Misdirections in slow,goal-directed arm movements are not primarily visually based

In a previous study we found that the initial direction of slow, goal-directed arm movements deviates consistently from the direction of the actual straight line between the starting position and the target position. We now investigate whether these deviations are caused by imperfections or peculiarities in the processing of vision-related spatial information, such as retinal information, and eye- and head-position information. This could lead to incorrect localization of the target relative to the starting position. Subjects were seated in front of a horizontal surface and had to move their arm slowly and accurately in the direction of target positions. We varied the amount of vision-related spatial information. In experiment 1, subjects were presented with visual targets and could see their moving arm. In experiment 2, the subjects were again presented with visual targets, but now they could not see their moving arm. In experiment 3, the subjects were blindfolded and had to move their arm towards tactile targets. In all three experiments we found comparable consistent deviations in the initial movement direction. We also instructed congenitally and early-blind subjects to move their arm towards tactile targets. Their performance showed deviations congruous with those found in the sighted subjects, and possibly somewhat larger. We conclude that the deviations in the initial movement direction of slow, goal-directed arm movements are not primarily visually based. The deviations are generated after all spatial information has been integrated.     

Spatial and temporal aspects of eye-hand coordination across different tasks

The way in which saccadic eye movements are elicited influences their latency and accuracy. Accordingly, different tasks elicit different types of saccades. Using the tasks steps, gap, memory, scanning and antisaccade, we analyzed combined eye and hand movements to determine whether both motor systems share control strategies. Errors and latencies were measured to examine whether changes in eye motor behavior are reflected in hand motor behavior. Directional and variable errors of eye and hand changed differently according to the tasks. Moreover, errors of the two systems did not correlate for any of the tasks investigated. Contrary to errors, mean latencies of eye movements were organized in the same pattern as hand movements. A correlation of latencies indicates that both motor systems rely on common information to initiate movement. Temporal coupling was stronger for intentional tasks than for reflexive tasks.     

Manual and oculomotor performance develop contemporaneously but independently during continuous tracking

The coordination of the oculomotor and manual effector systems is an important component of daily motor behavior. Previous work has primarily examined oculomotor/manual coordination in discrete targeting tasks. Here we extend this work to learning a tracking task that requires continuous response and movement update. Over two sessions, participants practiced controlling a computer mouse with movements of their arm to follow a target moving in a repeated sequence. Eye movements were also recorded. In a retention test, participants demonstrated sequence-specific learning with both effector systems, but differences between effectors also were apparent. Time series analysis and multiple linear regression were employed to probe spatial and temporal contributions to overall tracking accuracy within each effector system. Sequence-specific oculomotor learning occurred only in the spatial domain. By contrast, sequence-specific learning at the arm was evident only in the temporal domain. There was minimal interdependence in error rates for the two effector systems, underscoring their independence during tracking. These findings suggest that the oculomotor and manual systems learn contemporaneously, but performance improvements manifest differently and rely on different elements of motor execution. The results may in part be a function of what the motor learning system values for each effector as a function of its effector’s inertial properties.     

An fMRI study of optokinetic nystagmus and smooth-pursuit eye movements in humans

Both optokinetic nystagmus (OKN) and smooth-pursuit eye movements (SPEM) are subclasses of so-called slow eye movements. However, optokinetic responses are reflexive whereas smooth pursuit requires the voluntary tracking of a moving target. We used functional magnetic resonance imaging (fMRI) to determine the neural basis of OKN and SPEM, and to uncover whether the two underlying neural systems overlap or are independent at the cortical level. The results showed a largely overlapping neural circuitry. A direct comparison between activity during the execution of OKN and SPEM yielded no oculomotor-related area exclusively dedicated to one or the other eye movement type. Furthermore, the performance of SPEM evoked a bilateral deactivation of the human equivalent of the parietoinsular vestibular cortex. This finding might indicate that the reciprocally inhibitory visual–vestibular interaction involves not only OKN but also SPEM, which are both linked with the encoding of object-motion and self-motion. Moreover, we could show differential activation patterns elicited by look-nystagmus and stare-nystagmus. Look-nystagmus is characterized by large amplitudes and low-frequency resetting eye movements rather resembling SPEM. Look-nystagmus evoked activity in cortical oculomotor centers. By contrast, stare-nystagmus is usually characterized as being more reflexive in nature and as showing smaller amplitudes and higher frequency resetting eye movements. Stare-nystagmus failed to elicit significant signal changes in the same regions as look-nystagmus/SPEM. Thus, less reflexive eye movements correlated with more pronounced signal intensity. Finally, on the basis of a general investigation of slow eye movements, we were interested in a cortical differentiation between subtypes of SPEM. We compared activity associated with predictable and unpredictable SPEM as indicated by appropriate visual cues. In general, predictable and unpredictable SPEM share the same neural network, yet information about the direction of an upcoming target movement reduced the cerebral activity level.     

The influence of goals on movement kinematics during imitation

This study took a quantitative approach to investigate movement kinematics during the imitation of goal-directed and non-goal directed movements. Motion tracking equipment was used to record the hand movements of 15 healthy participants during an imitation task involving aiming movements that varied in speed. We predicted that movement kinematics would be most similar to the observed movements in the non-goal condition, as a result of direct visuomotor mapping of the action, and least similar in the goal-directed condition because more importance would be given to the end goal. We also predicted that precues (prior information about the movement) would increase imitation accuracy in the non-goal condition by reducing cognitive demand, and that precues would reduce accuracy in the goal-directed condition, as less attention would be paid to the movement. Results showed that imitation was modulated by the speed of the observed action in the non-goal condition only. Contrary to predictions, precues did not improve imitation in the non-goal condition or improve imitation accuracy in the goal-directed condition. These results demonstrate that visuomotor mapping is favoured in non-goal imitation, regardless of prior information, and that accurate imitation of movement detail is compromised by the presence of goals. Such differences in movement kinematics indicate that different processes mediate the imitation of non-goal and goal-directed actions.     

Optimizing rapid aiming behaviour: movement kinematics depend on the cost of corrective modifications

Recent studies have shown that the initial impulse associated with goal-directed aiming movements typically brings the limb to a position short of the target. This is because target overshooting is associated with greater temporal and energy costs than target undershooting. Presumably these costs can be expected to vary not only with the muscular forces required to move the limb, but also the gravitational forces inherent in the aiming task. In this study we examined the degree to which primary movement endpoint distributions depend on the direction of the movement with respect to gravity. We hypothesized that the magnitude of an undershoot bias would be greatest for downward movements because target overshooting necessitates a time and energy consuming movement reversal against gravity. Participants completed rapid aiming movements toward targets located above and below, as well as proximal and distal to a central home position. Movements were made both with and without additional mass attached to the limb. Although movement time did not vary with experimental condition, primary movement endpoint distributions were consistent with our predictions. Specifically, both greater undershooting and greater endpoint variability was associated with downward aiming movements. As well, a greater proportion of the overall movement time was spent in the corrective phase of the movement. These results are consistent with models of energy minimization that posit an inherent efficiency of control and hold that movements are organized to minimize movement time and energy expenditure and maximize mechanical advantages.     

人体运动的生物力学建模与计算机仿真进展

人体运动的生物力学建模与仿真,广泛应用于阐明不同运动的生理机理、探讨运动损伤机制、提高运动员运动成绩和降低损伤等诸多研究领域。他涉及人体骨骼、关节、肌肉和神经等组织的生理、解剖和力学特征的数学建模。利用数学模型,可以采用相关算法求解运动过程中不同肌肉的收缩力,同时还可通过计算机软件进行仿真实验并将仿真结果可视化。本文对人体运动的生物力学建模与仿真及其应用进行了综述。     

Control of eye movement reflexes

We investigated the effect of strategic suppression of reflexive eye movements on external control over fixation using a fixation offset paradigm. A visual signal at fixation facilitates the fixation reflex and inhibits eye movements. Certain preparatory states render the fixation reflex less reactive to visual stimulation at fixation, as evidenced by a reduction in the fixation offset effect (FOE). For example, past studies have suggested that the reduced FOE during anti-saccade tasks results from the requirement to inhibit reflexive eye movements. We tested whether suppressing reflexive saccades reduces external control over ocular fixation using a go-nogo saccade paradigm. During each trial, one of two targets appeared in the periphery. Participants were instructed to saccade to one target (go), but when the other target appeared they either had to maintain fixation (nogo) or move their eyes in the direction opposite the target (anti). When nogo trials were admixed with target-directed saccades a large FOE was observed compared to when target-directed saccades occurred alone (experiment 1); however, when anti-saccades were mixed with target-directed saccades, a small FOE was observed for both types of eye movements (experiment 2). We conclude that suppressing reflexive eye movements does not reduce external control over fixation. Further research is necessary to elucidate which other component of preparing to make an anti-saccade diminishes the FOE.     

Manual asymmetries in the directional coding of reaching: further evidence for hemispatial effects and right hemisphere dominance for movement planning

Directional coding of hand movements is of primary importance in the proactive control of goal-directed aiming. At the same time, manual reaction times are known to be asymmetric when reaching at lateralized targets. Generally, ipsilateral movements and left hand advantages are interpreted using the classical model of interhemispheric transmission for simple visuomotor integration, but the use of this model was recently challenged when applied to reaching movements, arguing that attentional and biomechanical effects could also account for such asymmetries. In this work, we aimed at controlling both visual attention orienting and movement mechanical constraints in order to clarify the origin of manual reaction time asymmetries and hemispatial effects in the directional coding of reaching. Choice reaction time pointing tasks were assessed in two experiments in which identical movements were compared in different conditions of target lateralization and different conditions of head, eye and hand position. Results suggested that biomechanical constraints could account for hemispatial effects for movement execution but not for movement direction coding. These results are discussed in the light of models of interhemispheric cooperation and the right hemisphere dominance for spatial processing. Electronic Publication     

Integration of visual and somatosensory target information in goal-directed eye and arm movements

 In this study, we compared separate and coordinated eye and hand movements towards visual or somatosensory target stimuli in a dark room, where no visual position information about the hand could be obtained. Experiment 1 showed that saccadic reaction times (RTs) were longer when directed to somatosensory targets than when directed to visual targets in both single- and dual-task conditions. However, for hand movements, this pattern was only found in the dual-task condition and not in the single-task condition. Experiment 1 also showed that correlations between saccadic and hand RTs were significantly higher when directed towards somatosensory targets than when directed towards visual targets. Importantly, experiment 2 indicated that this was not caused by differences in processing times at a perceptual level. Furthermore, hand-pointing accuracy was found to be higher when subjects had to move their eyes as well (dual task) compared to a single-task hand movement. However, this effect was more pronounced for movements to visual targets than to somatosensory targets. A schematic model of sensorimotor transformations for saccadic eye and goal-directed hand movements is proposed and possible shared mechanisms of the two motor systems are discussed. Received: 15 June 1998 / Accepted: 21 September 1998     

Anticipatory smooth-pursuit eye movements in man and monkey

A fundamental problem in the generation of goal-directed behaviour is caused by the inevitable latency of biological sensory systems. Behaviour which is fully synchronised with the triggering sensory event can only be executed if the occurrence of this event can be predicted based on prior information. Smooth-pursuit eye movements are a classical and well-established example of goal-directed behaviour. The execution of these eye movements is thought to be very closely linked to the processing of visual motion signals. Here, we show that healthy human subjects as well as trained rhesus monkeys are able to initiate smooth-pursuit eye movements in anticipation of a moving target. These anticipatory pursuit eye movements are scaled to the velocity of the expected target. Furthermore, we can exclude the possibility that anticipatory pursuit is simply an after-pursuit of the previous trial. Visually-guided pursuit is only marginally affected by the presence of a structured background. However, the presence of a structured background severely impedes the ability to perform anticipatory pursuit. More generally, our data provide additional evidence that the cognitive oculomotor repertoires of human and monkeys are similar, at least with respect of smooth-pursuit in the prediction of an appearing target.     

Agency, gait and self-consciousness

Agency is an important aspect of bodily self-consciousness, allowing us to separate own movements from those induced by the environment and to distinguish own movements from those of other agents. Unsurprisingly, theoretical frameworks for agency such as central monitoring are closely tied to computational models of sensorimotor control. Until recently agency research has largely focussed on goal-directed movements of the upper limbs. In particular, the influence of performance-related sensory cues and the relevance of prediction signals for agency judgements have been studied through a variety of spatio-temporal mismatches between movement and the sensory consequences of movement. However, agents often perform a different type of movement; highly automated movements that involve the entire body such as walking, cycling, and swimming with potentially different agency mechanisms. Here, we review recent work about agency for full-body movements such as gait, highlighting the effects of performance-related visual and auditory cues on gait agency. Gait movements differ from upper limb actions. Gait is cyclic, more rarely immediately goal-directed, and is generally considered one of the most automatic and unconscious actions. We discuss such movement differences with respect to the functional mechanisms of full-body agency and body-part agency by linking these gait agency paradigms to computational models of motor control. This is followed by a selective review of gait control, locomotion, and models of motor control relying on prediction signals and underlining their relevance for full-body agency.     

The role of online visual feedback for the control of target-directed and allocentric hand movements

Studies that have investigated how sensory feedback about the moving hand is used to control hand movements have relied on paradigms such as pointing or reaching that require subjects to acquire target locations. In the context of these target-directed tasks, it has been found repeatedly that the human sensory-motor system relies heavily on visual feedback to control the ongoing movement. This finding has been formalized within the framework of statistical optimality according to which different sources of sensory feedback are combined such as to minimize variance in sensory information during movement control. Importantly, however, many hand movements that people perform every day are not target-directed, but based on allocentric (object-centered) visual information. Examples of allocentric movements are gesture imitation, drawing, or copying. Here we tested if visual feedback about the moving hand is used in the same way to control target-directed and allocentric hand movements. The results show that visual feedback is used significantly more to reduce movement scatter in the target-directed as compared with the allocentric movement task. Furthermore, we found that differences in the use of visual feedback between target-directed and allocentric hand movements cannot be explained based on differences in uncertainty about the movement goal. We conclude that the role played by visual feedback for movement control is fundamentally different for target-directed and allocentric movements. The results suggest that current computational and neural models of sensorimotor control that are based entirely on data derived from target-directed paradigms have to be modified to accommodate performance in the allocentric tasks used in our experiments. As a consequence, the results cast doubt on the idea that models of sensorimotor control developed exclusively from data obtained in target-directed paradigms are also valid in the context of allocentric tasks, such as drawing, copying, or imitative gesturing, that characterize much of human behavior.     

Proportional myoelectric control of a virtual object to investigate human efferent control

We used proportional myoelectric control of a one-dimensional virtual object to investigate differences in efferent control between the proximal and distal muscles of the upper limbs. Eleven subjects placed one of their upper limbs in a brace that restricted movement while we recorded electromyography (EMG) signals from elbow flexors/extensors or wrist flexors/extensors during isometric contractions. By activating their muscles, subjects applied virtual forces to a virtual object using a real-time computer interface. The magnitudes of these forces were proportional to EMG amplitudes. Subjects used this proportional EMG control to move the virtual object through two tracking tasks, one with a static target and one with a moving target (i.e., a sine wave). We hypothesized that subjects would have better control over the virtual object using their distal muscles rather than using their proximal muscles because humans typically use more distal joints to perform fine motor tasks. The results indicated that there was no difference in subjects ability to control virtual object movements when using either upper arm muscles or forearm muscles. These results suggest that differences in control accuracy between elbow joint movements and wrist joint movements are more likely to be a result of motor practice, proprioceptive feedback or joint mechanics rather than inherent differences in efferent control.     

Primate area MST-l is involved in the generation of goal-directed eye and hand movements

The contributions of the middle superior temporal area (MST) in the posterior parietal cortex of rhesus monkeys to the generation of smooth-pursuit eye movements as well as the contributions to motion perception are well established. Here, we present the first experimental evidence that this area also contributes to the generation of goal-directed hand movements toward a moving target. This evidence is based on the outcome of intracortical microstimulation experiments and transient lesions by small injections of muscimol at identified sites within the lateral part of area MST (MST-l). When microstimulation was applied during the execution of smooth-pursuit eye movements, postsaccadic eye velocity in the direction of the preferred direction of the stimulated site increased significantly (in 93 of 136 sites tested). When microstimulation was applied during a hand movement trial, the hand movement was displaced significantly in the same direction (in 28 of 39 sites tested). When we lesioned area MST-l transiently by injections of muscimol, steady-state eye velocity was exclusively reduced for ipsiversive smooth-pursuit eye movements. In contrast, hand movements were displaced toward the contralateral side, irrespective of the direction of the moving target. Our results provide evidence that area MST-l is involved in the processing of moving targets and plays a role in the execution of smooth-pursuit eye movements as well as visually guided hand movements.     

Initiation of a goal-directed movement in the monkey

Summary The participation of the dentate nucleus (DN) in the initiation of a voluntary movement was investigated in five baboons (Papio papio). In these experiments, we have analyzed the effects of excluding the DN on the latency (reaction time, RT) of a learned goal-directed movement.Two techniques were used for excluding the DN. In three animals, the structure was cooled with a probe, stereotaxically implanted on the side ipsilateral to the active hand. In two others, a partial electrolytic destruction of the DN ipsilateral to the operant hand was undertaken. In one further animal, both DNs were destroyed electrolytically.A comparison was made of the effect of DN inactivation on the latency of stereotyped goal-directed movements of constant amplitude and direction, and of goal-directed movements whose amplitude and/or direction were varied in random fashion.The exclusion of DN brought about a prolongation of RTs in all animals. This prolongation was not accentuated by variation of different characteristics (amplitude and/or direction) of the impending goal-directed movement.A recovery of the RTs to their prelesion values was observed after irreversible unilateral DN lesion, but not so easily after bilateral destruction.These results show that in the monkey DN is concerned with the initiation of a goal-directed movement, but is not critically implicated in the encoding of direction and amplitude parameters. These findings are discussed in view of the role that is usually attributed to the neocerebellum in programming voluntary movements.This work was in part supported by C.N.R.S. grant (ATP 4187) and INSERM grants (ATP 80.79.112, CRL 79.4.346.6)     

Two eyes in action

Do relative binocular disparities guide our movements in depth? In order to find out we asked subjects to move a ‘cursor’ to a target within a simulated horizontal plane at eye height. They did so by moving a computer mouse. We determined how quickly subjects responded to the target jumping in depth. We found that it took subjects about 200 ms to respond to changes in binocular disparity. Subjects responded just as quickly if the cursor was temporarily only visible to one eye near the time that the target jumped in depth, and less vigorously, though just as quickly, if the cursor jumped rather than the target, so the fastest binocular responses cannot be based directly on the relative retinal disparity between the target and the cursor. Subjects reacted faster to changes in the target’s height in the visual field than to changes in binocular disparity, but did not react faster to changes in image size. These results suggest that binocular vision mainly improves people’s everyday movements by giving them a better sense of the distances of relevant objects, rather than by relative retinal disparities being used to directly guide the movement. We propose that relative disparities only guide parts of very slow movements that require extreme precision.     

Eye–hand coupling is not the cause of manual return movements when searching

When searching for a target with eye movements, saccades are planned and initiated while the visual information is still being processed, so that subjects often make saccades away from the target and then have to make an additional return saccade. Presumably, the cost of the additional saccades is outweighed by the advantage of short fixations. We previously showed that when the cost of passing the target was increased, by having subjects manually move a window through which they could see the visual scene, subjects still passed the target and made return movements (with their hand). When moving a window in this manner, the eyes and hand follow the same path. To find out whether the hand still passes the target and then returns when eye and hand movements are uncoupled, we here compared moving a window across a scene with moving a scene behind a stationary window. We ensured that the required movement of the hand was identical in both conditions. Subjects found the target faster when moving the window across the scene than when moving the scene behind the window, but at the expense of making larger return movements. The relationship between the return movements and movement speed when comparing the two conditions was the same as the relationship between these two when comparing different window sizes. We conclude that the hand passing the target and then returning is not directly related to the eyes doing so, but rather that moving on before the information has been fully processed is a general principle of visuomotor control.     

Similarities between digits’ movements in grasping, touching and pushing

In order to find out whether the movements of single digits are controlled in a special way when grasping, we compared the movements of the digits when grasping an object with their movements in comparable single-digit tasks: pushing or lightly tapping the same object at the same place. The movements of the digits in grasping were very similar to the movements in the single-digit tasks. To determine to what extent the hand transport and grip formation in grasping emerges from a synchronised motion of individual digits, we combined movements of finger and thumb in the single-digit tasks to obtain hypothetical transport and grip components. We found a larger peak grip aperture earlier in the movement for the single-digit tasks. The timing of peak grip aperture depended in the same way on its size for all tasks. Furthermore, the deviations from a straight line of the transport component differed considerably between subjects, but were remarkably similar across tasks. These results support the idea that grasping should be regarded as consisting of moving the digits, rather than transporting the hand and shaping the grip.     

Between-trial inhibition and facilitation in goal-directed aiming: manual and spatial asymmetries

Three experiments were conducted with right-handed participants to examine between-trial inhibition and facilitation effects in goal-directed aiming. Participants were required to execute rapid left-hand or right-hand aiming movements upon illumination of a target light in left or right space. Thus, from trial to trial, participants executed movements to either the same target location or a different target location with the either same hand or the other hand. Our reaction time results indicated that participants were particularly slow in initiating their movements when they were required to return to the same target location with the other hand. This was especially the case when the right hand was required to move to a target just occupied by the left hand. For both reaction time and movement time the right hand but not the left hand exhibited an advantage when it was required to perform the same movement two times in a row. Taken together these results suggest that inhibition of return, in a target-target paradigm, is more associated with the particular spatial location of the target than the organization of a specific movement to that location. Moreover, the between-trial facilitation observed for the right hand may reflect the ability of the left cerebral hemisphere to maintain an already parameterized motor program over a short intertrial interval.