Von: "Eickhoff, Simon"
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Gesendet: Donnerstag, den 4. Februar 2010, 14:05:59 Uhr
Betreff: [SPM] Strange behaviour of model evidence
Dear Klaas and DCM-experts
Recently, we encountered what we considered a rather strange behaviour of model evidence in DCM for fMRI (SPM8):
We tried to explain an (positive) interaction observed in the GLM analysis of a 2x2 design in region Z by different models of feed-forward from the two regions [X, Y] showing the main effects for the two factors [x and y].
So we set up and estimated several model.
In all models, the values obtained for the different parameters (driving input, intrinsic connections and modulations) were well plausible.
Strangely, however, the model evidence DCM.F as well
as AIC/BIC is exactly (!) identical across models.
Closer analysis showed, that in spite of the different connection patterns (e.g., different patterns of modulation such as X->Z modulated by y or Y->Z modulated by x) and the plausible parameters (in the range of 0.01 to 0.1), the residuals in Z were basically identical.
Any thoughts on this? Seems not logical to me!
Thanks in advance
Simon
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Forschungszentrum Juelich GmbH
52425 Juelich
Sitz der Gesellschaft: Juelich
Eingetragen im Handelsregister des Amtsgerichts Dueren Nr. HR B 3498
Vorsitzende des Aufsichtsrats: MinDir'in Baerbel Brumme-Bothe
Geschaeftsfuehrung: Prof. Dr. Achim Bachem (Vorsitzender),
Dr. Ulrich Krafft (stellv. Vorsitzender), Prof.
Dr.-Ing. Harald Bolt,
Prof. Dr. Sebastian M. Schmidt
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