Dear Peter:
As a follow-up to our previous discussion (https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=ind1901&L=SPM&F=&S=&X=C088331A853E80BB55&Y=guoshi_li%40med.unc.edu&P=775888), I have a few additional questions:
1) My DCM model has 27 ROIs, which has a total of 729 connections (A matrix). After PEB analysis and BMR, 380 connections (for group difference) remain with a posterior probability of greater than 0.99. Does it mean all these 380 connections are different between the two groups? Does it suggest that the PEB analysis may be more suitable for model with a smaller number of ROIs?
2) To find out the connections with the most salient group effect, should I focus on the connections with the largest size effect (beta value)? If the size effect (PEB model parameters) is not used for group comparison, why do they need to be estimated?
3) The fMRI data was collected from two different centers. To include the center and other (age, gender) information into the PEB model, I have the following design matrix (assuming each group only have 3 subjects).
mean diff. gender age center
[ 1 1 1 age1 1
1 1 2 age2 1
1 1 2 age3 2
1 -1 1 age4 2
1 -1 1 age5 2
1 -1 2 age6 2 ]
I will remove the mean for the last three columns. Is this correct?
Thank you very much for your help!
Best regards,
Guoshi Li
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