[Eeglablist] Ghost ICs on EEG data

[Tarik S Bel-Bahar]

Hi Maitane,

One or a few ghost ICs can show up
In ica solutions.(also twin ICs and zombie ICs, topics for another day).
Few notes below that may be helpful.

If you havent, You may need to drop rank by 1 because of the average
referencing.

Do they remain if you just sparse down the channels to 90 or so ,but no PCA
reduction?

Did you do something like cleanline or asr before? These and other
denoising approaches can impact ICA, and might contribute to ghost ICs.

Is there some form of slow drift remaining in the data before ICA?

Note that new eeglab automatically checks for rank, i believe, and perhaps
fixes it if necessary.

The rule of thumb about how much data ICA needs is a rule of thumb.
To circumvent the rule, you could try sparse down in both time and channels,
and then reviewing full ICA with no pca.

If you are a beginner with ICA, it can be very instructive do ICA several
different ways on a few files, so you have an empirical look at how
different pipelines may affect things. If you havent had a chance to, see
also Luca's iclabel website and plugin, and Artoni's RELICA.

You could drop all the eog before ica unless its central to your study.
Frontal HdEEG picks eye activity very well for ica eye artifact denoising.











On Fri, May 15, 2020, 3:26 AM Maitane Barrenetxea Carrasco <
mbarrenetxea at mondragon.edu> wrote:

> Hi all,
>
> I am working on a dataset that has 128 EEG channels and 4 EOG channels (2
> vertical + 2 horizontal) plus a nose-tip reference. Hence, 133 channels in
> total.
> After ICA decomposition I have noticed that there are 2 ghost ICs in one
> the datasets (the 13 files that I have preprocessed before were apparently
> fine). These ghost ICs appear in first two positions of the ICs and exhibit
> inverted time-course and activation maps.
>
> This seems to be a problem of low-rank data before ICA. The thing is that
> in this case my data is full rank when entering ICA (128 channels and rank
> 128 as this subject doesn't have any interpolated channels). Additionally,
> I am only computing 92 components (I use PCA for dimensionality reduction)
> as the EEG data length doesn't allow for the calculation of at least 30
> data points per ICA weight if all 128 components are to be estimated (data
> length=254458, 30*92^2=253920).
>
> So, is this a matter of runica() instability or is there something wrong in
> my pipeline? Here is the code I use to preprocess the EEG data:
>
> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
> %% STEP 2: Filtering  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
> ...
> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
> %% STEP 3: Remove initial and final segments of the data
> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>     ....
> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
> %% STEP 4: Import channel info
> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>
> if  there are bad channels then
>         %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>         %% STEP 5: Reject bad channels
>         %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>
>         EEG = pop_select(EEG,'nochannel',toremove);
>
>         %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>         %% STEP 6 : Interpolate
>         %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>         EEG = pop_interp(EEG, originalEEG.chanlocs, 'spherical');
> end
>
>     %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>     %% STEP 7: Average re-reference --> exclude EOG channels to avoid
> artifact propagation
>     %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>     EEG = pop_reref( EEG, [],'exclude',[129:132] );
>
>     %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>     %% STEP 8: ICA --> EOG and reference channel excluded
>     %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>
>     EEG = pop_runica(EEG, 'pca', 92,
> 'extended',1,'interupt','on','chanind',[1:128]);
>
> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>
> Thank you very much in advance,
>
> Maitane
>
> --
> Maitane Barrenechea Carrasco (PhD)
> Biomedikoa - BIO
> Mondragon Unibertsitateko Goi Eskola Politeknikoa
> Loramendi, 4; 20500 Arrasate - Mondragón (Gipuzkoa)
> Tel. : +(34) 647504294 / +(34) 943794700 + Ext. 8162
>
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