Dear Dr. McLaren,

I have a question regarding the interpretation of PPI results. Could enhanced coupling for experimental condition 1 vs. 2 between seed region A and a different region B reflect increased inhibition of region A by region B (or vice versa), or can it only reflect increased excitation?

Specifically, based on the results of a PPI analysis showing enhanced coupling between seed region A and another region B for condition 1 > 2, I performed a DCM analysis which showed that during condition 1, region B's influence on region A is strongly decreased, becoming negative/inhibitory. Are these results plausible?

Thanks a lot in advance and all the best,
Phil

PPI does not show causation.

While the results of A-->B is not the same as B-->A, the method isn't sensitive to the direction or cause of the effect.

All that we can say is the the slope of the of the relationship between A and B is greater in one task compared to another. This could be due to A being less active in one condition and B being relatively the same across both conditions. 

In your interpretation of the PPI results, I would not try to explain the underlying biology as there are numerous reasons the connectivity could change.