 |
 |
On Wed, 11 Nov 2009 21:09:57 +0000, Greg Burgess <[log in to unmask]> wrote: >Hi SPMers, > >I'm looking into running and interpreting PPI analyses, and I have a question. Rather than >a PPI analysis on the contrast between two conditions [1 -1 0], can one run a PPI analysis >looking at one condition versus baseline [1 0 0]? The former would give you differences >in functional connectivity with the seed VOI during Condition A versus Condition B. If I'm >interpreting things correctly, I believe that the latter would give you voxels in which the >functional connectivity with the seed VOI is greater for Condition A than the average >connectivity during the rest of the task (Condition B, Condition C, and resting blocks). Is >this interpretation correct? Sounds right. (A little too early in the morning here... :-) ) >From my quick search for existing studies using PPI analyses, I've only seen people use >contrasts of [1 -1]. Is there any reason that other contrasts aren't commonly used in the >literature (e.g, [1 0] or [1 -0.5 -0.5])? If I recall correctly, there's nothing in principle wrong with using the baseline condition in PPI. The problem is that you have to tell SPM two explicit conditions. So you have to go back to your "conventional" (non-PPI) analysis and remodel it with baseline _explicitly_ modeled. See for example this post by Darren Gitelman, who's one of the PPI gurus: https://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=SPM;w6Y61Q;20050503005207- 0500;ind05 Caveat: I tried doing this once, and as a sanity check, I compared the "conventional" model results with implicit and explicit baselines. They actually weren't that similar! (Kind of scary in its own right.) I'd recommend doing that before you proceed. >Thanks in advance for your responses, >--Greg > >_________________________________________________________________ ___ >Greg Burgess, Ph.D. >Research Associate, Institute of Cognitive Science >University of Colorado - Boulder >Email: [log in to unmask]