Dear SPMers,
I have run a group comparison using PPI, and am attempting to make sense of the results. I extracted the eigenvariate from a region that showed differential connectivity between the groups. My understanding is that these values represent the change in slopes (between the seed and this region) for each individual between psychological condition A and condition B. At the group level, one has a positive change, the other has a negative.
The difficulty (as others have noted in previous posts) is interpreting what this means. As I understand it, a positive change in slopes could mean that the regions are changing from being negatively associated in A to positively associated in B, a negative in A to a slightly less negative in B, or a positive in A to a more positive in B.
To clarify this ambiguity, I would like to extract the underlying slopes for each individual under conditions A and B (not just the change scores), which I could then combine at the group level. I followed the directions from the SPM tutorial to plot the slopes for each individual for each condition, but am not sure how to access the actual slope values that SPM uses to generate the lines of best fit. Does anyone have a suggestion for how to accomplish this? Obviously the data is in there somewhere, but I would like to figure out the easiest way to get it out.
Many thanks, Ben
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