[Cortico-basal ganglia circuits--parallel closed loops and convergent/divergent connections]

The basal ganglia play important roles not only in motor control but also in higher cognitive functions such as reinforcement learning and procedural memory. Anatomical studies on the neuronal connections between the basal ganglia, cerebral cortex, and thalamus have demonstrated that these nuclei and cortical areas are interconnected via independent parallel loop circuits. The association, motor, and limbic cortices project to specific domains in the striatum, which, in turn, project back to the corresponding cortical areas via the substantia nigra/globus pallidus and the thalamus. Likewise, subregions in the motor cortex representing different body parts project to specific regions in the putamen, which project back to the original motor cortical regions. These parallel loops have been thought to be the basic anatomical structures involved in the basal ganglia functions. Furthermore, neuronal projections communicating between different loops (or functional domains) have also been discovered. A considerable number of corticostriatal projections from functionally interrelated cortical areas (e. g., hand representations of the motor cortex and somatosensory cortex) converge at the striatum. It has also been suggested that the location of the substantia nigra is in such that it can transmit information from the 'limbic loop' to the 'association loop', and from the 'association loop' to the 'motor loop'. Furthermore, a recent transsynaptic neuronal tracing study conducted at our laboratory demonstrated that the ventral (limbic) striatum sends divergent outputs to multiple regions in the frontal cortex. These 'inter-loop' connections would be important for the integration of information to achieve goal-directed behaviors.