Humans and animals construct internal models of their environment in order to select
appropriate courses of action. The representation of uncertainty about the current state of the
environment is a key feature of these models that controls the rate of learning as well as directly
affecting choice behaviour. To maintain flexibility, given that uncertainty naturally decreases over
time, most theoretical inference models include a dedicated mechanism to drive up model
uncertainty. Here we probe the long-standing hypothesis that noradrenaline is involved in
determining the uncertainty, or entropy, and thus flexibility, of neural models. Pupil diameter,
which indexes neuromodulatory state including noradrenaline release, predicted increases (but not
decreases) in entropy in a neural state model encoded in human medial orbitofrontal cortex, as
measured using multivariate functional MRI. Activity in anterior cingulate cortex predicted pupil
diameter. These results provide evidence for top-down, neuromodulatory control of entropy in
neural state models