[Eeglablist] LPP and skin potentials

[Stephen Politzer-Ahles]

Hi Sahar,

Like Makoto, I have also sometimes used high-pass filtering to try to get
rid of skin potentials; if doing so, just be aware of the possible
consequences of high-pass filters (see Luck, 2005). Some people high-pass
filter the data for ICA decomposition and then apply those ICA weights to
the unfiltered data.

As for your concern about deleting data during artifact rejection: of
course, artifact removal always will remove data, and presumably the ERP
for every trial includes activation due to the component you are interested
in, so that data will be removed if that trial is removed during artifact
rejection. But if that trial is contaminated by artifacts then it has to be
removed anyway, unless you are confident that you can remove the artifact
from the trial while preserving the data (e.g., by using ICA or a filter).
I would not worry about accidentally removing LPPs because they look like
skin potentials, because in my experience the amplitude of these kinds of
activity is very different. Artifactual skin potentials have a very large
amplitude (it will depend on your EEG system and amplifier, but usually
they are very obvious when I view my raw data at a scale of 60 or 80
microvolts), whereas components related to cognition are much smaller (in
my experience such components are not visible to the naked eye without
averaging across a large number of trials, and even then I have to zoom
into a scale of 10 or 15 microvolts). I think it's very unlikely that
you'll "see" an LPP in your raw data and think it's a skin potential.

Hope this helps,
Steve Politzer-Ahles

On Mon, Jan 28, 2013 at 10:52 AM, Makoto Miyakoshi <mmiyakoshi at ucsd.edu>wrote:

> Dear Sahar,
>
> To control skin potential you need to control room temperature and
> moisture. I'm not sure if it is a common practice in recording EEG.
>
> Our group uses relatively high cutoff frequency for highpass filter
> (around 1 Hz) because suppressing the skin potential improves ICA
> performance.
>
> Makoto
>
> 2013/1/23 sahar azarang <saharazarang at gmail.com>:
> > Dear all,
> >
> > I have a problem with distinguishing skin potential artefacts and
> > positive/negative shift in EEG due to brain activity (such as LPP ...),
> > especially when i'm working with event related potentials. So I don't
> know
> > if I am doing correct atefact removing or i am deleting some valuabe data
> > from eeg data. is there any criterion for this?
> >
> > Thanks in advance,
> > Sahar
> >
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>
> --
> Makoto Miyakoshi
> JSPS Postdoctral Fellow for Research Abroad
> Swartz Center for Computational Neuroscience
> Institute for Neural Computation, University of California San Diego
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-- 
Stephen Politzer-Ahles
University of Kansas
Linguistics Department
http://people.ku.edu/~sjpa/
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