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On Mon, 29 Jun 2009 14:16:49 +0000, Marco Sperduti <[log in to unmask]> wrote: >Dear SPM users, > >I have some questions about PPIs: > >1) When I extract the time course with the VOI button I'm asked if I want to adjust it with a contrast. Which is the difference between adjusting or not? If I decide to adjust, which contrast should I use? Adjust for "effects of interest". It's probably an F-contrast created by SPM itself. By adjusting for effects of interest, you're _removing_ effects of no interest from the timecourse. See e.g. https://www.jiscmail.ac.uk/cgi-bin/wa.exe? A2=SPM;OEA31Q;20040406173202%2B0100;ind04 >2) In my experiment I have more than one session. By now I'm extracting the time course for each session and calculating the PPI separately for each session, then I use the parameters in each PPI as regressors for a GLM with multiples sessions. Is that correct? For some strange reason the issue of multiple sessions and PPI is somewhat obscure. What I've done in the past, if I recall correctly, is instead of three regressors and one session, used 3N regressors and N sessions. (The "three" is, per the SPM way of doing PPI, the psycho and physio regressors, and their interaction (the PPI).) >3) Once the single subject analysis is completed, how can I proceed for the group analysis? Usual SPM two-stage (heirarchical) way: test the PPI at the subject level, and then take that to the group level. To test the PPI at the subject level, put a +1 contrast weight for the PPI regressor for every session. NB: be careful if there are different numbers of sessions per subject; in that case you should take an average. (Otherwise subjects with more sessions will be artificially weighted more in the group averages.) >Thank's a lot, > >Marco >