Modeling the role of basal ganglia in saccade generation: is the indirect pathway the explorer?

We model the role played by the Basal Ganglia (BG) in the generation of voluntary saccadic eye movements. The BG model explicitly represents key nuclei like the striatum (caudate), Substantia Nigra pars reticulata (SNr) and compata (SNc), the Subthalamic Nucleus (STN), the two pallidal nuclei and Superior Colliculus. The model is cast within the Reinforcement Learning (RL) framework, with the dopamine representing the temporal difference error, the striatum serving as the critic, and the indirect pathway playing the role of the explorer. Performance of the model is evaluated on a set of tasks such as feature and conjunction searches, directional selectivity and a successive saccade task. Behavioral phenomena such as independence of search time on number of distractors in feature search and linear increase in search time with number of distractors in conjunction search are observed. It is also seen that saccadic reaction times are longer and search efficiency is impaired on diminished BG contribution, which corroborates with reported data obtained from Parkinson's Disease (PD) patients.