This research will examine the rhythmic coordination between two limbs in patterns that are exhibited by humans in simple (e.g., two limbs moving at one tempo), moderately complex (e.g., one limb moving at twice the frequency of another), and very complex (e.g., 7-ball juggling) activities. Its objective is twofold, (1) to identify general (dynamical) principles that govern the stability and variability of rhythmic interlimb coordinations, and (2) to understand how these principles apply in coordinating the limbs with respect to objects and other environmental influences. Four classes of experiments will use two tasks, swinging hand-held pendulums and cascade juggling. The experiments will manipulate the phase relation between the limbs (e.g., moving in the same or opposite directions), the frequencies at which they oscillate together when coordinated, and the frequencies at which they tend to oscillate when not coordinated. Spatial and temporal features of the environmental information constraining the rhythmic patterns will also be varied. Computerized motion recorders will analyze movement patterns in three dimensions, and theory and methods of nonlinear dynamics will be used to address them. The types of rhythmic interlimb coordination patterns under study are common and basic to very many everyday activities, as seemingly nondescript as walking and as skilled as playing a musical instrument. The behavioral simplicity of the basic coordination pattern is deceptive; its understanding requires that we use recent advances in a branch of physics to identify lawful regularities within which cognitive (psychological) constraints on movement can be defined nonarbitrarily. This so-called dynamical perspective promises a richer set of constraints on modeling what the brain has to do in order that coordinated rhythmic movements in general and locomotion in particular can be executed. It should also provide new hypotheses about the causes of movement disorders as well as ideas for designing prosthetic and robotic limbs.
