help > Second-level covariate of interest with gPPI
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Sep 4, 2019  01:09 AM | konstantina ki
Second-level covariate of interest with gPPI
Hi Alfonso!

I have a quick question with gPPI and covariates I would like to confirm the answer with you.

My design:
1) I have a 4 conditions (2x2 factorial design) repeated measures. I am interested in the interaction term.
2) I have a behavioural covariate inserted as a second-level covariate (one value per subject).
3) I perform gPPI with one seed region. 

What I am interested in:
I want to find if there are any voxels increasing their connectivity with my seed as a function of the behavioural covariate in the interaction contrast. I hypothesise that people with greater value in the covariate would show increased connectivity with the seed in this contrast than others.

How I test this:
- In the subject effects, I selected 'All subjects' and my behavioral covariate and I set the contast [0 1].
- In the conditions, I selected my 4 conditions and I set my interaction contrast [-1 1 1 -1].
- In the seeds, I selected my seed.

Is this the combination of contrasts that correctly addresses the question?
Moreover, could you please write the complete GLM model of gPPI with the covariate? Would the model be this one (gppi: with an extra regressor? Which coefficient is being tested in the above-mentioned contrast? I suppose the coefficient of the covariate. The significance of this coefficient is tested for remaining variance (after removing the variance explained by the interaction only) right? 

Thank you,