Striatal dopamine encodes the relationship between actions and reward

Although the role of striatal dopamine in Pavlovian conditioning and in habits has been reasonably well described, relatively little is known about its function in goal-directed action. In this study we trained hungry rats on two lever press actions for distinct food outcomes and recorded dopamine release in the dorsomedial striatum as these action-outcome associations were encoded and subsequently degraded. During initial training the lever press actions generated bilateral dopamine release that was found to reflect the predicted action value. This value was updated by the prediction error generated by the feedback produced by contact with the outcome, or its absence, after the press. Importantly, hemispheric dopamine release became increasingly lateralized across the course of training, with greater release in the hemisphere contralateral to the press. Using video analysis and multiple different measures, we could find no evidence that the degree of lateralized release was associated with movement; rather, we found that it tracked the strength of the action-outcome association, increasing and decreasing with increments and decrements in the contingency between specific actions and their consequences. Similar results emerged whether the rewards were delivered on ratio or interval schedules of reinforcement and whether we used unpaired outcome delivery or outcome-identity reversal to modify the specific contingencies. These findings suggest that, whereas moment-to-moment fluctuations in action value are reflected in bilateral dopamine release, a second signal broadcasts the overall strength of specific action-outcome relationships via the difference between contralateral and ipsilateral release during actions.