Dear DCMers -
A few, hopefully simple questions for DCM(10) for fMRI:
1. Does it make sense to have a model with 4 regions, in which 2 regions within each of 2 pairs are interconnected, but there are no connections across pairs (ie, two "isolated" subnetworks; eg, with regions E1<->E2 F1<->F2, DCM.a = [1 1 0 0; 1 1 0 0; 0 0 1 1; 0 0 1 1])?
2. A related question to above: does it make sense to compare the free energy of two models, one in which a region is "isolated" (ie has no connections apart from a self-connection) with one in which it has connections to other regions? And could this be used to ask whether that region needs to be in the model? (I realise one cannot use the free energy to compare models with different regions - ie different data - but wonder whether this approach could be a useful heuristic answer to that question?).
3. Does it make sense to have a model with no modulations (eg, DCM.b = zeros(4,4,2), for two inputs)?
4. If the GLM in an SPM.mat file has two event-related regressors for conditions G and H (and a jittered SOA so that responses vs the inter-event baseline are estimated efficiently):
4.1 does it make sense to use one of these as a driving input (eg, to both of two regions, eg, DCM.c = [1 0; 1 0]) and the other as a modulatory input (eg, DCM.b(:,:,1) = zeros(2,2); DCM.b(:,:,2) = [0 1; 1 0])?
4.2 if instead one wants to make the driving input both G and H (ie treating any event vs baseline equivalently), and modulate by just H, is it sufficient to set:
DCM.U.u = [full(SPM.Sess(ses).U(1).u)+full(SPM.Sess(ses).U(2).u) full(SPM.Sess(ses).U(2).u)];
DCM.U.name = [sprintf('%s+%s',SPM.Sess(ses).U(1).name{1},SPM.Sess(ses).U(2).name{1}) SPM.Sess(ses).U(2).name];
(and with DCM.c = [1 0; 1 0] and DCM.b(:,:,1) = zeros(2,2); DCM.b(:,:,2) = [0 1; 1 0], as above), rather than having to re-parameterise and refit the GLM? (ie, are there any other fields in the DCM structure that would be affected by this re-definition of inputs, hence causing different results compared to reparametrising and refitting the GLM?)
Apologies if some of the above questions illustrate an incorrect understanding of the DCM code.
BW,R
