hi Will & Klaas & others....
1) to repeat Amit's recent question - I think he didn't receive a
reply - how can one interpret a significant modulatory connection (B) when
the intrinsic connection (A) is not significant? is is something like: at
baseline, these regions show little or no coupling, but they begin to do
so following that particular task peturbation??
2) Am I right in assuming that inputs in the C matrix should pretty much
track GLM effects - for example, a region defined by its GLM repsonsivity
to a regressor coding all visual inputs (like 'photic' in the worked
example) should be modulated by that regressor in the DCM analysis (like
'connect photic to V1/V2' in the worked example? If this is *not* the
case...are the DCM and GLM results contradicting each other?
3) In the GLM analysis, variance which is not modelled in the design
matrix will either be captured by other regressors, or find its way
into the residuals. Does the same apply for DCM? I remember reading
something to the contrary on this list, or perhaps in the 2003 paper.
Practically, does this mean that it is not mandatory to model all visual
events? for example, in the worked example, there were presumably stimulus
events such as task instructions - were these modelled in 'photic'? or
were they just left out?
4) My DCM.B matrix is peppered with zeros (for those modulations specified in DCM.b). Why might the modulation be
exactly zero for a given subject/session? is it because the GLM data for
that subject/session/voxel don't reach a certain threshold? For some
modulations, there are more zeros than non-zero datapoints.
many thanks,
Chris
Christopher Summerfield
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