Dear experts,
I'm refering to my last post on the mailing-list (subject: vbm covariate in fmri analysis). Since nobody has commented on it and this problem bothers me a lot, I try it again and hope that somebody can help me.
Especially because the answer on my previous question confused me quite a lot (see below).
Here are the elements of the design:
subject factor
interaction group (3 levels)/ condition (2 levels)
covariate 'smoker' (bivariate coded)
I defined contrast vectors like the following:
condition1/group1 > condition1/group2:
[ones(1,nSubj1)/nSubj1 -ones(1,nSubj2)/nSubj2 zeros(1,nSubj3) 1 0 -1 0 zeros(1,nConds)]
nSubj: number of subjects in the group
nConds: number of conditions (which is 2)
Is that contrast valid? And is it possible to insert a covariate in a within-subject design?
Thank's a lot in advance!
Best
Anja
previos post and answer:
>> I performed a 2nd level fmri analysis (flexible factorial design) with a
>> subject factor (24 subs) and a group factor (3 groups). Regardig this I
>> found functional differences between the 3 groups. Further a colleague of
>> mine found differences in gray matter volume between the 3 groups and
>> relating to some functional relevant areas.
>
>>>> Group comparisons (unless its the group*condition interaction) are not
>>>> valid in the flexible factorial (or full factorial) or any GLM which
>>>> only has a single error term when you have repeated-measures. In my
>>>> previous posts, I've stated that between-subject effects are not
>>>> statistically valid in within-subject designs. If you only have one
>>>> condition per subject, then you don't need to enter subjects as a
>>>> factor.
>
>> Now I would like to analyze, if the functional differences between the
>> groups are a consequence of gray matter volume differences.
>> Regarding this I thought it should be a good idea to integrate the vbm
>> data as a covariate in the fmri analysis. I have heard of the bpm toolbox
>> (biological parametric mapping) which offers the opportunity to integrate
>> voxel-wise covariates. But I'm not sure if it is possible to realize the
>> flexible factorial design in the environment of this toolbox (the subject
>> factor seems to be important for my fmri analysis). I would be very
>> thankful if somebody could give me some advice. Maybe there are also other
>> opportunities in solving the problem.
>
>>>> Covariates should not be included in within-subject designs.
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