(1) You need the mean column in the model. If you don't then, your
saying if the value of all your covariates is 0, then the amount of
grey matter would be 0, which isn't plausible.
(2) Masking can be done in several ways. However, I generally like to
threshold after the model estimation and not before. If you mask
before, then you can never make your mask bigger; however, if you mask
the results, then you can make the mask bigger or smaller based on
your question.
Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General
Hospital and Harvard Medical School
Office: (773) 406-2464
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On Sun, Sep 26, 2010 at 12:43 PM, R Duke <[log in to unmask]> wrote:
> SPM Experts,
>
> I am performing a multiple regression analysis (VBM) and I was wondering if the mean is always modeled in the design matrix? I have 3 covariates that I have entered (1 of interest and 2 nuisance), yet when I load the design, the matrix adds a 4th mean column. How might this mean column impact the results?
>
> Also, I was wondering what the advantage was of using an Absolute Masking Threshold at 0.1? Would it be more advantageous to run 'None' (for the Masking) in combination with an Explicit GM tmp?
>
> Cheers!
>
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