Direct Connections Between the Cerebral Cortex and Spinal Cord Play a Fundamental Role in the Organization of Voluntary Movements
The Four Premotor Areas of the Primate Brain Also Have Direct Connections in the Spinal Cord
Motor Circuits Involved in Voluntary Actions Are Organized to Achieve Specific Goals
The Hand Has a Critical Role in Primate Behavior
The Joint Activity of Neurons in the Parietal and Premotor Cortex Encodes Potential Motor Acts
Some Neurons Encode the Possibilities for Interaction with an Object
Mirror Neurons Respond to the Motor Actions of Others
Potential Motor Acts Are Suppressed or Released by Motor Planning Centers
An Overall View
In Chapter 18 we surveyed the higher-order organization of sensory systems. In this chapter we turn to the higher-order functioning of the motor systems, by examining how the brain represents behavioral goals and how voluntary actions are planned to achieve those goals. To illustrate how the motor systems generate goal-oriented behavior, we focus on reaching and grasping, actions that are possible because of the prehensile hand.
The evolution of the prehensile hand greatly enriched the development of cognitive capacities in primates. Indeed, the two are interdependent. As the German philosopher Friedrich Engels wrote: "Man alone has succeeded in impressing his stamp on nature. He has accomplished this primarily and essentially by means of the hand. But step by step with the development of the hand went that of the brain."
The prehensile hand radically changed the way in which primates relate to the external world; but the change occurred slowly, and required the evolution of cortical circuits for a variety of new specialized movements adapted to different objects. In his book The Sensory Hand, Vernon Mountcastle, one of the pioneers in the study of the connection between sensation and action, quotes Herbert Spencer (Principles of Psychology, 1885) on why the hand is so critical to understanding action:
All that we need notice here is the extent to which in the human race a perfect tactual apparatus subserves the highest processes of the intellect. I do not mean merely that the tangible attributes of things rendered completely cognisable by the complex adjustments of the human hands, and the accompanying manipulative powers have made possible those populous societies in which alone a wide intelligence can be evolved. I mean the most far-reaching cognitions, and inferences (even those) most remote from perception, have their roots in the … impression which the human hands can receive.
The final neural pathway for all bodily actions, including movement of the hand, is through motor neurons in the ventral horn of the spinal cord. These motor neurons are not simply responding to independently generated sensory information, however. The sensory information needed for action is the product of interaction between the motor systems and sensory systems. Under many circumstances, therefore, action and perception are inseparable. Indeed, many sensory ...