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Dear Rik and Christoph, In response to the question whether you could use 2 event-related regressions as direct(G) and modulatory(H) inputs respectively: I think this is perfectly fine. It would allow you to model H-dependent effective connectivity from an area driven by G on a second area. The effects of the modulatory (B) inputs are simply additive to the fixed/endogenous connectivity, and there are no constraints for any durations for B. For example, suppose you have two regions, x1 and x2, and condition G is the direct input to x1, while H is the modulatory on x1 --> x2. This would test for an H-dependent influence of x1-->x2, where x1 is only active when G is present. (assuming positive input parameter estimates). best, Hanneke 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])? > > An event-type modulatory input would only modulate the intrinsic > connectivity for a very brief instant, while neuronal activity lasts much > longer. Mathematically it is ok but I don't think it makes much sense. > I would be happy to hear Klaas (or other experts) opinion about this. > > -- Hanneke den Ouden, PhD Donders Centre for Cognitive Neuroimaging PO Box 9101, 6500 HB Nijmegen The Netherlands Tel: +31 (0)24 36 11757 Fax: +31 (0)24 36 10989 Visiting address: Trigon, room 0.64 Kapittelweg 29, Nijmegen