ICA on just the correct trials, it would
> probably be better just to use the correct trials.  If you don't have
> enough data using just the correct trials though, you'll probably be
> fine using the all the trials, since surely a lot of the EEG activity
> is common to both sets of trials.
>         hope this helps,
>             -David Groppe
> On Wed, Nov 10, 2010 at 4:19 PM, Baris Demiral
> <demiral.007 at googlemail.com> wrote:
> > Hi everyone,
> > I am running a simple EEG experiment where I measure reaction times and
> > accuracies.
> > I want to use ICs for artifact removal, and I will report only the
> correct
> > trials (hits).
> > So would it be better to use the correct epochs for the ICA to correct
> for
> > the artifacts or is it OK to use all the epochs to detect the artifacts
> and
> > then run the artifact correction (pop_subcomp) followed by deleting the
> > incorrect epochs?
> > Thanks,
> > Baris
> >
> > --
> > SB Demiral, PhD.
> > Department of Psychology
> > 7 George Square
> > The University of Edinburgh
> > Edinburgh, EH8 9JZ
> > UK
> > Phone: +44 (0131) 6503063
> >
> > _______________________________________________
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> --
> David Groppe, Ph.D.
> dgroppe at cogsci.ucsd.edu
> http://www.cogsci.ucsd.edu/~dgroppe/

SB Demiral, PhD.
Department of Psychology
7 George Square
The University of Edinburgh
Edinburgh, EH8 9JZ
Phone: +44 (0131) 6503063
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