Motor Commands Arise Through Sensorimotor Transformations
The Central Nervous System Forms Internal Models of Sensorimotor Transformations
Movement Inaccuracies Arise from Errors and Variability in the Transformations
Different Coordinate Systems May Be Employed at Different Stages of Sensorimotor Transformations
Stereotypical Patterns Are Employed in Many Movements
Motor Signals Are Subject to Feedforward and Feedback Control
Feedforward Control Does Not Use Sensory Feedback
Feedback Control Uses Sensory Signals to Correct Movements
Prediction Compensates for Sensorimotor Delays
Sensory Processing Is Different for Action and Perception
Motor Systems Must Adapt to Development and Experience
An Overall View
In the preceding part of this book we considered how the brain constructs internal representations of the world around us. These internal representations have no intrinsic value and are behaviorally meaningful only when used to guide movement, whether foraging for food or attracting a waiter's attention. Thus the ultimate function of the sensory representations is to shape the actions of the motor systems. Sensory representations are the framework in which the motor systems plan, coordinate, and execute the motor programs responsible for purposeful movement.
In this part of the book we describe the principles of motor control that allow the brain and spinal cord to maintain balance and posture; to move our body, limbs, and eyes; and to communicate through speech and gesture.
Although movements are often classified according to function—eye movements, prehension (reach and grasp), posture, locomotion, breathing, and speech—many of these functions are subserved by overlapping groups of muscles. In addition, the same groups of muscles can be controlled voluntarily, rhythmically, or reflexively. For example, the muscles that control respiration can be used to take a deep breath voluntarily before diving under water, to breathe automatically and rhythmically in a regular cycle of inspiration and expiration, or to act reflexively in response to a noxious stimulus in the throat, producing a cough.
Voluntary movements are those that are under conscious control by the brain. Rhythmic movements can also be controlled voluntarily, but many such movements differ from voluntary movements in that their timing and spatial organization is to a large extent controlled autonomously by spinal or brain stem circuitry. Reflexes are stereotyped responses to specific stimuli that are generated by simple neural circuits in the spinal cord or brain stem. Although reflexes are highly adaptable to changes in behavioral goals, mainly because several different circuits exist to connect sensory and motor neurons, they cannot be directly controlled voluntarily.
In this chapter we focus on voluntary movements, using arm and hand movements to illustrate principles of sensorimotor control. Reflexes and rhythmic movements are discussed in detail in Chapters 35 and 36.
Conscious processes are not necessary for moment-to-moment control of movement. Although we may be aware ...