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Dear all, a rather basic question: I would like to do a DCM analysis using a model with 8 regions (4 in each hemisphere). I am interested both in the connectivity within one hemisphere and in the connectivity between the hemispheres. I am afraid however that due to the many regions, connections and posssible modulations, I will not get useful results. One approach that has been described in some articles is the following: First, models restricted to one hemisphere were constructed (i.e., with 4 regions) and only intrahemispheric connecitivity was analyzed. Often, this approach was used separately for each hemisphere to also investigate whether intrahemispheric information transfer mechanisms are similar between hemispheres or not. Second, in a next step models that include regions in both hemispheres were constructed and both intra- and interhemispheric connecitivity was assessed. Also it is of course possible to use both approaches separately, I am not sure whether this sequential approach is valid if the same data set is used. What can be done? If only one approach is possible (as I believe), I have to make at some point an informed decision how many regions I should include in my models. How do I do that a-priori? Any suggestions? Best wishes, Andreas