Dear SPM Users,

This is a repost, apologies.  But any insights would be really

Thanking you very much in advance,

Best Regards,


On Thu, 12 Apr 2007, Paul GRAVEL wrote:

> 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
> above.
> Best Regards,
> Paul
> ______________________________
> Paul Gravel
> Neurobiological Psychiatry Unit
> McGill University
> 1033 Pine Avenue West Room 203
> Montreal, Quebec, Canada, H3A 1A1
> Phone:  (514) 398-7301
> Fax:    (514) 398-4866