Basal ganglia mechanisms of reward-oriented eye movement

Expectation of reward facilitates motor behaviors that enable the animal to approach a location in space where the reward is expected. It is now known that the same expectation of reward profoundly modifies sensory, motor, and cognitive information processing in the brain. However, it is still unclear which brain regions are responsible for causing the reward-approaching behavior. One candidate is the dorsal striatum where cortical and dopaminergic inputs converge. We tested this hypothesis by injecting dopamine antagonists into the caudate nucleus (CD) while the monkey was performing a saccade task with a position-dependent asymmetric reward schedule. We previously had shown that: (1) serial GABAergic connections from the CD to the superior colliculus (SC) via the substantia nigra pars reticulata (SNr) exert powerful control over the initiation of saccadic eye movement and (2) these GABAergic neurons encode target position and are strongly influenced by expected reward, while dopaminergic neurons in the substantia nigra pars compacta (SNc) encode only reward-related information. Before injections of dopamine antagonists the latencies of saccades to a given target were shorter when the saccades were followed by a large reward than when they were followed by a small reward. After injections of dopamine D1 receptor antagonist the reward-dependent latency bias became smaller. This was due to an increase in saccade latency on large-reward trials. After injections of D2 antagonist the latency bias became larger, largely due to an increase in saccade latency on small-reward trials. These results indicate that: (1) dopamine-dependent information processing in the CD is necessary for the reward-dependent modulation of saccadic eye movement and (2) D1 and D2 receptors play differential roles depending on the positive and negative reward outcomes.