Stiffness control after fast goal-directed arm movements

Mechanical parameters of the effector system directly after the termination of fast goal-directed arm movements were studied.Subjects were asked to move their hand as fast as possible to a target the instant the target was presented. Only movements of the subjects' forearms were allowed. They were also instructed not to react actively to forces applied suddenly to their forearm after the movement. As a result of such a force pulse the arm moved to a new position. The apparent stiffness, i.e. the quotient of the applied force and the resultant change of position, was measured. This stiffness is a measure for the resistance of the forearm to externally applied mechanical disturbances.It was found that after the arm has reached the target the apparent stiffness decreases as a function of time. This is an agreement with the declining amplitude of the electromyographic activity of the muscles that effect the movement.Arguments are given to support the hypothesis that this apparent stiffness control is part of the motor programme for movements of the forearm, i.e. the stiffness is planned together with the movement.     

Characteristics of velocity profiles of speech movements

The control of individual speech gestures was investigated by examining laryngeal and tongue movements during vowel and consonant production. A number of linguistic manipulations known to alter the durational characteristics of speech (i.e., speech rate, lexical stress, and phonemic identity) were tested. In all cases a consistent pattern was observed in the kinematics of the laryngeal and tongue gestures. The ratio of maximum instantaneous velocity to movement amplitude, a kinematic index of mass-normalized stiffness, was found to increase systematically as movement duration decreased. Specifically, the ratio of maximum velocity to movement amplitude varied as a function of a parameter, C, times the reciprocal of movement duration. The conformity of the data to this relation indicates that durational change is accomplished by scalar adjustment of a base velocity form. These findings are consistent with the idea that kinematic change is produced by the specification of articulator stiffness.     

Kinematic analysis of multiple constraints on a pointing task

The current experiment suggests that the speed/accuracy tradeoff is composed of two classes of constraints, effector and task. We examined the effects of movement distance, target size, orientation of the movement in the workspace, and C-D gain on the kinematics of discrete pointing movements made with computer mouse. It was found that target size influenced the shape of velocity profiles by elongating the duration of the corrective sub-movement phase, while movement distance scaled the entire velocity curve without affecting its shape. C-D gain and orientation of the movement exhibited two kinds of effects: an overall scaling of the velocity curve and a change in its shape. We conclude that target size is a task constraint and movement distance is an effector constraint, while movement orientation exhibited characteristics of both. C-D gain by itself was not a constraint, but interacted with both task and effector constrains. These results highlight the roles of biomechanical and information processing factors in the speed/accuracy tradeoff.     

Adaptive programming of arm movements

Adaptations of goal-directed elbow movements of moderate speed, called "continuous" movements and recognized by their single-peaked velocity profiles, were studied for two monkeys that were learning to perform a motor task. The animals were rewarded for what they did, namely, to carry out a step-tracking and holding task by means of discrete elbow movements, but not for how they did it, that is, for any particular mode of movement execution. Yet, both animals increased the use of the programmed, continuous movements when they began to carry out the behavioral task requirements appropriately. Furthermore, continuous movements adapted with increases of peak and of average velocity such that the ratio of these parameters tended to be maintained or decreased. These velocity changes were incorporated into remembered movement programs late in motor learning when the animals approached their best performance proficiencies.     

The parameter preferences of acquired motor programs for rapid,discrete movements: I. Transfer of training

Sixty subjects made discrete movements to visual targets without concurrent visual feedback. After 160 acquisition trials with a single target/movement, subjects were divided into five groups for transfer. The major independent variable was the relationship between the acquisition and transfer movements. For three groups, extent, duration, or velocity was the same in acquisition and in transfer, with the other two parameters changing. For the remaining two groups, either all parameters were changed or no parameters were changed (i.e., subjects performed the same movement). It was found that, as long as one parameter remained the same, subjects performed as well as if nothing were changed. Only when all parameters were changed did performance deteriorate. The results are in direct contrast to those of Newell and Zelaznik (1980) and provide no support for their conclusion that velocity is a more important parameter than movement duration or extent.     

Can computer mice be used as low-cost devices for the acquisition of planar human movement velocity signals?

The main goal of this work is to determine whether a computer mouse can be used as a low-cost device for the acquisition of two-dimensional human movement velocity signals in the context of psychophysical studies and biomedical applications. A comprehensive overview of the related literature is presented, and the problem of characterizing mouse movement acquisition is analyzed and discussed. Then, the quality of velocity signals acquired with this kind of device is measured on horizontal oscillatory movements by comparing the mouse data to the signals acquired simultaneously by a video motion tracking system and a digitizing tablet. A synthesis of the information gathered in this work indicates that the computer mouse can be used for the reliable acquisition of biosignals in the context of human movement studies, particularly for many applications dealing with the velocity of the end effector of the upper limb. This paper concludes by discussing the possibilities and limitations of such use.     

Coordination Patterns in Ball Bouncing as a Function of Skill

By observing the coordination patterns of people of different ages and skill levels bouncing a ball, the authors addressed hypotheses regarding (a) the relative increase and decrease of degrees of freedom with learning and (b) the order of progression in the changing organization of those degrees of freedom with development and learning. The movement patterns of the dominant arm and hand of subjects at various skill levels were contrasted in a cross-sectional design so that the way movement organization changes as a function of practice could be examined. Nine subjects aged between 4 and 22 years bounced a basketball at a preferred rhythm while standing still. Each performance trial was videotaped, and motions of arm and ball were digitized. Statistical analyses were made of linear and angular body motions: pairwise correlations and multiple regressions, amplitude and period variability, phase relations, and spectral and coherency measures. The coordination patterns revealed that (a) the movement patterns of less skilled subjects were more variable than those of more skilled subjects and (b) the motions of the articulators showed directional changes as a function of skill level that began both proximally and distally and moved toward the center of the effector chain with practice. The findings point up the relevance of studying change of system organization for understanding control and coordination.     

Sequence learning: response structure and effector transfer.

Two experiments are reported that investigate the response structure and effector transfer of repeated movement sequences. Participants moved a lever to targets sequentially presented on the computer monitor. In Experiment 1 the learning of 10- and 16-element sequences (identical movement pattern) was contrasted. After 1 day of practice the 10-element sequence was organized into fewer subsequences and, thus, performed more rapidly than the 16-element sequence. The imposed organization appeared to be coded in a relatively abstract way, as evidenced by effector transfer that was as good as that on the retention test. In Experiment 2 the 16-element sequence was studied after more extensive practice. By the end of 4 days of practice the participants produced relatively seamless responses void of obvious transitions between subsequences, but the control of the movement was less effector independent than observed earlier in practice. The results suggest that the process of consolidating the sequence, which led to more fluent response production, also resulted in the utilization of effector specific information.     

The Relationship of Impulse to Response Timing Error

This paper examines the relationship between response impulse and timing error in 200 msec discrete timing responses over a range of movement velocities and system masses. The results from two experiments showed that variable timing error decreased as both movement velocity and the mass of the system to be moved increased. The variability of force proportional to force (measured either as impulse or peak force) decreased curvilinearly as force output increased. The correlations between each of these parameters and variable timing errors, calculated on a group mean basis, ranged between .91 and .95. The ability to predict the movement time outcome of each individual trial from impulse-related parameters was considerably reduced, although the relationship between the various kinematic and kinetic parameters did strengthen as the movement velocity approached maximum. Collectively, the findings show that size of impulse is related to movement timing error, although it is premature to argue that impulse variability is a causal agent of timing error.     

The relationship of impulse to response timing error

This paper examines the relationship between response impulse and timing error in 200 msec discrete timing responses over a range of movement velocities and system masses. The results from two experiments showed that variable timing error decreased as both movement velocity and the mass of the system to be moved increased. The variability of force proportional to force (measured either as impulse or peak force) decreased curvilinearly as force out-put increased. The correlations between each of these parameters and variable timing errors, calculated on a group mean basis, ranged between.91 and.95. The ability to predict the movement time outcome of each individual trial from impulse-related parameters was considerably reduced, although the relationship between the various kinematic and kinetic parameters did strengthen as the movement velocity approached maximum. Collectively, the findings show the size of impulse is related to movement timing error, although it is premature argue that impulse variability is a causal agent of timing error.     

Effector-dependent learning by observation of a finger movement sequence

Can observational learning be effector dependent? In 3 experiments, observers watched a model respond to a 6-item unique sequence in a serial reaction time task. Their sequence knowledge was then compared with that of controls who had performed an unrelated task or observed a model responding to random targets. Observational learning was indicated when the introduction of a new sequence was associated with more reaction time elevation in observers than in controls. The authors found evidence of observational learning only when observers used the finger movement sequence that they observed during training, not when they responded at the same sequence of locations using different digits. Free generation and recognition tests also detected observational learning. These results imply that observational learning can be both explicit and effector dependent.     

The direction of bilateral transfer depends on the performance parameter

To acquire a more comprehensive understanding of the learning benefits associated with bilateral transfer and to gain knowledge of possible mechanisms behind bilateral transfer, we investigated the transfer direction of several parameters which are assumed to represent important features of movement control in a visuo-motor task. During the study, participants learned a multidirectional point-to-point drawing task in which the visual feedback was rotated 45° and the gain was increased. Performance changes of the untrained hand in movement time, trajectory length, normalized jerk, initial direction error, ratio of the primary sub-movement time to the total movement time, and the accuracy of the aiming movement after the primary sub-movement were investigated as indices of learning from bilateral transfer. The results showed that performance parameters related to the initial production of the movement, such as the initial direction, ratio of primary sub-movement to the total movement time, and movement accuracy after the primary sub-movement, only transferred to the non-dominant, while hand performance variables related to the overall outcome, such as movement duration, movement smoothness, and trajectory length, transferred in both directions. The findings of the current study support the basic principle of the “dynamic dominance model” because it is suggested that overall improvements in the non-dominant system are controlled by trajectory parameters in visuo-motor tasks, which resulted in transference of the afore mentioned production parameters to rather occur to the non-dominant hand as opposed to transference to the dominant hand.     

The relative efficacy of different forms of knowledge of results for the learning of a new relative timing pattern

The goal of the present study was to determine the relative efficacy of verbal and auditory knowledge of results for promoting learning of a new constrained relative timing pattern. In a series of four experiments we compared the efficiency of verbal knowledge of results to that of auditory knowledge of results. The results of all four experiments revealed that verbal knowledge of result is a very effective source of information to promote learning of a new imposed relative timing pattern. Auditory knowledge of results favoured learning of a new relative timing pattern in a very limited set of circumstances. In the present study, this was only the case when movement velocity remained constant from one segment of the task to the next and if it resulted in an unfamiliar temporal pattern. The results of all four experiments also provided evidence that movement parameterization and relative timing are independent processes that can be developed in parallel.     

Relative damping improves linear mass-spring models of goal-directed movements

A limitation of a simple linear mass-spring model in describing goal directed movements is that it generates rather slow movements when the parameters are kept within a realistic range. Does this imply that the control of fast movements cannot be approximated by a linear system? In servo-control theory, it has been proposed that an optimal controller should control movement velocity in addition to position. Instead of explicitly controlling the velocity, we propose to modify a simple linear mass-spring model. We replaced the damping relative to the environment (absolute damping) with damping with respect to the velocity of the equilibrium point (relative damping). This gives the limb a tendency to move as fast as the equilibrium point. We show that such extremely simple models can generate rapid single-joint movements. The resulting maximal movement velocities were almost equal to those of the equilibrium point, which provides a simple mechanism for the control of movement speed. We further show that peculiar experimental results, such as an 'N-shaped' equilibrium trajectory and the difficulties to measure damping in dynamic conditions, may result from fitting a model with absolute damping where one with relative damping would be more appropriate. Finally, we show that the model with relative damping can be used to model subtle differences between multi-joint interceptions. The model with relative damping fits the data much better than a version of the model with absolute damping.     

Abstract levels of motor control in prehension: Normal and pathological performance

Voluntary motor pattern generation is typically assumed to be organized hierarchically, with an abstract (effector independent) upper level which constrains the lower levels. There are at least two advantages of such an organization: it allows the system to exploit its redundancy, and when controlling a very large number of degrees of freedom the burden of control can be distributed over a sequence of stages. Experiments are reviewed which provide a consistent body of evidence in support of the contention that there is an effector independent level of motor representation in prehension. This is consistent with a hierarchically organized motor pattern generator. One set of experiments demonstrated that during active movement of a hand-held load, the coupling of grip force with movement induced load force is independent of how the load is grasped or how it is actively moved. A second set of experiments demonstrated that the movement patterns observed during reach-to-grasp are preserved over changes in the effector system used. Extensions and replications of these experiments with Parkinson's patients illustrates the idea that neurological problems may affect different levels of the control hierarchy suggesting that these levels may be anatomically as well as functionally distinguishable.PsycINFO classification: 2330; 2520; 3297     

Initial-state dependency of learning in young infants

With the aim of investigating the effect of the initial state of pre-learning on subsequent infant learning (i.e., the initial-state dependency), we observed the limb movements in 3-month-old infants in the course of a motor learning task. The session comprised 2-min pre-learning and 4-min learning periods, and the infants learned to move a toy using a string attached to either an arm (arm-based learning, Experiment 1) or a leg (leg-based learning, Experiment 2). Infants were assigned to low- and high-state groups in the initial-state condition according to the average velocity of the arm (Experiment 1) or leg (Experiment 2) movements during the pre-learning period. The results revealed that, during the learning period, infants in the low-state group increased the movement of their limbs, whereas those in the high-state group showed no significant changes in the movement of most of their limbs. These results suggest that infants demonstrating a low average velocity of movement in the initial state easily observed and learned the circular causality between self-produced movements and environmental changes. On the other hand, it seemed that infants demonstrating a high average velocity of movement in the initial state could not or did not need to increase their limb movements (the toy would already be shaking enough to form striking movements).     

Movements or targets: What makes an action in action–effect learning?

According to ideomotor theory, actions become linked to the sensory feedback they contingently produce, so that anticipating the feedback automatically evokes the action it typically results from. Numerous recent studies have provided evidence in favour of such action–effect learning but left an important issue unresolved. It remains unspecified to what extent action–effect learning is based on associating effect-representations to representations of the performed movements or to representations of the targets at which the behaviour aimed at. Two experiments were designed to clarify this issue. In an acquisition phase, participants learned the contingency between key presses and effect tones. In a following test phase, key–effect and movement–effect relations were orthogonally assessed by changing the hand–key mapping for one half of the participants. Experiment 1 showed precedence for target–effect over movement–effect learning in a forced-choice RT task. In Experiment 2, target–effect learning was also shown to influence the outcome of response selection in a free-choice task. Altogether, the data indicate that both movement–effect and target–effect associations contribute to the formation of action–effect linkages—provided that movements and targets are likewise contingently related to the effects.     

The Schematic Representation of Effector Function Underlying Perceptual-Motor Skill

Conceptual and methodological problems related to Schmidt’s (1975) motor schema theory are discussed. In particular, the motor schema is interpreted as representing the dynamics of the system being controlled, which may or may not be associated with a referent movement pattern. Furthermore, it is suggested that prior familiarity with a control system’s dynamics is a critical but uncontrolled factor in tests of the theory, and largely accounts for their equivocal findings. These ideas are examined by two experiments in which subjects had to bimanually control the movement of a computer-displayed cursor along a track on a CRT screen. Different track orientations required different patterns of movement not entailing a single generalized motor program. Experiment 1 shows that variable track performance with a given control system, results in better transfer to novel tracks than does fixed practice. Experiment 2 demonstrates that altering the control system disrupts performance whether or not the required movements remain the same. These results indicate the need for a fundamental modification of schema theory, such that a schematic representation of effector-environment relations (effector function) is available independently of particular movement patterns used in its acquisition.     

Coordination dynamics of learning and transfer across different effector systems

If different effector systems share a common task-specific coordination dynamics, transfer and generalization of sensorimotor learning are predicted. Subjects learned a visually specified phase relationship with either the arms or the legs. Coordination tendencies in both effector systems were evaluated before and after practice to detect attractive states of the coordination dynamics. Results indicated that learning a novel relative phase with a single effector system spontaneously transferred to the other, untrained effector system. Transfer was revealed not only as improvements in performance but also as modifications of each system's initial (prelearning) coordinative landscape. What is learned, appears to be a high-level but neurally instantiated dynamic representation of skilled behavior that proves to be largely effector independent, at least across anatomically symmetric limbs.     

The schematic representation of effector function underlying perceptual-motor skill

Conceptual and methodological problems related to Schmidt' (1975) motor schema theory are discussed. In particular, the motor schema is interpreted as representing the dynamics of the system being controlled, which may or may not be associated with a referent movement pattern. Furthermore, it is suggested that prior familiarity with a control system's dynamics is a critical but uncontrolled factor in tests of the theory, and largely accounts for their equivocal findings. These ideas are examined by two experiments in which subjects had to bimanually control the movement of a computer-displayed cursor along a track on a CRT screen. Different track orientations required different patterns of movement not entailing a single generalized motor program. Experiment 1 shows that variable track performance with a given control system, results in better transfer to novel tracks than does fixed practice. Experiment 2 demonstrates that altering the control system disrupts performance whether or not the required movements remain the same. These results indicate the need for a fundamental modification of schema theory, such that a schematic representation of effector-environment relations (effector function) is available independently of particular movement patterns used in its acquisition.