Towards Automaticity in Reinforcement Learning: A Model-Based Functional Magnetic Resonance Imaging Study

Introduction: Previous studies showed that over the course of learning many neurons in the medial prefrontal cortex adapt their firing rate towards the options with highest predicted value reward but it was showed that during later learning trials the brain switches to a more automatic processing mode governed by the basal ganglia. Based on this evidence, we hypothesized that during the early learning trials the predicted values of chosen options will be coded by a goal directed system in the medial frontal cortex but during the late trials the predicted values will be coded by the habitual learning system in the dorsal striatum.

Methods: In this study, using a 3 Tesla functional magnetic resonance imaging scanner (fMRI), blood oxygen level dependent signal (BOLD) data was collected whilst participants (N=12) performed a reinforcement learning task. The task consisted of instrumental conditioning trials wherein each trial a participant choose one of the two available options in order to win or avoid losing money. In addition to that, depending on the experimental condition, participants received either monetary reward (gain money), monetary penalty (lose money) or neural outcome.

Results: Using model-based analysis for functional magnetic resonance imaging (fMRI) event related designs; region of interest (ROI) analysis was performed to nucleus accumbens, medial frontal cortex, caudate nucleus, putamen and globus pallidus internal and external segments. In order to compare the difference in brain activity for early (goal directed) versus late learning (habitual, automatic) trials, separate ROI analyses were performed for each anatomical sub-region. For the reward condition, we found significant activity in the medial frontal cortex (p<0.05) only for early learning trials but activity is shifted to bilateral putamen (p<0.05) during later trials. However, for the loss condition no significant activity was found for early trials except globus pallidus internal segment showed a significant activity (p<0.05) for later trials.

Conclusion: We found that during reinforcement learning activation in the brain shifted from the medial frontal regions to dorsal regions of the striatum. These findings suggest that there are two separable (early goal directed and late habitual) learning systems in the brain.