A crowd watches a tightrope walker. The specific movements and perilous position of the acrobat are thought to resonate with viewers by engaging their system of mirror neurons. These neurons are activated both when an individual performs a set of movements, and when he or she observes those same movements performed by others. Similar neural systems may underlie social cognition as well, allowing us to perceive and comprehend the mental states of others. (Reproduced, with permission, from Arthur Paxton.)
So far in this book we have examined the properties of individual nerve cells and how they communicate at synapses to produce simple reflex behaviors. We now begin to consider larger, interconnected networks of neurons, the complex circuits that give rise to mental activity: perception, planned action, and thought. The field of systems neuroscience aims to understand how these networks produce the cognitive functions of the brain, one of the ultimate challenges of science. We need to know how sensory information is perceived, and how perceptions are assembled into inner representations and recruited into plans for immediate behavior or concepts for future actions. It is still unclear how complex memories are made and how percepts, ideas, and feelings are transformed into language.
Neural science first emerged in the mid-1950s with the development of powerful techniques for exploring the cellular dynamics of the nervous system and with the convergence into a single discipline of several previously separate disciplines concerned with the brain and behavior: molecular biology, neuroanatomy, electrophysiology, and cell and developmental biology. The modern science of mind is the pragmatic result of the attempt to merge neural science with cognitive psychology. New techniques permit us to observe the system properties of the brain directly, not only in animal models, but in controlled behavioral experiments in alert, behaving people. As a result neural science is able to address testable hypotheses about how brain functions lead to mental processes such as perception, memory, decisions, and actions.
The aim of the new science of mind is to examine classical philosophical and psychological questions about mental functions in the light of modern cell and molecular biology. This is a bold undertaking. How do we begin to think about perception, ideas, action, and feelings in biological terms? So far, progress in understanding the major functional systems of the brain—the sensory, motor, motivational, memory, and attentional systems—has benefited from a reductionist approach to mental function. This approach is based on the assumption that these functions will emerge from the biological properties of nerve cells and of their pattern of interconnections. According to this view, which was introduced in Chapter 1, brain and mind are inseparable. Mind can be considered a set of operations carried out by the brain, an information-processing organ made powerful by the enormous number, variety, and interactions of its nerve cells and by the complexity of interconnection among these cells. In this section and later parts of this book, we describe the attempt to extend this cell biological approach beyond the neuron doctrine to the neuronal circuit doctrine, to the cognitive functions of the brain. We focus specifically on the major domains of cognitive neural science: perception, action, motivation, attention, learning, and memory.
An understanding of the biological basis of cognitive functions requires deep appreciation of the anatomy of the neural systems that subserve these functions in the brain. In the same way that the detailed structure of a protein reveals important principles of its action, knowledge of neuroanatomy and physiology can provide profound insight into how the nervous system functions. Just as many contemporary ideas about the dynamic mechanisms underlying the development of connectivity in the nervous system were anticipated a century ago by Ramón y Cajal on the basis of Golgi images of neurons in histological specimens, we predict that much of our understanding of higher brain function will depend on refined mapping of neuronal circuits and analysis of the signals that pass through those circuits.
Modern anatomical, physiological, and imaging techniques are revealing how neural circuits are organized. For example, the sensory pathways from one brain region to the next are organized in such a way that neighboring groups of neurons in the brain maintain the spatial relationship of sensory receptors in the periphery of the body. This topological organization is an important way of conveying spatial information about sensory events. In recent years, the study of connectivity in the brain has been advanced even further with new imaging techniques, such as diffusion-tensor imaging. These techniques have made visible the patterns of interconnectivity of different regions of the living human brain during specific behaviors. As a consequence, a much clearer idea of the brain regions involved in many complex cognitive functions is emerging.
In this part of the book, we first review in Chapters 15 and 16 the anatomical organization of the three major functional subdivisions of the nervous system: sensory, motor, and modulatory. We also take a closer look at the structural and functional organization of the central nervous system by following the flow of sensory information from the periphery into the spinal cord and brain, the transformation of that information into a motor command, and the effect of that command on muscle, the organ of behavior. We then examine in Chapters 17 and 18 the cognitive processes of the brain that are concerned with visual perception, planned action, memory, and selective conscious attention. Many of these activities are represented in higher-order-association regions of the cerebral cortex, areas that bring together information from various sensory systems to provide coordinated plans for action. A number of major insights in action and perception are now emerging. Perhaps most importantly, we examine in Chapter 19 why we no longer conceive of sensory and motor and cognitive processes as occurring sequentially. Rather perception and the planning for action occur simultaneously, and the higher motor systems have cognitive functions. One of the important questions that we shall examine in this section is how mental functions are represented in different regions of the brain. In doing so we shall explore what approaches can be used to render cognitive processes such as attention, motivation, and even consciousness accessible to rigorous physiological and anatomical analysis.
In later parts of the book we shall explore each functional system of the brain in detail, examining how the specific structure and cellular interconnections of a system determine its particular function.