Striatal dopamine D1 receptors are involved in the dissociation of learning based on reward-magnitude

Here we investigate the contribution of striatal dopamine receptors (D1) to the influence of reward-magnitude on learning. Pigeons (Columba livia) were trained on a discrimination-task with two pairs of stimuli; correct discrimination resulted in a large reward in one pair of stimuli and in a small reward in the other pair. Acquisition of the discrimination-task was accompanied by intracranial injections to the medial striatum, either of a dopamine-antagonist (Sch23390) or of vehicle. In the control-condition the rate of learning was modulated by the magnitude of the reward; discrimination was learned faster if contingent rewards were large and learning was slower if contingent rewards were small. Following injections of D1 antagonist this effect vanished even though the ability to discriminate between the rewards was unaffected. Interestingly, the mean rate of learning was indistinguishable between the control and antagonist conditions. Consequently, it appears that not learning per se but the effect of reward-magnitude on learning is mediated through D1 receptors in the striatum. We argue that the injections of dopamine-antagonist cause a shift in strategy underlying learning. In the control-condition animals rely on positive feedback and thus learning is affected by the magnitude of the contingent reward; in the antagonist-condition, however, learning might rely on negative feedback and is thus insensitive to reward-magnitude.