Steffi,

Q: - in oder to interpert this interaction can I just extract the PPI
values with standard tools such as rfx Plot/marsbar?

A: I don't believe rfx/marsbar can extract the PPI values. I'd suggest
using spm_summarise to extract values from the VOI.

Q:- and how do I interpret these values? For instance, I have seen in the
literature that e.g. positive values in HC vs. negative values in pat. (for
contrast A>B) are sometimes interpreted as greater connectivity in HC (and
no connectivity in pat) oder sometimes as positive interaction (of the
regions) in HC and negative interaction in pat.

A: This means that the slope difference between A and B is higher in HC,
than in in Patients. To know if the connectivity difference of A and B in
Patients is not significantly different than 0, you'd need to test whether
they are different from 0 or not. The same is true for the healthy
controls. As with task differences, you need to know what the effect is in
each group. I would also suggest looking at the connectivity difference
between groups for task A and task B to get a more complete picture of the
connectivity. The difference of A and B, doesn't tell if A and B are both
greater in HC or if both less or if A is greater. It is important to
recognize that the caveats of condition comparisons in task data also apply
to PPI data.