What is motor control? Edit
Motor control refers to how biological organisms (like people, dogs, or snails) and artificial constructs (like robots) control the motions of their bodies in response to stimuli in the environment. While motor control is clearly a set of physiological processes in organisms, it is often conceived of as a computational problem. How is something or someone able to move to achieve various environmental goals? In human motor control, there are many sources of evidence to suggest that the motor system is organized into multiple hierarchical levels (e.g.,the action of individual motor units, agonist/antagonist pairs of muscles around a joint, abstract representations of discrete actions). Successful motor control results in movements that bring about desired/intended outcomes. These outcomes may satisfy numerous and sometimes competing goals (e.g., to remain balanced, to reach a hot cup without getting burned, to avoid obstacles, to move quickly and accurately, to minimize energy expenditure, to avoid injury or uncomfortable positioning) and different goals may be represented at different levels within the nervous system.
Although the exact number and type of levels/processes is not clear [REF], motor behaviors clearly exist along a spectrum that ranges from voluntary or intentional movements to more automatic or reflexive movements. These movement types are not necessarily exclusive, however, and the automatic/reflexive properties of the motor system may be important components of volitional control as well. A major question in the neurophysiology of motor control is "what" is being controlled, prepared or programmed in different areas of the nervous system. Theoretical perspective on motor control range from information processing models that are computational [REF] or psychological in nature [REF] to more physiologically based theories that try representational or "symbolic" approaches to control [REF]. Many of these theoretical perspectives are not mutually exclusive, but are based on different levels of analysis or designed to explain different experimental effect (e.g., continuous versus discrete movements). Even for seemingly very simple movements like maintaining standing balance or walking there is considerable debate in the scientific literature and, arguably, less is known about specialized movements that characterize expert athletes and performers. Despite this ambiguity, centuries of research into the neural control of movement have greatly improved our understanding of how the healthy motor system functions and how to treat disorders of the motor system [REF].
Hierarchical control models Edit
Many of the processes underlying human movement take place without explicit awareness on the part of the actor, but many movements are still voluntary. That is, an actor makes a conscious decision to act and this desire ultimately leads to movement. Yet, the exact parameters of the movement are usually unknown and not directly controlled by the actor. Therefore, motor control is the problem of which transformations intervene between the thought of attaining a goal and the muscle activations that result in movement. Models and theories of motor control differ, but the majority if not all the models are hierarchical in nature, suggesting that the goal of the task is loosely set at a high level (sometimes referred to as the task-level), a sequence of movements is specified at a lower level (i.e., response programming), and at the lowest level a specific pattern of muscle activation allows for execution (execution-level). Although for clarity we treat these various levels as independent, it will be clear from the discussion below that these processes are neither serial nor mutually exclusive and our distinctions across levels serves primarily to aid thinking about the various problems involved in controlling movement.
Response selection: Intending and planning to act Edit
Response programming: Preparing the movement Edit
The history of response programming in motor control dates back to the late 19th/early 20th century. Programming is thought to be the process preceding voluntary actions, whereby action plans are organized and potentially stored in cortical or subcortical structures in the brain, ready to be released when a response is required. Evidence for programming is that people are unable to inhibit actions when they are close to their release point, studies of RT show that as complexity of a movement increases so does RT, stereotyped movement features are observed when an action is repeatedly executed, suggesting that at least part of the response is programmed in advance and is not influenced by feedback (so termed open-loop control), and features of a movement remain when the movement is unexpectedly blocked or is initiated early due to a startling acoustic stimulus (known as the start-react effect).
Execution: Making the movement Edit
At the execution-level, there is little or no access to awareness and conscious control as this level consists of detailed specification of motor unit recruitment in both time and space. The type of representation used at this level is not clear, but there is research to suggest that rather than programming individual muscle contractions or joint torques, the execution-level relies on stereotyped relationships/patterns of muscle activity referred to as synergies or “motor primitives”. Some neurophysiological data suggest that for simple movements much of execution-level programming occurs at the level of the spinal cord. Electrical stimulation of the spinal interneurons in animals leads to relatively smooth movements of the whole effector towards a consistent endpoint. In contrast stimulation of the alpha-motor neurons, the next level below interneurons, leads to motor-unit contractions within a single muscle at a constant force dependent on the magnitude of stimulation.
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