Human Motor Behavior: An Introduction

Hollis L. Fitch,
Betty Tuller,
M. T. Turvey
University of Connecticut
and
Haskins Laboratories

Following Bernstein, we have in the last two chapters proposed a view of motor control whose strategy is to leave as little explanatory power as possible residing in a "homunculus." In older models, this theoretical entity was asked to make an overwhelming number of decisions (about individual muscle states), requiring an overwhelming amount of information (about the state of the body and the world). The job of dealing with all possible combinations of perceptual states is equal in difficulty to (and just as intractable as) the job of dealing with all combinations of muscle states. Bernstein saw these as complementary problems; it is the perceptual side of the dilemma that we address in this chapter. We have seen how a coordinative structure style of organization can reduce the number of executive decisions that need to be made. Now we want to see how the perceptual information can modulate or tune the coordinative structures without intervention from a super-human homunculus.

Notice that it is not desirable to reduce the degrees of freedom of the muscle system to zero. With no degrees of freedom, an act would be insensitive to changes in its context. It would be a fixed pattern, so stereotyped that adjustments to changes in the environment in which the action occurred would be impossible. The organization that we desire is one that limits the possible combinations of muscle values but still allows some flexibility. Coordinative structures accomplish this: The equation of constraint defining the coordinative structure preserves a relationship among the variables but still allows its variables to take on different values. This reduces the number of degrees of freedom that must be controlled but allows the system to be sensitive to its context. The coordinative structure is flexible and may be "tuned" to its environment.

-271-