Comparison of initial and terminal temporal accuracy of 5 male top table tennis players performing attacking forehand drives led to the conclusion that because of a higher temporal accuracy at the moment of ball/bat contact than at initiation the players did not fully rely on a consistent movement production strategy. Functional trial-to-trial variation was evidenced by negative correlations between the perceptually specified time-to-contact at the moment of initiation and the mean acceleration during the drive; within-trial adaptation was also evident for two of the Ss. It is argued that task constraints provide the organizing principles for perception and action at the same time, thereby establishing a mutual dependency between the two. Allowing for changes in these parameters over time, a unified explanation is suggested that does not take recourse to large amounts of (tacit) knowledge on the part of the S.

Information specifying the future passing distance of an approaching object is available (in units of object size) in the ratio of optical displacement velocity and optical expansion velocity. Despite empirical support for the assumption that object size can serve as a metric in the perception of passing distance, the present series of experiments reveals that in catching a ball subjects do not rely on such "point-predictive" information. The angle at which (real and simulated) balls approached the subject systematically affected verbal and manual estimates of future passing distance, as well as the kinematic characteristics of catching movements. To catch a ball, the actor uses momentary action-related information instead of spatiotemporal estimates. The hand velocity is geared to information specifying the currently required velocity. This secures ending up at the right place in the right time, regardless of where this may be.To be successful in interceptive actions, like catching or hitting a ball, the movement behavior of the actor has to be very precisely attuned to the spatiotemporal characteristics of the event. The timing of interceptive acts is generally considered to be based on visual information about the time remaining until the object reaches the observer. In line with Gibson's (1966Gibson's ( , 1979 notion of optical specification of information, Lee (1976) derived an optical quantity that specifies this "time-to-contact" (t c ), namely, the inverse of the relative rate of dilation of the ball's optical contour that is generated in the optic array by the relative approach between ball and point of observation. This optical quantity, termed r (tau), specifies the time remaining until the ball makes contact with the point of observation if velocity of

In the present study a direct manipulation of the optical expansion pattern was carried out. What happens to the timing of the grasp movements involved in catching a ball when optical expansion information is not veridically provided? By using 2 luminescent balls of constant size and a luminescent ball that could change its diameter during flight, it was possible to manipulate the rate of optical expansion directly. The results of 2 experiments (binocular vision in Experiment 1 and monocular vision in Experiment 2) showed that the time of the maximal closing velocity of the hand--which conforms to the prediction if Ss use retinal expansion information--was later for the deflating ball than for the balls of constant size. Adjustments to the aperture of the hand in response to the different ball sizes, especially the adjustment of the hand to the deflating ball (even though Ss were not aware that the ball was deflating during its approach), point to a finely attuned perception-action coupling.

The coordination of multiple body segments (torso and legs) in the control of standing posture during a suprapostural task was studied. The analysis was motivated by dynamical theories of motor coordination. In 2 experiments it was found that multisegment postural coordination could be described by the relative phase of rotations around the hip and ankle joints. The effective length of the feet, the height of the center of mass, and the amplitude of head motions in a visual tracking task were varied. Across these variations, 2 modes of hip-ankle coordination were observed: in-phase and anti-phase. The emergence of these modes was influenced by constraints imposed by the suprapostural tracking task, supporting the idea that such tasks influence postural control in an adaptive manner. Results are interpreted in terms of a dynamical approach to coordination in which postural coordination modes can be viewed as emergent phenomena.

Based on the kinematics of goal-directed aiming movements in a reciprocal Fitts' task, a minimal limit cycle model is proposed that is capable of producing the behavior observed at levels of task difficulty ranging from 3 to 7. From graphical and statistical analyses of the phase planes, Hooke's planes and velocity profiles, we concluded that the minimal terms to be included in the model were (i) a nonlinear damping in the form of a self-sustaining, velocity-driven Rayleigh oscillator and (ii) a nonlinear stiffness in the form of a softening spring Duffing term. The model reproduced the kinematic patterns experimentally observed in rhythmical precision aiming, accounting for 95% of the variance. The coefficients in the model changed in a systematic way when distance and precision constraints were varied, and the meaning of these changes is discussed in the framework of the dynamical patterns approach.

In the present study, the authors examined transitions between postural coordination modes involved in human stance. The analysis was motivated by dynamical theories of pattern formation, in which coordination modes and transitions between modes are emergent, self-organized properties of the dynamics of animal-environment systems. In 2 experiments, standing participants tracked a moving target with the head. Results are consistent with the hypothesis that changes in body coordination follow typical nonequilibrium phase transitions, exhibiting multistability, bifurcation, critical fluctuations, hysteresis, and critical slowing down. The findings suggest that posture may be organized in terms of dynamical principles and favor the existence of general and common principles governing pattern formation and flexibility in complex systems.

In a forced-choice paradigm, human observers' sensitivity to visual information specifying a moving object's future time of arrival at a designated position in the field of view was evaluated. A geometrical analysis demonstrated that information specifying a first-order temporal relationship (i.e., without taking changes in velocity into account) is available in the combination of the relative rate of dilation of the optical contour of the moving object and the relative rate of constriction of the optical gap separating the moving object from the target position. Observers were sensitive to information contained in the relative rate of constriction of the optical gap if no contour dilation component was present and to the combination of information contained in the relative rates of dilation of the optical contour of the moving object and constriction of the optical gap if both were present albeit with a differential weighting of the 2 components.

Earlier studies have suggested that the size of an object to be grasped influences the time taken to complete a prehensile movement. However, the use of cylindrical objects in those studies confounded the effects of object size-extent orthogonal to the reach axis-and object width-extent along the reach axis. In separating these effects, the present study demonstrates that movement time is not affected by manipulation of object size, as long as the latter does not approach the maximal object size that can be grasped. Object width, on the other hand, is shown to exert a systematic influence on movement time: Smaller object widths give rise to longer movement times through a lengthening of the deceleration phase of the movement, thus reproducing the effect of target width on the kinematics of aiming movements. As in aiming, movement amplitude also affects the movement time in prehension, influencing primarily the acceleration phase (i.e. peak velocity attained). The effects of object width and movement amplitude were found to combine in a way predicted by Fitts' law, allowing a generalisation of the latter to the transport component in prehensile actions. With respect to the grasp component, both object size and object width are shown to affect peak hand aperture. Increasing object width thus lowers the spatial accuracy demands on the transport component, permitting a faster movement to emerge. At the same time, the hand opens to a larger grip in order to compensate for eventual directional errors that result.(ABSTRACT TRUNCATED AT 250 WORDS)