Dear Dr. McLaren,

I have managed to run the gPPI toolbox on my data, and the 1st level gPPI contrasts seem to be specified correctly, and I get no error messages. However, when I submitted these 1st level contrasts to a 2nd-level one-sample t-test in SPM8, there were no significant voxels, not even at very lenient uncorrected thresholds. In fact, when I set the threshold to p<.05 uncorrected, all I found was "clusters" of 1 voxel. This is very unusual to me, especially since I have a relatively large sample (n = 58), and in the one-sample t-test that I used to select the seed region for the gPPI, a large fronto-parietal network was active, that survived FWE-correction. I would expect that the parietal seed I selected would show functional connectivity with at least the other parietal regions (e.g., the region contralateral to the seed region). What makes it even more odd, is that I tried 3 different seed regions, and 2 different contrasts, and the 2nd-level results are similar, in that there are no significant voxels, not even at very liberal thresholds. When I checked the 1st-level results for each subject, there were definitely regions that were functionally connected to the seed region, but there was large variability in the exact regions across subjects (which might explain the null findings at the 2nd level). Again, this seems unusual to me, given that I would expect some degree of consistency in functional connectivity across subjects.

Therefore, I get the feeling that I did something wrong, but I can't figure out exactly what it might be. I am still in doubt whether I specified the tasks and contrasts correctly. I have an event-related design, with parametric regressors to model the delay and amount of rewards that were used in a temporal discounting task (choices between a small immediate reward, and larger delayed reward). For the gPPI contrast, I'm only interested in comparing choices, not the parametric modulators. I have tried both specifying the names of the parametric modulators under "P.tasks", and only specifying the name of the regressor to which they are added, but that doesn't seem to make any difference.

I have attached my script. Note that I used a separate script to define the list fo subjects (not included). 

Best,

Erik