[Eeglablist] Problem with channel detection in Run ICA

[Makoto Miyakoshi]

Dear Jason,

In short, Jan is asking why he sometimes has different ranks though
having the same number of channels. I'm interested in this question
too. I appreciate your help.

Makoto

2012/6/21 Remi, Jan Dr. <Jan.Remi at med.uni-muenchen.de>:
> Dear EEGLAB users,
>
> I am using EEGLAB to run an ICA on my EEG data that I acquire in an EEG-fMRI
> environment to ultimately get rid of the cardioballistogram artifact that is
> typical for recording EEG inside the strong magnet of an MRI machine.
>
> Recently I get a message that reads as follows:
> "EEGLAB has detected that the rank of your data matrix is lower [than] the
> number of input data channels. This might be because you are including a
> reference channel or because you are running a second ICA decomposition. The
> proposed dimension for ICA is 57 (out of 62 channels). Rank computation may
> be inaccurate so you may edit this number below. If you do not understand,
> simply press OK below."
>
> Besides being very thankful for the last sentence, I really do not
> understand the problem. Actually the number of channels that EEGLAB proposes
> varies between 57 and 60 (out of the actual 62 channels) for the 6 files I
> want to run the ICA on. These files differ only in the stimulus condition,
> the EEG properties are not changed at all, they are recorded on the same EEG
> machine (Neuroscan Maglink), with the exact same setup for approximately the
> same time (about 9:45 minutes each). So while I of course do expect the EEG
> to differ in some properties of the EEG signal, i.e. changes in gamma band
> etc., the recording setup conditions are the same. So I do not see where
> there would be a systematic mistake in the recording, especially since I
> have had the same failure notice on a data set, where I had used the ICA
> before without any problem and then 2 weeks later, when I wanted to redo the
> ICA on the same EEG data, where I had only applied a different filter in the
> Neuroscan software before running the ICA analysis (a different low
> frequency filter), I get the same failure notice.
> More over, the channels that are not displayed in the channel selection
> dialog before running the ICA is not systematic, once it was for example the
> EEG channel F5, once the EEG channel P7.
>
> The ICA itself gets me great decomposition, I can get rid of the artifact
> very nicely, I am happy with the resulting data, but I don't like the idea,
> that I am possibly systematically missing data. I do read the EEG in a
> clinical way, I am a medical researcher.
>
> Any ideas where my mistake could be?
> A similar question had been asked in 2011 and 2009, mainly pertaining to a
> problem of displaying all channels in a 32 bit dataset.
>
> In case you need screenshots of my problem I will be happy to answer emails
> to my email-adress directly.
>
> Thank you all, I enjoy EEGLAB and its community a lot,
>
> Jan Rémi
> Epilepsy and Sleep Center, Department of Neurology, University of Munich
> currently: Department of Neurology, University of Coimbra, Portugal
>
>
>
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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