[Eeglablist] Problem with channel detection in Run ICA
jdesjardins at brocku.ca
Tue Jun 26 14:08:15 PDT 2012
I have been dealing with this as well.
When the rank of the scalp data is smaller than the number of channels
it means that there are highly predictable channels in the data (they
do not have information that could not be derived from other channels
in the data).
Re-referencing reduces the rank of the data by 1 (e.g. in average
referenced data each channel is perfectly negatively correlated with
the average of all other channels).
Interpolating data channels does not increase the rank of the data. If
you have 128 channels and you interpolate 5 bad channels your rank
will be 128-5 at best.
Linked channels can be overly correlated. electrolyte bridges cause
neighbouring channels to share the same information resulting in each
bridged pair decreasing the rank of the data by 1. I have started
checking my data for unusually large and invariant correlation
coefficients across neighbouring sites.
James Desjardins, MA
Technician, Cognitive and Affective Neuroscience Lab
Department of Psychology, Behavioural Neuroscience
500 Glenridge Ave.
St. Catharines, ON, Canada
Quoting Makoto Miyakoshi <mmiyakoshi at ucsd.edu>:
> 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.
> 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
> Eeglablist page: http://sccn.ucsd.edu/eeglab/eeglabmail.html
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