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Abstract

One of the aggravations in motor research is the large number of different human movements. Although everyday language provides well-established categories to manage this manifold, there is no well-defined and generally accepted classification system that satisfies scientific criteria. In the face of an almost unlimited variability, it is comforting to have some invariant characteristics. Invariant features can be used as defining characteristics for sets of movements. More importantly, they suggest inferences about the nature of internal representations or structures that underlie movement production. This chapter is about an invariance that has received a great deal of attention, the invariance of relative timing. The chapter describes the phenomenon and its incorporation into a particular kind of theory, the notion of a generalized motor program. Several points of criticism are discussed in the chapter that can be raised against the phenomenon and its theoretical underpinnings. Finally, a relaxed concept of a generalized motor program is outlined in the chapter.
... total and relative times have long been used for assessing different dimensions of movement patterns, including inferring the role of the central nervous system on the variable (parameterization) and invariant (structure) aspects of movement patterns [32][33][34][35]. In the present study, both variable and invariant dimensions were assumed as levels of a single structure, that is, a complex system of hierarchical organization [15][16][17][18][19]. ...
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... Perceptual invariance has been studied and found in several domains of cognition, including speech (Perkell & Klatt, 1986), motor behaviour (Heuer, 1991) and object motion (Shepard, 2002). It has also been the topic of several studies in music perception. ...
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... One such distinction is between absolute and relative timing (cf. Heuer, 1991;Schmidt, 1985). In the study of Lüttgen and Heuer (2012a), the challenge of the task was the required relative timing. ...
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The last several years have seen a number of approaches to robot assistance of motor learning. Experimental studies have produced a range of findings from beneficial effects through null-effects to detrimental effects of robot assistance. In this review we seek an answer to the question under which conditions which outcomes should be expected. For this purpose we derive tentative predictions based on a classification of learning tasks in terms of the products of learning, the mechanisms involved, and the modulation of these mechanisms by robot assistance. Consistent with these predictions, the learning of dynamic features of trajectories is facilitated and the learning of kinematic and dynamic transformations is impeded by robotic guidance, whereas the learning of dynamic transformations can profit from robot assistance with error-amplifying forces. Deviating from the predictions, learning of spatial features of trajectories is impeded by haptic guidance, but can be facilitated by divergent force fields. The deviations point to the existence of additional effects of robot assistance beyond the modulation of learning mechanisms, e.g., the induction of a passive role of the motor system during practice with haptic guidance. Copyright © 2015. Published by Elsevier Ltd.
... Accuracy and consistency of performance result from the structure of skill [2,15]. Relative time represents a structure of the skill showing the relationship between components of the task and tends to become fairly consistent but not rigid with practice [26]. This measure aims to identify changes in the relationship between components of the skill [27] by comparing the blocks immediately prior to with those during exposure to the perturbation. ...
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Chapter
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For many tasks it is not only important what one does, but also when one does it. Catching is an example. A grasp that is too early or too late will result in a failure; the timing is an integral and critical part of the action. Timing is controlled, and the time window in which the action can be performed successfully is specified by environmental variables. Thus the timing is extrinsic in addition to being controlled.
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Visual errors in the two horizontal directions and of three different sizes were produced instantaneously and presented in random order to 10 Ss. The errors were corrected through the movement of a joy stick. The joy stick responses were analyzed electrically into time plots of position, rate, acceleration and the third derivative of acceleration (|D acceleration)." The following generalizations seem justified: "a. As the visual error increases, (1) the S applies more force in the direction of motion and also more braking force; (2) he applies and removes these forces at a greater rate; and (3) he applies these forces over a slightly greater period of time. b. Throughout the course of any one motion, force varies continuously The time relations of these corrective motions are such that it appears that once started, the motions run off without visual or kinesthetic guidance The latter finding suggests that control in target tracking is an intermittent rather than a continuous process.
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According to Fitts (1954), movement time (MT) is a function of the combined effects of movement amplitude and target width (index of difficulty). Aiming movements with the same index of difficulty and MT may have different planning and control processes depending on the specific combination of movement amplitude and target size. Trajectories were evaluated for a broad range of amplitudes and target sizes. A three-dimensional motion recording system (WATSMART) monitored the position of a stylus during aiming movements. MT results replicated Fitts' Law. Analysis of the resultant velocity profiles indicated the following significant effects: As amplitude of movement increased, so did the time to peak resultant velocity; peak resultant velocity increased slightly with target size, and to a greater extent with increases in the amplitude of movement; the time after peak resultant velocity was a function of both amplitude and target size. Resultant velocity profiles were normalized in the time domain to look for scalar relation in the trajectory shape. This revealed that: the resultant velocity profiles were not symmetrical; the proportion of time spent prior to and after peak speed was sensitive to target size only, i.e. as target size decreased, the profiles became more skewed to the right, indicating a longer decelerative phase; for a given target size, a family of curves might be defined and scaled on movement amplitude. These results suggest that a generalized program (base trajectory representation) exists for a given target width and is parameterized or scaled according to the amplitude of movement.