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Dear Simon, This is a rather bizarre finding. Without more information on the type of models you compared it is difficult, however, to say what might have gone wrong. If you could send me two of your bilinear DCMs, I am happy to look into this problem as soon as I find some time. If you could provide me with further information on how you constructed the models (wrt. choice of inputs and model space), that would also be helpful. Best wishes, Klaas ________________________________ Von: "Eickhoff, Simon" <[log in to unmask]> An: SPM@JISCMAIL.AC..UK 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 ------------------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------------------ 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 ------------------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------------------ __________________________________________________ Do You Yahoo!? Sie sind Spam leid? Yahoo! Mail verfügt über einen herausragenden Schutz gegen Massenmails. http://mail.yahoo.com