questions on ICA to remove EOG artifacts from MEG data

[Elisabeth May]

Dear Fieldtrip users,

I am trying to use independent component analysis (ft_componentanalysis)
to remove eye artifacts from my MEG data. The dataset was recorded with
the Neuromag 306 system and contains two conditions with 40 trials each
(one where a painful laser stimulation was applied to the subject's
right hand and one where the same stimulation was applied to the left
hand). So far, I am trying to do the ICA just for the gradiometers on
preprocessed data (after removal of bad channels) and then plotting the
topographies of the components to identify and then reject components
representing the eye artifacts (as suggested in the example script on
the Fieldtrip webpage). I have used different settings with the default
option cfg.method = 'runica' (no PCA before ICA, different numbers of
principal components to be retained (cfg.runica.pca), both conditions
sepparately and together...). However, I have never gotten component
topographies that I can clearly identify as components representing eye
artifacts (showing two sources right above the eyes), although there are
almost 200 eye artifacts in the 80 trials of this data set (i.e. there
was usually more than one EOG artifact in every single trial). So I am
still wondering about a few questions on the exact settings for the ICA
in my case and if I am missing something really obvious:

    * Is it correct to do the ICA on both horizontal and vertical
      gradiometers together and then plot them sepparately to identify
      artifacts?
    * Should I do the PCA before the ICA and if so, how many principal
      components should I retain?
    * Should I do the ICA on both conditions together or sepparately
      (since I expect different sources to be activated in both
      conditions, e.g. one of the two primary somatsensory cortices
      activated according to stimulation of the right or left hand)?
    * Is it accurate enough to use complete trials for the component
      analysis or do I have to extract and use just the parts containing
      EOG artifacts?
    * And finally, are the component topographies usually sufficient to
      identify eye artifacts?

Any help or advise on this would be greatly appreciated! Thanks in advance,
Elisabeth


--
Dipl.-Psych. Elisabeth May

Universitätsklinikum Düsseldorf
Institut für Klinische Neurowissenschaften und Medizinische Psychologie
Universitätsstr. 1
40225 Düsseldorf

Tel: +49 211 81-18075

http://www.uniklinik-duesseldorf.de/med-psychologie

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