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R J Bootsma

Publications and source records attributed to R J Bootsma.

14 recordsLinked to original sources
PubMed1998

Coordination in prehension. Information-based coupling of reaching and grasping.

Prehension involves the coordination of a reaching and a grasping movement, such that the hand opens and closes in tune with the transport of the hand to the object to be grasped. To investigate this coordination, we focused on the transition from hand opening to hand closing in the grasping component of prehension. Earlier research has suggested that the time taken to close the hand remains constant over varying reaching amplitudes. In the present experiment, in which subjects reached for objects at six different distances and for objects that moved away from them at three different, constant speeds, hand-closure time was found to vary as a function of experimental conditions. Moreover, initiation of hand closure did not occur at a constant value of the (perceptually available) first-order time remaining until contact with the object. However, the variations observed, occurring as a function of initial hand-object distance and object speed, could be accounted for by an abstract dynamical model of perceptually driven postural changes.

Adult↗
PubMed1997

The effects of task constraints on the organization of interception movements.

In the light of the intensity coupling effects reported in the literature, subjects' capacities for independently controlling the absolute velocity of their movement at the point of interception was evaluated by asking them to learn to propel orthogonally approaching balls, varying in their speed of approach, into target boxes placed at various distances from the interception point. As required for successful accomplishment of the task, movement velocity was found to be adapted to the distance to be covered, with the near target giving rise to lower velocities and the far target giving rise to higher velocities even when the conditions were presented in random order. Nevertheless, even though target distance accounted for a substantial proportion of the total variance, a small but significant effect of ball approach speed on movement velocity was found, suggesting that intensity coupling is a persistent but modulable phenomenon in interception tasks.

Adaptation, Physiological↗
PubMed1994

Fitts' law in two-dimensional task space.

A traditional continuous Fitts' task may be described as a one-dimensional oscillation between two targets. The combination of two such oscillations along intersecting axes gives rise to a two-dimensional aiming task, allowing the study of the speed-accuracy trade-off in two-dimensional task space. In two experiments subjects were asked to draw as many ellipses as possible while passing through four targets, arranged around the extreme points of the two major axes of a model ellipse. In the first experiment, task difficulty was manipulated simultaneously along the two axes of the ellipse. Regardless of ellipse eccentricity and orientation, movement time (MT) was found to depend linearly on Fitts' index of difficulty (ID), which combines between-target distance and target width. In the second experiment, ID was manipulated independently for the short and the long axes of the ellipse. There was a strong linear relation between MT and ID averaged over the two axes, with the two independent measures of task difficulty exerting interactive effects on MT: the higher the ID on one axis, the smaller the effect of the ID on the other. The present results demonstrate that Fitts' law, only examinated so far in one-dimensional aiming tasks, generalises to two-dimensional task space.

Adult↗
PubMed1994

The speed-accuracy trade-off in manual prehension: effects of movement amplitude, object size and object width on kinematic characteristics.

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)

Adult↗
PubMed1994

Catching balls: how to get the hand to the right place at the right time.

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.

Adult↗
PubMed1993

Visual information about time-to-collision between two objects.

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.

Adult↗
PubMed1991

Grasping tau.

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.

Adult↗
PubMed1991

Acquiring an attacking forehand drive: the effects of static and dynamic environmental conditions.

Two groups of 10 novice subjects each were trained to perform attacking forehand drives in table tennis and land the balls as fast and as accurately as possible onto a target on the opposite side of the net under two different training conditions. Under the static training condition, the balls were to be struck from a constant position, and under the dynamic training condition, balls approached the subjects in a normal way. Both groups were tested under dynamic conditions prior to and after four days of training, during which they received 1,600 practice trials. Both groups of subjects were shown to increase the number of balls that landed on the target, and learning was also evident from an increased consistency of the direction of travel of the bat at the moment of ball/bat contact. However, no increase in consistency was found for the location of the bat at the moment of ball/bat contact and for the movement times. Thus, learning can occur in the absence of externally generated time-to-contact information, but this is not due to the establishment of a consistent movement form. Learning appears to progress from control at the moment of ball/bat contact backward, toward the moment of initiation.

Adolescent↗
PubMed1989

The underarm throw for accuracy in children.

Forty boys and girls of two age groups (mean ages 52 and 70 months, respectively) performed 35 trials on a throwing task demanding accuracy. As might be expected, the older children were significantly more accurate than the younger ones but there were no sex differences. Film analysis of the throws showed that, although both age groups attempted to employ the adult strategy of varying velocity while keeping ball-release angle constant, the younger group was less successful in the latter aspect. The manner in which consistent release angles were arrived at by the older group was through the use of a movement pattern in which the elbow angle did not change during the forward swing. Such a movement pattern appeared less frequently in the younger age group. The results of this study provide support for a mode of research identifying functionally relevant kinematic variables that can subsequently be shown to follow from distinctive execution categories. In such a way, changes in outcome measures with increasing age and/or skill level can be functionally understood and identified.

Arm↗
PubMed1989

The effect of video-feedback on the learning of the tennis service by intermediate players.

Effects of video-feedback on improvement of the tennis service were investigated in subjects having at least 2 years of playing experience in tennis. The experiment was carried out in an indoor tennis hall under normal training conditions. Subjects were randomly divided into three groups: a video-feedback training (VFT) group, a traditional training (TT) group and a control group, each consisting of 22 subjects. While the subjects of the control group received no training at all, subjects in both the VFT and TT groups were trained twice weekly during 5 consecutive weeks. Each training session lasted 40 min, of which 30 min was spent on actual practice in training of the service. The remaining 10 min was spent on watching, analysing and discussing video recordings of either their own service performed during the training session (VFT group), or ground strokes and volleys of top level players (TT group). Both the VFT and TT group showed significantly greater improvements in both achievement scores and form (technique) scores than did the control group. No differences, however, could be demonstrated between the VFT and TT group, indicating that the subjects of the former group did not benefit from the video-feedback they received. While form scores correlated significantly (P less than 0.001) with the velocity of the served ball, no such relation between form scores and spatial accuracy was apparent.

Adult↗
PubMed1989

Accuracy of perceptual processes subserving different perception-action systems.

The accuracy of perceptual processes subserving different perception-action systems was evaluated by presenting subjects (N = 17) with similar optic flow patterns, while requiring different actions from them. Squash balls were dropped along a fixed trajectory, and subjects were asked to (a) hit the ball using their own arm, (b) release an artificial arm to hit the ball, and (c) indicate when the ball was at the point of contact of conditions (a) and (b). The variability of the temporal initiation point of the actions, which served as an operationalization of perceptual accuracy, was compared under all three conditions. The results indicated that the variability of the temporal initiation point was smallest under the natural arm condition, even though movement time was more variable here than under the artificial arm condition. It is argued that, because perception and action are intimately interwoven components of a perceiving/acting system, it is not an extrinsically (experimenter-) determined simplicity of perceptual and/or motor aspects of the task, but the intrinsic make-up of this overarching system that determines which couplings lead to a more accurate performance.

Adolescent↗
PubMed1985

The effect of video-modelling and video-feedback on the learning of the tennis service by novices.

This project is addressed to the effectiveness of video mediated instruction on the learning of the tennis service by novices. The research was carried out in an indoor tennis hall under normal training conditions. Three experimental groups were used: a video-model (VMT), a video-feedback (VFT) and a group with a combination of video modelling and video feedback (VMFT). Two control groups - based on different training periods-were also used: a traditional training group (TT1) which practised for a period of 45 min per training session and a second traditional training group (TT2) which practised for a period of only 30 min per training session. All subjects took part in five successive training sessions. No clear advantages of using video mediated instruction methods in teaching novices the tennis service could be demonstrated. Five sessions of 30 min practical training were shown to be as effective as five sessions of 45 min practical training in producing improvements in both form and achievement scores of novice tennis players. While, in the early stages, form and achievement scores were not shown to be significantly related, after five sessions a significant correlation (P less than 0.05) was shown.

Achievement↗