Dear SPM'ers,
I have a question concerning the correction for movement artifacts using the
realignment parameters.
We have several fMRI-datasets which show deviations from the reference image over
the time course of the experiment, as can be seen in the "ps"-file graphs after
the realignment procedure.
However, those deviations are considerably different from subject to subject.
They range from a maximum of about 1.5 mm in one subject to about 5 mm in another
subject. Unfortunately, we are not able to completely eliminate head movements by
better fixation of the head so that we just have to deal with the movements.
Now my questions are:
Is there any rule of thumb as to when to include the realignment parameters in
the design matrix ?
And if I decide to include them, do I always have to use all six of them or just
the ones exceeding some criteria, in other words showing the greatest
translations ?
Moreover, if I include them for one subject, don't I have to include them
automatically for all others, since, as far as my understanding goes, those
additional covariates change the overall model ?
I analyzed several datasets both with and without inclusion of the realignment
parameters and got considerably different results for some of them.
So, I am not sure about how to proceed because of that.
So any comments on this issue would be very helpful and be greatly appreciated.
Thanks a lot in advance,
Tobias Flaisch
Tobias Flaisch
Dep. of Psychology
University of Konstanz
P.O Box D25
D-78457 Konstanz, Germany
Phone +49 (0)7531 88-4706
Fax: +49 (0)7531 88-2891
e-mail: [log in to unmask]
http://www.clinical-psychology.uni-konstanz.de/
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