Just to clarify why I bring this up: In the SPM5 GUI, when you select
variational Bayes estimation, it gives you options for spatial priors on the AR
coefficients and states that the GMRF priors were favored by Bayesian model
comparison, which I don't think is a correct statement (even if GMRF priors are
in fact an accurate representation of one's prior beliefs). That said, in practice
these priors are probably reasonable most of the time, and something similar is
being done by spm_reml in the classical inference scheme anyways.
Sam Gershman
Department of Psychology
Princeton University
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