Hello,
I have encountered a problem and would greatly appreciate some direction in
resolving it.
In my study design there are two drug conditions, Pre and Post-drug scans.
During each scan subjects are presented with a task that shows drug-related
and neutral cues, as well as a rest fixation cross in a blocked design. As I have
one subject group and the subjects vary in their level of tobacco dependence
(TD), my goal is to create a model that examines how TD influences
differential activation between the two scan sessions. Therefore, in essence I
would like to create a paired-samples t-test with TD as a covariate of
interest.
So far I have tried the following:
To best look at how drug cues elicit brain activation in my population of
interest, for every subject during each of the two scans, I have created a 1st
level drug cue > neutral cue contrast. These contrasts were later taken into a
2nd level paired t-test design (with TD as a covariate for each pair), however
the job would not compute. I’m wondering if there is a way to look at the
effect of TD on the difference between the two scans, or as a covariate of
interest.
Then I tried creating a full factorial design with one factor (Drug) that has two
levels (pre and post). There was no independence between the two levels. TD
was the covariate, but it was entered twice repeatedly, once for the pre-drug
level and once for the post-drug. I am not sure if this is the most appropriate
design.
In an alternative method, I took every subject’s pre and post drug scans into
1st level and created a drug cue pre > drug cue post (and vice versa)
contrast. I then took these contrast images into a 2nd level multiple regression
analysis, where TD was the covariate. I am also not sure if this is the most
appropriate design, especially since neutral cues are not taken into context.
Please advise.
Thank you in advance,
Vlad Kushnir
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