One-Handed Juggling: A Dynamical Approach to a Rhythmic Movement Task

The skill of rhythmically juggling a ball on a racket was investigated from the viewpoint of nonlinear dynamics. The difference equations that model the dynamical system were analyzed by means of local and nonlocal stability analyses. These analyses showed that the task dynamics offer an economical juggling pattern that is stable even for open-loop actuator motion. For this pattern, two types of predictions were extracted: (a) Stable periodic bouncing is sufficiently characterized by a negative acceleration of the racket at the moment of impact with the ball, and (b) a nonlinear scaling relation maps different juggling trajectories onto one topologically equivalent dynamical system. The relevance of these results for the human control of action was evaluated in an experiment in which subjects (N = 6) performed a comparable task of juggling a ball on a paddle. Task manipulations involved different juggling heights and gravity conditions of the ball. The following predictions were confirmed: (a) For stable rhythmic performance, the paddle's acceleration at impact is negative and fluctuations of the impact acceleration follow predictions from global stability analysis; and (b) for each subject, the realizations of juggling for the different experimental conditions are related by the scaling relation. These results permit one to conclude that humans reliably exploit the stable solutions inherent to the dynamics of the given task and do not overrule these dynamics by other control mechanisms. The dynamical scaling serves as an efficient principle for generating different movement realizations from only a few parameter changes and is discussed as a dynamical formalization of the principle of motor equivalence.     

Rhythmic movement disorder in children

How should sleep-related rhythmic movements in children be assessed and treated? Rhythmic movement disorder (RMD) represents an unusual variety of childhood parasomnia characterized by repetitive motion of the head, trunk, or extremities, which usually occurs during the transition from wakefulness to sleep or arises during sustained sleep. Although the condition most often affects infants and toddlers in a transient and self-limited fashion, the condition occasionally persists in a problematic fashion, which may nevertheless be amenable to treatment. Since RMD may occasionally cause injury or resemble nocturnal seizure, prompt recognition, and appropriate management on the part of the clinician is essential. This article will examine the spectrum of RMD in children, including their common clinical manifestations; data regarding their epidemiology and natural history; the role of polysomnography, electroencephalography; and other diagnostic testing. Potential causes of the condition and available methods of treatment are also examined.     

A Muscle Awareness Model for Changes in Rorschach Human Movement Responses

The present series of studies sought to provide evidence that M would increase under any conditions which make an S more aware of his muscles. The studies demonstrated that at least in women, M increased over a variety of conditions involving heightened muscle awareness. Specifically M increased in conditions utilizing muscle activation, deactivation, hypnosis, and focusing thoughts on the body musculature. The muscle awareness model unlike the sensory-tonic model accounts for increases in M following hyperactivity as well as inhibition.     

Effects of Object Texture on Precontact Movement Time in Human Prehension

Using kinematic data in a precision-grip reaching task, Weir, MacKenzie, Marteniuk, and Cargoe (1991) concluded that prior to contact with an object, its texture does not affect the course of grasping. The present study used their task of reaching for and lifting a slippery-, normal- (polished metal), or rough-surfaced dowel. This occurred under the original, blocked condition, in which textures were held constant within a series of trials, and under a new, randomized condition, in which textures varied randomly from trial to trial. Performance was also examined over more extended periods of practice. Reaction time and precontact movement time were directly measured. In contrast to the results of Weir et al., 1991, reaching for the slippery dowel resulted in slower movement time. This effect was found both early and late in practice for the randomized condition; it was found only in late practice for the blocked condition. These effects can be attributed to the greater geometric and dynamic precision required for lifting a slippery object.     

Temporal Orientation,Human Movement Responses and Time Estimation

The relationship between temporal orientation, human movement (M) responses on the Barron inkblots, and brief time estimates was investigated. Knapp's Time Metaphor Test and the Barron inkblots were administered to 110 Ss. 40 Ss were selected on the basis of their scores on the Time Metaphor Test to participate in an individual time estimation experiment This produced two criterion groups: 20 Dynamic-Hasty (D-H) Ss and 20 Naturalist-Passive (N-P) Ss. Results indicated that N-P Ss had a lower M threshold, perceived more M and overestimated the passage of time. Discussion focused on the relationship between temporal orientation, fantasy behavior, and impulse control.     

Resonance Tuning in Rhythmic Arm Movements

The hypothesis was tested that the preferred frequency of rhythmic movement corresponds to the resonant frequency of the muscle-limb system, as proposed by the hybrid spring-pendulum model (Kugler Turvey, 1987). In contrast to previous studies, the resonant frequency and stiffness of the system were estimated independently, which permitted quantitative predictions of the preferred frequency to be made. Human subjects (N = 5) were asked to oscillate their forearms in the vertical plane at their preferred frequency under conditions of added mass and external spring loading. Subjects also oscillated their arms at frequencies below and above the preferred frequency, which enabled the investigators to estimate the resonant frequency and stiffness of the elbow joint by using the phase transfer method (Viviani, Soechting, Terzuolo, 1976). The preferred frequency corresponded to the resonant frequency of the muscle-limb system under each condition, as predicted. The oscillation amplitude varied inversely with the preferred frequency, which was also predicted. Finally, the internal joint stiffness was modulated so that it matched the impedance of the external springs but was unaffected by added mass. The results are consistent with an autonomous oscillator model that incorporates proprioception about the dynamics of the periphery.     

Chaos in physiology

A bird’s eye view of the role of nontrivial dynamical phenomena in physiological systems is presented. Many levels in the physiological hierarchy are affected. It appears that physiology has a particularly high affinity to chaos. The old central place of physiology in the circle of sciences can perhaps be regained if the two disciplines join resources. Even physics may turn out to be dependent on the chaotic micro dynamics of the brain.     

Rhythmic context modulates foreperiod effects

Two experiments examined hypotheses about the roles of probabilistic uncertainty and rhythmic context on attentional preparation as reflected by choice response times (RTs) to the final tone of auditory sequences. Nonisochronous sequences with tone timings either arranged metrically or scrambled were linked with one of three different sequence-final time intervals, or foreperiods (FPs), which varied randomly from trial to trial. Two primary results emerged. First, RTs were faster to target tones ending metrical rhythms than to targets ending scrambled rhythms. Second, metrical contexts elicited RTs that increased with FP duration, whereas scrambled contexts elicited RTs that often decreased with FP duration, despite equivalent variability of time intervals in metrical and scrambled contexts. The results suggest that time relations implied by metrical rhythms systematically modulate preparatory responses to sequence-final FPs.