Are you using DCM8 or DCM10?
If your connection R1==>R2 is not significant at the group level, then
it is possible that (1) your modulatory parameter MI1 is significant,
which may suggest that the inter-regional interaction between R1 and R2
increased "specifically" during your context MI1; or (2) you have a
large inter-subject variability on this connection. For a similar
situation, see Figure 24 in Karl's 2003 paper (Friston 2003 p1298:
connection V1-to-V5 not significant but motion modulation was
I hope this helps,
On 25/02/2011 16:23, Stephen J. Fromm wrote:
> I'm working on a DCM project. So far the best fitting model looks like this:
> DI--->R1 ==> R2 ==> R3
> ^ ^
> | |
> MI1 MI2
> That is:
> * there are three regions, with intrinsic connections from R1 to R2 and R2 to R3
> * there is one driving input DI acting on R1
> * there are two modulatory connections MI1 and MI2 acting on the two between-regions intrinsic connections
> (The experimental variables for MI1 and MI2 are identical.)
> At the group level, the second connection (R2 ==> R3) is significant, but the first (R1 ==> R2) isn't. Conceptually, this doesn't make sense, insofar as the only way the system perturbations introduced by the DI can get to (R2 ==> R3) is through (R1 ==> R2).
> Does this reduce the credibility of the model? Or is it alone not enough to do that because of the possible vagaries of what is significant?