Figure 1.

Reinforcement learning experiments and involved learning circuit. (A) Sequence of events in the one direction rewarded saccade task (1DR). The monkey first fixated at the central spot (the dotted circle indicates the eye position). As the fixation point disappeared, a target appeared randomly on the right or left and the monkey was required to make a saccade to it immediately. Correct saccades in one direction were followed by a tone and juice reward; saccades in the other direction followed by a tone alone. The rewarded direction was fixed in a block of 24 trials, and was changed in the following block. (B) Distribution of saccade latencies in reward trials (in red) and in no-reward trials (in blue). (C) Illustration of D1 and D2 antagonist experiments. D1 or D2 antagonist was administrated in the caudate to examine the behavioral consequence in the 1DR task. Black, red, and purple connections indicate excitatory, inhibitory, and dopaminergic modulatory connections, respectively. (D) Hypothesized circuit involving D1 and D2 mediated plasticities. D1 and D2 mediated plasticities in direct and indirect pathways are assumed to contribute to eye movements. The purple arrows indicate dopaminergic modulatory connections. The lines with rectangle ends indicate inhibitory connections. Arrow ends indicate excitatory connections. Figures (A) and (B) are from Hong and Hikosaka (2008). Abbreviations: CD, caudate nucleus; D1, D2, D1, and D2 receptors; SC, superior colliculus; SNc/SNr, substantia nigra pars compacta/reticulata; GPe, globus pallidus external segment; FEF, frontal eye field; SEF, supplementary eye field; DLPF, dorsolateral prefrontal cortex; LIP, lateral intraparietal area; STN, subthalamic nucleus.