The preparation of a set of instructions for movement. Borrowed from computing, the term encapsulates the idea that the high-level decision to move must be translated into low-level commands for synchronized contractions and relaxations of muscles. It is unlikely that the program itself contains specification of muscle activity: rather, like a computer program which must be compiled into machine code, the motor program has a representation of current (start) state and goal state and it contains the specification of the translation of one to the other via muscular activity. Thus, when ‘compiled’ it specifies which muscles contract or relax in order, the relative timings of these contractions (specifically, the onset and duration of specific muscular activity) and finally, the degree of force.
Although it appears to make intuitive sense that these aspects of the program would be hierarchically organized (for example, with the first specification being which muscles to move, the second being the sequence to move them in and the third being the duration or force of their activity), this does not appear to be the case. Rather, any of these aspects of the program can be composed in advance. For example, when subjects are given knowledge, prior to the signal to move, of one or more of the parameters (limb to be moved, direction of movement or extent of movement), the movement is initiated with a shorter latency following the signal to move (that is to say, there is a REACTION TIME advantage). This indicates that it is possible to program, say, the extent of a movement in advance of specifying which muscles will move. By measuring the reaction time advantage and manipulating the availability of information about the forthcoming movement prior to the signal to move, it is possible to infer the nature and timing of motor programming processes. For example, it is possible to measure how long it takes to program sequences of movements of different complexity or length.
From such experiments, it has been demonstrated that although it is not possible to state exactly how long motor programming takes, it takes longer to make a more complex or a longer motor program.
It is a widely held view that once initiated, the program runs through to the goal state without modification. Indeed, some definitions of a motor program refer to the control of movements executed in the absence of peripheral feedback. Although it is possible to use feedback to change movement during the execution of a motor program, these kinds of modifications are most likely to be themselves defined within the program as a potential contingency. Thus, the program still runs to completion and could be said to be unmodified. For example, when lifting an object, if the anticipated weight is greater than the actual weight, the object will initially be moved with the force required to lift the anticipated weight and, therefore, will fly up. Within about 50 msec (too short a period to halt execution and initiate a revised program) there will be a correction of the force and compensation for the unanticipated outcome of the movement. Other kinds of corrections require the termination of the running motor program and the initiation of an entirely new program. Therefore, these corrections take longer (perhaps as long as 200 msec) to make. Indeed, even before the series of muscular contractions begins, there is a point, estimated to be about 100 msec prior to movement onset, after which initiation cannot be terminated and execution of the program is inevitable. Slips of action (such as pouring orange juice on one’s breakfast cereal, or heading for the telephone when the door bell is ringing) are often compounded because the unfortunate subject cannot swiftly terminate the inappropriately initiated motor program.
VERITY J.BROWN
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