Dear PPI experts,
We'm running a PPI analysis on some fMRI data.
In this 2X2 design we would like to explore how the planned contrast A1B1>A1B2 is represented, using the Main Effect of B as search region (e.g. a 6mm sphere is created per subject around the group level peak).
Here is the problem:
Our PPI output graphics (here attached, ppi_screenshot) look quite different from the ones shown in the spm8 manual (ppi_manual). The graph in the lower left shows the task condition plot, dotted green line, and the convolved task
conditions (psych variable), while the lower right is the PPI interaction term.
From what we can tell, task plot and convolved task conditions in our analysis are not consistent with each other at all.
Anyone having experience on this?
Also: when creating subject-specific VOIs, is it batter to make sure that each subject has the same number of voxels at a specific threshold (e.g. the canonical 33 voxels in a 6mm sphere, but with an extremely liberal threshold), or it does not matter that subjects have different amount of voxels in their VOIs, insofar they are all thresholded with a much stringent threshold?
Any help would be highly appreciated!
Sincerely,
Emiliano
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