Dear Sasha,

If I understand your question correctly, then you would like to apply a linear contrast to the posterior densities of two groups that resulted from using the fixed effects Bayesian averaging routine in SPM.  In principle, this should be possible.  However, you would have to compute this manually.  As far as I remember, the SPM_DCM_average function only stores the posterior means in the DCM file it writes to disc, not the posterior covariances; you would need both, however, to compute the contrast. 

However, you should ask yourself whether for the particular question you are interested in a fixed effects or random effects analysis is more appropriate.  Ultimately, this should determine which analysis you are using at the second level.  There were numerous emails on this topic in the recent past which you should find easily in the archives. 

Best wishes,
Klaas



Von: Sascha Fruehholz <[log in to unmask]>
An: [log in to unmask]
Gesendet: Samstag, den 18. Juli 2009, 18:08:23 Uhr
Betreff: [SPM] DCM parameter comparison between groups

Dear SPMers,

 

I have two groups of subjects and I applied the same DCM model for both groups. I got DCM parameters for each group which are in accordance with our hypothesis. I want to do statistical comparisons for these parameters (i.e. DCM.B) between both groups. Therefore, I entered all single subject parameters in a classical statistical inference test (t-test). The resulting statistical group parameters are not the same as those which I  get when I use DCM averaging (spm_dcm_average) where group parameters are actually computed using a  Bayesian fixed effects analysis. More importantly, the group parameters which result from the classical inference approach did not fit with our hypothesis as good as those parameters from the Bayesian fixed effects analysis.

 

My question is: instead of using classical statistical inference tests for between group comparisons, is there a possibility to use for example the same algorithm which is implemented in spm_dcm_review (which computes the posterior density for contrasts of connections)  for between group comparisons? This algorithm obviously compares parameters within a DCM model using the DCM.Ep and DCM.Cp parameters. Can I also take the group specific DCM.Ep and DCM.Cp parameters and compare them by using the algorithm implemented in spm_dcm_review?

 

Any help is highly appreciated.

Best, Sascha.