Relative salience signaling within a thalamo-orbitofrontal circuit governs learning rate

Learning to predict rewards is essential for the sustained fitness of animals. Contemporary views suggest that such learning is driven by a reward prediction error (RPE)—the difference between received and predicted rewards. The magnitude of learning induced by an RPE is proportional to the product of the RPE and a learning rate. Here we demonstrate using two-photon calcium imaging and optogenetics in mice that certain functionally distinct subpopulations of ventral/medial orbitofrontal cortex (vmOFC) neurons signal learning rate control. Consistent with learning rate control, trial-by-trial fluctuations in vmOFC activity positively correlate with behavioral updating when the RPE is positive, and negatively correlates with behavioral updating when the RPE is negative. Learning rate is affected by many variables including the salience of a reward. We found that the average reward response of these neurons signals the relative salience of a reward, because it decreases after reward prediction learning or the introduction of another highly salient aversive stimulus. The relative salience signaling in vmOFC is sculpted by medial thalamic inputs. These results support emerging theoretical views that prefrontal cortex encodes and controls learning parameters. •Novel approach to test whether neural activity abides by learning rate control•Outcome response in orbitofrontal cortex (OFC) is consistent with learning rate control•OFC outcome response signals relative salience of an outcome•Medial thalamic input to OFC causally mediates encoding in OFC By developing a theoretical approach to test neuronal control of learning rate (a key reinforcement learning variable), Namboodiri et al. show that outcome responses in a thalamo-orbitofrontal circuit guide learning rate by signaling the relative salience of outcomes.