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  • Author: Jeroen B.J. Smeets x
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Jeroen B.J. Smeets and Eli Brenner

Reaching out for an object is often described as consisting of two components that are based on different visual information. Information about the object's position and orientation guides the hand to the object, while information about the object's shape and size determines how the fingers move relative to the thumb to grasp it. We propose an alternative description, which consists of determining suitable positions on the object—on the basis of its shape, surface roughness, and so on—and then moving one's thumb and fingers more or less independently to these positions. We modeled this description using a minimum-jerk approach, whereby the finger and thumb approach their respective target positions approximately orthogonally to the surface. Our model predicts how experimental variables such as object size, movement speed, fragility, and required accuracy will influence the timing and size of the maximum aperture of the hand. An extensive review of experimental studies on grasping showed that the predicted influences correspond to human behavior.

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Jeroen B.J. Smeets and Eli Brenner

We begin our response by discussing the commentators' arguments concerning our proposal to abandon the classical distinction between transport and grip. In the second section, we argue that the minimum-jerk model is not fundamental to our approach, but very convenient. In the third section, we discuss how the experimental results that the commentators mention fit into our new approach. We conclude that the predictive capacity of our model, combined with its simplicity, makes it very useful for understanding grasping.

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Jeroen B.J. Smeets and Eli Brenner

We agree with Robertson that our new view on grasping is a description of motor behavior rather than an exploration into the nature of the neural processing underlying this behavior. However, neurophysiologists might be inspired by our new view to ask other questions, perform other experiments, and analyze these differently. In this way, they could generate new insights about the neural control of grasping.

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Jeroen B.J. Smeets, Leonie Oostwoud Wijdenes and Eli Brenner

We can adjust an ongoing movement to a change in the target’s position with a latency of about 100 ms, about half of the time that is needed to start a new movement in response to the same change in target position (reaction time). In this opinion paper, we discuss factors that could explain the difference in latency between initiating and adjusting a movement in response to target displacements. We consider the latency to be the sum of the durations of various stages in information processing. Many of these stages are identical for adjusting and initiating a movement; however, for movement initiation, it is essential to detect that something has changed to respond, whereas adjustments to movements can be based on updated position information without detecting that the position has changed. This explanation for the shorter latency for movement adjustments also explains why we can respond to changes that we do not detect.

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Jeroen B.J. Smeets, Leonie Oostwoud Wijdenes and Eli Brenner

We begin our response by clarifying the concept of detection, and explaining why this is needed for initiating, but not for adjusting a movement. We present a simulation to illustrate this difference. Several commentators referred to studies with results that might seem in conflict with our proposal that movement adjustments have short latencies because there is no need to detect anything. In the last part of our response, we discuss how we interpret these studies as being in line with our proposal.

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Anne-Marie Brouwer, Eli Brenner and Jeroen B.J. Smeets

Before an aspect of a movement that is predicted by a control theory can be considered as evidence for that theory, it should be clear that this aspect is not the result of some other property of the movement. We investigate whether this condition is met in studies that claim to provide evidence for the tau-coupling theory. This theory proposes that moving targets are intercepted at a specified goal zone by maintaining a constant ratio between the tau (time to closure) of the gap between the hand and the goal zone and the tau of the gap between the hand and the moving target. In line with the theory, previous research has found a linear relationship between these two decreasing taus during the last part of such a movement. To investigate whether this linear relationship was a side-effect of smooth successful movements, we modeled smooth ballistic hand movements that were independent of the target's movement but led to successful interception. We found that the resulting taus of decreasing gaps were also related linearly. We conclude that this relationship cannot be considered as evidence for the tau-coupling theory.

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Leonie Oostwoud Wijdenes, Eli Brenner and Jeroen B.J. Smeets

This study set out to determine whether the fastest online hand movement corrections are only responses to changing judgments of the targets’ position or whether they are also influenced by the apparent target motion. Introducing a gap between when a target disappears and when it reappears at a new position in a double-step paradigm disrupts the apparent motion, so we examined the influence of such a gap on the intensity of the response. We found that responses to target perturbations with disrupted apparent motion were less vigorous. The response latency was 10 ms shorter when there was a gap, which might be related to the gap effect that has previously been described for initiating eye and hand movements.

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Josje van Houwelingen, Sander Schreven, Jeroen B.J. Smeets, Herman J.H. Clercx and Peter J. Beek

In this paper, a literature review is presented regarding the hydrodynamic effects of different hand and arm movements during swimming with the aim to identify lacunae in current methods and knowledge, and to distil practical guidelines for coaches and swimmers seeking to increase swimming speed. Experimental and numerical studies are discussed, examining the effects of hand orientation, thumb position, finger spread, sculling movements, and hand accelerations during swimming, as well as unsteady properties of vortices due to changes in hand orientation. Collectively, the findings indicate that swimming speed may be increased by avoiding excessive sculling movements and by spreading the fingers slightly. In addition, it appears that accelerating the hands rather than moving them at constant speed may be beneficial, and that (in front crawl swimming) the thumb should be abducted during entry, catch, and upsweep, and adducted during the pull phase. Further experimental and numerical research is required to confirm these suggestions and to elucidate their hydrodynamic underpinnings and identify optimal propulsion techniques. To this end, it is necessary that the dynamical motion and resulting unsteady effects are accounted for, and that flow visualization techniques, force measurements, and simulations are combined in studying those effects.