Movement Endpoint Research Articles

People are better at maximizing expected gain in a manual aiming task with rapidly changing probabilities than with rapidly changing payoffs

Previous research has shown that humans can select movements that achieve their goals, while avoiding negative outcomes, by selecting an "optimal movement endpoint." This optimal endpoint is modeled based on the participants' endpoint variability and the payoffs associated with the target and penalty regions within the environment. Although the values associated with our goals vary on a moment-to-moment basis in our daily interactions, the adaptation of endpoint selection to changing payoffs in laboratory-based tasks has been examined by varying contexts between blocks of trials. The present study was designed to determine whether participants adjust endpoints and aim to optimal endpoints and whether performance differs when probability or payoff parameters change from trial to trial. Participants aimed to a target circle that was partially overlapped by a penalty circle. They received 100 points for hitting the target and lost points for hitting the penalty area. The magnitude of the penalty value or the distance between the centers of the circles (related to the probability of target and penalty contact) was changed randomly from trial to trial in separate blocks. Results revealed that participants shifted their endpoint and generally aimed optimally when the distance between the circles was varied but did not optimally shift their endpoints when the penalty value was varied. The results suggest that participants rapidly adapted endpoints when the probabilities associated with the task change, because the spatial parameters are an intrinsic property of the visual stimuli that are tightly linked with the motor system, whereas consistent feedback may be necessary to adjust to value parameters effectively.

On the limited effect of stimulus boundaries on saccade metrics

How the endpoint of saccadic eye movements is determined out of many potential peripheral locations is a crucial issue in the field of vision. Models of saccade generation account for this seemingly selective process in terms of competitive interactions between populations of neurons that encode respectively for different saccade amplitudes and directions. However, these models do not specify which visual stimulus properties other than the relative location of the stimuli are involved and how these properties contribute to ultimately determine a single saccade endpoint. We addressed this issue by contrasting the respective contributions of the 2-D spatial extent of the stimuli and the location of their boundaries in a global-effect paradigm. Participants were presented a to-be-looked-at peripheral target stimulus with or without a less eccentric visually invariant distractor. The extent of the target stimulus was manipulated in either one or two dimensions, such that targets differed either by their 2-D spatial extent (small, medium, or large circle) or the location of their boundaries (circle vs. horizontal or vertical ellipse of medium size). Results showed that the distractor deviated the eyes away from the target with the deviation varying with the 2-D spatial extent of the target but not the location of its boundaries. This finding suggests that the spatial distribution of luminance contrast and/or the number of elementary features that compose the stimuli prevails over visual boundaries in specifying the saccade endpoint. Implications for models of saccade generation are discussed.

Effet de l’action motrice sur les asynchronies perceptives

In agreement with the hypothesis of differential treatment of the intrinsic (color, shape, category…) and extrinsic (position, orientation…) visual characteristics along the ventral and dorsal pathways of the visual system (Milner & Goodale, 1995), the study of temporal order judgment (TOJ) of the variation of two visual attributes highlighted the perceptual asynchrony even though these changes occur synchronously. In this context, we investigated the role of action in the perception of perceptual asynchrony, especially the effect of a reaching movement on the TOJ of position and color changes of a target occurring at different times of movement execution. In the absence of voluntary action, the point of subjective simultaneity (PSS) shows that the color changes must occurr 46.6ms before the change of position to give rise to a synchronous perception of these two changes. Performing a reaching movement reduced significantly the PSS (12.4ms), but only if the changes occur near movement end-point. If changes occur during movement execution, the PSS (40.2ms) was not different from that obtained in the perceptual condition. These results suggest that endogenous signals associated with voluntary motor action contribute to the reduction of perceptual asynchrony in relation to the goal of the action. We discuss the possibility that, in the context of the action, the motor system contributes to the binding of objects sensory attributes as well as to the sense of agency.

Posture-movement responses to stance perturbations and upper limb fatigue during a repetitive pointing task

Localized muscle fatigue and postural perturbation have separately been shown to alter whole-body movement but little is known about how humans respond when subjected to both factors combined. Here we sought to quantify the kinematics of postural control and repetitive upper limb movement during standing surface perturbations and in the presence of fatigue. Subjects stood on a motion-based platform and repetitively reached between two shoulder-height targets until noticeably fatigued (rating of perceived exertion=8/10). Every minute, subjects experienced a posterior and an anterior platform translation while reaching to the distal target. Outcomes were compared prior to and with fatigue (first vs. final minute data). When fatigued, regardless of the perturbation condition, subjects decreased their shoulder abduction and increased contralateral trunk flexion, a strategy that may relieve the load on the fatiguing upper limb musculature. During perturbations, kinematic adaptations emerged across the trunk and arm to preserve task performance. In contrast to our expectation, the kinematic response to the perturbations did not alter in the presence of fatigue. Kinematic adaptations in response to the perturbation predominantly occurred in the direction of the reach whereas fatigue adaptations occurred orthogonal to the reach. These findings suggest that during repetitive reaching, fatigue and postural perturbation compensations organize so as to minimize interaction with each other and preserve the global task characteristics of endpoint motion.

Immediate compensation for variations in self-generated Coriolis torques related to body dynamics and carried objects

We have previously shown that the Coriolis torques that result when an arm movement is performed during torso rotation do not affect movement trajectory. Our purpose in the present study was to examine whether torso motion-induced Coriolis and other interaction torques are counteracted during a turn and reach (T&R) movement when the effective mass of the hand is augmented, and whether the dominant arm has an advantage in coordinating intersegmental dynamics as predicted by the dynamic dominance hypothesis (Sainburg RL. Exp Brain Res 142: 241-258, 2002). Subjects made slow and fast T&R movements in the dark to just extinguished targets with either arm, while holding or not holding a 454-g object. Movement endpoints were equally accurate at both speeds, with either hand, and in both weight conditions, but subjects tended to angularly undershoot and produce more variable endpoints for targets requiring greater torso rotation. There were no changes in endpoint accuracy or trajectory deviation over repeated movements. The dominant right arm was more stable in its control of trajectory direction across targets, whereas the nondominant left arm had an improved ability to stop accurately on the target for higher levels of interaction torques. The trajectories to more eccentric targets were straighter when performed at higher speeds but slightly more deviated when subjects held the weight. Subjects did not slow their torso velocity or change the timing of the arm and torso velocities when holding the weight, although there was a slight decrease in their hand velocity relative to the torso. The delay between the onsets of torso and finger movements was almost twice as large for the right arm than the left, suggesting the right arm was better able to account for torso rotation in the arm movement. Holding the weight increased the peak Coriolis torque by 40% at the shoulder and 45% at the elbow and, for the most eccentric target, increased the peak net torque by 12% at the shoulder and 34% at the elbow. In accordance with Sainburg's dynamic dominance hypothesis, the right arm exhibited an advantage for coordinating intersegmental dynamics, showing a more stable finger velocity in relation to the torso across targets, decreasing error variability with movement speed, and more synchronized peaks of finger relative and torso angular velocities in conditions with greater joint torque requirements. The arm used had little effect on the movement path and the magnitude of the joint torques in any of the conditions. These results indicate that compensations for forthcoming Coriolis torque variations take into account the dynamic properties of the body and of external objects, as well as the planned velocities of the torso and arm.

Effective intracortical microstimulation parameters applied to primary motor cortex for evoking forelimb movements to stable spatial end points

High-frequency, long-duration intracortical microstimulation (HFLD-ICMS) applied to motor cortex is recognized as a useful and informative method for corticomotor mapping by evoking natural-appearing movements of the limb to consistent stable end-point positions. An important feature of these movements is that stimulation of a specific site in motor cortex evokes movement to the same spatial end point regardless of the starting position of the limb. The goal of this study was to delineate effective stimulus parameters for evoking forelimb movements to stable spatial end points from HFLD-ICMS applied to primary motor cortex (M1) in awake monkeys. We investigated stimulation of M1 as combinations of frequency (30-400 Hz), amplitude (30-200 μA), and duration (0.5-2 s) while concurrently recording electromyographic (EMG) activity from 24 forelimb muscles and movement kinematics with a motion capture system. Our results suggest a range of parameters (80-140 Hz, 80-140 μA, and 1,000-ms train duration) that are effective and safe for evoking forelimb translocation with subsequent stabilization at a spatial end point. The mean time for stimulation to elicit successful movement of the forelimb to a stable spatial end point was 475.8 ± 170.9 ms. Median successful frequency and amplitude were 110 Hz and 110 μA, respectively. Attenuated parameters resulted in inconsistent, truncated, or undetectable movements, while intensified parameters yielded no change to movement end points and increased potential for large-scale physiological spread and adverse focal motor effects. Establishing cortical stimulation parameters yielding consistent forelimb movements to stable spatial end points forms the basis for a systematic and comprehensive mapping of M1 in terms of evoked movements and associated muscle synergies. Additionally, the results increase our understanding of how the central nervous system may encode movement.

Optimal weighting of costs and probabilities in a risky motor decision-making task requires experience.

Previous research has revealed that people choose to aim toward an "optimal" endpoint when faced with a movement task with externally imposed payoffs. This optimal endpoint is modeled based on the magnitude of the payoffs and the probability of hitting the different payoff regions (endpoint variability). Endpoint selection, however, has only been studied after people had experience with the aiming task. The present study examined initial endpoint selection and how it changed as a function of experience with performing the task. Participants completed 300 movements to a target that was overlapped by a penalty region. Mean endpoint was analyzed in intervals of 50 trials. Predictions based on the optimal model would indicate that the mean endpoint should be farther from the penalty region early in practice when endpoint variability is higher-increasing target misses but decreasing costly penalty hits. As variability decreases, however, it is predicted that the endpoint should shift closer to the optimal location. In contrast to these predictions, participants' mean movement endpoints started closer to the penalty region and shifted away with increasing practice, even as endpoint variability decreased. This pattern of endpoint selection leads to suboptimal gains early in experience but more optimal gains by the end of the trials. These findings suggest that, similar to results found in cognitive decision-making tasks, people need to receive performance-based feedback to weight their estimations of probability and payoffs in motor tasks.

Testing Whether Humans Have an Accurate Model of Their Own Motor Uncertainty in a Speeded Reaching Task

In many motor tasks, optimal performance presupposes that human movement planning is based on an accurate internal model of the subject's own motor error. We developed a motor choice task that allowed us to test whether the internal model implicit in a subject's choices differed from the actual in isotropy (elongation) and variance. Subjects were first trained to hit a circular target on a touch screen within a time limit. After training, subjects were repeatedly shown pairs of targets differing in size and shape and asked to choose the target that was easier to hit. On each trial they simply chose a target – they did not attempt to hit the chosen target. For each subject, we tested whether the internal model implicit in her target choices was consistent with her true error distribution in isotropy and variance. For all subjects, movement end points were anisotropic, distributed as vertically elongated bivariate Gaussians. However, in choosing targets, almost all subjects effectively assumed an isotropic distribution rather than their actual anisotropic distribution. Roughly half of the subjects chose as though they correctly estimated their own variance and the other half effectively assumed a variance that was more than four times larger than the actual, essentially basing their choices merely on the areas of the targets. The task and analyses we developed allowed us to characterize the internal model of motor error implicit in how humans plan reaching movements. In this task, human movement planning – even after extensive training – is based on an internal model of human motor error that includes substantial and qualitative inaccuracies.

What autocorrelation tells us about motor variability: insights from dart throwing.

In sports such as golf and darts it is important that one can produce ballistic movements of an object towards a goal location with as little variability as possible. A factor that influences this variability is the extent to which motor planning is updated from movement to movement based on observed errors. Previous work has shown that for reaching movements, our motor system uses the learning rate (the proportion of an error that is corrected for in the planning of the next movement) that is optimal for minimizing the endpoint variability. Here we examined whether the learning rate is hard-wired and therefore automatically optimal, or whether it is optimized through experience. We compared the performance of experienced dart players and beginners in a dart task. A hallmark of the optimal learning rate is that the lag-1 autocorrelation of movement endpoints is zero. We found that the lag-1 autocorrelation of experienced dart players was near zero, implying a near-optimal learning rate, whereas it was negative for beginners, suggesting a larger than optimal learning rate. We conclude that learning rates for trial-by-trial motor learning are optimized through experience. This study also highlights the usefulness of the lag-1 autocorrelation as an index of performance in studying motor-skill learning.

Assessing age differences in touchscreen pointing movements using the method of ballistic movement

To enhance the use of touchscreens for the elderly population, this study tested the ballistic movement method for assessing age-related differences when executing pointing movements. Six old and six young healthy participants performed ballistic movements of specific distances on a modified touchscreen monitor. The measured data of constant errors, variable errors, and movement time were statistically analyzed and fitted using the ballistic movement models. Except that the older participants aimed at approximately 4 mm above where the young participants did, no significant aging effect was found on ballistic movement time and endpoint variable errors using analysis of variance. The two ballistic movement models predicted well the measured data of movement time and variable errors, except for the vertical variable error committed by the older participants. The ballistic movement method can reveal detailed differences in age and separately predict motor properties of speed and accuracy, helping design target sizes for specific populations.

Verification of models for ballistic movement time and endpoint variability

A hand control movement is composed of several ballistic movements. The time required in performing a ballistic movement and its endpoint variability are two important properties in developing movement models. The purpose of this study was to test potential models for predicting these two properties. Twelve participants conducted ballistic movements of specific amplitudes using a drawing tablet. The measured data of movement time and endpoint variability were then used to verify the models. This study was successful with Hoffmann and Gan's movement time model (Hoffmann, 1981; Gan and Hoffmann 1988) predicting more than 90.7% data variance for 84 individual measurements. A new theoretically developed ballistic movement variability model, proved to be better than Howarth, Beggs, and Bowden's (1971) model, predicting on average 84.8% of stopping-variable error and 88.3% of aiming-variable errors. These two validated models will help build solid theoretical movement models and evaluate input devices. Practitioner summary: This article provides better models for predicting end accuracy and movement time of ballistic movements that are desirable in rapid aiming tasks, such as keying in numbers on a smart phone. The models allow better design of aiming tasks, for example button sizes on mobile phones for different user populations.

The Mechanism of Saccade Motor Pattern Generation Investigated by a Large-Scale Spiking Neuron Model of the Superior Colliculus

The subcortical saccade-generating system consists of the retina, superior colliculus, cerebellum and brainstem motoneuron areas. The superior colliculus is the site of sensory-motor convergence within this basic visuomotor loop preserved throughout the vertebrates. While the system has been extensively studied, there are still several outstanding questions regarding how and where the saccade eye movement profile is generated and the contribution of respective parts within this system. Here we construct a spiking neuron model of the whole intermediate layer of the superior colliculus based on the latest anatomy and physiology data. The model consists of conductance-based spiking neurons with quasi-visual, burst, buildup, local inhibitory, and deep layer inhibitory neurons. The visual input is given from the superficial superior colliculus and the burst neurons send the output to the brainstem oculomotor nuclei. Gating input from the basal ganglia and an integral feedback from the reticular formation are also included.We implement the model in the NEST simulator and show that the activity profile of bursting neurons can be reproduced by a combination of NMDA-type and cholinergic excitatory synaptic inputs and integrative inhibitory feedback. The model shows that the spreading neural activity observed in vivo can keep track of the collicular output over time and reset the system at the end of a saccade through activation of deep layer inhibitory neurons. We identify the model parameters according to neural recording data and show that the resulting model recreates the saccade size-velocity curves known as the saccadic main sequence in behavioral studies. The present model is consistent with theories that the superior colliculus takes a principal role in generating the temporal profiles of saccadic eye movements, rather than just specifying the end points of eye movements.

Does risk-sensitivity transfer across movements?

An intriguing finding in motor control studies is the marked effect of risk on movement decision making. However, there are inconsistent reports of risk-sensitivity across different movements and tasks, with both risk-seeking and risk-averse behavior observed. This raises the question of whether risk-sensitivity in movement decision making is context dependent and specific to the movement or task being performed. We investigated whether risk-sensitivity transfers between dissimilar movements within a single task. Healthy young adults made arm-reaching movements or whole-body leaning movements to move a cursor as close to the edge of a virtual cliff as possible without moving beyond the edge. They received points on the basis of the cursor's final proximity to the cliff edge. Risk was manipulated by increasing the point penalty associated with the cliff region and/or adding Gaussian noise to the cursor. We compared subjects' movement endpoints with endpoints predicted by a subject-specific, risk-neutral model of movement planning. Subjects demonstrated risk-seeking behavior in both movements that was consistent across risk environments, moving closer to the cliff than the model predicted. However, subjects were significantly more risk-seeking in whole-body movements. Our results present the first evidence of risk-sensitivity in whole-body movements. They also demonstrate that the direction of risk-sensitivity (i.e., risk-seeking or risk-averse) is similar between arm-reaching and whole-body movements, although degree of risk-sensitivity did not transfer from one movement to another. This finding has important implications for the ability of quantitative descriptions of decision making to generalize across movements and, ultimately, decision-making contexts.

Predictive Ability of the Auditory System during Smooth and Abrupt Movements of Low-Intensity Sound Images

The predictive ability of the human auditory system in locating the positions of the end points of movement trajectories was studied in comparison with stationary fused sound sources. Sound images moved smoothly or abruptly from the midline of the head towards one of the ears. Sound source movement was created by altering interaural stimulus time differences. The perceived positions of the end points of movement trajectories, like the positions of stationary sound images, depended on the final interaural time delay. With final interaural time delays of less than 200 μsec, the movement end points of sound images were displaced further from the midline of the head relative to the position of the corresponding stationary sound images; at delays of greater than 400 μsec, they were closer. The resulting anticipatory displacement can be regarded as a manifestation of the predictive ability of the auditory system in locating moving sound images. For smoothly moving sound images with interaural delays of greater than 400 μsec, longer-lasting stimuli were found to induce greater displacement of end points than those with shorter stimuli.

Kinematic characteristics of tenodesis grasp in C6 quadriplegia

Descriptive control case study. To analyze the kinematics of tenodesis grasp in participants with C6 quadriplegia and healthy control participants in a pointing task and two daily life tasks involving a whole hand grip (apple) or a lateral grip (floppy disk). France. Four complete participants with C6 quadriplegia were age matched with four healthy control participants. All participants were right-handed. The measured kinematic parameters were the movement time (MT), the peak velocity (PV), the time of PV (TPV) and the wrist angle in the sagittal plane at movement onset, at the TPV and at the movement end point. The participants with C6 quadriplegia had significantly longer MTs in both prehension tasks. No significant differences in TPV were found between the two groups. Unlike control participants, for both prehension tasks the wrist of participants with C6 quadriplegia was in a neutral position at movement onset, in flexion at the TPV, and in extension at the movement end point. Two main kinematic parameters characterize tenodesis grasp movements in C6 quadriplegics: wrist flexion during reaching and wrist extension during the grasping phase, and increased MT reflecting the time required to adjust the wrist's position to achieve the tenodesis grasp. These characteristics were observed for two different grips (whole hand and lateral grip). These results suggest sequential planning of reaching and tenodesis grasp, and should be taken into account for prehension rehabilitation in patients with quadriplegia.

Hand-held tools with complex kinematics are efficiently incorporated into movement planning and online control

Certain hand-held tools alter the mapping between hand motion and motion of the tool end point that must be controlled in order to perform a task. For example, when using a pool cue, the motion of the cue tip is reversed relative to the hand. Previous studies have shown that the time required to initiate a reaching movement (Fernandez-Ruiz J, Wong W, Armstrong IT, Flanagan JR. Behav Brain Res 219: 8-14, 2011), or correct an ongoing reaching movement (Gritsenko V, Kalaska JF. J Neurophysiol 104: 3084-3104, 2010), is prolonged when the mapping between hand motion and motion of a cursor controlled by the hand is reversed. Here we show that these time costs can be significantly reduced when the reversal is instantiated by a virtual hand-held tool. Participants grasped the near end of a virtual tool, consisting of a rod connecting two circles, and moved the end point to displayed targets. In the reversal condition, the rod translated through, and rotated about, a pivot point such that there was a left-right reversal between hand and end point motion. In the nonreversal control, the tool translated with the hand. As expected, when only the two circles were presented, movement initiation and correction times were much longer in the reversal condition. However, when full vision of the tool was provided, the reaction time cost was almost eliminated. These results indicate that tools with complex kinematics can be efficiently incorporated into sensorimotor control mechanisms used in movement planning and online control.

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 20days 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.

Effect of a visual distractor on line bisection

Previous studies have shown that a flanking distractor produces a spatial bias during line bisection. In the present study, we investigated whether that bias depends on perceptual or motor components. Participants were asked to bisect a horizontal line, or to reach towards a dot, with or without vision of their hand. The line and the target could be flanked by a distractor. Movement trajectories and endpoints were consistently deviated away from the location of the distractor in the bisection task, but not in the reaching task, irrespectively of whether the participants had online visual feedback from their moving hand. It is suggested that flanking distractors influence perceptual localization of the subjective mid-point during line bisection.

The influence of the dynamic transformation of a sliding lever on aiming errors

Human movements are quickly adjusted to variations of inertial load. However, this adjustment does not always imply a full compensation, so that kinematic movement characteristics vary. The present experiment served to explore the consequences of a complex dynamic transformation, implemented by a sliding first-order lever, on the endpoint distributions of goal-directed movements. Whereas the endpoint distributions were clearly affected by the inertial anisotropy of the arm, there was no effect of the dynamic transformation of the lever, neither on the parameters of endpoint distributions nor on the covariations of endpoints of successive movements (error propagation). However, when the lever was used, the effect of the inertial anisotropy of the arm on movement amplitudes was reduced, accompanied by a longer movement time overall, in particular for movements with higher inertial load of the arm. These observations suggest an interaction of the use of internal models and impedance control in the presence of variable inertial loads. Most likely the influence of the dynamic transformation of the sliding lever is absorbed by increased joint impedance, which also reduces the influence of the inertial anisotropy of the arm which otherwise is (incompletely) compensated based on an internal model of the dynamic transformation of the arm.

Response: The apparent reorganization of gesture in the evolution of verb agreement in signed languages

Article Response: The apparent reorganization of gesture in the evolution of verb agreement in signed languages was published on August 10, 2012 in the journal Theoretical Linguistics (volume 38, issue 1-2).