  In this paper we introduce a new hierarchical model for the simultaneous
detection of brain activation and estimation of the shape of the hemodynamic
response in multi-subject fMRI studies. The proposed approach circumvents a
major stumbling block in standard multi-subject fMRI data analysis, in that it
both allows the shape of the hemodynamic response function to vary across
region and subjects, while still providing a straightforward way to estimate
population-level activation. An efficient estimation algorithm is presented, as
is an inferential framework that not only allows for tests of activation, but
also for tests for deviations from some canonical shape. The model is validated
through simulations and application to a multi-subject fMRI study of thermal
pain.
