Dear Melina,
This sounds odd, and I am not sure what the problem could be. Here
are a few things that you may wish to check (unless you have already done so):
1. How many steps does the estimation take? Does it converge after a
single step? If so, have you double checked that you specified the
driving inputs correctly? Also, do you get any warnings or errors
messages during estimation?
2. Are you sure that the values in DCM.A are really zero, or not
just close to zero? You can check this by printing the values at
higher precision, using something like the sprintf command in Matlab.
3. Did you manually specify the DCM that you are trying to estimate,
or was it created algorithmically? If the latter, have you double
checked whether anything went wrong in transferring the timeseries,
i.e. does DCM.xY.u look like it contains your time series?
This is all I can think of at the moment. You may also want to try
estimating the model with the code in SPM 5. Generally, I would
always recommend to use SPM 5 for DCM. The code is much more robust
than in SPM 2.
Best wishes,
Klaas
At 18:57 13/06/2007, you wrote:
>Hi Klaas,
>
>I'm a postdoc working with Mick Rugg in California, and I've an odd (and
>hopefully quick) DCM question for you. In running my fourth set of DCMs, I'm
>running into a problem I've not encountered before. I'm looking at how
>regions that exhibit sustained activity interact with those showing
>transient activity, but when both sets of regions are included in the same
>model, the parameters in the A matrix corresponding to the connection
>strength from the sustained to the transient regions do not seem to get
>estimated. The DCM.a is appropriate, but the DCM.A contains 0s in the
>relevant cells (yet has appropriate values in the cells for the visual
>region to the transient regions).
>
>I should qualify this by stating I'm still using SPM2, as all my prior DCMs
>have been estimated in SPM2. I've tried to run diagnostics by systematically
>excluding regions or otherwise changing the A matrix, yet I find no rhyme or
>reason to when the parameters are estimated and when they're not. Have you
>encountered this before?
>
>Any help would be greatly appreciated. Mick sends his best.
>
>Thanks in advance,
>~Melina Uncapher
>
>*****************************************
>Melina R. Uncapher, Ph.D.
>Functional NeuroImaging of Memory Group
>Center for the Neurobiology of Learning and Memory
>Department of Neurobiology and Behavior
>University of California at Irvine
>http://fnim.bio.uci.edu
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>
>
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