The classical model of blood oxygen level dependent (BOLD) responses by Buxton et al. (1998) has been very important in providing a biophysically plausible framework for explaining different aspects of hemodynamic responses. We have derived a generalized model that has been included in SPM8 as part of DCM for fMRI. Bayesian model selection using empirical fMRI data from a group of twelve subjects showed that this model outperforms other variants of hemodynamic models. This extended hemodynamic model has a non-linear BOLD function and includes a new parameter per region. This parameter represents the region-specific ratio of intra- and extravascular signals and makes DCM more robust for applications to data acquired at higher field strengths.

K.E. Stephan, N. Weiskopf, P.M. Drysdale, P.A. Robinson, K.J. Friston. Comparing hemodynamic models with DCM. NeuroImage 38: 387-401, 2008.
 
These descriptions of the new features are taken from the SPM8 Release Notes
 
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