Hi Feng
I think that is exactly what Stephen means. If you use a group analysis to define your VOI for a PPI analysis using the same dataset, you are guilty of double dipping. There have been several recent pertinent discussions regarding the topic of circularity in neuroimaging research, specifically related to fMRI activation / behavior correlations, but would also be relevant to VOI definition as it relates to PPI analysis.
My approach with PPI seed VOI selection has been to obtain seed VOIs from multiple different studies in the literature, which avoids issues with circularity.
Hope this helps
Mehul Trivedi
References:
1) Vul et al (2009) Puzzlingly High Correlations in fMRI Studies of Emotion, Personality, and Social Cognition, Perspectives on Psychological Science 2009 4: 274
NOTE: There are several commentaries associated with this article that are worth reading as well.
2) Poldrack and Mumford (2009) Independence in ROI analysis: where is the voodoo? Social, Cognitive, and Affective Neuroscience, 4, 208-213
3) Kriegeskorte et al (2009) Circular analysis in systems neuroscience: the dangers of double dipping. Nature Neuroscience 12, 535 - 540 (2009)
________________________________________
From: SPM (Statistical Parametric Mapping) [[log in to unmask]] on behalf of Feng Lu [[log in to unmask]]
Sent: Wednesday, December 01, 2010 7:01 AM
To: [log in to unmask]
Subject: Re: [SPM] Is there the circularity of data in PPI?
Hi, Stephen.
Do you mean that there is a circularity if I used the same dataset to define functional regions and perform the PPI? Maybe I didn't elucidate my study clearly. Or maybe I misunderstand Donald's words.
Best Wishes!
Lu Feng
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-----Original Message-----
From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] On Behalf Of Stephen J. Fromm
Sent: 2010年12月1日 20:46
To: [log in to unmask]
Subject: Re: [SPM] Is there the circularity of data in PPI?
I might be misunderstanding something here, but I would assume a distinction re circularity has to be made between (a) using the same dataset to define the functional region and to perform the PPI, and (b) using different datasets. That is, there should be such a distinction even if the task is the same for the two datasets. (This is apart from Karl Friston's point re nominal orthogonality and functional localizers.)
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