Dear SPM Experts,
We are currently doing a PET study comparing 17 healthy controls with 17
patients. We would like to do an spm analysis that takes into account the
differences between the 2 groups while covarying a high-novelty seeking
score. I have looked through the spm archives, spm manuals, PubMed
archives, to find a similar study, and the closest I got is a post by Sean
Colloby (posted: Tue, 19 Jun 2001 13:21:41 +0100) with the reply of Dr.
Friston (posted: Tue, 19 Jun 2001 17:25:43 +0100). However, I am still
confused about how to setup this analysis, and then the contrast(s);
apologies! I would mainly have 4 questions...
Under PET models, I chose "Single-subject: conditions & covariates", then
I selected the 34 images and set 17 conditions to 1 for the controls, and
17 to 2 for the patients. Then I chose one set of covariates and input
the 34 high-novelty seeking scores. This is where the confusion starts:
Q1 - Covariate Interaction: By default SPM selects "none" for covariate
interaction, however intuitively I would have assumed an interaction with
the condition. How do we decide on what type of interaction to choose?
Q2 - Covariate Centering: By default SPM selects "around overall mean" for
covariate centering, Again, I would have assumed a centering around
condition means. How do we decide on what type of centering to choose?
Q3 - Contrast: Based on the different options selected above, the design
matrix will be formed of either 4 or 5 columns. Therefore, how does one
test for main effect, group interaction, etc...?
Q4 - Or should I be using a different type of analysis?
Again, I do apologize for these questions, but at this point my
statistical knowledge is not up to par. It would really be appreciated if
anyone could help or refer me to a good source in order to understand the
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