[Eeglablist] ICA "adds" noise?

[Kristina Borgström]

Hi,

I have an issue regarding ICA for artifact correction that I really would appreciate some help with.

Here is some background information: I have recorded child data (2 years old) with EGI, 128 channels. Before export to EEGLab, the data has been band-pass filtered 1-30 Hz, epoched, clearly bad epochs (with more artifacts than just eye artifacts) were removed, and bad channels in the remaining epochs were interpolated. The data was rereferenced to average mastoid reference. I then imported into EEGlab, which treats the data as continuous, but has all the event information. I have then performed ICA in order to correct for eye artifacts.



The problem (please see the image files located at following links):
 https://dl.dropbox.com/u/7016081/DataPlots.jpg

     https://dl.dropbox.com/u/7016081/ICAComponents.jpg

In (at least) two data files, besides some clear eye artifact components (components 1 & 11: blink; component 7: horizontal eye movement), the ICA also found two components that look like pure high frequency noise (components 2 & 3).

When I remove the eye artifact components (1,7 & 11), the eye artifacts are in fact removed, BUT the data looks generally “noisier”, i.e. the channels overall are fuzzier (see the image “DataPlots” for comparisons). When I calculated individual averages with this data, it indeed contained massive amounts of high frequency noise that was not present in the averages where I did not use ICA at all but instead removed all epochs containing eye artifacts.

I then continued and tested removing the “noise components” (2 & 3), and the data then looked like it did originally, minus the eye artifacts. It didn’t seem to have a major effect on the ERP components either, but of course removed the high frequency noise.

My main questions are: How can noise be “added” to the data, after removal of certain components? How can I determine what type of noise components 2 & 3 consist of? I’ve looked at the frequency plots, but I don’t think it’s very clear. Can it be line noise, or EMG? The scalp topographies are very widespread, and EMG is usually more laterally located. Should it be ok to just remove these two components when they appear, or is there a risk that they contain cognitive components?

Many thanks for any input you can give me!

Regards,

Kristina Borgström
PhD Student
Department of Psychology
Lund University
Sweden
+46-46-2223638

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