Hello everyone,
I'm new to SPM (and neuroimaging analysis in general), and I'm having some
difficulty in setting the contrast for my analysis.
I've tried to search the archives to find an answer to my question, and have
found some similar questions but I'm afraid I still don't understand.
I'm working on the analysis of a resting-state experiment.
We are interested in the effect of the polymorphisms of the DAT gene on
resting-state activity, and we are using the striatum (putamen & caudate) as
our seed region.
We already looked at areas that are correlated with the activity of the striatum
in the two different DAT groups.
Now we are interested in finding out if alcohol use has an influence on the
results we found.
I have added this variable as a covariate in the ANOVA (full factorial with 1
factor, in SPM8), and selected 'interaction with Factor 1'.
We do not want to control for the effect of this variable, but we want to see
how alcohol use is related to the activity we found in the two groups.
Intuitively, I would set the contrast like this:
(DAT group1/ DAT group2/ Alcohol use group1/ Alcohol use group2)
1/-1/1/-1
I assumed that this would give me the activity in areas that are both related to
the activity in the striatum in group 1 and to their alcohol use.
(and -1/1/-1/1 for group2).
But someone else suggested:
0/0/1/-1
To me it seemed as if this contrast would only look at the effect of alcohol use
and not take into account the activity of the seed striatum.
But maybe I misunderstand what the first two zero's mean.
I hope that I've explained things properly and I really hope that someone is
able to answer this question for me.
Nadia
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