help > PPI Interpretation
Feb 24, 2019  03:02 AM | Phil Kuhnke
PPI Interpretation
Dear all,

I have a couple of questions regarding the interpretation of PPI results.

- A significant PPI effect in voxel Y with seed region X for task A > B means that the regression slope between the time courses of X and Y is significantly higher during A than B, above and beyond the activation of both X and Y for A > B, and their task-independent correlation, correct?
Now, does this mean that we would find a significant PPI effect if the correlation between X and Y during B is highly negative, and it becomes less negative or even 0 during A? Can we really interpret such effects as an increase in coupling / functional connectivity between X and Y for A > B? It seems to me that the coupling even rather decreases (from an anti-correlation to no correlation).

- Related to this, aren't strong negative correlations also interesting, in that they could reflect inhibitory influences between brain regions? Maybe what we should truly analyze is the absolute regression slope?!

- My PPI analysis revealed that seed region X (6mm sphere around group activation peak for task A > B) couples more strongly with region Y during A than B, although Y was not (significantly) activated for A > B in the group univariate analysis. How can such a result be interpreted? How is it possible that X and Y functionally interact more strongly during task A than B, although Y shows no significant activation differences for A > B?

Thanks a lot in advance and all the best,

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PPI Interpretation
Phil Kuhnke Feb 24, 2019
Phil Kuhnke Feb 24, 2019
Donald McLaren Feb 24, 2019