When learning sequential movements, such as those involved in speech production, handwriting, typing, drumming, or sports skills, performers exhibit the ability to modify a learned movement sequence from execution to execution in some ways but not in others. This is thought to occur because a generalized motor program (GMP), which can be used to produce a specific class of movements, has developed and been stored in memory. When the GMP is retrieved, movement parameters must be specified in an effort to scale the program output to meet the specific demands at hand. Because the movement output can be altered by the premovement specification of parameters, the program was termed generalizable by Richard “Dick” Schmidt. The notion of a generalizable program is quite different from the more traditional notion of motor program as a prestructured set of central commands that can be used to produce a specific movement.
A generalized motor program is thought to develop over practice and provides the basis for generating movement sequences within a class of movements that share the same invariant features, such as sequence order, relative timing, and relative force. Specific movements are produced by the premovement specification of movement parameters like absolute timing, absolute force, and effectors. For example, when a movement situation requires a learned sequence to be produced either faster or slower than typically practiced, the invariant features remain unchanged, but the absolute timing parameter is changed to accommodate the rate at which the movement is produced. Thus, the relative times used to produce the elements in the sequence remain unchanged, but the absolute time is rescaled to meet the specific demands. Changing the absolute timing parameter results in slower movements that could be considered stretched out (in time) copies of faster movements. Likewise, a lower specification of the force parameter would result in a movement sequence generating reduced forces, which could be thought of as a compressed (with respect to force) copy of a more forceful movement.
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A common example used to exemplify the notion of a GMP is writing one’s signature. Each of us can write our signature under a variety of conditions. According to the GMP perspective, we do this by specifying the movement parameters needed to meet the requirements at hand while maintaining the invariant features of the GMP. For example, if I were asked to write my name (Charles Shea) very quickly on a sheet of paper, the signature maintains the relative timing characteristics invariant to the GMP, but the specification of absolute timing would be reduced. This will result in me taking the same proportion of the total time to write the Ch, for example, in my first name when writing quickly as when writing under normal time constraints, but the absolute time used would be reduced. Similarly, if I were asked to write my name in a small box on a sheet of paper or much larger on a white board in the classroom, the invariant features would not change, but the actual timing, actual forces, and even the specific effectors used to produce the movement would change. In the smaller situation, one would use primarily finger movements to produce a signature, while in the larger situation one may use shoulder and arm movements with minimal or no movements of the fingers.
The notion of a GMP has a great deal of intuitive appeal. It seems efficient for each motor program in our movement repertoire to be able to generate a class of movement sequences. This reduces not only the potential storage problems that would result if different programs were needed each time the movement requirements changed but also would reduce potential retrieval problems that would be associated with selecting from among a group of similar motor programs. The notion of a GMP is also consistent with our experience with computer programs and electronic video devices. Indeed, the record player analogy often used to describe the invariant and variant features of the GMP feeds this intuitive appeal. In the record player analogy, a phonograph record is used to illustrate the invariant and variant feature of the GMP. For example, a phonograph record could be played at different speeds (331⁄3 rpm or 78 rpm), played with different settings on the volume control, or the output directed to different speakers while maintaining the invariant features of the recording. In this analogy, speed is used to indicate absolute time, volume to indicate force, and speakers used to illustrate different effectors.
There is, however, also a good deal of empirical evidence to support the notion of a GMP. Research has demonstrated that learned movement sequences when scaled in time or force exhibit a pervasive tendency toward approximately proportional scaling. Whereas participants are able to rescale learned movement sequences in time and force, variable practice, in which the learner is exposed to various parameter requirements, appears important for the learner to accurately specify the time or force parameter. There is also a good deal of evidence that participants can execute movement sequences using different effectors. As noted earlier, one’s signature can be executed with the dominant limb using a variety of difference muscle groups. This literature is, however, a little cloudy especially in terms of transfer to homologous and nonhomologous limbs. That is, when asked to produce a movement sequence with the left hand after learning with the right hand, for example, transfer is not always very effective. Thus, there do seem to be some limits to parameterizing a GMP.
References:
- Schmidt, R. A. (1975). A schema theory of discrete motor skill learning. Psychological Review, 82, 225–260.
- Schmidt, R. A. (1976). Control processes in motor skills. Exercise and Sport Sciences Reviews, 4, 229–261.
- Shea, C. H., & Wulf, G. (2005). Schema theory: A critical appraisal and reevaluation. Journal of Motor Behavior, 37, 85–101.
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