Dear Will,
> Given that you know the true parameters you should check (i) are they within the range of the priors (See spm_dcm_fmri_priors.m) - if not the optimisation algorithm will never find them,
I am a little bit confused about that. What do you mean with "within the
range of the priors"? The neural priors seem to be encoded as
% enforce self-inhibition
%----------------------------------------------------------------------
A = A > 0;
A = A - diag(diag(A));
% prior expectations
%----------------------------------------------------------------------
pE.A = A/(64*n) - eye(n,n)/2;
pE.B = B*0;
pE.C = C*0;
pE.D = D*0;
% prior covariances
%----------------------------------------------------------------------
pC.A = A*a/n + eye(n,n)/(8*n);
pC.B = B;
pC.C = C;
pC.D = D;
% and add hemodynamic priors
%==========================================================================
pE.transit = sparse(n,1);
pE.decay = sparse(n,1);
pE.epsilon = sparse(1,1);
pC.transit = sparse(n,1) + exp(-6);
pC.decay = sparse(n,1) + exp(-6);
pC.epsilon = sparse(1,1) + exp(-6);
pC = diag(spm_vec(pC));
For example, in a model with four regions, the prior expectation for
intrinsic connections is 0.0039 (and self-inhibition is set to -0.5?)
But when I look into my estimated DCMs I see (significant) neural
coupling strengths between 0.01 to 2.0, which seems to be ok. So why
shouldn't DCM (along with BMS) not be able to correctly estimate neural
parameters within a larger range?
Furthermore, if I understand it correctly, the prior for the time
scaling parameter is set to zero. Isn't this a bit unrealistic as this
value causes a delay / smoothing effect in the neural (and hrf) signal
in the range of seconds (given identical hemodynamic parameters for all
regions?).
> (ii) are the parameter estimates DCM.Ep close to the true values, esp important for the connections that are different between models.
>
Does this imply that, in general, the sensitivity of DCM should be
checked for various neural and HRF parameters?
Best,
Martin
--
SPM for programmers
http://spm.martinpyka.de
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