Dear DCMers,
in a DCM analysis I want to fix a "parameter" at a pre-specified value,
so that it is no longer a (variable) parameter. In order to do so, I
supplied the structure "P" in the options-field of the DCM structure,
where the respective value for an endogenous connection was set to 0.5
or 1 (I tried both separately). The respective value for the covariance
in DCM.a was set to zero, so that the value was indeed fixed and did not
vary during inversion of the model.
After inversion, however, the structure containing the estimated
parameters (DCM.Ep) indicates that the value for that specific
endogenous connection was zero (and not 0.5 or 1 as specified in the
options). Nevertheless, other parameters of the models indeed changed
compared to the model without an initialization, indicating that the
initialization had an impact on parameter estimation. In addition,
however, the parameters of two structurally identical models with
different fixed values in options.P did not differ from each other. In
other words, it seemed as if it did not matter if the value was fixed at
0.5 or 1 with respect to the impact on estimation of other free
parameters (in contrast to an omission of any initialization as
mentioned above).
Against this background I have basically two questions:
1) Is our procedure suited for the described purpose in the first place?
That is to say, is our assumption correct that initializing a parameter
in options.P and setting its covariance in DCM.a to zero will fix this
connection to the pre-specified value? (If this is not correct, what
would be the right way?)
2) If our procedure is correct, I wonder why the respective value in
DCM.Ep is rather indicating zero instead of the value that has been
specified in options.P.
Thanks a lot in advance and
best wishes,
Thilo
--
Thilo Kellermann
Department of Psychiatry, Psychotherapy and Psychosomatics
Uniklinik RWTH Aachen
JARA Translational Brain Medicine
Pauwelsstr. 30
52074 Aachen
Germany
Tel.: +49 (0)241 / 8089977
Fax.: +49 (0)241 / 8082401
E-Mail:[log in to unmask]
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