> We are close to finish the analysis and would like to make sure the stat
> model that you described:
> 'Do the stats, with proportional scaling looking for interactions
> between modality and group, like (PET_1 - PET_2) - (GM_1 - GM_2)'
> For this step, I assume that we need to use 'Multi subj:
> interaction&covariates'from stat model, as Example2 in the SPM99 manual ,
> then choose '2 subjects' (for PET and GM), enter images of both comparison
> groups (PET_1 and PET_2 to subject 1, GM_1 and GM_2 to subject 2) with
> conditions 1 or 2, choose 'proportional scaling', and threshold (0 for
> GM?). In the stat analysis of Results, make contrasts '1 -1 -1 1'(PET_1,
> PET_2, GM_1, GM_2, respectively) or '-1 1 1 -1'. By these contrasts, we can
> see the interaction of PET and GM, which is the outcome of this group
> comparison (1 vs 2).
>
> I would appreciate your correction if anything wrong.
I think the setup you describe should produce the right design matrix, and
the contrasts seem to be correct. I would possibly use a slightly higher
threshold though (approximately 0.05 of the value that the data is
proportionally scaled to), as non-normality of smoothed grey matter images
is greatest close to either zero or one.
Comments from anyone else are most welcome.
Best regards,
-John
--
Dr John Ashburner.
Functional Imaging Lab., 12 Queen Square, London WC1N 3BG, UK.
tel: +44 (0)20 78337491 or +44 (0)20 78373611 x4381
fax: +44 (0)20 78131420 http://www.fil.ion.ucl.ac.uk/~john
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