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THE TRADITIONAL VIEW THAT THE BASAL ganglia play a role in movement arises primarily because diseases of the basal ganglia, such as Parkinson and Huntington disease, are associated with prominent disturbances of movement, and from the belief that basal ganglia neurons send their output exclusively to the motor cortex by way of the thalamus. However, we now know that the basal ganglia also project to wide areas of the brain stem and via the thalamus to nonmotor areas of the cerebral cortex and limbic system, thereby providing a mechanism whereby they contribute to a wide variety of cognitive, motivational, and affective operations. This understanding also explains why diseases of the basal ganglia are frequently associated with complex cognitive, motivational, and affective dysfunction in addition to the better-known motor disturbances.

This chapter provides a perspective on the fundamental contributions of the basal ganglia (Figure 38–1) to overall brain function. Recent advances in the fields of artificial neural networks and robotics emphasize that behavioral function is an emergent property of signal processing in physically connected networks (Chapter 5). Thus, how components of networks are connected and how their input signals are transformed into output signals impose important constraints on final behavioral outputs. We first describe the principal anatomical and physiological features of the basal ganglia network and consider the constraints these might impose on their function. We consider the extent to which the basal ganglia have been conserved during vertebrate brain evolution and, based on these insights, review evidence suggesting that the basal ganglia’s normal functions are to select between incompatible behaviors and to mediate reinforcement learning. We conclude by examining important insights into how the system can malfunction in some of the major diseases involving the basal ganglia.

Figure 38–1

The basal ganglia and surrounding structures. The nuclei of the basal ganglia are identified on the right in this coronal section of a human brain. (Adapted from Nieuwenhuys, Voogd, and van Huijzen 1981.)

The Basal Ganglia Network Consists of Three Principal Input Nuclei, Two Main Output Nuclei, and One Intrinsic Nucleus

The striatum (a collective term for the caudate nucleus and putamen; see Figure 38–1), subthalamic nucleus, and substantia nigra pars compacta/ventral tegmental area are the three major input nuclei of the basal ganglia, receiving signals directly and indirectly from structures distributed throughout the neuraxis (Figure 38–2).

Figure 38–2

The principal input, intrinsic, and output connections of the mammalian basal ganglia. The main input nuclei are the striatum (STR), subthalamic nucleus (STN), and substantia nigra pars compacta (not shown). They receive input directly from the thalamus, cerebral cortex, and limbic structures (amygdala and hippocampus). The main output nuclei are the substantia nigra pars reticulata (SNr) and internal globus pallidus/entopeduncular nucleus (not shown). ...

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