Subregions of the ventral striatum show preferential coding of reward magnitude and probability.

As shown in non-human primate and human fMRI studies the probability and magnitude of anticipated rewards modulate activity in the mesolimbic dopaminergic system. Importantly, non-human primate data have revealed that single dopaminergic neurons code for both probability and magnitude of expected reward, suggesting an identical system. Using a guessing task that allowed the independent assessment of the factors probability and magnitude we were able to assess the impact of reward probability and magnitude in ventral striatal subregions in a large sample (n=98). We observed more anterior and lateral peak activation foci in the ventral striatum for reward probability and a more posterior and medial activation peak for reward magnitude, suggesting a functional segregation at the mesoscopic level. Importantly, this functional bias observed for the group average was also tested in each individual subject, allowing for proper random effects inference for the spatial dissociation. Taken together, our data point toward a functional dissociation of neuronal assemblies suggesting that certain populations of neurons are more sensitive to expected reward probability and other populations are more sensitive to reward magnitude.