Dynamic causal models of brain responses
Functional magnetic resonance imaging (fMRI) is used to investigate where the neural implementation of specific cognitive processes occurs. The standard approach uses linear convolution models relating experimentally designed inputs, through a hemodynamic response function (HRF), to observed blood oxygen level dependent (BOLD) signals. However, these models are blind to the causal mechanisms that underlie observed BOLD responses, both in terms of neural dynamics and neurovascular coupling. This article reviews current developments towards causal models of how BOLD responses are generated. We focus on (i) biophysical input-state-output models with neural and hemodynamic state equations, and (ii) models of functional integration that explain local dynamics through interactions with remote areas. Forward models with parameters at the neural level, e.g. Dynamic Causal Modeling, combine both approaches, modeling the entire causal chain from external stimuli, via induced neural dynamics, to observed BOLD responses.