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Dagmar Sternad

Publications and source records attributed to Dagmar Sternad.

9 recordsLinked to original sources
PubMed2003

Actively tracking 'passive' stability in a ball bouncing task.

This study investigates the control involved in a task where subjects rhythmically bounce a ball with a hand-held racket as regularly as possible to a prescribed amplitude. Stability analyses of a kinematic model of the ball-racket system revealed that dynamically stable solutions exist if the racket hits the ball in its decelerating upward movement phase. Such solutions are resistant to small perturbations obviating explicit error corrections. Previous studies reported that subjects' performance was consistent with this 'passive' stability. However, some 'active' control is needed to attune to this passive stability. The present study investigates this control by confronting subjects with perturbations where stable behavior cannot be maintained solely from passive stability. Six subjects performed rhythmic ball bouncing in a virtual reality set-up with and without perturbations. In the perturbation trials the coefficient of restitution of the ball-racket contact was changed at every fifth contact leading to unexpected ball amplitudes. The perturbations were compensated for within 2-3 bouncing cycles such that ball amplitudes decreased to initial values. Passive stability was reestablished as indicated by negative racket acceleration. Results revealed that an adjustment of the racket period ensured that the impacts occurred at a phase associated with passive stability. These findings were implemented in a model consisting of a neural oscillator that drives a mechanical actuator (forearm holding the racket) to bounce the ball. Following the perturbation, the oscillator's period is adjusted based on the perceived ball velocity after impact. Simulation results reproduced the major aspects of the experimental results.

Acceleration↗
PubMed2003

Control of ball-racket interactions in rhythmic propulsion of elastic and non-elastic balls.

Ball-racket interactions were investigated in a task where participants propelled a ball rhythmically into the air. The study contrasted two ball-racket conditions: (1) an elastic impact where the ball was able to rebound due to the elasticity of the colliding objects and participants bounced the ball, and (2) a non-elastic impact where the coefficient of restitution was zero and the ball did not rebound such that the participants had to throw the ball. The goal of the study was to contrast the situations where haptic information about the ball-racket interactions is either secondary (elastic bouncing) or becomes a primary factor for control (non-elastic propulsion). In the elastic condition, the performers controlled the parameters for ball-racket contact prior to contact: In agreement with the criteria for dynamical stability defined by a model, racket accelerations immediately before the contacts were negative, racket positions and velocities at the instant of the initial contact correlated negatively, contact durations were short (30+/-9 ms), and during the collision interval racket velocity and acceleration decreased monotonically. In the non-elastic condition, the parameters of ball release were primarily controlled during the collision phase: Racket accelerations before contact were positive, racket positions and velocities at initial contact showed weak correlations, and the contact intervals were significantly longer (116+/-15 ms) with a clear segmentation into two segments. Negative correlations were observed between the integrals of the velocity and acceleration computed over the two consecutive segments, giving evidence that in the non-elastic condition the CNS is able to introduce corrections during the very short collision interval. The results are discussed with respect to physiological mechanisms of movement corrections available during such short time intervals.

Acceleration↗
PubMed2003

A randomization method for the calculation of covariation in multiple nonlinear relations: illustrated with the example of goal-directed movements.

A randomization method is developed for the calculation of covariation between multiple variables that are linked nonlinearly to a dependent variable. Covariation is a phenomenon often invoked in the study of movement coordination to capture the fact that in coordinated movement the outcome shows greater than expected consistency from the variability in the component processes. However, in most cases, the problem is that more than two variables covary in a nonlinear fashion, which makes quantification with the bivariate linear covariation and correlation coefficient inapplicable. This paper presents a generalization of the calculation of linear bivariate covariance using a variant of a randomization method that is based on the comparison between the empirically measured variability in the outcome and a covariation-free variability. The latter can be estimated by permuting data sets. A generalized correlation coefficient is derived, and it is shown how errors of estimation can be quantified. The permutation method can also quantify partial multiple nonlinear covariation. The calculations are illustrated in a numerical example of an arm-reaching task. However, the method is applicable to all cases where the internal organization of a nonlinear system of multiple variables needs to be quantified. The relation and applicability of the permutation method compared to other methods using regression and principal component analysis are discussed and illustrated with a numerical example.

Arm↗
PubMed2003

Task-effector asymmetries in a rhythmic continuation task.

Variability in rhythmic movements has been interpreted as a signature of internal or peripheral noise processes. Grounded in an oscillator interpretation, this study hypothesized that period variability and drift arises from the asymmetry between target period and the limb's intrinsic dynamics. Participants synchronized to 7 target periods, swinging 1 of 3 pendulums in a continuation paradigm; 3 periods were longer, 3 shorter, and 1 identical to the preferred period. Results supported 5 predictions: Drift toward the preferred period was observed that scaled with the asymmetry. Variability was lowest for symmetry conditions and increased with the asymmetry. Variability decreased concomitant with the approach toward the preferred period. Periods exponentially approached the preferred period with positive autocorrelations up to 10 cycles.

Adult↗
PubMed2002

Interaction of discrete and rhythmic movements over a wide range of periods.

This study investigates a complex task in which rhythmic and discrete components have to be combined in single-joint elbow rotations. While previous studies of similar tasks already reported that the initiation of the discrete movement is constrained to a particular phase window of the ongoing rhythmic movement, interpretations have remained contradictory due to differences in paradigms, oscillation frequencies, and data analysis techniques. The present study aims to clarify these findings and further elucidate the bidirectional nature of the interaction between discrete and rhythmic components. Participants performed single-degree-of-freedom elbow oscillatory movements at five prescribed periods (400, 500, 600, 800, 1,000 ms). They rapidly switched the midpoint of oscillation to a second target after an auditory signal that occurred at a random phase of the oscillation, without stopping the oscillation. Results confirmed that the phase of the discrete movement initiation is highly constrained with respect to the oscillation period. Further, the duration, peak velocity, and the overshoot of the discrete movement varied systematically with the period of the rhythmic movement. Effects of the discrete-onto-rhythmic component were seen in a phase resetting of the oscillation and a systematic acceleration after the discrete movement, which also varied as a function of the oscillation period. These results are interpreted in terms of an inhibitory bidirectional coupling between discrete and rhythmic movement. The interaction between discrete and rhythmic movement elements is discussed in comparison to sequential and gating processes suggested previously.

Adult↗
PubMed1999

Dynamics of 1:2 Coordination: Generalizing Relative Phase to n:m Rhythms.

Interlimb rhythmic movements can be modeled as coupled oscillators, with stable performance characterized by the relative phase between the limbs. In the present study, that modeling strategy, verified previously for 1:1 coordination, was generalized to 1:2 coordination with a view to n:m coordination. The generalized model predicted interactions between coordination (specifically, 1:1 vs. 1:2) and the frequency asymmetry between the limbs determining mean relative phase and its variability. The predicted interactions were evaluated with bimanual 1:2 and 1:1 rhythmic tasks in which participants (N = 8) oscillated hand-held pendulums whose uncoupled frequencies could be adjusted so that different interlimb asymmetries were produced. The authors needed new analytic procedures to verify stable 1:2 coordination and to resolve stochastic and deterministic sources of variability in the component oscillations. The major expectations from the generalized model were confirmed, and the implications of additional but unpredicted findings for the modeling of multifrequency behavior are discussed.

Journal Article↗
PubMed1999

Dynamics of 1:2 Coordination: Sources of Symmetry Breaking.

Three asymmetries in the dynamics of 1:2 interlimb coordination were examined: the asymmetry in uncoupled frequencies, the asymmetry in coupled frequencies, and the left-right functional asymmetry of the body. In a bimanual 1:2 task, participants (N = 8) oscillated hand-held pendulums whose uncoupled frequencies were adjusted so that the first kind of asymmetry could be manipulated. For any given pendulum pair, the pendulum assuming the faster motion in the 1:2 coordination was oscillated in the right and the left hands. By assigning combinations of uncoupled eigenfrequencies and coupled task-specified frequencies across hands, the authors studied the interaction of all 3 asymmetries. The results confirm the appropriateness of generalized relative phase as a collective variable for 1:2 coordination. Additionally, they suggest that the generalized form of the detuning parameter represents the first asymmetry and that the coupling function expresses the second asymmetry. In 1:2 coordination, the body's functional asymmetry plays a limited role.

Journal Article↗
PubMed1999

Dynamics of 1:2 Coordination: Temporal Scaling, Latent 1:1, and Bistability.

The simplest interlimb multifrequency coordination of 1:2 can be performed at different speeds and in at least two different styles or modes. The effects of speed and mode (in-phase or antiphase) were evaluated in a bimanual 1:2 rhythmic task in which participants (N = 8) oscillated hand-held pendulums with identical or different uncoupled frequencies. A motion equation in relative phase that captures the asymmetries of components and task predicted the 1:2 coordination equilibria resulting from temporal scaling. According to the experimental results, both coordination modes proved to be equally stable. More detailed analyses of individual trials showed signs that the more fundamental 1:1 coordination intruded into the 1:2 coordination.

Journal Article↗