Modeling the organization of the basal ganglia.

INTRODUCTION

Several models have been elaborated to describe the structure and function of the basal ganglia, but its different levels of architectural organization, macroscopic anatomy, connectivity, functional territorial subdivision and neuronal morphology have rarely been considered. In this review, I present some of these models and I analyze the functioning of the basal ganglia at the light of its architectural properties.

STATE OF THE ART

The basal ganglia form an important neuronal system, which interacts with the cerebral cortex through a complex series of loop circuits. While the morphological, electrophysiological and biochemical properties of this system are progressively known better and better, they have led to various interpretations from which different, sometimes contradictory, models have been constructed. The basal ganglia are often analyzed as homogeneous nuclei that communicate through excitatory or inhibitory connections, the so-called "box and arrows" models. Among them, the dual-, triple- and five circuit models are the most popular. Analysis of the "inside of the boxes" provides, however, important data such as the functional subdivision into three, motor, associative and limbic territories and the demonstration that integrative properties are a characteristic of the basal ganglia.

PERSPECTIVES

Considering these properties, the way cortical information is processed in the basal ganglia can be analyzed, which leads to modeling its organization and functioning. The striatum receives a compressed version of cortical information and transforms it through complex processes of activation/deactivation under a double dopaminergic and cholinergic control that enables behavioral reinforcement learning. The globus pallidus behaves as a keyboard on which various behavioral repertoires can be coded, from the simplest movement of a single joint to the most complex motor sequence involving the entire body and expressing an emotional content in a cognitive context. The role of the subthalamic nucleus must be considered at different scales. At the macroscopic scale, it works as a thermostat that regulates the level of execution of cortical commands. At a territorial scale, it can process separately motor, cognitive and emotional information. At the neuronal scale, it assures a much finer neuronal representation of cortical commands and can integrate motor, cognitive and emotional aspects. New experiments in both animal models and human clinical-research protocols are needed to demonstrate the neuronal mechanisms of these processes.

CONCLUSION

A model is proposed that considers how neural information is processed in the basal ganglia during the execution of motor, cognitive and emotional activities.