Dear Dr Gitelman,
>I want to set up an fmri design which will basically just allow me to
>remove a variety of covariates of no interest. Once these covariates
>are removed from the data I will use the data in another way. Karl
>answered a previous question about how to remove the covariates using
>spm_regions. This question is one step before that.
>
>What I want to know is how do I set up a design matrix so there are
>basically no covariates of interest? spm97 allowed one to do this by
>answering 0 to the covariates of interest question. I don't know how
>to do this in spm99.
I don't really understand your question. There is no distinction between
'covariates of interest' and 'covariates of no interest' in SPM99, and in
SPM97 they were only treated as different for the purposes of reducing the
data using the F map. Given that what you want to do (as far as I can
tell) is extract the residuals (the 'data') after the covariates of no
interest have been modelled out, you can surely do this in the same way as
you did when you were using SPM97.
>The resulting design matrix has all 0's as it's first column,
>followed by the covariates [of no interest] followed by a mean vector
>of all 1's.
>
>I then looked at a series of F contrasts-
>The first column of the design (all 0's) is not uniquely estimable so
>I can't define a contrast as just 1. Likewise choosing columns for
>the reduced design of either 1 or 2:112 doesn't give the right answer.
This is the puzzling part. If what you want to do is use the data once the
covariates of no interest have been modelled out, why are you interested in
obtaining a contrast, or a parameter estimate? From what you wrote, it
seems that you only want the 'covariates of no interest' modelled, so the
parameter estimates will surely also be of no interest.
It may be worth clarifying this for the SPM people (apologies if I am
missing something obvious!),
Best wishes,
Richard.
from: Dr Richard Perry,
Clinical Research Fellow, Wellcome Department of Cognitive Neurology,
Darwin Building, University College London, Gower Street, London WC1E 6BT.
Tel: 0171 504 2187; e mail: [log in to unmask]
Pager: 04325 253 566.
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