[Eeglablist] Lower Number of ICs than others

[Makoto Miyakoshi]

Dear Hamed,

20 x 64 = 1280 ICs you had in the beginning, but you have in the end 305.
305/20 = 15 ICs per subject, which does not seem very strange for me.

There are several empirical facts.

   1. More channels ~= More ICs.
   2. Number of ICs to reject ~= amount of variance to reject

About 1, regardless of the number of channels you have, to certain extent,
the final number of 'brain ICs' are consistent to 10-20 per subject. There
is no dedicated study on this (I know we SCCN are responsible for
investigating this kind of ICA problems, sorry!), but empirically this is
the case. I even think that the true degrees of freedom on scalp EEG
recording is limited from the beginning.

About 2, ICs are sorted by variance (i.e., power). Even if you reject
80/100 ICs, if the remaining ICs are the first 10 ICs, namely IC1, IC2,
..., IC10, you probably keep 80-90% of original variance. If you want to
report exactly how much data rejection was performed, the variance rejected
should be reported (which you can't do from GUI, sorry... but what you need
to do is subtract EEG.data before and after IC rejection, and compute
1-100*mean(var(data_beforeRejection)-var(data_afterRejection))/mean(var(data_beforeRejection))
see evntopo() function for this calculation, or Lee et al. 2014 IEEE conf
proc) rather than the number of ICs since the latter is poor estimator of
the former. A single blink IC may even account for 30-40% of variance
easily!

Makoto


On Mon, Jul 9, 2018 at 11:10 AM Hamed Taheri <hamedtaheri at yahoo.com> wrote:

> Hello all,
>
> I have EEG data from 20 subjects which were recorded using 64 channels EEG
> cap. I've used Mokoto's preprocessing pipeline. When I make a STUDY and
> remove the dipoles with < 15% residual variance and the dipoles outside
> the brain, the number of remained ICs compared to other studies are very
> lower.
> For instance, a Paper with 21 Subjects and the same recording system (64
> Channels) reached 897 ICs while my ICs are just 305.
> Based on your experience do you think what is my mistake?
> Do you think it can be related to the quality of my recording system
> because our cap is a bit old or I have a problem with my processing?
> I would be so grateful if you could help me with this issue.
>
> Best Regards,
> Hamed
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-- 
Makoto Miyakoshi
Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego
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