Neural circuits and topographic organization of the basal ganglia and related regions

The present review was attempted to analyze the multiple channels of basal ganglia-thalamocortical connections, and the connections of their related nuclei. The prefrontal and motor areas consist of a number of modules, which seem to provide multiple subloops of the basal ganglia-thalamocortical connections in subhuman primates. There may be a great degree of convergence of the limbic, associative and motor loops at the level of the striatum, substantia nigra, pallidum, and the subthalamic nucleus as well as the pedunculopontine nucleus. Nigral dopaminergic neurons receive limbic input directly as well as indirectly through the striosomes in the striatum. Dopamine contributes to behavioral learning by signaling motivation and reinforcement. The pedunculopontine nucleus might be involved in behavioral state control, learning and reinforcement processes, locomotion and autonomic functions. Each subdivision of the motor areas receives a mixed and weighted transthalamic input from both the cerebellum and basal ganglia. In particular, based on the author's data, the hand/arm motor area and adjacent premotor area receive strong superficial basal ganglia-thalamocortical projections as well as the deep cerebello-thalamocortical projections. These areas, have very dense corticocotrical connections with other cortical areas, receive polymodal afferents from the parietal and temporal cortices, and integrated information, via multiple routes, from the prefrontal cortex. The author suggests that the ventrolateral part of the caudal medial pallidal segment (GPi) and the ventromedial part of the GPi are linked directly to these areas by ways of the oral part of ventral lateral nucleus (VLo) and the ventral part of the parvicellular part of ventral anterior nucleus (VApc), respectively. These connections are thought to be involved in the acquisition and coordination of motor sequences.