Dear SPM-experts,
I have several questions regarding the results I obtained using a standard PPI (sPPI) and a generalized form of context-dependent PPI (gPPI) analysis and I was hoping someone could help me.
I am doing PPI analyses with the bilateral dorsolateral prefrontal cortex, bilateral inferior parietal cortex and bilateral pre-supplementary motor area as seed regions (6 mm sphere around individual peak voxel). I have two groups, healthy controls and patients with Parkinson’s disease. I want to determine group differences in connectivity during a set-shifting task. I have used both sPPI and gPPI.
Question 1: At group level, we find that all seed regions in both groups are positively connected with the default mode network (i.e. precuneus and medial PFC) during the psychological condition (switch_minus_repeat). My question is: why do I find connectivity with the default mode network during a task condition in which I expect my participants to be cognitively active? Did others find this as well?
Question 2: What also confuses me is that I always find connectivity with the default mode network, indifferent of which seed-region I choose. Is this what I should expect?
Question 3: When using gPPI, we don’t find any negative connectivity between the seed regions and other brain areas. When using sPPI, however, we find that all seed-regions show huge clusters of negative connectivity with approximately the entire brain. Why can’t I find any negative connectivity with gPPI, and why do I find so much negative connectivity with sPPI?
I hope anyone can help me!
Thanks in advance,
Corine
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