Human movements, from the simplest pointing movement to more elaborate multi-gesture movements, follow directions contained in pre-existing mental plans, often called motor programs. Although we are seldom aware of the existence of such programs, their purpose is to structure commands to the muscles so that movements can be made with minimal reliance on slow, feedback-based, error- correcting processes. While there is general agreement on this point, the structure of motor programs is far from clear. Motor control theorists have speculated that motor programs can be generalized. This idea, which stems from an analogy to subprograms in computer programming, suggests that motor programs in memory have formal parameters that are replaced with actual, situation-specific values when a movement is about to be made. This research will explore the empirical reality of motor-program generalization as well as the implications of various proposals describing how such generalizations might be implemented. These issues will be addressed in the domains of handwriting and drawing movements. While this research will investigate motor program generalization across several movement parameters, e.g., movement speed or size, special emphasis will be given to a particular form of motor- program generalization known as effector independence. This term describes the ability of a single motor program to drive any of several effector systems to produce movement associated with that program. Effector independence is claimed to draw empirical support from several informal comparisons of the shapes of words written using only the wrist and fingers versus those same words written with the whole arm, as on a chalkboard. In this context, different effector systems might include the right hand, the left hand, the whole right arm, or even a foot. It is this diversity of the movement systems involved, with their different joint configurations, muscle systems, and types of nerve innervation, that makes the claim of effector independence so tantalizing. This research will proceed along two lines. The first will examine, for previously learned movements, the disruptive effects on the movement's shape, timing, force, and velocity caused by manipulating a number of aspects of the experimental situation. In the second line of research, people will learn novel movements under one set of conditions, and their ability to transfer that learning to new conditions will be studied. The insights about the structure and function of motor programs gained from this research will have potential application to the training of skills requiring coordinated movement, the diagnosis and treatment of movement disorders, and the planning and representation of movements in robotic systems.
