Chapter 5 Motor Programs 5 Motor Programs C H A P T E R

Objectives This chapter will help you to understand the following: –Motor control as an open-loop system and the role of motor programs –Experimental evidence for motor programs –Limitations and problems in the simple motor program concept –Generalized motor programs and evidence for this expanded concept

Open-Loop Control The basic open-loop system consists of two parts: an executive and an effector. Without feedback, the open-loop system is not sensitive to whether the actions generated in the environment were effective in meeting the goal. Modifications to the action cannot be made while the action is in progress.

Motor Programs as Open-Loop Systems Open-loop control seems especially important when the environmental situation is predictable and stable. Under these circumstances, human movements appear to be carried out without much possibility of, or need for, modification.

Figure 5.2

Evidence of Reaction Time RT is affected by several features of the movement to be performed, presumably by influencing the complexity and duration of the movement programming stage. RT increases when –additional elements in are added to an action, –more limbs must be coordinated, and –the duration of the movement becomes longer. (continued)

Evidence of Reaction Time (continued) RT can be dramatically shortened under certain conditions. A loud acoustic signal usually produces the typical startle indicators. A prepared movement is also produced normally but with an RT that may be up to 100 ms shorter than on control trials without the extra startle stimulus.

Deafferentation Experiments A surgical technique that involves cutting an animal’s afferent nerve bundle where it enters the cord. The central nervous system no longer can receive information from some portion of the periphery. These studies show that sensory information from the moving limb is not absolutely critical for movement production.

Central Pattern Generator It is a centrally located control mechanism that produces mainly genetically defined actions. The concept of a central pattern generator is used to describe simple, genetically defined activities such as walking, whereas motor program theory applies to learned skills such as riding a bicycle.

Inhibiting Actions Considerable evidence suggests that a motor program is released that is responsible for initiating the action in tasks and serves to carry out the entire action unless a second stop signal program is initiated in time to arrest its completion. The stop signal paradigm is the method most frequently used for studying action inhibition.

Muscle Response Patterns A limb’s electrical muscle activity patterns are unaffected for 100 to 120 ms when the limb is blocked by a mechanical perturbation. These findings support the motor program idea that the movement activities are organized in advance and run off unmodified by sensory information for 100 to 120 ms.

Motor Programs and the Conceptual Model Motor programs are a critical part of the conceptual model, operating in the system, sometimes in conjunction with feedback, to produce flexible skilled actions. The open-loop part of these actions provides the organization, or pattern, that the feedback processes can later modify if necessary.

Major Roles of Open-Loop Organizations To define and issue the commands to musculature that determine when, how forcefully, and for how long muscles are to contract and which ones To organize the many degrees of freedom of the muscles and joints into a single unit (continued)

Major Roles of Open-Loop Organizations (continued) To specify and initiate preliminary postural adjustments necessary to support the upcoming action To modulate the many reflex pathways to ensure that the movement goal is achieved

Integration of Central and Feedback Control The motor program is responsible for the major events in the movement pattern. However, there is considerable interaction with sensory processes (e.g., the organization of various reflex processes to generate rapid corrections). It makes the movement flexible in the face of changing environmental demands.

Problems in Motor Program Theory Storage: How (or where) do humans store the nearly countless number of motor programs needed for future use? Novelty: How do performers produce truly novel behavior that cannot be represented in an already stored motor program?

Generalized Motor Program Theory A GMP underlies a class of movements and is structured in memory with a rigidly defined temporal organization. GMP structure is characterized by its relative timing, a movement’s deep, fundamental structure. Variations in movement time, movement amplitude, and the limb used represent the movement's surface structure.

Invariant Features of a GMP GMP structure is characterized by its relative timing, which can be measured by a set of ratios among the durations of various events in the movement. Relative timing represents a movement’s deep, fundamental structure. Relative timing remains invariant, and relative timing structure is very difficult to alter.

Parameters Added to the GMP Relative timing may be carried out with different surface features (e.g., duration, amplitude). Surface features are very easy to alter by parameter adjustment. Parameters change only how the GMP is expressed at any given time.

Classes of Actions Motor programs are thought to be generalized to account for a class of actions, such as throwing. Parameters must be supplied to define the way in which the pattern is to be executed (e.g., throwing either rapidly or slowly).

Figure 5.13