THE HUMAN BRAIN carries out actions in ways no current computer can begin to approach. Merely to see—to look onto the world and recognize a face or facial expression—entails amazing computational achievements. Indeed, all our perceptual abilities—seeing, hearing, smelling, tasting, and touching—are analytical triumphs. Similarly, all of our voluntary actions are triumphs of engineering. Sensation and movement, while wondrous in their own right, pale in comparison to complex cognitive behaviors such as forming memories or understanding social conventions.
The brain accomplishes these computational feats because its nerve cells are wired together in very precise functional circuits. The brain is hierarchically organized such that information processed at one level is passed to higher-level circuits for more complex and refined processing. In essence, the brain is a network of networks. Different brain areas work in an integrated fashion to accomplish purposeful behavior.
In this chapter, we outline the neuroanatomical organization of some of the circuits that enable the brain to process sensory input and produce motor output. We focus on touch as a sensory modality because the somatosensory system is particularly well understood and because touch clearly illustrates the interaction of sensory processing circuits at several levels, from the spinal cord to the cerebral cortex. Our purpose here is to illustrate the basic principles of how circuits control behavior. In the next chapter, we consider the functional properties of these circuits, including the computations by which they process information. In subsequent chapters, we consider in more detail the anatomy and function of the various sensory modalities and how sensory input regulates movement.
Finally, we provide a preview of the brain circuits that are instrumental in producing the memories of our daily lives, called explicit memory (see Chapters 52 and 54). We do this to make the point that while many of the neurons in the memory circuits are similar to those in the sensory and motor circuits, not all are. Moreover, the organization of the pathways between circuits is different in the memory system than it is in the motor and sensory systems. This highlights a basic neurobiological tenet that different circuits of the brain have evolved an organization to most efficiently carry out specific functions.
Comprehending the functional organization of the brain might at first seem daunting. But as we saw in the previous chapter, the organization of the brain is simplified by three anatomical considerations. First, there are relatively few types of neurons. Each of the many thousands of spinal motor neurons or millions of neocortical pyramidal cells has a similar anatomical structure and serves a similar function. Second, neurons in the brain and spinal cord are clustered in functional groups called nuclei or discrete areas of the cerebral cortex, which form networks or functional systems. Third, the discrete areas of the cerebral cortex are specialized for sensory, motor, or associative functions such as memory.