Hi,
just a follow-up question: fsl_motion_outliers is likely to generate a different number of confound EVs for every subject. Prior to higher level analyses, would we be expected to fill up confounding outlier EVs of every subject exhibiting less than the maximum outliers by dummy EVs so that every first level design matrix contains the same total number of EVs (keeping the DoFs constant across subjects)? Does that pertain to fixed as well as mixed effect analyses, and also to OLS vs. FLAME?
Cheers-
Andreas
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Von: FSL - FMRIB's Software Library [[log in to unmask]] im Auftrag von Jesper Andersson [[log in to unmask]]
Gesendet: Donnerstag, 21. Mai 2009 20:21
An: [log in to unmask]
Betreff: Re: [FSL] FSL-MotionCorrection
Dear Zarrar,
> I was interested in doing something just like this and was glad to
> see that Klara asked this question. I have another clarification
> question regarding the use of fsl_motion_outliers. When you add the
> confound file produced by fsl_motion_outliers to your FEAT model,
> does this impact the inclusion of the motion parameters as
> covariates? So if you previously included the motion parameters as
> covariates, will they no longer be necessary with the inclusion of
> the confound matrix from fsl_motion_outliers?
No, they are pretty much complementary. Including the motion
parameters removes (as a first order approximation) effects that
depend ~linearly on subject position, such as e.g. distortion-by-
position and dropout-by-position interactions.
The covariates you get from motion_outliers on the other hand removes
effects related to "subject velocity" (again as a first order
approximation). For example if someone makes a sudden movement in the
middle of the acquisition of a volume the rigid-body transformation is
no longer valid, and such a volume would be picked up by
motion_outliers.
Good Luck Jesper
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