Frank T. J. M. Zaal

The information for catching fly balls : Judging and intercepting virtual balls in a CAVE

Visually guided action implies the existence of information as well as a control law relating that information to movement. For ball catching, the Chapman Strategy--keeping constant the rate of change of the tangent of the elevation angle (d(tan(alpha))/dt)--leads a catcher to the right location at the right time to intercept a fly ball. Previous studies showed the ability to detect the information and the consistency of running patterns with the use of the strategy. However, only direct manipulation of information can show its use. Participants were asked to intercept virtual balls in a Cave Automated Virtual Environment (CAVE) or to judge whether balls would pass behind or in front of them. Catchers in the CAVE successfully intercepted virtual balls with their forehead. Furthermore, the timing of judgments was related to the patterns of changing d(tan(alpha))/dt. The advantages and disadvantages of a CAVE as a tool for studying interceptive action are discussed.

Prehension is really reaching and grasping.

Prehension has traditionally been seen as the act of coordinated reaching and grasping. However, recently, Smeets and Brenner (in Motor Control 3:237–271, 1999) proposed that we might just as well look at prehension as the combination of two independently moving digits. The hand aperture that has featured prominently in many studies on prehension, according to Smeets and Brenner’s “double-pointing hypothesis”, is really an emergent property related to the time course of the positions of the two digits moving to their respective end points. We tested this double-pointing hypothesis by perturbing the end position of one of the digits while leaving the end position of the opposing digit unchanged. To this end, we had participants reach for and grasp a metallic object of which the side surfaces could be made to slide in and out. We administered the perturbation right after movement initiation. On several occasions, after perturbing the end position of one digit, we found effects also on the kinematics of the opposing digit. These findings are in conflict with Smeets and Brenner’s double-pointing hypothesis.

Virtual reality as a tool for the study of perception-action: The case of running to catch fly balls

Virtual reality (VR) holds great promise for the study of perception-action. The case of studying the outfielder problem is presented as an example of how VR has contributed to our understanding of perception-action, and of the potential and pitfalls of using VR in such a task. The outfielder problem refers to the situation in a baseball game (and analogous situations) in which an outfielder has to run to get to the right location at the right time to make a catch. Several experimental studies are discussed in which participants had to intercept real or virtual balls. The biggest added value of using VR is the fact that the virtual world is completely in the hands of the experimenter, which allows studying situations that do not exist outside of VR, thus enabling strong hypothesis testing. A number of factors related to the success of the VR experiments are identified, such as the lack of haptic feedback in VR setups used in this paradigm until now, the specifics of the optics presented to the participants, and the available space for locomotion. We argue that it is important to make a close comparison of task behavior in VR with that outside of VR, but conclude having great expectations of the role of VR in perception-action research.

Effects of Changing Object Size During Prehension

ABSTRACT The authors tested how fast the grasp component of prehension was able to adjust to a sudden change in object size. Participants grasped an object, the size of which could suddenly increase. Whereas previous researchers usually applied perturbations through a change in illumination at movement onset, the present perturbations involved a change in the objects physical size at 1 of 4 moments during the movement (125, 200, 275, and 350 ms after movement onset). The results showed that grasp adjustments came in many forms and could be as fast as 120 ms. The implications for the understanding of the coordination of reaching and grasping in prehension are discussed.

Rowing Crew Coordination Dynamics at Increasing Stroke Rates

In rowing, perfect synchronisation is important for optimal performance of a crew. Remarkably, a recent study on ergometers demonstrated that antiphase crew coordination might be mechanically more efficient by reducing the power lost to within-cycle velocity fluctuations of the boat. However, coupled oscillator dynamics predict the stability of the coordination to decrease with increasing stroke rate, which in case of antiphase may eventually yield breakdowns to in-phase. Therefore, this study examined the effects of increasing stroke rate on in- and antiphase crew coordination in rowing dyads. Eleven experienced dyads rowed on two mechanically coupled ergometers on slides, which allowed the ergometer system to move back and forth as one ‘boat’. The dyads performed a ramp trial in both in- and antiphase pattern, in which stroke rates gradually increased from 30 strokes per minute (spm) to as fast as possible in steps of 2 spm. Kinematics of rowers, handles and ergometers were captured. Two dyads showed a breakdown of antiphase into in-phase coordination at the first stroke rate of the ramp trial. The other nine dyads reached between 34–42 spm in antiphase but achieved higher rates in in-phase. As expected, the coordinative accuracy in antiphase was worse than in in-phase crew coordination, while, somewhat surprisingly, the coordinative variability did not differ between the patterns. Whereas crew coordination did not substantially deteriorate with increasing stroke rate, stroke rate did affect the velocity fluctuations of the ergometers: fluctuations were clearly larger in the in-phase pattern than in the antiphase pattern, and this difference significantly increased with stroke rate. Together, these results suggest that although antiphase rowing is less stable (i.e., less resistant to perturbation), potential on-water benefits of antiphase over in-phase rowing may actually increase with stroke rate.

Prospective Control in Catching: The Persistent Angle-of-Approach Effect in Lateral Interception

In lateral interception tasks balls converging onto the same interception location via different trajectories give rise to systematic differences in the kinematics of hand movement. While it is generally accepted that this angle-of-approach effect reflects the prospective (on-line) control of movement, controversy exists with respect to the information used to guide the hand to the future interception location. Based on the pattern of errors observed in a task requiring visual extrapolation of line segments to their intersection with a second line, angle-of-approach effects in lateral interception have been argued to result from perceptual biases in the detection of information about the balls future passing distance along the axis of hand movement. Here we demonstrate that this account does not hold under experimental scrutiny: The angle-of-approach effect still emerged when participants intercepted balls moving along trajectories characterized by a zero perceptual bias with respect to the balls future arrival position (Experiment 4). Designing and validating such bias-controlled trajectories were done using the line-intersection extrapolation task (Experiments 2 and 3). The experimental set-up used in the present series of experiments was first validated for the lateral interception and the line-intersection extrapolation tasks: In Experiment 1 we used rectilinear ball trajectories to replicate the angle-of-approach effect in lateral interception of virtual balls. Using line segments extracted from these rectilinear ball trajectories, in Experiment 2 we replicated the reported pattern of errors in the estimated locus of intersection with the axis of hand movement. We used these errors to develop a set of bias-free trajectories. Experiment 3 confirmed that the perceptual biases had been corrected for successfully. We discuss the implications on the information-based regulation of hand movement of our finding that the angle-of-approach effect in lateral interception cannot not explained by perceptual biases in information about the balls future passing distance.

Fractional-Order Information in the Visual Control of Lateral Locomotor Interception

Previous work on locomotor interception of a target moving in the transverse plane has suggested that interception is achieved by maintaining the targets bearing angle (often inadvertently confused and/or confounded with the target heading angle) at a constant value. However, dynamics-based model simulations testing the veracity of the underlying control strategy of nulling the rate of change in the bearing angle have been restricted to limited conditions of target motion, and only a few alternatives have been considered. Exploring a wide range of target motion characteristics with straight and curving ball trajectories in a virtual reality setting, we examined how soccer goalkeepers moved along the goal line to intercept long-range shots on goal, a situation in which interception is naturally constrained to movement along a single dimension. Analyses of the movement patterns suggested reliance on combinations of optical position and velocity for straight trajectories and optical velocity and acceleration for curving trajectories. As an alternative to combining such standard integer-order derivatives, we demonstrate with a simple dynamical model that nulling a single informational variable of a self-tuned fractional (rather than integer) order efficiently captures the timing and patterning of the observed interception behaviors. This new perspective could fundamentally change the conception of what perceptual systems may actually provide, both in humans and in other animals. (PsycINFO Database Record

Shaping decisions in volleyball An ecological approach to decision-making in volleyball passing

To extend research on decision-making in sport we addressed the choices volleyball-players are faced with in a simple volleyball pass-return task. We manipulated the distance that eight experienced volleyball players had to cover for successful ball passing, and mapped their passing technique (i.e., overhead or underhand) and ball return accuracy in a choice condition. Passing accuracy was then compared with conditions in which reception technique was imposed by instruction. When players were free to choose their technique the landing zone of the ball influenced the choice of technique: When a ball landed further away, the adoption of underhand technique increased, especially for balls that landed in front of the participants. Furthermore, in all conditions the accuracy of the pass decreased with increasing distance to be covered. These results are discussed vis-a-vis the idea that player behavior is shaped by affordances (i.e., possibilities for action). It is argued that to understand decision-making in dynamic sport situations we need to understand how players deal with competing affordances.

Chapter 17 The use of time-to-contact information for the initiation of hand closure in natural prehension

Abstract In prehension, the opening and closing of the hand need to be coordinated with the transport of that same hand. We provide evidence for the hypothesis that this coordination is based on the use of first-order time-to-contact information. This information acts to affect the relative stability of a hand opening regime and a hand closing regime, leading to a smooth and stable transition from the one to the other, rather than simply triggering closure of the hand upon passing a critical value. Whereas manipulations of target width, target size, and target distance led to a pattern of data that could not be captured by a number of competing models for the coordination of reaching and grasping, the appropriately parameterized dynamic timing model that we adopted for this study fitted the data quite well. In this chapter we pursue two goals at the same time. We build a case for the use of first-order time-to-contact information in the coordination in prehension. At the same time, we use that case as a vehicle for discussing various aspects of the concept of first-order time to contact and the application of dynamical-systems models.

Trajectory formation and speed-accuracy trade-off in aiming movements.

The trade-off between speed and accuracy and the patterning of movement kinematics have been central issues for theories of human movement for almost a century. In the present contribution experimentally obtained kinematics of reciprocal aiming movements, performed under different task conditions, are modelled as resulting from a single non-linear dynamical system whose parameters vary so as to respond to the task demands. Providing a unified account of speed-accuracy trade-off and trajectory formation phenomena, the model offers a theoretical framework in which both discrete and continuous movements, performed along one or more dimensions can be understood.