Dear SPMers,
we recently implemented the analysis of functional connectivity in our lab using standard (standard SPM 8 PPI & gPPI from Donald McLaren (http://www.ncbi.nlm.nih.gov/pubmed?term=PMID%3A%2022484411). In this context we particularly discussed the way to validely define seed regions. A recent scan paper from O’Reilly et al. (http://www.ncbi.nlm.nih.gov/pubmed?term=PMID%3A%2022569188) addresses this issue & concludes that defining seed regions based on (a) strongest task effect in a group analysis & (b) selecting each participants voxel with the strongest effect in a predefined anatomical volume of interest does not represent a case circularity “as long as we model the main effect of task when we run the PPI analysis. In this case the PPI will only detect functional connectivity effects over & above (orthogonal to) the main effect of task (p. 606, legend to figure 1)”
My questions:
- sPPI: Does the standard SPM8 PPI analysis account for this? However, it only allows to account for two task conditions – what about more complex tasks employing more task conditions? Does the main effect have to be specified in a separate model? (& perhaps serve as a mask during the analysis of PPI Effects?)
- gPPI: gPPI allows to adjust the data for the Omnibus F-test for PPI Analyses – does this correspond to modeling the main effect of task?
Thanks in advance & best regards
Ben
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