It takes the motion correction parameters (number of time points by 6), performs an SVD (this is what is stored in mcextreg file). It will then use the top 3. I don't know of any guidance as to whether to use them or not. I've seen it help, I've seen it hurt. Sorry I can give you more.
doug
Gabriel A. Tobon wrote:
Hi,
I noticed that mc-sess produces a file for external mc regressors that can be used for analysis with "mcparams2extreg". I can see the added regressors in my design matrix if I decide to include them. How are these external regressors actually calculated? Are the same set of motion parameters used at every voxel? The design matrix looks different than the design matrix produced by FSL's FEAT process when motion parameters are added there: http://www.fmrib.ox.ac.uk/fsl/feat5/detail.html#stats
FSL documentation says that adding these parameters is not really recommended. Is this the case for mcextreg in FsFast analysis?
Thank you!
Gabe
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