Applying the Dynamic Systems Model to a Motor Development Problem
Perhaps the best way to appreciate how the characteristics of dynamic systems can be used to model motor development is to see how a research team used dynamic systems to address a particular developmental issue. Let’s consider the case of the reflexes seen in infancy. Recall that a reflex is a particular automatic response to a specific stimulus. It is a response that is carried out without thought or intention. Developmentalists have long observed the existence of reflexes that disappear later on in infants.
Developmentalists have been particularly interested in the walking or stepping reflex for its similarity to voluntary walking (see table 3.1). The reflex is elicited by holding an infant upright and lowering the infant to a horizontal surface.
The reflexive response is stepping movements of the legs. The stepping reflex is typically observed up to 5 months of age, so it appears much earlier than voluntary walking and disappears months before voluntary walking.
Developmentalists have offered several explanations for the disappearance of the stepping reflex. The maturationists suggested that the disappearance reflects the maturation of the cortex. These researchers proposed that as higher brain centers mature they are able to inhibit the lower brain centers that mediate reflexes. With continued maturation, the centers control voluntary walking. The existence and disappearance of reflexes reflect the status of nervous system development.
Peiper (1963) suggested instead that reflexes have a purpose because they allow practice of movement patterns before the higher brain centers are mature enough to control those patterns. Zelazo et al. (Zelazo, 1983; Zelazo, Konner, Kolb, & Zelazo, 1974; Zelazo, Zelazo, & Kolb, 1972) offered yet another viewpoint. They purposely increased the number of times the stepping reflex was elicited in a small group of infants. Later, these infants were observed to start walking at a younger age than average. The researchers concluded that reflexes can be subsumed into voluntary movements. For example, the walking reflex is transformed into voluntary walking.
Unconvinced by these various hypotheses, Esther Thelen and her colleagues (Thelen & Fisher, 1982; Thelen, Fisher, & Ridley-Johnson, 1984) developed an ingenious series of experiments to examine different potential causes for the disappearance of the stepping reflex. They began with the notion that multiple systems—rather than just one system—play a role in the disappearance. They had observed that one of the changes in infants correlating with the disappearance of the stepping reflex is an increase in subcutaneous fat that occurs during infancy. Could this gain in fat and the resultant change in the strength required to move heavier legs be related to the disappearance of the stepping reflex? In order to test whether the muscle and adipose tissue systems are involved, the researchers had to find a way to make a light and stepping infant resemble a nonstepping and heavier infant and, just as importantly, to get a non-stepping, heavier infant to resemble a lighter, stepping infant. The only variable that could be changed was leg strength. In a first experiment, the researchers added tiny weights to the legs of lighter infants. The added weight was proportional to the amount of weight the infant would add between 4 and 6 weeks of age. If maturation of the nervous system were the only cause of reflex stepping, then the infants should continue to step regardless of the small weights. Actually, the infants reduced the number of reflex steps taken while wearing the weights.
The next experiment was designed to regress the leg weight of heavier infants to that of younger, lighter infants. The researchers achieved this by holding the infants upright in an aquarium filled with water up to their hips. The water reduced the pull of gravity on the legs and made them relatively lighter. The water was room temperature, and the researchers noted that often the infants did not respond in any way to being held in the water. When the infants’ feet touched the bottom surface of the tank, though, the number of steps increased (Thelen & Fisher, 1982). If maturation of the nervous system is the only explanation
for the reflex disappearing, then nothing should have been able to increase the rate of stepping, as the higher brain centers would have inhibited it. Thus the researchers demonstrated that change in other systems plays a role in the disappearance of the stepping reflex.
Thelen et al. demonstrated that the systems approach can be used to examine the processes underlying development. While maturation of the nervous system is important in motor development, other subsystems are important to the change observed in the stepping reflex. As well as studying multiple subsystems, Thelen et al. demonstrated that change in a subsystem can function as a control variable. The role of a control variable in bringing about change is another characteristic of dynamic systems. More of Thelen’s work will be discussed throughout the text.
The systems approach has much to offer researchers interested in understanding developmental change. Dynamic systems evolve over time and undergo change that is not linear. These characteristics describe the body as it grows and ages. Dynamic systems evolve by transitioning from times of stability, or attractor states, through times of instability to other attractor states. We observe this pattern in motor development, too. Dynamic systems are sensitive to the conditions initially in place when they begin to evolve. Movement scientists have long realized that models of movement control must allow for the same movement goal to be achieved from variable environmental conditions and different starting positions of the body and limbs. Most importantly, dynamic systems have been shown to self-organize and to use methods of control such as entrainment and the application of constraints. The demonstration of these possibilities is important, as they provide explanations for how the large number of degrees of freedom in movement can be reduced for the reasonable control of complex movements.
The systems approach provided a heuristic function for movement scientists by focusing researchers’ attention on new ways to solve existing questions in motor development. There is value in examining multiple theories. Researchers using differing approaches all serve to gain from alternative views, as both theorists and empiricists are forced to question behaviors their theories can’t account for and improve their models until a satisfactory explanation can be found (for example, see Schmidt & Lee, 2011 on generalized motor programs). Ultimately, these challenges and changes serve to enhance our understanding of motor development.