Sensorimotor control and linear visuomotor gains.

In everyday life, we often use graphical interfaces where the visual space is mapped to the motor space with a visuomotor gain called the control display gain. One of the key objectives in the field of Human Computer Interaction is to design this control display gain so as to enhance users' performance. Although the control display gain involved in operating systems has been found to improve users' pointing performance, the reasons for this improvement have not yet been fully elucidated, especially because the control display gains on operating systems are both non-constant and non-linear. Here, we tested non-constant but linear velocity-based control display gains to determine which parameters were responsible for pointing performance changes based on analyses of the movement kinematics. Using a Fitts' paradigm, constant gains of 1 and 3 were compared with a linearly increasing gain (i.e., the control display gain increases with the motor velocity) and a decreasing gain (i.e., the control display gain decreases with the motor velocity). Three movements with various indexes of difficulty (ID) were tested (3, 5 and 7 bits). The increasing gain was expected to increase the velocity of the initial impulse phase and decrease that of the correction phase, thus decreasing the movement time (MT), and the contrary in the case of the decreasing gain. Although the decreasing gain increased MT at ID3, the increasing gain was found to be less efficient than the constant gain of 3, probably because a non-constant gain between the motion and its visual consequences disrupted the sensorimotor control. In addition, the kinematic analyses of the movements suggested that the motion profile was planned by the central nervous system based on the visuomotor gain at maximum motor velocity, as common features were observed between the constant gain of 1 and the decreasing gain, and between the constant gain of 3 and the increasing gain. By contrast, the amplitude of the velocity profile seemed to be specific to each particular visuomotor mapping process.

"It ain't what you do, it's the way that you do it": does obesity affect perceptual motor control ability of adults on the speed and accuracy of a discrete aiming task?

The ability to control speed and accuracy of goal directed aiming tasks underpins many activities of daily living. Recent evidence has begun to suggest that obesity can affect the control of movement. This study evaluated perceptual motor control of 183 normal weight, overweight, and obese participants using a discrete Fitts' task on a digital tablet. In addition, we manipulated tablet orientation to determine whether tablet orientation influences task difficulty with the view to increase the task's constraints. Our study found that the traditional relationship between target distance and target width hold true for each of the three weight groups in both tablet orientations. Interestingly, no significant differences were found for movement time between the groups, while movement kinematics differed between weight groups. Obese participants demonstrated significantly higher peak acceleration values in the horizontal tablet orientation when compared to their normal weight and overweight counterparts. Further to this, obese participants made significantly more errors than normal weight and overweight groups. These findings suggest that obese individuals have altered control strategies compared to their normal weight peers.

A multi-level approach to investigate the control of an input device: application to a realistic pointing task.

This study investigates the subjects' performance during realistic conditions of control of a joystick. An adapted reciprocal aiming task consisting in driving a virtual vehicle along a slalom course as fast as possible was performed while accuracy constraints were manipulated. Realistic dynamical Interface Screen Relationship between the joystick displacements and the displacements of the vehicle was simulated. Vehicle displacements and motor activity (muscle activity and joint kinematics) were recorded. The results highlighted the applicability of the Fitts' law to more realistic conditions where the use of an input device is performed in an intensive control situation. Besides, biomechanical results suggested that neuromuscular responses were different regarding the direction of movement, whereas the performance at a behavioural level were not affected. Thus, this study demonstrates the interest in considering two different aspects of the user's performance (behavioural and biomechanical ones) to make a better agreement between the device design and users' needs. PRACTITIONER SUMMARY: This study considered two different aspects of the subject's performance in a realistic situation of speed­accuracy trade-off: the behavioural and motor activity. The necessity for the design of the future ergonomics pointing devices to meet the expectations of the neuromuscular system in order to facilitate their uses is highlighted.

Influence of daily physical activity on fine motor skills of adults around a Fitts task.

INTRODUCTION: Achieving our daily tasks depends on the speed-accuracy conflict. Physical activity plays a role in the development of our motor skills. However, the relationship between physical activity level (PAL) and fine motor skills remains largely unexplored.

Studying the timescale of perceptual-motor (re)calibration following a change in visual display gain.

Experiencing a non-1:1 mapping between perception and action in everyday life is not common. It could be considered as a problem for our perceptual-motor system because of the need to adapt our goal-directed movement to different gains between movement and task spaces. In the Human Computer Interface domain, the main example of such a situation consists in switching from one operating system to another which requires to adapt our movement to different Control Display gains. The aim of the study was to characterize the perceptual-motor calibration process following a sudden change in control display gain. Sixteen participants manipulated a mouse computer to move a cursor on the screen. The discrete aiming task consisted on reaching the target from a starting target position as fast and as accurately as possible. Our methodology consisted in suddenly manipulating the gain between both spaces following a three-step adaptation methodology (baseline condition followed by a perturbation and return to baseline condition). Results demonstrated that not only participants produce adaptive behavior following several types of perturbations, but they were also able to do it at a very short timescale. As the calibration process described in the present study may play a significant role in the acquisition of accurate perceptual-motor skills involving the use of devices that augment human fine motor capabilities (e.g., telesurgery, mouse and joystick), we conclude that this study could have important implications in the domain of Human-Computer Interaction (HCI) as well as in the domain Human Equipment Interaction.

Fitts' law is not continuous in reciprocal aiming.

It takes longer to accomplish difficult tasks than easy ones. In the context of motor behaviour, Fitts' famous law states that the time needed to successfully execute an aiming movement increases linearly with task difficulty. While Fitts' explicit formulation has met criticism, the relation between task difficulty and movement time is invariantly portrayed as continuous. Here, we demonstrate that Fitts' law is discontinuous in reciprocal aiming owing to a transition in operative motor control mechanisms with increasing task difficulty. In particular, rhythmic movements are implemented in easy tasks and discrete movements in difficult ones. How movement time increases with task difficulty differs in both movement types. It appears, therefore, that the human nervous system abruptly engages a different control mechanism when task difficulty increases.

Movement Speed-Accuracy Trade-Off in Parkinson's Disease.

Patients with Parkinson's disease (PD) often have difficulties generating rhythmic movements, and also difficulties on movement adjustments to accuracy constraints. In the reciprocal aiming task, maintaining a high accuracy comes with the cost of diminished movement speed, whereas increasing movement speed disrupts end-point accuracy, a phenomenon well known as the speed-accuracy trade-off. The aim of this study was to examine how PD impacts speed-accuracy trade-off during rhythmic aiming movements by studying the structural kinematic movement organization and to determine the influence of dopamine replacement therapy on continuous movement speed and accuracy. Eighteen patients with advanced idiopathic Parkinson's disease performed a reciprocal aiming task, where the difficulty of the task was manipulated through target width. All patients were tested in two different sessions: ON-medication and OFF-medication state. A control group composed of healthy age-matched participants was also included in the study. The following variables were used for the analyses: Movement time, Error rate, effective target width, and Performance Index. Percentage of acceleration time and percentage of non-linearity were completed with kinematics patterns description using Rayleigh-Duffing model. Both groups traded off speed against accuracy as the constraints pertaining to the latter increased. The trade-off was more pronounced with the PD patients. Dopamine therapy allowed the PD patients to move faster, but at the cost of movement accuracy. Surprisingly, the structural kinematic organization did not differ across group nor across medication condition. These results suggest that PD patients, when involved in a reciprocal aiming task, are able to produce rhythmic movements. PD patients' overall slowing down seems to reflect a global adaptation to the disease in the absence of a structurally altered kinematic organization.

Control of fast-reaching movements by muscle synergy combinations.

How the CNS selects the appropriate muscle patterns to achieve a behavioral goal is an open question. To gain insight into this process, we characterized the spatiotemporal organization of the muscle patterns for fast-reaching movements. We recorded electromyographic activity from up to 19 shoulder and arm muscles during point-to-point movements between a central location and 8 peripheral targets in each of 2 vertical planes. We used an optimization algorithm to identify a set of time-varying muscle synergies, i.e., the coordinated activations of groups of muscles with specific time-varying profiles. For each one of nine subjects, we extracted four or five synergies whose combinations, after scaling in amplitude and shifting in time each synergy independently for each movement condition, explained 73-82% of the data variation. We then tested whether these synergies could reconstruct the muscle patterns for point-to-point movements with different loads or forearm postures and for reversal and via-point movements. We found that reconstruction accuracy remained high, indicating generalization across these conditions. Finally, the synergy amplitude coefficients were directionally tuned according to a cosine function with a preferred direction that showed a smaller variability with changes of load, posture, and endpoint than the preferred direction of individual muscles. Thus the complex spatiotemporal characteristics of the muscles patterns for reaching were captured by the combinations of a small number of components, suggesting that the mechanisms involved in the generation of the muscle patterns exploit this low dimensionality to simplify control.

Kinematic adaptation to sudden changes in visual task constraints during reciprocal aiming.

In a series of three experiments the visual modulation of movement during a reciprocal aiming task was examined when participants were confronted with sudden changes in visually specified task constraints. Amplitude and precision constraints were manipulated independently in Experiments 1 and 2, respectively, while their simultaneous effects were analyzed in Experiment 3. Analysis of the evolution of kinematic characteristics following a sudden change in task constraints revealed two different times scales of adaptation: a rapid adjustment occurring during the deceleration phase of the first movement following change and a more gradual adaptation, affecting the kinematic pattern as a whole, occurring over the next few movements. Overall, the results indicate that visual information with respect to the adequacy of the unfolding movement is continuously monitored, even under the least constraining conditions, and serves to modulate the pattern of movement to (a) comply with the (new) task constraints and (b) optimally tailor the pattern of movement to the situation at hand. We interpret these findings in the framework of a dynamical perspective on movement organization, with information modulating the parameters of an otherwise invariant underlying dynamical structure.

Informational constraints in human precision aiming.

Twelve human subjects performed a reciprocal precision aiming task of varying difficulty (index of difficulty=4, 5, or 6) while vision of the ongoing movement was available either continuously or intermittently. In the intermittent conditions, vision of the moving end-effector was available at regular intervals (equal to 100%, 125%, or 150% of the movement durations measured under continuous visibility conditions) for varying amounts of time (75%, 50%, or 25% of the duration of the interval). Movement time (MT) increased with both increasing task difficulty and decreasing availability of visual information. Increases in MT were brought about by the same systematic changes in the kinematic characteristics of movement, whether task difficulty increased or availability of visual information decreased. At higher levels of task difficulty, subjects organized their movements so as to make visual information available at particular instances (at the start and at the end of the aiming movement).

Influence of task constraints and device properties on motor patterns in a realistic control situation.

The influences of task difficulty (index difficulty: 2-4), input device of different length, range of motion and mode of resistance (joystick or rotorcraft stick), and directions of movement (leftward rightward) on motor patterns in a realistic control situation were examined with a multilevel analysis (joint kinematics and muscular variables, and global task performance). Eight subjects controlled the displacements of a virtual object during a slalom task characterized by a realistic inertial model. Pilots adapted the endpoint kinematic organization to increasing accuracy constraints to preserve task success whatever the device and the direction. However, the rotorcraft stick manipulation remains highly complex in comparison to the joystick due to poorer proprioceptive information, higher inertial constraints, and an asymmetrical muscle control.

Top-level players' visual control of interceptive actions: Bootsma and van Wieringen (1990) 20 years later.

Using a two-step approach, Van Soest et al. (2010) recently questioned the pertinence of the conclusions drawn by Bootsma and Van Wieringen (1990) with respect to the visual regulation of an exemplary rapid interceptive action: the attacking forehand drive in table tennis. In the first step, they experimentally compared the movement behaviors of their participants under conditions with and without vision available during the execution of the drive. In the second step, through simulation they evaluated the extent to which a preprogrammed pattern of muscle stimulation acting on the dynamical characteristics of the musculoskeletal system could explain the patterns of movement observed, including the phenomena of kinematic convergence and compensatory variability. In this contribution, we show how methodological and conceptual shortcomings, pertaining to both parts of Van Soest et al.'s study, severely limit the impact of their findings. We argue that their conclusion-denying the possibility of visual regulation of rapid interceptive actions-cannot be upheld in the light of the existing evidence, while Bootsma and Van Wieringen's conclusion-in favor of the visual regulation of rapid interceptive actions in top-level players- still holds strong, even after 20 years. Irrespective of the trends of the moment, we suggest that both appropriate experimentation and principled theorization need to be deployed before a model-based predictive architecture can be considered as a serious alternative to a (more parsimonious) information-based control architecture.

Linear and logarithmic speed-accuracy trade-offs in reciprocal aiming result from task-specific parameterization of an invariant underlying dynamics.

The authors examined the origins of linear and logarithmic speed-accuracy trade-offs from a dynamic systems perspective on motor control. In each experiment, participants performed 2 reciprocal aiming tasks: (a) a velocity-constrained task in which movement time was imposed and accuracy had to be maximized, and (b) a distance-constrained task in which accuracy was imposed and movement time had to be minimized. In Experiment 1, accuracy was constant across the 2 tasks; in Experiment 2, movement time was kept constant. Behavior in both tasks could be modeled with a single nonlinear equation of motion. Model coefficients captured the particulars of each task, especially apparent for the slowest or most difficult conditions. The distance-constrained task revealed a strong contribution of nonlinear stiffness with a moderate degree of nonlinear damping, favoring local control of speed. The velocity-constrained task revealed weaker nonlinear stiffness with stronger nonlinear damping, favoring global stabilization of the movement with a more constant rate of phase progression. In this way, the different speed-accuracy trade-offs emerged from the task-specific parameterization of the underlying dynamics.

Self-controlled concurrent feedback and the education of attention towards perceptual invariants.

The present study investigates the effects of different types of concurrent feedback on the acquisition of perceptual-motor skills. Twenty participants walked through virtual corridors in which rhythmically opening and closing sliding doors were placed. The participants aimed to adjust their walking speed so as to cross the doors when the doors were close to their maximal aperture width. The highest level of performance was achieved by learners who practiced the task with unambiguous self-controlled concurrent feedback, which is to say, by learners who could request that feedback at wish. Practice with imposed rather than self-controlled feedback and practice without concurrent feedback were shown to be less effective. Finally, the way in which the self-controlled concurrent feedback was presented was also found to be of paramount importance; if the feedback is ambiguous, it may even prevent participants from learning the task. Clearly, unambiguous self-controlled feedback can give rise to higher levels of performance than other feedback conditions (compared to imposed schedule) but, depending on the way it is presented, the feedback can also prevent the participants from learning the task. In the discussion it is argued that unambiguous self-controlled concurrent feedback allows learners to more rapidly educate their attention towards more useful perceptual invariants and to calibrate the relation between perceptual invariants and action parameters.

Optic variables used to judge future ball arrival position in expert and novice soccer players.

Although many studies have looked at the perceptual-cognitive strategies used to make anticipatory judgments in sport, few have examined the informational invariants that our visual system may be attuned to. Using immersive interactive virtual reality to simulate the aerodynamics of the trajectory of a ball with and without sidespin, the present study examined the ability of expert and novice soccer players to make judgments about the ball's future arrival position. An analysis of their judgment responses showed how participants were strongly influenced by the ball's trajectory. The changes in trajectory caused by sidespin led to erroneous predictions about the ball's future arrival position. An analysis of potential informational variables that could explain these results points to the use of a first-order compound variable combining optical expansion and optical displacement.

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.

Modulation of phasic and tonic muscle synergies with reaching direction and speed.

How the CNS masters the many degrees of freedom of the musculoskeletal system to control goal-directed movements is a long-standing question. We have recently provided support to the hypothesis that the CNS relies on a modular control architecture by showing that the phasic muscle patterns for fast reaching movements in different directions are generated by combinations of a few time-varying muscle synergies: coordinated recruitment of groups of muscles with specific activation profiles. However, natural reaching movements occur at different speeds and require the control of both movement and posture. Thus we have investigated whether muscle synergies also underlie reaching at different speeds as well as the maintenance of stable arm postures. Hand kinematics and shoulder and elbow muscle surface EMGs were recorded in five subjects during reaches to eight targets in the frontal plane at different speeds. We found that the amplitude modulation of three time-invariant synergies captured the variations in the postural muscle patterns at the end of the movement. During movement, three phasic and three tonic time-varying synergies could reconstruct the time-normalized muscle pattern in all conditions. Phasic synergies were modulated in both amplitude and timing by direction and speed. Tonic synergies were modulated only in amplitude by direction. The directional tuning of both types of synergies was well described by a single or a double cosine function. These results suggest that muscle synergies are basic control modules that allow generating the appropriate muscle patterns through simple modulation and combination rules.

Judging where a ball will go: the case of curved free kicks in football.

This study examined whether adding spin to a ball in the free kick situation in football affects a professional footballer's perception of the ball's future arrival position. Using a virtual reality set-up, participants observed the flight paths of aerodynamically realistic free kicks with (+/-600 rpm) and without sidespin. With the viewpoint being fixed in the centre of the goal, participants had to judge whether the ball would have ended up in the goal or not. Results show that trajectories influenced by the Magnus force caused by sidespin gave rise to a significant shift in the percentage of goal responses. The resulting acceleration that causes the ball to continually change its heading direction as the trajectory unfolds does not seem to be taken into account by the participants when making goal judgments. We conclude that the visual system is not attuned to such accelerated motion, which may explain why goalkeepers appear to misjudge the future arrival point of such curved free kicks.

Effects of biomechanical and task constraints on the organization of movement in precision aiming.

Nine participants performed a reciprocal precision aiming task under different experimental conditions. Due to the anisotropy of the work space, varying the direction of motion (from 0 degrees to 315 degrees in steps of 45 degrees ) allowed exploration of the effects of biomechanical constraints that were found to affect the duration of movement but not the shape of the kinematic pattern. Varying the size of the targets to be attained (W: 2.5, 1.25, and 0.625 cm, for a constant intertarget distance of 10 cm) and the nature (linear or non-linear) of the mapping between effector space (motion of a handheld stylus on a graphics tablet) and task space (motion of a pointer between targets on a computer screen) also led to changes in movement duration. However, the latter type of constraint gave rise to systematic changes in the pattern of movement, with progressively more difficult tasks being characterized by progressively less harmonic motion patterns. We conclude that in contrast to (biomechanical) constraints at the level of the effector, (informational) constraints at the level of the task affect the processes underlying movement organization. For the range of values studied, the effects of these two types of constraint can be considered to be independent.