. Three sources of interlimb interactions have been postulated to underlie the stability characteristics of bimanual coordination but have never been evaluated in conjunction: integrated timing of feedforward control signals, phase entrainment by contralateral afference, and timing corrections based on the perceived error of relative phase. In this study, the relative contributions of these interactions were discerned through systematic comparisons of five tasks involving rhythmic flexion-extension movements about the wrist, performed bimanually (in-phase and antiphase coordination) or unimanually with or without comparable passive movements of the contralateral hand. The main findings were the following. 1) Contralateral passive movements during unimanual active movements induced phase entrainment to interlimb phasing of either 0°(in-phase) or 180°(antiphase). 2) Entrainment strength increased with the passive movements' amplitude, but was similar for in-phase and antiphase movements. 3) Coordination of unimanual active movements with passive movements of the contralateral hand (kinesthetic tracking) was characterized by similar bilateral EMG activity as observed in active bimanual coordination. 4) During kinesthetic tracking the timing of the movements of the active hand was modulated by afference-based error corrections, which were more pronounced during in-phase coordination. 5) Indications of in-phase coordination being more stable than antiphase coordination were most prominent during active bimanual coordination and marginal during kinesthetic tracking. Together the results indicated that phase entrainment by contralateral afference contributed equally to the stability of in-phase and antiphase coordination, and that differential stability of these patterns depended predominantly on integrated timing of feedforward signals, with only a minor role for afference-based error corrections.

This article examines the status of dynamical models of movement coordination qua phenomenological models. After a brief outline of the aims, methods and strategic assump tions of the dynamical systems approach, a survey is provided of the theoretical and empirical progress it has made in identifying general principles of coordination. Although dynamical models are constructed for phenomena at a particular level of analysis for which they provide descriptive explanations, their dynamics can sometimes he linked to or associated with the dynamics of processes at other levels of analysis. The article concludcs with a tentative scheme to clarify the position of the dynamical approach relative to other ex! an I approaches in movement science, « W ipBodily movements occur in the context of the everyday functioning of people while realizing specific task goals. As a rule, such movements involve the participation of multiple joints and limbs. When in action, these body parts are coordinated, that is, they are brought into proper relation to one another as well as to the surrounding layout of surfaces (cf. Turvey, 1990). To the naked eye, this coordination may look relatively simple, as in picking up an object, or relatively complicated, as in juggling, performing an attacking forehand drive in table tennis or playing the drums. To the ( 'orrcspoiKling author. Tel.: +31 20 444 8532. Fax: +31 20 444 5867. E-mail: 1' .1 HeekWl'BW .VU.NL (l|li7-lM 5 7 /l)5/$(W.50 < v> 1W5 I Use vier Science U.V. A ll lights reserved S S D I 0 I ()7-t) 4 5 7 ( t>5 MI0028-3 movement scientist, however, all coordination is complex in that he or she is confronted with the challenge to explain coordinated movements as the orderly products of a hybrid biological organization involving a very large number of different subsystems (e.g., vascular, neural, muscular, skeletal). These subsystems are operating at different rates and are connected in intricate ways. Due to this compositional complexity, the problem of movement coordination is extremely difficult to resolve in a scientifically satisfactory way. Finding an adequate solution is hampered by the fact that the field of motor control is still very much partitioned according to the traditional disciplines of movement science (mechanics, neuropliysiology, psychology, and so on), whereas a multidisciplinary or interdisciplinary approach is required.Broadly speaking, two types of approaches may be distinguished in movement science: structural and phenomenological approaches (ef, Otten, 1991). Structural approaches seek causal explanations of movement in terms of dedicated structures within the human body. Phenomenological approaches, in contrast, seek noncausal explanations in terms of phe nomenological laws and principles without reference to dedicated mecha nisms and structures within the human body.Structural models of motor control are typically (neuro)physiological models which attempt to explain different aspects of motor behaviour on the basis of hypothetical (neuro)physiologieal m...

Acoustic rhythms are frequently used in gait rehabilitation, with positive instantaneous and prolonged transfer effects on various gait characteristics. The gait modifying ability of acoustic rhythms depends on how well gait is tied to the beat, which can be assessed with measures of relative timing of auditory-motor coordination. We examined auditory-motor coordination in 20 healthy elderly individuals walking to metronome beats with pacing frequencies slower than, equal to, and faster than their preferred cadence. We found that more steps were required to adjust gait to the beat, the more the metronome rate deviated from the preferred cadence. Furthermore, participants anticipated the beat with their footfalls to various degrees, depending on the metronome rate; the faster the tempo, the smaller the phase advance or phase lead. Finally, the variability in the relative timing between footfalls and the beat was smaller for metronome rates closer to the preferred cadence, reflecting superior auditory-motor coordination. These observations have three practical implications. First, instantaneous effects of acoustic stimuli on gait characteristics may typically be underestimated given the considerable number of steps required to attune gait to the beat in combination with the usual short walkways. Second, a systematic phase lead of footfalls to the beat does not necessarily reflect a reduced ability to couple gait to the metronome. Third, the efficacy of acoustic rhythms to modify gait depends on metronome rate. Gait is coupled best to the beat for metronome rates near the preferred cadence.

The cerebral cortex contains circuitry for continuously computing properties of the environment and one's body, as well as relations among those properties. The success of complex perceptuomotor performances requires integrated, simultaneous use of such relational information. Ball catching is a good example as it involves reaching and grasping of visually pursued objects that move relative to the catcher. Although integrated neural control of catching has received sparse attention in the neuroscience literature, behavioral observations have led to the identification of control principles that may be embodied in the involved neural circuits. Here, we report a catching experiment that refines those principles via a novel manipulation. Visual field motion was used to perturb velocity information about balls traveling on various trajectories relative to a seated catcher, with various initial hand positions. The experiment produced evidence for a continuous, prospective catching strategy, in which hand movements are planned based on gaze-centered ball velocity and ball position information. Such a strategy was implemented in a new neural model, which suggests how position, velocity, and temporal information streams combine to shape catching movements. The model accurately reproduces the main and interaction effects found in the behavioral experiment and provides an interpretation of recently observed target motion-related activity in the motor cortex during interceptive reaching by monkeys. It functionally interprets a broad range of neurobiological and behavioral data, and thus contributes to a unified theory of the neural control of reaching to stationary and moving targets.

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The circle map provides a general mathematical model for the mode-locking behavior observed in systems of coupled oscillators. From this theoretical perspective, multifrequency tapping was studied. Three experiments were conducted in which skilled drummers participated. The results were in qualitative agreement with the dynamical features of the circle map. The stability of behavior was affected by the movement frequency at which the multifrequency relations were performed. Attraction to lower order ratios (predominantly showing Farey relations) was observed. In some situations bistability and hysteresis occurred, implying that the system was situated in the supercritical domain of the circle map where resonance zones overlap. Furthermore, the results suggest that multifrequency tapping is characterized by an asymmetrical coupling in that the influence of the fast hand on the slow hand is the strongest.Tasks in which two limbs move at different frequencies, so-called multifrequency tasks, tend to be very difficult. Tapping polyrhythms is a case in point. Polyrhythms are frequency ratios that cannot be simplified to ratios with one as a numerator or denominator (e.g., 2:3, 3:5, and 4:11). Tapping a 3:5 polyrhythm implies that the hands move at frequencies that are related in such a way that one hand taps three times in the same interval in which the other hand taps five times. The performance of polyrhythms has been investigated in a number of studies (e.g.

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Abstract. Based on recent brain-imaging data and congruent theoretical insights, a dynamical model is derived to account for the patterns of brain activity observed during stable performance of bimanual multifrequency patterns, as well as during behavioral instabilities in the form of phase transitions between such patterns. The model incorporates four dynamical processes, defined over both motor and premotor cortices, which are coupled through inhibitory and excitatory inter-and intrahemispheric connections. In particular, the model underscores the crucial role of interhemispheric inhibition in reducing the interference between disparate frequencies during stable performance, as well as the failure of this reduction during behavioral transitions. As an aside, the model also accounts for in-and antiphase preferences during isofrequency movements. The viability of the proposed model is illustrated by magnetoencephalographic signals that were recorded from an experienced subject performing a polyrhythmic tapping task that was designed to induce transitions between multifrequency patterns. Consistent with the model's dynamics, contra-and ipsilateral cortical areas of activation were frequency-and phase-locked, while their activation strength changed markedly in the vicinity of transitions in coordination.