Hello, 

I realized after estimating all of my gPPI models that I only included the contrasts that examine positive connectivity (e.g., Faces > Shapes), and now I need to add the negative connectivity contrasts (e.g., Faces < Shapes). Is it possible to re-compute the contrasts without re-estimating the entire gPPI model??

Any advice is appreciate

Thank you for this resource

Best,
Arianna

Hi Arianna,

your con-images for Faces > Shapes from the first-level PPI should "include" the negative correlations as they reflect the difference between the parameter estimates (betas) for Faces minus Shapes - a value in a con-image can be negative. Thus, at the group level, to get Faces < Shapes (or equivalently, Shapes > Faces) you can simply set '-1' as the contrast weight when estimating a one-sample t-test across your Faces > Shapes con-images (assuming this is what you want to do).

If, for some reason, you want the subject-level con-images for Shapes > Faces, you could multiply the Faces > Shapes con-images by -1 (e.g. using SPM's ImCalc).

Maybe someone else can confirm whether my reasoning here is correct.

Hope this helps!

All the best,
Phil

Arianna,

Phil is correct.

And unfortunately, it doesn't look like the current code has an option to only estimate contrasts.

It wouldn't be too hard for someone to add an option in PPPI.m to only estimate the contrasts.

Best,
Donald

Phil and Donald - 

Thank you so much, that makes perfect sense

Again, thanks for the prompt reply 
Best,
Arianna