[FieldTrip] ICA+frqanalysis questions

[David Groppe]

Hi Odelia,
   When you use ICA (or any other spatial filter) to correct for EEG
artifacts, you're going to distort your data some by removing true EEG
activity in addition to the artifact (for an explanation, see:
http://www.cogsci.ucsd.edu/%7Edgroppe/PUBLICATIONS/GroppeCSO2008.pdf).
 So to minimize distortion, it would be better not to apply ICA
artifact correction to artifact-free data.  However, if the frequency
of the artifact differs across experimental conditions, it could
confound your analysis.  For example, I suspect people blink more
often to targets in an oddball experiment than standards.  Thus if you
apply ICA only to blinky trials, you could find a difference between
the EEG response to standards and targets that simply reflects the
fact ICA removed more EEG activity in the target trials (i.e., it
wouldn't reflect a true difference in neural processing).
      hope this helps,
         -David

On Mon, May 23, 2011 at 1:44 AM, odelia nakar <odidodi at hotmail.com> wrote:
> Hi all,
>
> I'm troubled by the fact that when I use ICA for blinks\eyes movements
> removal, I remove the relevant components also from trials that do not
> contain blinks\eyes movements. In order to avoid this bias we thought to
> combine the data before ICA ("data" structure) with the data after ICA
> ("dataica" structure), only in specific trials, as follows:
>
> datall=dataica;
> datall.trial=data.trial;
> datall.time=data.time;
> blinks=[2 4 5 8 bla bla 156];
> for ind=1:length(blinks)
>      datall.trial{1,blinks(ind)}=dataica.trial{1,blinks(ind)};
> end
>
> To my first question: I just wanted to check that there is no problem with
> that, or any reason not to use it.
>
> Another issue- I use motor learning task, and I'm trying to understand what
> happens through the process, in terms of power-frequency changes through the
> process. How would you recommend that I'd use the ft_freqanalysis function?
> What method to use (or what do I need to consider when choosing the method
> field)?
>
>
> Thanks a lot,
> Odelia.
>
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-- 
David Groppe, Ph.D.
Postdoctoral Researcher
Kutaslab
Dept. of Cognitive Science
University of California, San Diego
http://www.cogsci.ucsd.edu/~dgroppe/