Christophe and others,
I would like to get further clarification on #2 below- is it ok to have
disconnected regions and to compare the models.
So if I understand what you are saying it is OK to disconnect regions and
then test that model vs. ones with that region connected? I guess these
models would be considered to have the "same data" even though the region
was disconnected?
I understand the disconnected region wouldn't have any activity since it
isn't being driven by anything, but I presume this would be reflected in a
reduced free energy term?
Would it be possible to use the change in the free energy term to decide if
a region truly belonged in the model?
Darren
On Thu, Mar 17, 2011 at 9:11 AM, Christophe Phillips
<[log in to unmask]>wrote:
> Hi Rik,
>
> Let me have a go at your questions.
> See interleaved text here under.
>
> Le 17/03/2011 11:09, Rik Henson a écrit :
>
>
> 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])?
>
> Yes it does.
> You're "simply" modelling 2 parrallel and independent processes a single
> model.
>
> 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?).
>
> The residual term of the isolated region will be its own signal (no drive
> -> flat modelled activity).
> So your question would rather be: is the activity in my last/isolated
> region better explained, or not, when it is driven by the rest of the
> network? The model comparison will thus be between a simple model where the
> activity of one area is not explained at all, and another one with an extra
> parameter and a bit more signal explained.
> This doesn't really answer your question whether the region should (or not)
> be part of the network though...
> I would say that choice of regions to include in the model is more
> empirical: you should include the areas necessary to build a model which
> models as accurately as possible (or sufficiently realistically) the "brain
> function" you want to study.
>
>
>
> 3. Does it make sense to have a model with no modulations (eg, DCM.b =
> zeros(4,4,2), for two inputs)?
>
> Yes.
> You would then be comparing the possible different intrinsic connectivity
> of a network.
>
>
> 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.
>
>
>
> 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?)
>
>
> Yes from a coding point of view this seems ok. Only 1 driving input which
> are the 2 types of events put together, i.e. with the same drive on the
> network, and the 2nd input made of only one sort of events.
> Though the same worry as previous comment applies here: Does it make sense
> to have an instantaneous modulation of connectivity ?
>
>
> Apologies if some of the above questions illustrate an incorrect
> understanding of the DCM code.
>
> Sounds rather like deep understanding of DCM machinery!
>
> HTH,
> Chris
>
>
> BW,R
>
>
>
>
>
--
Darren Gitelman, MD
710 N. Lake Shore Dr., 1122
Chicago, IL 60611
Ph: (312) 908-8614
Fax: (312) 908-5073
