The effect of visuomotor displacements on arm movement paths

The gently curved paths evident in point-to-point arm movements have been attributed to both an imperfect execution of a planned straight-hand path or as an emergent property of a control strategy in which an intrinsic cost, dependent on arm dynamics, is minimised. We used a virtual visual feedback system to test whether path curvature was mainly determined by the visually perceived or actual location of the moving limb. Hand paths were measured for movements between three pairs of targets under both veridical and uniformly translated visual feedback. This allowed us to decouple the actual and perceived hand location during movement. Under different conditions of visual feedback the curvature of the hand paths did not correlate with either the visually perceived location of the limb or the actual location but rather with the relative displacement between the actual and visually perceived limb locations. The results are consistent with the hypothesis that in planning a movement the internal estimate of intrinsic coordinates, such as joint angles, is at least partially derived from visual information. Received: 31 August 1998 / Accepted: 8 March 1999     

The time course of cross-talk during the simultaneous specification of bimanual movement amplitudes

 We investigated the time course of the amplitude specification of rapid bimanual reversal movements (lateral displacements on two digitizers). To this end we used the timed-response paradigm in which the response has to be initiated synchronously with an auditory signal. Information about the required amplitudes was presented at various times before the synchronization signal. Consistent with previous results, the progression of amplitude specification was reflected in the dependence of the amplitudes of the reversal movements on the time interval between amplitude information and synchronization signal. Same or different amplitudes for the hands were used to examine cross-talk at the programming level of the two-level model of intermanual interference. The results indicate the existence of cross-talk in particular at short intervals between information about amplitude and movement initiation. This is consistent with the notion that cross-talk between concurrent processes of amplitude specification is transient and vanishes as the time available for motor programming increases. Received: 28 August 1996 / Accepted: 10 July 1997     

Bimanual adaptation: internal representations of bimanual rhythmic movements

From tying your shoes and clipping your tie to the claps at the end of a fine seminar, bimanual coordination plays a major role in our daily activities. An important phenomenon in bimanual coordination is the predisposition toward mirror symmetry in the performance of bimanual rhythmic movements. Although learning and adaptation in bimanual coordination are phenomena that have been observed, they have not been studied in the context of adaptive control and internal representations—approaches that were successfully employed in the arena of reaching movements and adaptation to force perturbations. In this paper we examine the dynamics of the learning mechanisms involved when subjects are trained to perform a bimanual non-harmonic polyrhythm in a bimanual index finger tapping task. Subjects are trained in this task implicitly, using altered visual feedback, while their performance is continuously monitored throughout the experiment. Our experimental results indicate the existence of significant (p<<0.01) learning curves (i.e., error plots with significantly negative slopes) during training and aftereffects with a washout period after the visual feedback ceases to be altered. These results confirm the formation of internal representations in bimanual motor control. We present a simple, physiologically plausible, neural model that combines feedback and adaptation in the control process and which is able to reproduce key phenomena of bimanual coordination and adaptation.     

Evidence for an eye-centered spherical representation of the visuomotor map

During visually guided movement, visual coordinates of target location must be transformed into coordinates appropriate for movement. To investigate the representation of this visuomotor coordinate transformation, we examined changes in pointing behavior induced by a local visuomotor remapping. The visual feedback of finger position was limited to one location within the workspace, at which a discrepancy was introduced between the actual and visually perceived finger position. This remapping induced a change in pointing that extended over the entire workspace and was best captured by a spherical coordinate system centered near the eyes.     

Contribution of execution noise to arm movement variability in three-dimensional space

Reaching movements are subject to noise associated with planning and execution, but precisely how these noise sources interact to determine patterns of endpoint variability in three-dimensional space is not well understood. For frontal plane movements, variability is largest along the depth axis (the axis along which visual planning noise is greatest), with execution noise contributing to this variability along the movement direction. Here we tested whether these noise sources interact in a similar way for movements directed in depth. Subjects performed sequences of two movements from a single starting position to targets that were either both contained within a frontal plane ("frontal sequences") or where the first was within the frontal plane and the second was directed in depth ("depth sequences"). For both sequence types, movements were performed with or without visual feedback of the hand. When visual feedback was available, endpoint distributions for frontal and depth sequences were generally anisotropic, with the principal axes of variability being strongly aligned with the depth axis. Without visual feedback, endpoint distributions for frontal sequences were relatively isotropic and movement direction dependent, while those for depth sequences were similar to those with visual feedback. Overall, the results suggest that in the presence of visual feedback, endpoint variability is dominated by uncertainty associated with planning and updating visually guided movements. In addition, the results suggest that without visual feedback, increased uncertainty in hand position estimation effectively unmasks the effect of execution-related noise, resulting in patterns of endpoint variability that are highly movement direction dependent.     

Interlimb coupling strength scales with movement amplitude

The relation between movement amplitude and the strength of interlimb interactions was examined by comparing bimanual performance at different amplitude ratios (1:2, 1:1, and 2:1). For conditions with unequal amplitudes, the arm moving at the smaller amplitude was predicted to be more strongly affected by the contralateral arm than vice versa. This prediction was based on neurophysiological considerations and the HKB model of coupled oscillators. Participants performed rhythmic bimanual forearm movements at prescribed amplitude relations. After a brief mechanical perturbation of one arm, the relaxation process back to the initial coordination pattern was examined. This analysis focused on phase adaptations in the unperturbed arm, as these reflect the degree to which the movements of this arm were affected by the coupling influences stemming from the contralateral (perturbed) arm. The thus obtained index of coupling (IC) reflected the relative contribution of the unperturbed arm to the relaxation process. As predicted IC was larger when the perturbed arm moved at a larger amplitude than did the unperturbed arm, indicating that coupling strength scaled with movement amplitude. This result was discussed in relation to previous research regarding sources of asymmetry in coupling strength and the effects of amplitude disparity on interlimb coordination.     

The effect of visual transformation on bimanual circling movement

In order to examine the effect of visual transformation on bimanual movements of various difficulty, fourteen participants performed bimanual circling tasks in three asymmetric movement modes—90° (the left hand precedes the right hand by 1/4 cycle), 180° (the delay between two hands is 1/2 cycle), and 270° (the left hand precedes the right hand by 3/4 cycle)—under the normal vision condition and the visual transformation condition. In the visual transformation condition, movement of the right hand was transformed so that the required bimanual movement was always presented visually as a symmetric pattern. Additionally, the participants also performed a 0° mode (in-phase symmetric) movement. Results revealed that the visual transformation increased the movement accuracy, with the variability of the right–left difference unchanged. Thus, proper visual transformation can improve the accuracy of a movement task. The 0° mode was performed with higher stability and accuracy than any other movement modes of the visual transformation condition and normal vision conditions. In addition, the constant error associated with the 90° and 270° modes indicated that, in the normal vision condition, the executed movement was shifted to the 180° mode, whereas in the visual transformation condition it stayed around the required mode and was slightly shifted to the 0° mode. This result suggests that visual transformation can change the relationship between the intention to realize the required mode and the intrinsic neuromuscular dynamics. The effect size of visual transformation was larger in the 90° and 270° modes than in the 180° mode. It is thus concluded that the effect of visual transformation depends upon the difficulty of the movement task.     

Intermanual interactions during initiation and production of rhythmic and discrete movements in individuals lacking a corpus callosum

Three individuals lacking a corpus callosum, two due to callosotomy and one agenesis, and three age-matched healthy controls were tested on a bimanual task in which a discrete or rhythmic arm movement was initiated following a visual signal while the other arm produced continuous, rhythmic movements. The control participants initiated the secondary, rhythmic movement in phase with the ongoing rhythmic base movement and the two limbs were coupled in an inphase mode across the duration of the trial. In contrast, the acallosal individuals failed to show phase entrainment at the initiation of the secondary, rhythmic movements. Moreover, the callosotomy patients exhibited weak coupling between the rhythmically moving limbs while the individual with callosal agenesis consistently synchronized in an antiphase mode. The control participants exhibited increased perturbation of the ongoing base movement when initiating a discrete movement; for the acallosal participants, the base movement was similarly perturbed in both secondary movement conditions. These results are consistent with the hypothesis that intermanual interactions observed during bimanual movements arise from various levels of control, and that these are distinct for discrete and rhythmic movements. Temporal coupling during rhythmic movements arises in large part from transcallosal interactions between the two hemispheres. The imposition of a secondary movement may transiently disrupt an ongoing rhythmic movement even in the absence of the corpus callosum. This may reflect subcortical interactions associated with response initiation, or, due to dual task demands, a transient shift in attentional resources.     

Timing of bimanual movements and deafferentation: implications for the role of sensory movement effects

In a repetitive tapping task, the within-hand variability of intertap intervals is reduced when participants tap with both hands instead of single-handedly. This bimanual advantage has been attributed to timer as opposed to motor variance (according to the Wing-Kristofferson model; Helmuth and Ivry 1996) and related to the additional sensory consequences of the movement of the extra hand in the bimanual case (Drewing et al. 2002). In the present study the effect of sensory feedback of the movement on this advantage was investigated by comparing the results of a person (IW) deafferented below the neck with those of age-matched controls. IW showed an even more pronounced bimanual advantage than controls, suggesting that the bimanual advantage is not due to actual sensory feedback. These results support another hypothesis, namely that bimanual timing profits from the averaging of different central control signals that relate to each effectors movements.     

Unimanual and bimanual voluntary movement in Huntington's disease

Unimanual and bimanual cyclical forearm movements were studied in 15 Huntington's disease (HD) patients and 15 healthy, gender- and age-matched controls. Whereas the unimanual task was only performed at maximal speed, the bimanual movements were performed according to the in-phase and anti-phase mode at different cycling frequencies. The HD patients also performed the tasks after 12 months of follow-up. Findings revealed that maximal cycling frequency during unimanual movement was significantly lower in HD patients as compared with controls. In addition, measures of relative phasing established that bimanual cyclical movements were performed with lower accuracy and higher variability in HD patients. The differential variability between both groups was magnified by increasing the cycling frequency and coordinative complexity whereas only coordinative complexity differentially affected the accuracy of relative phasing. The obtained performance measures were found to be significantly correlated with disease duration (unimanual) and with the score on the total motor scale, the Mini-Mental State Examination and the Stroop Interference Test (uni- and bimanual). After 12 months, maximal cycling frequency of unimanual elbow flexion–extension was significantly decreased in HD patients whereas the quality of the in-phase and anti-phase movement patterns remained stable. Electronic Publication     

Are arm trajectories planned in kinematic or dynamic coordinates? An adaptation study

There are several invariant features of pointto-point human arm movements: trajectories tend to be straight, smooth, and have bell-shaped velocity profiles. One approach to accounting for these data is via optimization theory; a movement is specified implicitly as the optimum of a cost function, e.g., integrated jerk or torque change. Optimization models of trajectory planning, as well as models not phrased in the optimization framework, generally fall into two main groups-those specified in kinematic coordinates and those specified in dynamic coordinates. To distinguish between these two possibilities we have studied the effects of artificial visual feedback on planar two-joint arm movements. During self-paced point-to-point arm movements the visual feedback of hand position was altered so as to increase the perceived curvature of the movement. The perturbation was zero at both ends of the movement and reached a maximum at the midpoint of the movement. Cost functions specified by hand coordinate kinematics predict adaptation to increased curvature so as to reduce the visual curvature, while dynamically specified cost functions predict no adaptation in the underlying trajectory planner, provided the final goal of the movement can still be achieved. We also studied the effects of reducing the perceived curvature in transverse movements, which are normally slightly curved. Adaptation should be seen in this condition only if the desired trajectory is both specified in kinematic coordinates and actually curved. Increasing the perceived curvature of normally straight sagittal movements led to significant (P<0.001) corrective adaptation in the curvature of the actual hand movement; the hand movement became curved, thereby reducing the visually perceived curvature. Increasing the curvature of the normally curved transverse movements produced a significant (P<0.01) corrective adaptation; the hand movement became straighter, thereby again reducing the visually perceived curvature. When the curvature of naturally curved transverse movements was reduced, there was no significant adaptation (P>0.05). The results of the curvature-increasing study suggest that trajectories are planned in visually based kinematic coordinates. The results of the curvature-reducing study suggest that the desired trajectory is straight in visual space. These results are incompatible with purely dynamicbased models such as the minimum torque change model. We suggest that spatial perception-as mediated by vision-plays a fundamental role in trajectory planning.     

Proprioceptive control of cyclical bimanual forearm movements across different movement frequencies as revealed by means of tendon vibration

The effect of unilateral tendon vibration on the performance of cyclical bimanual forearm movements was investigated across different cycling frequencies (from 0.67 to 2.53 Hz). The spatiotemporal features of the individual limb motions as well as their coordination were studied. Tendon vibration was found to result in a substantial reduction in the amplitude of the vibrated arm, leaving the nonvibrated arm unaffected. The vibration-induced amplitude reduction decreased from 26% to 11% as cycling frequency increased even though significant reductions were still observed at the highest cycling frequencies. Tendon vibration was also found to result in an increase of the phase lead of the dominant arm with respect to the nondominant arm, but this effect was not modulated by cycling frequency. The data argue in favor of a closed-loop mode of movement control during cyclical high-speed movements. It is suggested that kinesthetic afferent information is processed and used to guide action up to near-maximal movement speeds, reinforcing recent claims with respect to visual information processing. Electronic Publication     

Instabilities during antiphase bimanual movements: are ipsilateral pathways involved?

The spatial and temporal coupling between the hands is known to be very robust during movements which use homologous muscles (in-phase or symmetric movements). In contrast, movements using nonhomologous muscles (antiphase or asymmetric movements) are less stable and exhibit a tendency to undergo a phase transition to in-phase movements as movement frequency increases. The instability during antiphase movements has been modeled in terms of signal interference mediated by the ipsilateral corticospinal pathways. In this study we report that participants in whom distal ipsilateral motor-evoked potentials could be elicited with transcranial magnetic stimulation (TMS), exhibited higher variability during a bimanual circling task than participants whose ipsilateral pathways could not be transcranially activated. These results suggest that ipsilateral control of the limb affects the level of bimanual coupling, and may contribute to uncoupling phenomena observed during asymmetric coordination.     

Contralateral manual compensation for velocity-dependent force perturbations

It is not yet clear how the temporal structure of a voluntary action is coded allowing coordinated bimanual responses. This study focuses on the adaptation to and compensation for a force profile presented to one stationary arm which is proportional to the velocity of the other moving arm. We hypothesised that subjects would exhibit predictive coordinative responses which would co-vary with the state of the moving arm. Our null hypothesis is that they develop a time-dependent template of forces appropriate to compensate for the imposed perturbation. Subjects were trained to make 500 ms duration reaching movements with their dominant right arm to a visual target. A force generated with a robotic arm that was proportional to the velocity of the moving arm and perpendicular to movement direction acted on their stationary left hand, either at the same time as the movement or delayed by 250 or 500 ms. Subjects rapidly learnt to minimise the final end-point error. In the delay conditions, the left hand moved in advance of the onset of the perturbing force. In test conditions with faster or slower movement of the right hand, the predictive actions of the left hand co-varied with movement speed. Compensation for movement-related forces appeared to be predictive but not based on an accurate force profile that was equal and opposite to the imposed perturbation.     

Combined effects of planning and execution constraints on bimanual task performance

In this study we investigated the relative impact of planning and execution constraints on discrete bimanual task performance. In particular, in a bimanual CD-placement task, we compared people’s preference to end movements comfortably with their preference to move symmetrically. In “Experiment 1” we examined the degree of interlimb coupling as participants repositioned two CDs in a CD rack by simultaneously moving their arms mirror-symmetrically or asymmetrically into comfortable or uncomfortable end postures. Interlimb coupling was stronger when the arms moved symmetrically towards uncomfortable end postures. In “Experiment 2” participants were asked to realize specific end orientations of the CDs but they were free to choose an initial grip type and subsequent direction of forearm rotation. Surprisingly, the participants did not move their arms symmetrically but preferred to end in a comfortable posture with their right hand but not with their left hand. We conclude that in discrete bimanual task performance the tendency to end movements in a comfortable posture dominates over the tendency to synchronously activate homologous muscle pairs. The lateralized end-state comfort effect suggests a hemispheric specialization for motor planning.     

Primary motor cortex excitability is modulated with bimanual training

Bimanual visuomotor movement has been shown to enhance cortical motor activity in both hemispheres, especially when movements require simultaneous activation of homologous muscle groups (in-phase movement). It is currently unclear if these adaptations are specific to motor preparatory areas or if they also involve changes in primary motor cortex (M1). The present study investigated the representation of wrist muscles within motor cortex before and following bimanual movement training that was in-phase, anti-phase with or without motor preparation. Motor evoked potentials (MEPs) for the extensor carpi radialis muscle (ECR) cortical territory were acquired and analyzed before and following bimanual movement. The cortical representation was quantified and compared in terms of spatial extent and MEP amplitude, in two different experiments involving distinct movement training types. In Experiment 1, participants performed bimanual wrist flexion/extension movements to targets which involved in-phase movements, either following a 2s preparation period (In-phase preparation), or without the preparation period (In-phase no preparation). In Experiment 2, training involved antagonist muscle groups activated simultaneously (Anti-phase) with the addition of the 2s preparation period. In-phase bimanual movement enhanced the spatial representation of ECR in M1, and did not show a difference in MEP amplitude of the cortical area. It may be that simultaneous activation of homologous M1 representations in both hemispheres, in combination with activity from premotor areas, leads to a greater increase in plasticity in terms of increased M1 spatial extent of trained muscles.     

The role of visual reafferents during a pointing movement: comparative study between open-loop and closed-loop performances in monkeys before and after unilateral electrolytic lesion of the sustantia nigra

Summary In order to elucidate the compensatory role of visual feedback during movement, two experiments were designed to compare the motor performances of Papio papio baboons depending on whether the animals were able to visually control the limb trajectory (visual closed-loop condition) or not (visual open-loop condition). The visuomotor task used consisted of making trained pointing movements towards a stationary target. In experiment A, the baboons were successively presented with these two experimental conditions. The abolition of visual control was found to cause no change in either reaction time (RT) or movement time (MT), but brought about extensive pointing errors. It was also associated with a conspicuous increase in the mean velocity and the mean length of the trajectories. In experiment B, two groups of baboons were used. The monkeys in the first group were required to perform under closed loop conditions. The second group performed the pointing movement under open loop conditions. Once criterion was reached by each animal, a unilateral electrolytic lesion of the substantia nigra (SN) was performed. A comparison between the post operative performances of the animals in the two groups showed that suppression of visual cues resulted in a lengthening of the RT and a slowing of the movement speed. Moreover when visual feedback was lacking, the amplitude of the movement decreased and the finger fell short of the target. During the last post operative period, suppression of visual feedback brought about a more rapid return of RTs to their preoperative level and a more durable slowing of movement speed than with normal vision. The discussion deals with the role of visual feed-back in the control of movement preparation and execution, and with the change in mode of motor control caused by lesion of the SN. Partial exclusion of the SN might bring about a shift from the feedforward to a feedback mode relying more heavily on visual cues.     

Coordination and concurrency in bimanual rotation tasks when moving away from and toward the body

In the present series of experiments we investigated how object transport and rotate movements are performed when they are directed away from (Experiment 1) and toward (Experiment 2) the body under both unimanual and bimanual conditions. Our results indicated that unimanual conditions are faster and more efficiently produced than bimanual movements in far peripersonal space, suggesting that there is a cost to performing bimanual movements. However, in near peripersonal space, bimanual same movements were performed in a manner similar to unimanual movements, indicating that there is no significant cost associated with similar bimanual movements that are performed using the lower visual field and in near peripersonal space. Both experiments also indicate that the two hands are tightly synchronized when the two movements being performed require the same rotation. However, when performing bimanual movements where the rotation being performed by the two hands is different, this synchronization is weaker. Finally, the combined results from the two experiments indicated that movements made toward the body are not performed in a similar manner to movements that are made away from the body. Specifically, it is clear from the current studies that movements toward the body are performed faster and possibly that the hands are less synchronized for bimanual movements requiring different rotations by the two hands.

Latency of saccades and vergence eye movements in dyslexic children

We investigated the ability to adjust to nonlinear transformations that allow people to control external systems like machines and tools. Earlier research (Verwey and Heuer 2007) showed that in the presence of just terminal feedback participants develop an internal model of such transformations that operates at a relatively early processing level (before or at amplitude specification). In this study, we investigated the level of operation of the internal model after practicing with continuous visual feedback. Participants executed rapid aiming movements, for which a nonlinear relationship existed between the target amplitude seen on the computer screen and the required movement amplitude of the hand on a digitizing tablet. Participants adjusted to the external transformation by developing an internal model. Despite continuous feedback, explicit awareness of the transformation did not develop and the internal model still operated at the same early processing level as with terminal feedback. Thus with rapid aiming movements, the type of feedback may not matter for the locus of operation of the internal model.     

Dual transplantation of human neural stem cells into cervical and lumbar cord ameliorates motor neuron disease in SOD1 transgenic rats

During gain adaptation, participants must learn to adapt to novel visuo-motor mappings in which the movement amplitudes they produce do not match the visual feedback they receive. The aim of the present study was to investigate the neural substrates of gain adaptation by examining its possible disruption following left hemisphere stroke. Thirteen chronic left hemisphere stroke patients and five healthy right-handed control subjects completed three experimental phases involving reaching with the left hand, which was the less-affected hand in patients. First, participants reached without visual feedback to six different target locations (baseline phase). Next, in the adaptation phase, participants executed movements to one target under conditions in which the perceived movement distance was 70% of the produced movement distance. Last, in order to test the generalization of this new visuomotor mapping, participants made movements without visual feedback to untrained target locations (generalization phase). Significant between-patient differences were observed during adaptation. Lesion analyses indicated that these between-patient differences were predicted by the amount of damage to the supramarginal gyrus (Brodmann area 40). In addition, patients performed more poorly than controls in the generalization phase, suggesting that different processes are involved in adaptation and generalization periods.