IN THIS CHAPTER, WE DESCRIBE HOW the cerebral cortex uses sensory information from the external world to guide motor actions that allow the individual to interact with the surrounding environment. We begin with a general description of what we mean by the term voluntary movement and some theoretical frameworks for understanding its control, followed by the basic anatomy of the cortical circuits involved in voluntary motor behavior. We then consider how information related to the body, external space, and behavioral goals is combined and processed in parietal cortical regions. This is followed by a discussion of the role of premotor cortical regions in selecting and planning motor actions. Finally, we examine the role played by the primary motor cortex in motor execution.
Voluntary Movement Is the Physical Manifestation of an Intention to Act
Animals, including humans, have a nervous system not just so that they can sense their world or think about it, but primarily to interact with it to survive and reproduce. Understanding how purposeful actions are achieved is one of the great challenges in neuroscience. We focus here on the cerebral cortical control of voluntary motor behavior, in particular voluntary arm and hand movements in primates.
In contrast to stereotypical fixed-latency reflexive responses that are automatically triggered by incoming sensory stimuli (Chapter 32), voluntary movements are purposeful, intentional, and context-dependent, and are typically accompanied, at least in humans, by a sense of “ownership” of the actions, the sense that the actions have been willfully caused by the individual. Decisions to act are often made without an external trigger stimulus. Moreover, the continuous flux of events and conditions in the world presents changing opportunities for action, and thus voluntary action involves choices between alternatives, including the choice not to act. Finally, the same object or event can evoke different actions at different times, depending on the current context.
Throughout evolution, these features of voluntary behavior have become increasingly prominent in higher primates, especially in humans, indicating that the neural circuits controlling voluntary behavior in primates are adaptive. In particular, evolution has resulted in an increasing degree of dissociation of the physical properties of sensory inputs from their behavioral salience to the individual. Adaptation of the control circuits also enhances the repertoire of voluntary motor actions available to a species by allowing individuals to remember and learn from prior experience, to predict the future outcomes of different action choices, and to adopt new strategies and find new solutions to attain their desired goals. Volitional self-control over how, when, and even whether to act endows primate voluntary behavior with much of its richness and flexibility and prevents behavior from becoming impulsive, compulsive, or even harmful.
Voluntary behavior is the physical manifestation of an individual’s intention to act on the environment, usually to achieve a goal immediately or at some point in the ...