[Eeglablist] ICA does not start running properly
arno at salk.edu
Tue Apr 19 14:40:04 PDT 2005
>after successfully running ICA on epoched data to remove eyemovement
>artifacts, I want to run ICA again on the filtered epochs to remove
>other artifact components. I do not want to work with the remaining
>components of the first ICA as these epochs can differ a lot from the
>task related epochs I want to use.
Since you seem to have different data epochs (between the first ICA pass
on all your data, and the second one on your task related epochs), it is
OK to rerun ICA (it is useless to rerun ICA on the same data epochs; see
the tutorial for more details). In theory, it should not be necessary to
change any parameter since you do not use exactly the same data epochs.
However, since the algorithm does not seem to converge with your data,
you probably have a large amount of common data epoch, so you should use
the 'pca' option to reduce the number of dimensions. If you removed 10
artifactual components and have 64 electrodes, use the option " 'pca', 54 ".
Note that you choose to run ICA on your task related epochs only. We
usually use all the data available to perform ICA decomposition and then
study components that vary between experimental conditions. The reason
for using as much data as possible is that we realized that ICA (at
least Infomax ICA, the default option) returns "nicer" components (i.e.
component which can be modeled using a single equivalent dipole) as the
amount of data increases. The reason why ICA finds more cognitively
related components as you increase the amount of data is that cognitive
components tend to be expressed over the whole data. In contrast,
electrical artifacts are usually only expressed transiently, so, as you
increase the amount of data, the relative proportion between cognitive
and electrical artifacts is increasing. As a consequence, the number of
component ICA will dedicate to the cognitive components will also
increase. This is our hypothesis at least.
Hope this help,
>However, for some unclear reason the ICA procedure starts stalling at
>the "Beginning ICA training..." print. Sometimes it gets to lowering its
>learning rate and starting again, but I tend to abort the thing after
>half an hour.
>Running the ICA for removing the eyemovements was no problem at all. It
>never took more than one ore two minutes. Even when I used it on more
>epochs than what I'm trying to now.
>What could be the problem?
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*Arnaud Delorme, Ph.D.*
Swartz Center for Computational Neuroscience, INC, University of San
La Jolla, CA92093-0961, USA
*Tel* :/(+1)-858-458-1927 ext 15/
*Web page*: sccn.ucsd.edu/~arno <http://www.sccn.ucsd.edu/%7Earno>
*To think upon*:
The ultimate measure of a man is not where he stands in moments of
comfort and convenience, but where he stands at times of challenge
/Martin Luther King/
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