We quickly learn that some of our actions result in reward. But reward comes in different magnitudes and we should be able to repeat those behaviors that are associated with the highest amount of reinforcement. In a new study, Biopsychologists from Boston, Oxford, and Bochum investigated the contribution of striatal dopamine receptors (D1) on the learning of reward-magnitude. To this end, pigeons 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. 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. The intriguing possibility is this: It is conceivable that the injections of the dopamine-antagonist caused a shift in learning strategy. In the control-condition animals possibly relied on positive feedback and thus their learning was affected by the magnitude of the contingent reward; in the antagonist-condition, however, learning might have relied on negative feedback and was thus insensitive to reward-magnitude.

Rose, J., Schiffer, A.-M., Güntürkün, O. (2013). Striatal dopamine D1 receptors are involved in the dissociation of learning based on reward-magnitude. Neuroscience, 230, 132-138.