Generalized Motor Program

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.

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:

  1. Schmidt, R. A. (1975). A schema theory of discrete motor skill learning. Psychological Review, 82, 225–260.
  2. Schmidt, R. A. (1976). Control processes in motor skills. Exercise and Sport Sciences Reviews, 4, 229–261.
  3. Shea, C. H., & Wulf, G. (2005). Schema theory: A critical appraisal and reevaluation. Journal of Motor Behavior, 37, 85–101.

 

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