[Eeglablist] channel location file

David Groppe dgroppe at cogsci.ucsd.edu
Wed Aug 25 22:43:41 PDT 2010


Hi Michael & Lazlo,
    Because there are more EEG sources than EEG sensors, it is
impossible for ICA (or any other linear spatial filter) to perfectly
filter out EEG artifacts.  Thus just using ICA to correct for
artifacts will distort the data to some extent.  See the following for
the explanation:

Groppe, D.M., Makeig, S., &  Kutas, M. (2008) Independent component
analysis of event-related potentials. Cognitive Science Online, 6.1,
pp. 1-44. http://cogsci-online.ucsd.edu/6/6-1.pdf

That being said, the amount of distortion may be negligible.  The most
careful study of this I know of is a recent paper by Mognon et al. in
Psychophysiology:
http://onlinelibrary.wiley.com/doi/10.1111/j.1469-8986.2010.01061.x/abstract

They found that the distortion caused by ICA artifact correction
ranged from around 75% to 25% the magnitude of the ERP components of
interest.  Personally, I've used ICA artifact correction for several
years and have found that my main ERP effects of interest (e.g., N170,
P300, N400, P600, auditory N1) look and behave as they should.  I
would be a bit nervous though about using any type of artifact
correction to study frontal ERPs unless the components were rather
robust (like the auditory N1's projection to frontal channels).
    hope this helps,
       -David

David M. Groppe, Ph.D.
Department of Cognitive Science, 0515
University of California, San Diego
9500 Gilman Dr.
La Jolla, CA 92093-0515
dgroppe at cogsci.ucsd.edu



On Mon, Jul 12, 2010 at 6:43 AM, Laszlo Balazs <balazs at cogpsyphy.hu> wrote:
> Hello Michael,
> To your second question: We have just started to try ICA based EOG
> correction and I have the same concern about throwing out some baby with
> the bath water. Once I have tested the regression based method provided
> by Neuroscan. I chose a few subjects with frequent blinks but still high
> enough number of clean sweeps. Then I compared uncorrected and corrected
> sweeps by visual inspection and also the averaged ERP to targets in a
> visual oddball (blink free epochs only).  I have seen marked decrease in
> both EEG and ERP amplitude over Fp1/2. We plan to repeat this test soon
> including ICA and hope it will outperform the regression method. I
> wonder if there are papers or other ideas for validation tests around?
> Kind regards,
> Laszlo
>
>> Dear all,
>>
>> are there any channel location files for the Neuroscan QuickCap40 (40
>> channels) available? On the Neuroscan ftp I only found "dave.zip" which
>> contains 32 and 64 channels + I get an error when I read those in, anyway.
>>
>> And a second question:
>> I'm playing around with ICA to reject ocular artifacts from continuous
>> data. Up to now I could find only few papers about the validity of
>> ICA-corrected data. I'm asking myself that question because very often I
>> get components representing the ocular artifacts, but they also seem to
>> contain eeg-information, especially when I use the fastica algorithm.
>> This results in quiet different looking signals in sections where no
>> artifacts occur - mainly in frontal channels of course.
>>
>> Any help is highly appreciated.
>>
>> Thanks!
>>
>> Michael
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>
> --
>
> ----------------------------------------------------------------------------
>     Laszlo Balazs, Ph.D.          /      dr. Balázs László
> Institute for Psychology HAS       /   MTA Pszichol¢giai Kutatóitézet
> Budapest, P O B 398, H-1394 Hungary / Tel:+36(1)354-2410  Fax:+36(1)354-2416
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