Discrete and cyclical units of action in a mixed target pair aiming task

How thresholding in segmentation affects the regression performance of the linear model

Evaluating any model underlying the control of speech requires segmenting the continuous flow of speech effectors into sequences of movements. A virtually universal practice in this segmentation is to use a velocity-based threshold which identifies a movement onset or offset as the time at which the velocity of the relevant effector breaches some threshold percentage of the maximal velocity. Depending on the threshold choice, more or less of the movement's trajectory is left in for model regression. This paper makes explicit how the choice of this threshold modulates the regression performance of a dynamical model hypothesized to govern speech movements.

Haptic Training: Which Types Facilitate (re)Learning of Which Motor Task and for Whom? Answers by a Review

The use of robots has attracted researchers to design numerous haptic training methods to support motor learning. However, investigations of new methods yielded inconclusive results regarding their effectiveness to enhance learning due to the diversity of tasks, haptic designs, participants' skill level, and study protocols. In this review, we developed a taxonomy to identify generalizable findings out of publications on haptic training. In the taxonomy, we grouped the results of studies on healthy learners based on participants' skill level and tasks' characteristics. Our inspection of included studies revealed that: i) Performance-enhancing haptic methods were beneficial for novices, ii) Training with haptics was as effective as training with other feedback modalities, and iii) Performance-enhancing and performance-degrading haptic methods were useful for the learning of temporal and spatial aspects, respectively. We also observed that these findings are in line with results from robot-aided neurorehabilitation studies on patients. Our review suggests that haptic training can be effective to foster learning, especially when the information cannot be provided with other feedback modalities. We believe the findings from the taxonomy constitute a general guide, which can assist researchers when designing studies to investigate the effectiveness of haptics on learning different tasks.

The influence of accuracy constraints on bimanual and unimanual sequence learning

An experiment was designed to determine whether accuracy constraints can influence how unimanual and bimanual motor sequences are produced and learned. The accuracy requirements of the task were manipulated using principles derived from Fitts' Law to create relatively low (ID = 3) and high (ID = 5) accuracy demands. Right-limb dominant participants (N = 28, age = 21.9 yrs; 15 females and 13 males) were required to produce unimanual left, unimanual right or bimanual movement sequences using elbow extension and flexion movements to hit a series of illuminated targets. The targets were illuminated in a repeating sequence of 16 elements. Participants performed 20 practice trials. Thirty minutes following the practice trials participants performed a retention test. Element duration (time interval between target hits) and segment harmonicity (hesitations/adjustments in movement pattern) were calculated. The results indicate longer element duration and lower harmonicity values (more adjustments) when the task required higher accuracy demands (ID = 5) compared to low accuracy demands (ID = 3). Element duration was shorter and harmonicity was higher at ID = 5 for both unimanual groups than the bimanual group. However, element duration was shorter and harmonicity was higher at ID = 3 for the bimanual group than for both unimanual groups. These results indicate that the accuracy demands of the task can influence both performance and learning of motor sequences and suggest differences between unimanual and bimanual motor sequence learning. It appears there is a bimanual advantage for tasks with lower accuracy demands whereas performance is more accurate with unimanual performance, regardless of limb, with higher accuracy demands. These results are consistent with recent research indicating that accuracy requirements change the control processes for bimanual performance differently than for unimanual tasks.

Trajectory evolution and changes in the structure of movement amplitude time series

With increases in the index of difficulty [ID = log₂(2A/W)], the time-series structure of movement amplitude values shift from pink to white noise. The appearance of pink noise at low-ID levels may be attributed to the dominance of feedforward control processes, while the appearance of white noise at high-ID levels may be attributed to increased reliance on visuomotor feedback processes needed to guide movement into the target region. Such within-movement corrections may disrupt the pink-noise time-series correlations that exist in the absence of feedback processing. In our prior work, movement amplitude was defined as the distance moved from movement start until its end. In contrast, in the current study we examined the time-series structure of movement amplitude values at each of 10 different percentages of time into the movement trajectory-ranging between 10 and 100% of the movement time (%MT)-at a low (2 bits) and a high (5 bits) ID level. We hypothesized that at both ID levels a pink-noise time-series structure would be seen during the early portions of the movement trajectory when feedforward control should dominate, but during later portions of the trajectory, increased whitening of time-series structure would emerge only under ID 5 as there would be an increased need to engage visuomotor feedback processes. Under ID 2, the same level of pink noise should be maintained across all %MT levels as movement should be under the same level of feedforward control throughout the trajectory. The only unpredicted result occurred at ID 2 where the pink-noise level increased with increases in %MT. We hypothesize that such strengthening of pink noise as a function of %MT reflects the engagement of early trajectory corrections superimposed on the initial feedforward signal, but, once such initial adjustments were made, feedforward processes increasingly took over as the trajectory neared its goal.

Implicit task switching in Parkinson's disease is preserved when on medication

People with Parkinson's disease have been shown to have difficulty switching between movement plans. In the great majority of studies, the need to switch between tasks was made explicitly. Here, we tested whether people with Parkinson's disease, taking their normal medication, have difficulty switching between implicitly specified tasks. We further examined whether this switch is performed predictively or reactively. Twenty five people with Parkinson's disease continuously increased or decreased the frequency of their arm movements, inducing an abrupt-but unaware-switch between rhythmic movements (at high frequencies) and discrete movements (at low frequencies). We tested whether that precipitous change was performed reactively or predictively. We found that 56% of participants predictively switched between the two movement types. The ability of people with Parkinson's disease, taking their regular medication, to predictively control their movements on implicit tasks is thus preserved.

Proportional and nonproportional transfer of movement sequences

Two experiments were designed to determine participants’ ability to transfer a learned movement sequence to new spatial locations. A 16-element dynamic arm movement sequence was used in both experiments. The task required participants to move a horizontal lever to sequentially projected targets. Experiment 1 included 2 groups. One group practised a pattern in which targets were located at 20, 40, 60, and 80° from the start position (long sequence). The other group practised a pattern with targets at 20, 26.67, 60, and 80° (mixed sequence). Both groups were tested 24 hours later on the long, mixed, and short sequence. The short sequence was considered a proportional transfer for the long acquisition group because all the amplitudes between targets were reduced by the same proportion. Nonproportional transfer occurred when the amplitudes between targets did not have the same proportions as those for their practice sequence (e.g., long sequence to mixed sequence or vice versa). The results indicated that participants could effectively transfer to new target configurations regardless of whether the transfer required proportional or nonproportional spatial changes to the movement pattern. Experiment 2 assessed the effects of extended practice on proportional and nonproportional spatial transfer. The data indicated that while participants can effectively transfer to both proportional and nonproportional spatial transfer conditions after 1 day of practice, they are only effective at transferring to proportional transfer conditions after 4 days of practice. The results are discussed in terms of the mechanism by which response sequences become increasingly specific over extended practice in an attempt to optimize movement production.

Changes in Predictive Task Switching with Age and with Cognitive Load

Predictive control of movement is more efficient than feedback-based control, and is an important skill in everyday life. We tested whether the ability to predictively control movements of the upper arm is affected by age and by cognitive load. A total of 63 participants were tested in two experiments. In both experiments participants were seated, and controlled a cursor on a computer screen by flexing and extending their dominant arm. In Experiment 1, 20 young adults and 20 older adults were asked to continuously change the frequency of their horizontal arm movements, with the goal of inducing an abrupt switch between discrete movements (at low frequencies) and rhythmic movements (at high frequencies). We tested whether that change was performed based on a feed-forward (predictive) or on a feedback (reactive) control. In Experiment 2, 23 young adults performed the same task, while being exposed to a cognitive load half of the time via a serial subtraction task. We found that both aging and cognitive load diminished, on average, the ability of participants to predictively control their movements. Five older adults and one young adult under a cognitive load were not able to perform the switch between rhythmic and discrete movement (or vice versa). In Experiment 1, 40% of the older participants were able to predictively control their movements, compared with 70% in the young group. In Experiment 2, 48% of the participants were able to predictively control their movements with a cognitively loading task, compared with 70% in the no-load condition. The ability to predictively change a motor plan in anticipation of upcoming changes may be an important component in performing everyday functions, such as safe driving and avoiding falls.

A novel approach to enhancing limb control in older adults

Two recent experiments have demonstrated that young adult participants were able to make faster and more harmonic movements in a typical reciprocal Fitts task (ID = 6) following a practice session of sine wave tracking (Boyle et al. in Exp Brain Res 223:377-387, 2012; J Mot Behav 46:277-285, 2014). The purpose of the present experiment was to replicate these findings with a young adult population (age 18-25) and determine whether sine wave tracking also enhances goal-directed limb movements in an older adult population (age 65-90). To establish a performance baseline, all participants were first pretested on a typical ID = 6 Fitts task. Participants in each age group were then randomly assigned to one of the two training conditions where they practiced (45 trials) on a typical Fitts task (ID = 6) or they were asked to track a sine wave template (45 trials). Following practice, all participants were then posttested under the ID = 6 Fitts conditions. The results demonstrated that both young and older adult participants that practiced under the sine wave conditions enhanced their Fitts task performance compared to participants in their respective age groups who practiced under the Fitts conditions. These enhancements included faster movement times, smaller dwell times, and more harmonic movements, all without decreases in movement accuracy. These results replicate our previous findings with young adults and extend the finding to older adult participants. Interestingly, the performances of the older adults following sine wave practice were as fast and as accurate as the young adults following Fitts task practice.

The Effect of Haptic Guidance on Learning a Hybrid Rhythmic-Discrete Motor Task

Bouncing a ball with a racket is a hybrid rhythmic-discrete motor task, combining continuous rhythmic racket movements with discrete impact events. Rhythmicity is exceptionally important in motor learning, because it underlies fundamental movements such as walking. Studies suggested that rhythmic and discrete movements are governed by different control mechanisms at different levels of the Central Nervous System. The aim of this study is to evaluate the effect of fixed/fading haptic guidance on learning to bounce a ball to a desired apex in virtual reality with varying gravity. Changing gravity changes dominance of rhythmic versus discrete control: The higher the value of gravity, the more rhythmic the task; lower values reduce the bouncing frequency and increase dwell times, eventually leading to a repetitive discrete task that requires initiation and termination, resembling target-oriented reaching. Although motor learning in the ball-bouncing task with varying gravity has been studied, the effect of haptic guidance on learning such a hybrid rhythmic-discrete motor task has not been addressed. We performed an experiment with thirty healthy subjects and found that the most effective training condition depended on the degree of rhythmicity: Haptic guidance seems to hamper learning of continuous rhythmic tasks, but it seems to promote learning for repetitive tasks that resemble discrete movements.

Effect of task-specific execution on accuracy of imagined aiming movements

Ideomotor theory states that the neural codes that represent action and the perceptual consequences of those actions are tightly bound in a common code. For action imagination, bound action, and perceptual codes are thought to be internally activated at a sub-threshold level through action simulation. In support of this hypothesis, previous research revealed that imagined movement times (MTs) for reciprocal aiming movements were closer to actual execution MTs after the participants gained experience executing the task. The current study examined the task-specific nature of the effects of experience on imagination by determining if improvements in accuracy in the imagination of reciprocal aiming movements occur only with experience of the reciprocal aiming task or with any aiming task. To this end, one group of participants executed a reciprocal pointing task, whereas a second group executed a discrete aiming task with comparable accuracy requirements before and after imagining reciprocal aiming movements. Influence of task specificity on imagination was assessed by evaluating the changes in imagined MTs before and after execution. Consistent with previous findings, there was a reduction in imagined MTs following task execution. Critically, there was a significant time by group interaction revealing a significant pre/post reduction in imagined MTs for the group that executed the reciprocal aiming movements, but not for the group that executed the discrete aiming movements. These data support ideomotor accounts of action imagination because it appears that the imagination of a movement is affected by task-specific experience with that movement.

Reaction time in ankle movements: a diffusion model analysis

Reaction time (RT) is one of the most commonly used measures of neurological function and dysfunction. Despite the extensive studies on it, no study has ever examined the RT in the ankle. Twenty-two subjects were recruited to perform simple, 2- and 4-choice RT tasks by visually guiding a cursor inside a rectangular target with their ankle. RT did not change with spatial accuracy constraints imposed by different target widths in the direction of the movement. RT increased as a linear function of potential target stimuli, as would be predicted by Hick-Hyman law. Although the slopes of the regressions were similar, the intercept in dorsal-plantar (DP) direction was significantly smaller than the intercept in inversion-eversion (IE) direction. To explain this difference, we used a hierarchical Bayesian estimation of the Ratcliff's (Psychol Rev 85:59, 1978) diffusion model parameters and divided processing time into cognitive components. The model gave a good account of RTs, their distribution and accuracy values, and hence provided a testimony that the non-decision processing time (overlap of posterior distributions between DP and IE < 0.045), the boundary separation (overlap of the posterior distributions < 0.1) and the evidence accumulation rate (overlap of the posterior distributions < 0.01) components of the RT accounted for the intercept difference between DP and IE. The model also proposed that there was no systematic change in non-decision processing time or drift rate when spatial accuracy constraints were altered. The results were in agreement with the memory drum hypothesis and could be further justified neurophysiologically by the larger innervation of the muscles controlling DP movements. This study might contribute to assessing deficits in sensorimotor control of the ankle and enlighten a possible target for correction in the framework of our on-going effort to develop robotic therapeutic interventions to the ankle of children with cerebral palsy.

Pointing with the ankle: the speed-accuracy trade-off

This study investigated the trade-off between speed and accuracy in pointing movements with the ankle during goal-directed movements in dorsal-plantar (DP) and inversion-eversion (IE). Nine subjects completed a series of discrete pointing movements with the ankle between spatial targets of varying difficulty. Six different target sets were presented, with a range of task difficulty between 2.2 and 3.8 bits of information. Our results demonstrated that for visually evoked, visually guided discrete DP and IE ankle pointing movements, performance can be described by a linear function, as predicted by Fitts' law. These results support our ongoing effort to develop an adaptive algorithm employing the speed-accuracy trade-off concept to control our pediatric anklebot while delivering therapy for children with cerebral palsy.

The effects of rhythmicity and amplitude on transfer of motor learning

We perform rhythmic and discrete arm movements on a daily basis, yet the motor control literature is not conclusive regarding the mechanisms controlling these movements; does a single mechanism generate both movement types, or are they controlled by separate mechanisms? A recent study reported partial asymmetric transfer of learning from discrete movements to rhythmic movements. Other studies have shown transfer of learning between large-amplitude to small-amplitude movements. The goal of this study is to explore which aspect is important for learning to be transferred from one type of movement to another: rhythmicity, amplitude or both. We propose two hypotheses: (1) Rhythmic and discrete movements are generated by different mechanisms; therefore we expect to see a partial or no transfer of learning between the two types of movements; (2) Within each movement type (rhythmic/discrete), there will be asymmetric transition of learning from larger movements to smaller ones. We used a learning-transfer paradigm, in which 70 participants performed flexion/extension movements with their forearm, and switched between types of movement, which differed in amplitude and/or rhythmicity. We found partial transfer of learning between discrete and rhythmic movements, and an asymmetric transfer of learning from larger movements to smaller movements (within the same type of movement). Our findings suggest that there are two different mechanisms underlying the generation of rhythmic and discrete arm movements, and that practicing on larger movements helps perform smaller movements; the latter finding might have implications for rehabilitation.

Extended practice of reciprocal wrist and arm movements of varying difficulties

An experiment was designed to determine the degree to which reciprocal aiming movements of the wrist and arm with various accuracy requirements (Fitts' tasks) are enhanced by extended practice. The vast majority of research on motor learning shows performance improvement over practice. However, literature examining the effect of practice on Fitts' task performance is limited and inconclusive. Participants were asked to flex/extend their limb/lever in the horizontal plane at the wrist (arm stabilized) or elbow joint (wrist stabilized) in an attempt to move back and forth between two targets as quickly and accurately as possible. The targets and current position of the limb were projected on the screen in front of the participant. Target width was manipulated with amplitude constant (16°) in order to create indexes of difficulty (ID) of 1.5, 3, 4.5, and 6. Contrary to the earlier reports, after 20 days of practice, we found minimal changes in movement time or the movement time-ID relationships for the arm and wrist over practice. However, the variability in the movement endpoints decreased over practice and wrist movements at ID=6 were characterized by shorter movement times and longer dwell times relative to arm movements with dwell time for the wrist increasing over practice. These data are consistent with the notion that Fitts' tasks provide a stable measure of perceptual-motor capabilities.

Wrist and arm movements of varying difficulties

Although a great deal of experimental attention has been directed at understanding Fitts' law, only a limited number of experiments have attempted to determine if performance differs across effectors for a given movement difficulty. In three experiments reciprocal wrist and arm movements were compared at IDs of 1.5, 3, 4.5 and 6. When absolute movement requirements and visual display were constant, participants' movement times and response characteristics for the arm and wrist were remarkably similar (Experiment 1). However, when amplitude for wrist movements was reduced to 8° and the gain (4×) for the visual display increased participants' movement time, defined on the basis of kinematic markers (movement onset-movement termination), was increasingly shorter relative to arm movements as movement difficulty was increased (Experiment 2). Experiment 3 where the arm was tested at 32° and 8° with the 8° movements provided the same gain (4×) that was used for the 8° wrist movements in Experiment 2, no advantage was observed for the arm at the shorter amplitude. The results are interpreted in terms of the advantages afforded by the increased gain of the visual display, which permitted the wrist, but not the arm, to more effectively preplan and/or correct ongoing movements to achieve the required accuracy demands. It was also noted that while the wrist was more effective during the actual movement production this was accompanied by an offsetting increase in dwell time which presumably is utilized to dissipate the forces accrued during movement production and plan the subsequent movement segment.

Parameter value switching in discrete and continuous aiming movements

The effect of practice variations on spatial and temporal accuracy was investigated in both discrete and continuous aiming movements in the preferred hand of college-aged participants (N=25). In a completely within-subject design, participants made rapid reversal movements with a lightweight lever in the sagittal plane, practicing 20 degrees and 60 degrees movements in repeated (same distance) and alternating (switching between 20 degrees and 60 degrees) conditions. Movements were also made one at a time (discretely) or in sequences of 20 movements (continuously). Spatial constant error, spatial variable error, spatial overall error, the coefficient of variation, movement time, and the relative timing were calculated for each set of 20 movements and analyzed by within-subject analyses of variance. Movements in the repeated conditions for both discrete and continuous movements were more accurate and consistent compared to the alternating condition where the short movements were overshot and the long movements were undershot. Discrete movements were more spatially and temporally variable than continuous movements. The discrete and continuous movements showed different relative timing patterns, suggesting that the temporal structure of the motor program is affected by task characteristics.

Effect of terminal accuracy requirements on temporal gaze-hand coordination during fast discrete and reciprocal pointings

Background

Rapid discrete goal-directed movements are characterized by a well known coordination pattern between the gaze and the hand displacements. The gaze always starts prior to the hand movement and reaches the target before hand velocity peak. Surprisingly, the effect of the target size on the temporal gaze-hand coordination has not been directly investigated. Moreover, goal-directed movements are often produced in a reciprocal rather than in a discrete manner. The objectives of this work were to assess the effect of the target size on temporal gaze-hand coordination during fast 1) discrete and 2) reciprocal pointings.

Methods

Subjects performed fast discrete (experiment 1) and reciprocal (experiment 2) pointings with an amplitude of 50 cm and four target diameters (7.6, 3.8, 1.9 and 0.95 cm) leading to indexes of difficulty (ID = log₂[2A/D]) of 3.7, 4.7, 5.7 and 6.7 bits. Gaze and hand displacements were synchronously recorded. Temporal gaze-hand coordination parameters were compared between experiments (discrete and reciprocal pointings) and IDs using analyses of variance (ANOVAs).

Results

Data showed that the magnitude of the gaze-hand lead pattern was much higher for discrete than for reciprocal pointings. Moreover, while it was constant for discrete pointings, it decreased systematically with an increasing ID for reciprocal pointings because of the longer duration of gaze anchoring on target.

Conclusion

Overall, the temporal gaze-hand coordination analysis revealed that even for high IDs, fast reciprocal pointings could not be considered as a concatenation of discrete units. Moreover, our data clearly illustrate the smooth adaptation of temporal gaze-hand coordination to terminal accuracy requirements during fast reciprocal pointings. It will be interesting for further researches to investigate if the methodology used in the experiment 2 allows assessing the effect of sensori-motor deficits on gaze-hand coordination.

Early switching between movement types: indication of predictive control?

In everyday life, we frequently alternate between performing discrete and rhythmic movements. When performing a periodic movement, two distinct movement types can be distinguished: highly harmonic vs. discrete-like. The harmonicity of the movement is used to classify it as one or the other. We asked: (1) whether the frequency at which a periodic movement is performed affects the harmonicity of the resultant movement; and (2) what underlies switching between these movement types. To answer these questions, we studied horizontal flexion/extension forearm movements in 13 young adults over a wide range of frequencies. Movements were performed either at a fixed frequency, or at gradually increasing or decreasing target frequencies. We found movement harmonicity to depend on the frequency of the movement. Furthermore, we found a reverse hysteresis behavior, where participants switched movement type in anticipation of the future-required frequency. These findings suggest that predictive control is employed in switching between movement types.

Movement characteristics of upper extremity prostheses during basic goal-directed tasks

BACKGROUND

After an upper limb amputation a prosthesis is often used to restore the functionality. However, the frequency of prostheses use is generally low. Movement kinematics of prostheses use might suggest origins of this low use. The aim of this study was to reveal movement patterns of prostheses during basic goal-directed actions in upper limb prosthetic users and to compare this with existing knowledge of able-bodied performance during these actions.

METHODS

Movements from six users of upper extremity prostheses were analyzed, three participants with a hybrid upper arm prosthesis, and three participants with a myoelectric forearm prosthesis. Two grasping tasks and a reciprocal pointing task were investigated during a single lab session. Analyses were carried out on the kinematics of the tasks.

FINDINGS

When grasping, movements with both prostheses showed asymmetric velocity profiles of the reach and had a plateau in the aperture profiles. Reach and grasp were decoupled. Kinematics with the prostheses differed in that the use of upper arm prostheses required more time to execute the movements, while the movements were less smooth, more asymmetric, and showed more decoupling between reach and grasp. The pointing task showed for both prostheses less harmonic movements with higher task difficulty.

INTERPRETATION

Characterizing prosthetic movement patterns revealed specific features of prosthetic performance. Developments in technology and rehabilitation should focus on these issues to improve prosthetic use, in particular on improving motor characteristics and the control of the elbow, and learning to coordinate the reach and the grasp component in prehension.

Asymmetric transfer of visuomotor learning between discrete and rhythmic movements

As long as we only focus on kinematics, rhythmic movement appears to be a concatenation of discrete movements or discrete movement appears to be a truncated rhythmic movement. However, whether or not the neural control processes of discrete and rhythmic movements are distinct has not yet been clearly understood. Here, we address this issue by examining the motor learning transfer between these two types of movements testing the hypothesis that distinct neural control processes should lead to distinct motor learning and transfer. First, we found that the adaptation to an altered visuomotor condition was almost fully transferred from the discrete out-and-back movements to the rhythmic out-and-back movements; however, the transfer from the rhythmic to discrete movements was very small. Second, every time a new set of rhythmic movements was started, a considerable amount of movement error reappeared in the first and the following several cycles although the error converged to a small level by the end of each set. Last, we observed that when the discrete movement training was performed with intertrial intervals longer than 4 s, a significantly larger error appeared, specifically for the second and third cycles of the subsequent rhythmic movements, despite a seemingly full transfer to the first cycle. These results provide strong behavioral evidence that different neuronal control processes are involved in the two types of movements and that discrete control processes contribute to the generation of the first cycle of the rhythmic movement.

Non-monotonicity on a spatio-temporally defined cyclic task: evidence of two movement types?

We tested 23 healthy participants who performed rhythmic horizontal movements of the elbow. The required amplitude and frequency ranges of the movements were specified to the participants using a closed shape on a phase-plane display, showing angular velocity versus angular position, such that participants had to continuously control both the speed and the displacement of their forearm. We found that the combined accuracy in velocity and position throughout the movement was not a monotonic function of movement speed. Our findings suggest that specific combinations of required movement frequency and amplitude give rise to two distinct types of movements: one of a more rhythmic nature, and the other of a more discrete nature.

Evidence for slowing as a function of index of difficulty in young adults with Down syndrome

Speed-accuracy trade-offs in persons with Down syndrome and typically developing controls were tested with a Fitts' task. Movement time scaled linearly with index of difficulty in both groups, and there were no accuracy differences. Persons with Down syndrome were slower than typically developing individuals. Regression analysis on movement time and index of difficulty showed a nearly two-fold higher regression coefficient and a nearly three-fold larger intercept value in the Down syndrome group. The dwell time on a target was much longer for Down syndrome persons but scaled with index of difficulty in about the same percentage for participants in both groups. Because of differences primarily related to scaling, we conclude that mechanisms of motor control are similar in Down syndrome and typically developing groups.

Integration deficiencies associated with continuous limb movement sequences in Parkinson's disease

The present study examined the extent to which Parkinson's disease (PD) influences integration of continuous limb movement sequences. Eight patients with idiopathic PD and 8 age-matched normal subjects were instructed to perform repetitive sequential aiming movements to specified targets under three-accuracy constraints: 1) low accuracy (W = 7 cm) - minimal accuracy constraint, 2) high accuracy (W = 0.64 cm) - maximum accuracy constraint, and 3) mixed accuracy constraint - one target of high accuracy and another target of low accuracy. The characteristic of sequential movements in the low accuracy condition was mostly cyclical, whereas in the high accuracy condition it was discrete in both groups. When the accuracy constraint was mixed, the sequential movements were executed by assembling discrete and cyclical movements in both groups, suggesting that for PD patients the capability to combine discrete and cyclical movements to meet a task requirement appears to be intact. However, such functional linkage was not as pronounced as was in normal subjects. Close examination of movement from the mixed accuracy condition revealed marked movement hesitations in the vicinity of the large target in PD patients, resulting in a bias toward discrete movement. These results suggest that PD patients may have deficits in ongoing planning and organizing processes during movement execution when the tasks require to assemble various accuracy requirements into more complex movement sequences.

Non-linear gaining in precision aiming: making Fitts' task a bit easier

The role of information in the processes underlying kinematic trajectory-formation was examined by manipulating the relation between effector space (movement of a hand-held stylus on a graphics tablet) and task space (movement of a cursor on a screen where targets were presented) in a precision aiming task with five different levels of task difficulty. Movement patterns were found to evolve as a function of the flow of information in task space, with participants (N=13) producing more rapid and more fluent movements when the mapping between spaces included the softening-spring characteristics typical of behavioural patterns at higher levels of task difficulty. We conclude that the kinematic changes (movement time and pattern) observed when task difficulty increases result from informational influences. Information affects behavioural dynamics at the level of the parameters without affecting the underlying dynamical structure.

Perceptual influences on Fitts' law

The linear relationship between movement time (MT) and index of difficulty (ID) for Fitts' type tasks has proven ubiquitous over the last 50+ years. A reciprocal aiming task (IDs 3, 4.5, 6) was used to determine if an enlarged visual display (visual angle 5.1 degrees , 7.4 degrees , or 13.3 degrees) would alter this relationship. With ID = 6, a condition typically associated with discrete action control, the largest visual display (13.3 degrees) allowed the motor system to exploit features of cyclical action control, e.g., shorter dwell times, more harmonic motion, less time decelerating the limb. The large visual display resulted in a quadratic relationship between MT and ID. For the IDs of 3 and 4.5, the visual displays did not alter the underlying control processes. The results are discussed in terms of the preference of the motor system to assemble movements from harmonic basis functions when salient visual information is provided.

Hypothesis regarding the transformation of the intended direction of movement during the production of graphic trajectories: a study of drawing movements in 8- to 12-year-old children

Children from 8 to 12 years of age drew figure-eights and ellipses at a self-chosen tempo on a digitizing tablet. Global aspects (perimeter and average speed) and local aspects (relation between instantaneous speed and curvature) of performance were analyzed across age groups and types of figures. We tested the predictions of the transformation model, which is based on the hypothesis that changing the intended direction of movement is a time-consuming process that affects the evolution in time of the movement trajectory, and compared how well it fitted the data relative to the power law. We found that the relation between speed and curvature was typically better described by the transformation model than by the power law. However, the power law provided a better description when ellipses were drawn at a fast speed. The analyses of the parameters of the transformation model indicate that processing speed increased linearly with age. In addition, the results suggest that the effects of the spring-like properties of the arm were noticeable when ellipses were drawn at a fast speed. This study indicates that both biomechanical properties and central processes have an effect on the kinematics of continuous movements and particularly on the relation between speed and curvature. However, their relative importance varies with the type of figure and average movement speed. In conclusion, the results support the hypothesis that a time-consuming process of transformation of the intended direction of movement is operating during the production of continuous movements and that this process increases in speed between 8 to 12 years of age.

The dynamics of reciprocal aiming with a steering wheel

The study of speed-accuracy trade-offs has a long history in scientists' attempts to understand human movement control. In most such studies of reciprocal aiming, participants have been required to make reaching or pointing movements in space to targets of varying size. We wished to extend this body of work to a situation in which participants had to use a steering wheel in order to move a cursor on a computer monitor. Our results revealed a positive linear relationship between movement times and movement difficulty. We also observed an increased contribution of nonlinear dynamical terms as the movement difficulty increased. These results are consistent with the claim that a linear speed-difficulty relationship is a general feature of human motor control and one which is effector-independent. These results have relevant application to the study of human driving performance.

Distinct timing mechanisms produce discrete and continuous movements

The differentiation of discrete and continuous movement is one of the pillars of motor behavior classification. Discrete movements have a definite beginning and end, whereas continuous movements do not have such discriminable end points. In the past decade there has been vigorous debate whether this classification implies different control processes. This debate up until the present has been empirically based. Here, we present an unambiguous non-empirical classification based on theorems in dynamical system theory that sets discrete and continuous movements apart. Through computational simulations of representative modes of each class and topological analysis of the flow in state space, we show that distinct control mechanisms underwrite discrete and fast rhythmic movements. In particular, we demonstrate that discrete movements require a time keeper while fast rhythmic movements do not. We validate our computational findings experimentally using a behavioral paradigm in which human participants performed finger flexion-extension movements at various movement paces and under different instructions. Our results demonstrate that the human motor system employs different timing control mechanisms (presumably via differential recruitment of neural subsystems) to accomplish varying behavioral functions such as speed constraints.

A fundamental question in motor control research is whether distinct movement classes exist. Candidate classes are discrete and continuous movement. Discrete movements have a definite beginning and end, whereas continuous movements do not have such discriminable end points. In the past decade there has been vigorous, predominantly empirically based debate whether this classification implies different control processes. We present a non-empirical classification based on mathematical theorems that unambiguously sets discrete and continuous rhythmic movements apart through their topological structure in phase space. By computational simulations of representative modes of each class we show that discrete movements can only be executed repetitively at paces lower than approximately 2.0 Hz. In addition, we performed an experiment in which human participants performed finger flexion-extension movements at various movement paces and under different instructions. Through a topological analysis of the flow in state space, we show that distinct control mechanisms underwrite human discrete and fast rhythmic movements: discrete movements require a time keeper, while fast rhythmic movements do not. Our results demonstrate that the human motor system employs different timing control mechanisms (presumably via differential recruitment of neural subsystems) to accomplish varying behavioral functions such as speed constraints.

The preparation and control of reversal movements as a single unit of action

Previous research has demonstrated that movement time and kinematic properties of limb trajectories to the first target of a two-target reversal movement differ to that of single-target responses. In the present study we investigated whether two-target reversal movements are organized as a single unit of action or two separate components by perturbing the number of targets prior to and during movement execution. In one experiment, participants performed single-target movements and on one-third of the trials a second target was presented either at target presentation, movement onset or peak velocity. On those trials in which a second target was presented, participants were required to complete their movement to the first target and then move to the second target. In a second experiment, the reverse was the case with participants performing two-target movements that changed to single-target movement on one-third of the trials. A two-target movement time advantage was observed only when the required response was specified prior to movement initiation. Also, participants failed to prevent movement towards the second target when the requirements of the task changed from a two-target to single-target response at movement onset or later. These results indicate that two-target reversal movements were organized as a single unit of action prior to response initiation.

Visual and musculoskeletal underpinnings of anchoring in rhythmic visuo-motor tracking

Anchoring, that is, a local reduction in kinematic (i.e., spatio-temporal) variability, is commonly observed in cyclical movements, often at or around reversal points. Two kinds of underpinnings of anchoring have been identified-visual and musculoskeletal-yet their relative contributions and interrelations are largely unknown. We conducted an experiment to delineate the effects of visual and musculoskeletal factors on anchoring behavior in visuo-motor tracking. Thirteen participants (reduced to 12 in the analyses) tracked a sinusoidally moving visual target signal by making flexion-extension movements about the wrist, while both visual (i.e., gaze direction) and musculoskeletal (i.e., wrist posture) factors were manipulated in a fully crossed (3 x 3) design. Anchoring was affected by both factors in the absence of any significant interactions, implying that their contributions were independent. When gaze was directed to one of the target turning points, spatial endpoint variability at this point was reduced, but not temporal endpoint variability. With the wrist in a flexed posture, spatial and temporal endpoint variability were both smaller for the flexion endpoint than for the extension endpoint, while the converse was true for tracking with the wrist extended. Differential anchoring effects were absent for a neutral wrist posture and when gaze was fixated in between the two target turning points. Detailed analyses of the tracking trajectories in terms of velocity profiles and Hooke's portraits showed that the tracking dynamics were affected more by wrist posture than by gaze direction. The discussion focuses on the processes underlying the observed independent effects of gaze direction and wrist posture on anchoring as well as their implications for the notion of anchoring as a generic feature of sensorimotor coordination.

On rhythmic and discrete movements: reflections, definitions and implications for motor control

At present, rhythmic and discrete movements are investigated by largely distinct research communities using different experimental paradigms and theoretical constructs. As these two classes of movements are tightly interlinked in everyday behavior, a common theoretical foundation spanning across these two types of movements would be valuable. Furthermore, it has been argued that these two movement types may constitute primitives for more complex behavior. The goal of this paper is to develop a rigorous taxonomic foundation that not only permits better communication between different research communities, but also helps in defining movement types in experimental design and thereby clarifies fundamental questions about primitives in motor control. We propose formal definitions for discrete and rhythmic movements, analyze some of their variants, and discuss the application of a smoothness measure to both types that enables quantification of discreteness and rhythmicity. Central to the definition of discrete movement is their separation by postures. Based on this intuitive definition, certain variants of rhythmic movement are indistinguishable from a sequence of discrete movements, reflecting an ongoing debate in the motor neuroscience literature. Conversely, there exist rhythmic movements that cannot be composed of a sequence of discrete movements. As such, this taxonomy may provide a language for studying more complex behaviors in a principled fashion.

Targeted Aiming Movements Are Compromised in Nonaffected Limb of Persons With Stroke

Background. Research has shown that movement impairments following stroke are typically associated with the limb contralateral to the side of the stroke. Prior studies identified ipsilateral motor declines across a variety of tasks. Objective. Two experiments were conducted to better understand the ipsilateral contributions to organization and execution of proximal upper extremity multisegment aiming movements in persons with right-hemispheric stroke. Methods. Participants performed reciprocal aiming (Experiment 1) and 2-segment aiming movements (Experiment 2) on a digitizing tablet. In both experiments, target size and/or target orientation were manipulated to examine the influence of accuracy constraints on the planning and organization of movements. Results. Kinematic measures, submovement analysis, and harmonicity measures were included in this study. Declines in organization and execution of multisegment movements were found to contribute to performance decrements and slowing in stroke patients. Furthermore, stroke patients were unable to efficiently plan multisegment movements as one functional unit, resulting in discrete movements. Conclusions . Results suggest the importance of considering ipsilateral contributions to the control and organization of targeted aiming movements as well as implications for rehabilitation and recovery.

Kinematic invariants during cyclical arm movements

It has been observed that the motion of the arm end-point (the hand, fingertip or the tip of a pen) is characterized by a number of regularities (kinematic invariants). Trajectory is usually straight, and the velocity profile has a bell shape during point-to-point movements. During drawing movements, a two-thirds power law predicts the dependence of the end-point velocity on the trajectory curvature. Although various principles of movement organization have been discussed as possible origins of these kinematic invariants, the nature of these movement trajectory characteristics remains an open question. A kinematic model of cyclical arm movements derived in the present study analytically demonstrates that all three kinematic invariants can be predicted from a two-joint approximation of the kinematic structure of the arm and from sinusoidal joint motions. With this approach, explicit expressions for two kinematic invariants, the two-thirds power law during drawing movements and the velocity profile during point-to-point movements are obtained as functions of arm segment lengths and joint motion parameters. Additionally, less recognized kinematic invariants are also derived from the model. The obtained analytical expressions are further validated with experimental data. The high accuracy of the predictions confirms practical utility of the model, showing that the model is relevant to human performance over a wide range of movements. The results create a basis for the consolidation of various existing interpretations of kinematic invariants. In particular, optimal control is discussed as a plausible source of invariant characteristics of joint motions and movement trajectories.

Target width scaling in a repetitive aiming task: switching between cyclical and discrete units of action

An aiming task was used to identify the processes whereby the motor system adapted a repetitive aiming action to systematic changes in ID (ID = log(2 )(2A/W), Fitts in J Exp Psychol 47:381-391, 1954) within a single trial. Task ID was scaled in a trial by moving the outside edge of two stationary targets to produce nine different target IDs in a trail. The ID within a trial was scaled in one of two directions: (1) an increasing ID condition, starting with an ID = 3.07 and ending with an ID = 5.91; and (2) a decreasing ID condition, starting with an ID = 5.91 and ending with an ID = 3.07. An index of movement harmonicity (Guiard in Acta Psychol 82:139-159, 1993) revealed that the repetitive aiming action was harmonic in nature when task ID was 3.07, and consisted of a series of discrete segments when task ID was 5.91. This finding provides evidence for the existence of discrete and cyclical units of action that are irreducible and that may be employed independently to assemble longer continuous actions. The scaling of ID within a trial promoted a transition in repetitive aiming motions assembled from discrete and cyclical units of action. A variety of kinematic measures (e.g., movement harmonicity, time spent accelerating the limb) revealed a critical ID (ID(c)) region (4.01-4.91) separating aiming motions governed by the different units of action. Enhancement of fluctuations before the transition were found in the movement harmonicity data and in the distance traveled to peak velocity data, with variability in these measures highest in the ID(c) region. The enhancement of fluctuations indicates that loss of stability in the limb's motion acted as a key mechanism underlying the transition between units of action. The loss of stability was associated with the transition from cyclical to discrete actions and with the transition from discrete to cyclical actions. The transition between units of action may be conceptualized as a transition from a limit cycle attractor (cyclical unit of action) to a shift between two fixed-point attractors (discrete unit of action) when ID was increased, with the transition occurring in the opposite direction when ID was decreased.

The role of different submovement types during pointing to a target

The present study extends our previous findings in challenging the traditional interpretation of irregularities in the velocity profile of pointing movements as corrective submovements performed to improve accuracy of target achievement. The study is driven by a hypothesis that pointing includes at least two subtasks, accurate target achievement and motion termination, each of which can cause submovements (Dounskaia et al. Exp Brain Res 164:505-516, 2005). To investigate submovements associated with these subtasks, two tasks were performed in the experiment. Task 1 was used to examine the contribution of the two subtasks on submovement production by comparing submovements in discrete movements that include motion termination and in cyclic movements during which motion termination is not performed. Target size manipulations emphasized submovements related to the accuracy subtask. The results confirmed that both subtasks included in pointing cause submovements. Gross types of submovements (types 1 and 2) were associated with motion termination and fine submovements (type 3) with accuracy regulation. Task 2 further investigated sources of the accuracy-associated type 3 submovements by including only cyclic movements performed at two levels of frequency. Most (97.6%) of the submovements in task 2 were of type 3. Submovement incidence was strongly (inversely) associated with cyclic frequency, and it was independent of target size. This result questions the accuracy subtask as a primary source for type 3 submovements, and it raises the possibility that these submovements are an inherent property of low-speed movements. Together, results of the two tasks support our previous finding that gross submovements are not necessarily related to accuracy regulation. They also provide evidence that challenges the interpretation of fine submovements as corrections performed voluntarily to improve pointing accuracy. Alternative interpretations of accuracy regulation mechanisms, such as regulation of muscle stiffness and of the muscle co-contraction level are discussed in light of the present results.

The advantage of cyclic over discrete movements remains evident following changes in load and amplitude

Previous studies suggested that the advantage in speed accuracy trade-off of cyclic over discrete aiming tasks with the upper limbs may be associated with the operation of spinal neural oscillators, as in locomotion. Similar to the locomotor rhythm that is fairly robust and can accommodate changes in loading or stride length, we predicted that cyclic aiming tasks would be equally resistant to changes in load or amplitude, thereby preserving the advantage over discrete tasks. To test the hypothesis, cyclic and discrete aiming movements were performed with and without loading of the hand. Furthermore a "complex" condition was introduced in which the distance between the targets that the participants moved to alternated between 2.5 and 5 cm. In all cases, two target sizes were used to test spatial accuracy and to be able to calculate the Index of Performance (IP). Findings revealed that even though part of the advantage of the cyclic over the discrete regime was lost during the complex movement pattern and with addition of weight, the former remained superior to the latter. Furthermore, adding weight did not change the oscillation frequency in the cyclic movements. It is concluded that the superiority of cyclic movements over discrete ones is fairly robust, consistent with the high degree of flexibility that is typically observed in neural oscillators.

Influence of biomechanical factors on substructure of pointing movements

Irregularities in the velocity profile near the end of pointing movements have been interpreted as corrective submovements whose purpose is to provide accuracy of pointing to the target. The purpose of the present study was to investigate whether two additional factors related to biomechanical properties of the arm also cause submovements. First, motion termination and stabilization of the limb in the final position required by a discrete pointing task may contribute to submovements. Second, inaccurate regulation of interactive torque at the joints may also cause submovements. To investigate the contributions of these two biomechanical factors and the traditionally considered factor of pointing accuracy, the incidence of submovements was analyzed during three types of experimental manipulations. In addition to target size manipulations (small and large targets), conditions for motion termination were manipulated by examining discrete movements (which terminated at the target) and reciprocal movements (which reversed direction without dwelling on the target). Interaction torques were varied by using targets that require different shoulder-elbow coordination patterns. Submovements were detected in 41% of all analyzed movements. Data supported influences from the accuracy and motion termination factors but not from the interactive torque regulation factor on submovement incidence. Gross submovements were associated with motion termination; fine submovements primarily with accuracy demands. These findings and the analysis of temporal movement characteristics suggest that motion termination is an extra movement component that makes control of discrete movements different to control of reciprocal movements. Implications of the findings to a noise-related interpretation of Fitts' law are discussed. The study emphasizes the influence of arm biomechanics on endpoint kinematics.

Discrete and cyclical movements: unified dynamics or separate control?

In the literature on motor control, three theoretical perspectives on the relation between discrete and cyclical movements may be discerned: (a) cyclical movements are concatenated discrete movements; (b) discrete movements are a limiting case of cyclical movements, and (c) discrete and cyclical movements are motor primitives that may be combined but are irreducible to each other. To examine the tenability of these perspectives, 16 participants performed cyclical and discrete (flexion and extension) reaching movements of various amplitudes to differently sized targets. The kinematic properties of the recorded movements were analyzed and compared in detail. The cyclical, ongoing movements differed markedly from the discrete movements as well as from the first and last half-cycles of a bout of cyclical movements, especially in terms of their symmetry ratio. These effects were largely independent of amplitude, target size and movement direction (flexion-extension). The results obtained ruled out perspective (a) and, in principle, left open perspectives (b) and (c). However, the observed kinematic features were not readily accounted for by the specific dynamical models that have been proposed under perspectives (b) and (c). Future modeling attempts should explicate the dynamics of initiation and abortion of both discrete and cyclical movements.

Systematic scaling of target width: dynamics, planning, and feedback

The target width of a single target in a two-target reciprocal aiming task was scaled from small (ID = 5.85) to large (ID = 2.85) and large-to-small within individual trials with movement amplitude fixed. Scaling target width produced a transition in the end-effector's dynamics and based on a measure of movement harmonicity, the transition was sensitive to the initial conditions but not to the direction of target width scaling. Hysteresis emerged in a variety of kinematic measures suggesting that the interdependency of planning and feedback control processes was sensitive to initial conditions as well as the direction of target width scaling. Practice increased the efficiency of the reciprocal movements and produced changes in movement time and the measure of harmonic motion that revealed a tuning of the end-effector's dynamics to cyclical motion over as large of range of IDs as possible. The tuning occurred through the modulation of time spent accelerating and decelerating the end-effector for IDs outside the range of 3.85-4.26. The results are discussed with reference to a critical ID boundary that separates regions of parameter space wherein the end-effector's dynamics are more cyclical (limit-cycle) or discrete (fixed-point) in nature.