On Wed, Oct 17, 2012 at 6:50 AM, Anja Dietrich <[log in to unmask]> wrote:
> Dear experts,
>
> 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.
>
> Thank's in advance!
>
> Anja
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