[Eeglablist] Different filters, different matrix ranks?

[Arnaud Delorme]

Dear Thomas,

if your data is 64 electrodes + reference, you should never get rank 65 even after your re-introduce the reference channel CPz. The sum of all of your channel at each time point should be 0 under the average reference assumption which is used to re-introduce your reference channel. The fact that you are getting a rank of 65 when you do

rank(EEG.data(:,:,1))

is puzzling though.
Best,

Arno

On Jun 22, 2011, at 12:12 PM, Thomas Schüller wrote:

> Hi eeglab list gurus
> 
> I have a problem that I really can't figure out on my own. I have eeg data recorded from 64 electrodes + reference  (CPz).  I find that using different filters changes the rank of the matrix in ways I would not have anticipated.
> 
> First, I used a Butterworth highpass (.5Hz) and a Chebychew lowpass (25Hz) filter  on the raw data and then reconstructed CPz in the process of re-referencing to average, and then epoched the file resulting in a EEG.data(:,:,1) matrix with rank 65.
> When computing the ICA using runica() I get a warning: "fixing rank computation inconsitency (65 vs 64) most likely because running under Linux 64-bit Matlab".
> The server that I ran it on is indeed 64-bit, but it's a windows server... the ICA creates 65 ICs, as many as there are Channels!
> 
> Second, I used the eegfilt function (highpass: .5 / lowpass:25) on the raw data and repeated the process as described above. Now, after reconstructing and re-referencing EEG.data(:,:,1) has a rank of 64. 
> When computing ICA i get the warning: "Data rank (64) is smaller than the number of channels (65)"
> and sure enough ICA composes only 64 ICs!
> 
> Does any one know why this is happening? 
> 
> Many thanks! 
> 
> Thomas
> _______________________________________________
> Eeglablist page: http://sccn.ucsd.edu/eeglab/eeglabmail.html
> To unsubscribe, send an empty email to eeglablist-unsubscribe at sccn.ucsd.edu
> For digest mode, send an email with the subject "set digest mime" to eeglablist-request at sccn.ucsd.edu

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://sccn.ucsd.edu/pipermail/eeglablist/attachments/20110709/90ff1eb1/attachment.html>