38: Voluntary Movement: The Parietal and Premotor Cortex

Introduction

• Voluntary Movement Expresses an Intention to Act

• Voluntary Movement Requires Sensory Information About the World and the Body

• Reaching for an Object Requires Sensory Information About the Object's Location in Space

  • Space Is Represented in Several Cortical Areas with Different Sensory and Motor Properties

  • The Inferior Parietal and Ventral Premotor Cortex Contain Representations of Peripersonal Space

  • The Superior Parietal Cortex Uses Sensory Information to Guide Arm Movements Toward Objects in Peripersonal Space

  • Premotor and Primary Motor Cortex Formulate More Specific Motor Plans About Intended Reaching Movements

• Grasping an Object Requires Sensory Information About Its Physical Properties

  • Neurons in the Inferior Parietal Cortex Associate the Physical Properties of an Object with Specific Motor Acts

  • The Activity of Neurons of the Inferior Parietal Cortex Is Influenced by the Purpose of an Action

  • The Activity of Neurons in the Ventral Premotor Cortex Correlates with Motor Acts

  • The Primary Motor Cortex Transforms a Grasping Action Plan into Appropriate Finger Movements

• The Supplementary Motor Complex Plays a Crucial Role in Selecting and Executing Appropriate Voluntary Actions

• The Cortical Motor System Is Involved in Planning Action

  • Cortical Motor Areas Apply the Rules That Govern Behavior

  • The Premotor Cortex Contributes to Perceptual Decisions That Guide Motor Behavior

  • The Premotor Cortex Is Involved in Learning Motor Skills

• Cortical Motor Areas Contribute to Understanding the Observed Actions of Others

• The Relationship between Motor Acts, the Sense of Volition, and Free Will Is Uncertain

• An Overall View

In this chapter we describe how the cerebral cortex uses sensory information about the external world in deciding on which actions to take and how to organize voluntary movements to accomplish those actions. Studies over the past 25 years have shown that the cortical motor system is not an unthinking, passive circuit controlled by more intelligent parts of the brain. Instead, it is intimately involved in the many interrelated neural processes required to choose a plan of action, including processes that appear to be more perceptual and cognitive than motor in nature. The motor system also contributes to cognitive processes that appear unrelated to motor control, such as understanding the actions of others and the potential outcomes of observed events.

Voluntary Movement Expresses an Intention to Act

Voluntary behavior is the physical expression of an intention to act on the environment to achieve a goal. Let us say you want a cup of coffee. There may be many reasons why: You may wish to enjoy the stimulating effect of caffeine or may simply be thirsty. Whatever its origin, your behavioral goal is established by your motivational state but is fulfilled by voluntary motor behavior. The motor system has to transform your intention into action.

How you achieve your goal depends on the circumstances in which you find yourself. If the cup of coffee is already prepared and sitting in front of you, you can simply reach out, grasp the cup, ...