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Hi Adrian
I'm not SPM developper but it seems to me that mean centering and orthogonality are kind of separate issues - if you use parametric modulation, regresors are orthogonalized i.e. in geometrical term theta = 90 and therefore the correlation which corresponds to the cos(theta) is 0. Whatever centered or not the correlation is still 0.
Hope this makes sense.
Cyril
> Dear SPMers,
>
> While
checking the design orthogonality matrix of my study, I've found that
>
the mean values of the regressors have a remarkable impact on the design
> orthogonality matrix in SPM8. SPM8 does not mean-center the regressors
to
> zero (as SPM99 apparently did), and therefore, the values of the
design
> orthogonality matrix do not correspond with the correlations
between the
> regressors. So the exact same design leads to quite
different orthogonality
> matrices when comparing SPM8's regressors
(attached image1.png) with
> mean-centered regressors (image2.png).
>
> So I'm not quite sure whether the design matrix given by SPM8
(image1)
> contains any meaningful information. Should I judge my design
based on the
> orthogonality matrix design given by SPM8 or by the
orthogonanlity matrix
> based on mean-centered regressors?.
>
> The following post nicely states the same problem and remained unanswered
on
> the SPM mailing-list:
>
> https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=ind04&L=SPM&a
mp;P=R98293&1=SPM&9=A&J=on&X=00793331D7D673CA9A&Y=wunibald%4
0gmx.ch&d=No+Match%3BMatch%3BMatches&z=4
>
> Any
clarifications are highly appreciated,
>
> Adrian
>