[Morphological Re-evaluation of the Basal Ganglia Network]

Electrophysiological studies in monkeys have shown that dopaminergic neurons respond to the reward prediction error. In addition, striatal neurons alter their responsiveness to cortical or thalamic inputs in response to dopamine signals, via dopamine-regulated synaptic plasticity. These findings have led to the hypothesis that the striatum exhibits synaptic plasticity under the influence of reward prediction error and conducts reinforcement learning throughout the basal ganglia circuits. The reinforcement learning model is useful; however, the mechanism by which such a process emerges in the basal ganglia needs to be anatomically explained. The actor-critic model has been previously proposed and extended by the existence of role sharing within the striatum, with particular focus on the striosome and matrix compartments. However, this hypothesis has been difficult to confirm morphologically, partly because of the complex structure of the striosome and matrix compartments. Here, we review recent morphological studies that elucidate the input/output organization of the striatal compartments.