Hi Steve
Thanks a lot for this clarification.
But why should I worry about co-linearity at all, since ist does not alter
my results? Though this way we allow the regressors to show activation that
is not independent, which is rather not the proper analysis. Anyway, why are
there no reports on orthogonalization in the papers?
I need my two almost inverted boxcar functions for separate parametric
modulation. Though I dont feel good with this correlation in the GLM.
Best regards, Rafael
On Wed, 20 Jan 2010 16:47:38 +0000
Steve Fleming <[log in to unmask]> wrote:
Hi Rafael
spm_orth is only called to orthogonalise regressors within trial-type
("condition" in Specify 1st level). Have a look within spm_fMRI_design.m
at
line 283 to see this.
So SPM will only orthogonalise regressors attached to the same onset
function (i.e. separate basis functions or parametric modulators).
This is why your stats have not changed when you change the order, as you
are looking at two separate trial types/conditions.
Best
Steve
2010/1/20 Rafael Lüchinger <[log in to unmask]>
> Dear SPM user
>
> The order of regressors within the designmatrix of spm (spm5) does
matter
> because all regressors get orthogonalized to the first one (spm_orth.m).
> I'm
> interested in the severity of this effect and tested this having two
>simple
> boxcar function with high colinearity of r=.7 within the design matrix
and
> run two separate analysis with switched regressor order. Surprisingly
the
> two regressor had the identical activation independet of their order in
>the
> designmatrix. I', pretty sure not done any mistakes. How is this to
> explain?
>
> Thanks for any comments
> RL
>
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Rafael Lüchinger
Brain Mapping Research, UZH Department of Child and Adolescent Psychiatry
Neumuensterallee 9, 8032 Zuerich, Switzerland. Phone: (+41) 43 499 26 50
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