[Eeglablist] margaux.izac at u-bordeaux.fr sent you Rank issue via WeTransfer

[Margaux Izac]

Hello,

Would someone please know :
- Why does the window before ICA warns us about a rank issue although our rank is 32 ?
- What could impact the data's rank when some files have a rank of 29, other than two electrodes recording the same signal ?

(see detailed version of the issue further down)

Best regards,
Margaux IZAC


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Message:
Hello,

We have recently used an eegosport 32 channels headset (ANT Neuro) for an experiment and
are now analysing the data with eeglab. However we're struggling with what seems to be a
rank issue.
For the experiment we used OpenVibe and created a channel list of 34 electrodes where CPz
and AFz were at the very end so that OpenVibe could identify them as Reference and Ground.
When importing our data from a GDF file on eeglab we therefore ended up with 34 electrodes
in the Channel locations window.
After confirmation from ANT Neuro, we manually rejected CPz and AFz to then follow the
"Append Channel", "Look up locks", "Set reference" procedure from the section 5b-Retaining
the reference channel chapter of the eeglab documentation. After these steps, we ended up
with 32 electrodes and CPz set as a Reference in the blue eeglab window.
We also tried to only reject AFz and directly set CPz as Reference but this procedure was
slightly different as we ended up with 33 electrodes and CPz set as a Reference in the
blue eeglab window.

In both cases we tried launching ICA after only applying a bandpass filter and resampling
the data to 250Hz and got a message (see ICA rank 0 message.png) saying the rank of the
data matrix was lower than the number of input data channels. Hyeonseok et al., 2023 paper
was useful to better understand this issue and suggested we check the rank by using the
function rank(EEG.data).

To our suprise, our rank was 30 (for Test_1.gdf).
As two channels that have identical data induce a data rank of n-1 we checked our EEG.data
matrix but values seemed correct. We therefore decided to take a look at a matrix
correlation with corrcoef(EEG.data') and saw that sometimes, the values were close to 0,99
(see Covariance_Test_1.mat).

We tried different GDF files and some had a rank of 32 (see Test_2.gdf), others a rank of
29 (see Test_7.gdf). The amount of correlation values close to 0.99 varied from 0 to 4 in
all our tested files.
We tried launching an ICA everytime but got the same message whatever the rank. 

Therefore our questions are the following :
- What could impact rank data ?
- Why does the ICA error message still pop up when our rank is 32 ?

I will attach to this message our code (ICA_rank_issue.m), three GDF files (Test_1.gdf;
Test_2.gdf & Test_7.gdf) and their corresponding correlation matrix
(Covariance_Test_1.mat, Covariance_Test_2.mat & Covariance_Test_7.mat).

Thanking you in advance for your help. 

Best regards,
Camille JEUNET-KELWAY, Emeline PIERRIEAU and Margaux IZAC



1 item

Rank issue eeglab.zip - 33.4 MB


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