Juggling is a complex rhythmic activity requiring a high degree of coordination between hand movements and the objects being kept aloft in the air or bounced against the floor. As the task constraints of juggling are rather severe, particularly when juggling many objects, there is little room for error, even though human motor performance is intrinsically variable. The key question how this is accomplished is addressed along two lines. First, it is discussed how coordination constraints such as frequency and phase locking may lead to a reduction of the number of dimensions (i.e., principal components) that are necessary for describing a particular juggling pattern (e.g., the three-ball cascade), and thus for producing it. How the control problem is being confronted depends on juggling speed, which affects both the number of relevant principal components as well as the smoothness of the projections of the signal onto the time evolution of the eigenvectors, particularly around the catches. Besides this global and time-dependent aspect, it is shown that, whereas the throws are highly reproducible, particularly the catches are used to correct errors in the timing of the act (i.e., to stabilize the phase difference between the objects).
Second, it is discussed to what extent, and how, jugglers use advanced visual information for the control of catching. In a one-ball juggling task jugglers look at the ball in the air at certain preferred times (in the order of 360 ms before catching) rather than at preferred places such as the zenith of the ball flight. In this task, expert jugglers tend to look at the ball earlier than less experienced jugglers, allowing them more time for making the interception. In ‘real’ juggling patterns (i.e., where the number of objects exceeds the number of hands), such as the cascade, expert jugglers often adopt a so-called "gaze through" (or "distant stare") strategy, suggesting that the direction of gaze is restricted to a small area and that information about the flights of the balls is being picked up peripherally rather than centrally. If expert jugglers perform less standard patterns, such as the reverse cascade, however, they may abandon the "gaze through" strategy. If clear saccadic eye movements are present in intermediate and expert jugglers alike, they are often mode locked to the objects being circulated. (The effects of tempo and skill on these mode-locked patterns are currently analyzed and will be discussed at the conference.)
In sum, jugglers appear to accomplish their amazing feats by creating task-specific, tempo-dependent couplings between the movements of the hands, objects and eyes, such that the errors induced by the throws are annihilated
References
Amazeen, E. L., Amazeen, P. G., Post, A. A., & Beek, P. J. (1999). Timing the selection of information during rhythmic catching. Journal of Motor Behavior, 31, 2792-289.
Post, A. A., Dafferthshofer, A., & Beek, P. J. (in press). Principal components in three-ball cascade juggling. Biological Cybernetics.