ICA with averaged data

Stefan Debener s.debener at UKE.UNI-HAMBURG.DE
Fri Mar 9 13:11:30 CET 2007

Hi Niki,

It does, in my opinion, not make much sense to average first and then
run ICA, you unlikely obtain a robust decomposition this way and may not
be able to remove the stimulator artefact. ICA needs quite a few
training data to find a decent decomposition, and this is not provided
by ERPs, even though some commercial software packages may allow you to
apply ICA to ERPs/ERFs. Only if (!) all your data are recorded such that
there was no substantial head movement (that is, the position between
sensors and sources did not change), you may get a decent artefact IC
(or several ICs) from combined data (there is a few more ifs involved,
please see previous ICA discussions here and in the EEGLAB mailing
list). YI'd suggest using the pca option and decomposse, say, about 50
components, to start with, and then try to optimize the result by
playing with the pca option and by running ICAs for separate recording

Niki Ray wrote:
> Hi,
>   I'm very new to fieldtrip. I have three questions. I have collected MEG
> data from patients with a deep brain stimulator implanted. I'd like to do an
> ICA to reject the stimulator related artifacts. Sometimes the stimulator is
> on and sometimes off. So my first question is, do i combine all the data
> together and then perform the ICA, or do i do different ICA analyses for
> each condition(on and off stimulation), and hence reject different
> components for each condition?
>  One possibility for deciding wich of the artifacts were related to the
> stimulator was to use the example matlab script you provide for rejecting
> ECG components. Some of the channels (about 13, i'm using a ctf 275 system)
> have a lot of very distinct artifact that is obviously from the stimulator,
> so i wanted to use one of these channels as the "ECG" channel. Is this
> sensible, or not??
>   Finally, I've been getting conflicting advice about whether it is okay to
> first average data, and then perform an ICA. What do you suggest is the
> right thing to do?
> Many thanks!
> Niki

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