On Thu, 16 Jul 2009 21:16:12 +0100, Thomas Gladwin
<[log in to unmask]> wrote:
>We're having some trouble with the interpretation of a PPI result. We found
>a positive significant interaction (this was in an event-related design),
>but wanted to know what kind of relationships this resulted from. For
>instance, was there a positive correlation in one condition and a negative
>correlation in the other; or a positive and a zero correlation; or a more
>versus less positive correlation, etc.
First you should ask how meaningful these other PPIs are. That is, if the
original PPI used "A - B" as the input contrast (the "psycho-" in "PPI"), then
taking it apart would mean doing PPIs for A and B separately. But everything
in fMRI and (some of) PET is based on differences, meaning you can't interpret
absolute levels. So a PPI for "A" would really mean a PPI for "A - (some
[Maybe this doesn't apply to your case; maybe your original PPI was for
something like "(A1 - A2) - (B1 - B2)", in which case the two component
contrasts are themselves differences.]
That can be problematic, in that if I recall correctly SPM insists on a
difference contrast (enforced by the PPI user interface), meaning that the
baseline would have to be explicitly modeled. And then there's the issue of
conceptual interpretations of these "sub-PPIs," which could possibly be
>What one of our colleagues tried was to run the PPI with each condition
>separately, and then use the T- and contrast values to determine whether
>relationship in that condition was positive, negative or around zero. A
>negative regression weight was found, which we at first thought might imply
>some sort of negative interaction.
>However, we're not quite sure whether this is a valid approach. We also used
>simulated data to try to understand how the analysis works, and this
>resulted in the expected PPIs, but the per-condition didn't seem to bear any
>absolute relationship to the simulated regression coefficients. For
>instance, in one condition we modelled impulses in a certain "dependent"
>voxel as 10 times an impulse in the independent voxel (which we used for the
>VOI). In the other condition, the scaling factor was 1. But this resulted in
>a positive and negative "partial PPI" rather than more and less positive values.
I didn't quite follow the above paragraph.
>So our primary question is, should we even be expecting these per-condition
>PPIs to provide a meaningful value, in the sense that negative values for
>one condition imply a true negative physiological interaction in that condition?
>Thanks very much in advance.