Visuo-motor control of the ipsilateral hand: evidence from right brain-damaged patients

Enhanced efficiency in visually guided online motor control for actions redirected towards the body midline

Reaching objects in a dynamic environment requires fast online corrections that compensate for sudden object shifts or postural changes. Previous studies revealed the key role of visually monitoring the hand-to-target distance throughout action execution. In the current study, we investigate how sensorimotor asymmetries associated with space perception, brain lateralization and biomechanical constraints, affect the efficiency of online corrections. Participants performed reaching actions in virtual reality, where the virtual hand was progressively displaced from the real hand to trigger online corrections, for which it was possible to control the total amount of the redirection and the region of space in which the action unfolded. The efficiency of online corrections and the degree of awareness of the ensuing motor corrections were taken as assessment variables. Results revealed more efficient visuo-motor corrections for actions redirected towards, rather than away from the body midline. The effect is independent on the reaching hand and the hemispace of action, making explanations associated with laterality effects and biomechanical constraints improbable. The result cannot either be accounted for by the visual processing advantage in the straight-ahead region. An explanation may be found in the finer sensorimotor representations characterizing the frontal space proximal to body, where a preference for visual processing has been documented, and where high-value functional actions, like fine manipulative skills, typically take place. This article is part of a discussion meeting issue 'New approaches to 3D vision'.

Interhemispheric interactions between the right angular gyrus and the left motor cortex: a transcranial magnetic stimulation study

The interconnection of the angular gyrus of right posterior parietal cortex (PPC) and the left motor cortex (LM1) is essential for goal-directed hand movements. Previous work with transcranial magnetic stimulation (TMS) showed that right PPC stimulation increases LM1 excitability, but right PPC followed by left PPC-LM1 stimulation (LPPC-LM1) inhibits LM1 corticospinal output compared with LPPC-LM1 alone. It is not clear if right PPC-mediated inhibition of LPPC-LM1 is due to inhibition of left PPC or to combined effects of right and left PPC stimulation on LM1 excitability. We used paired-pulse TMS to study the extent to which combined right and left PPC stimulation, targeting the angular gyri, influences LM1 excitability. We tested 16 healthy subjects in five paired-pulsed TMS experiments using MRI-guided neuronavigation to target the angular gyri within PPC. We tested the effects of different right angular gyrus (RAG) and LM1 stimulation intensities on the influence of RAG on LM1 and on influence of left angular gyrus (LAG) on LM1 (LAG-LM1). We then tested the effects of RAG and LAG stimulation on LM1 short-interval intracortical facilitation (SICF), short-interval intracortical inhibition (SICI), and long-interval intracortical inhibition (LICI). The results revealed that RAG facilitated LM1, inhibited SICF, and inhibited LAG-LM1. Combined RAG-LAG stimulation did not affect SICI but increased LICI. These experiments suggest that RAG-mediated inhibition of LAG-LM1 is related to inhibition of early indirect (I)-wave activity and enhancement of GABA_B receptor-mediated inhibition in LM1. The influence of RAG on LM1 likely involves ipsilateral connections from LAG to LM1 and heterotopic connections from RAG to LM1.NEW & NOTEWORTHY Goal-directed hand movements rely on the right and left angular gyri (RAG and LAG) and motor cortex (M1), yet how these brain areas functionally interact is unclear. Here, we show that RAG stimulation facilitated right hand motor output from the left M1 but inhibited indirect (I)-waves in M1. Combined RAG and LAG stimulation increased GABA_B, but not GABA_A, receptor-mediated inhibition in left M1. These findings highlight unique brain interactions between the RAG and left M1.

Highs and Lows in Motor Control Development

Motor control is classically described as relying on two components: anticipatory control (feedforward processing) and online control (feedback processing). Here we aimed to unveil the developmental steps of both feedback and feedforward control in 5-10 years old children, using a simple and ecological task. We manipulated object's weight in a reach-to-displace paradigm. When the weight was known before lifting it, anticipatory processes were quantifiable during the reaching phase. Conversely, an unknown weight triggered online corrections during the displacing phase. Movement kinematics revealed that children anticipate this objet property as young as 5 y-o. This anticipation becomes adequate around 7 y-o and is paralleled by poor online corrections. This simple yet relevant paradigm should allow quantifying deviations from neurotypical patterns in disorders of motor control.

Deficit of Motor Skill Acquisition on the Upper Limb Ipsilesional to the Injured Hemisphere in Individuals with Stroke

Background

Movement deficits in limbs ipsilesional to the damaged hemisphere in individuals with stroke have been established through various motor tasks. Nevertheless, there has been little evidence regarding hindrance of motor skill acquisition on the ipsilesional limb in patients with stroke. Therefore, we attempted to demonstrate whether the characteristics of ipsilesional deficits involved motor learning insufficiency in stroke survivors with unilateral brain damage.

Material/Methods

Thirty-six participants (18 patients with stroke and 18 normal individuals) were recruited. Patients with stroke performed a visuo-spatial tracking task in the upper limb ipsilesional to the injured hemisphere, and normal participants did the same task with the upper limb matched for the same side. The participants were required to track a target sine wave as accurately as possible while the wave was displayed on the computer screen for 15 seconds. An accuracy index was calculated for each of the trials.

Results

We found that motor skill learning improved in both stroke and normal groups with repetitive practice. However, the normal group exhibited greater motor skill acquisition than in comparison the stroke group for motor skill improvement. Statistical analyses revealed significant differences in time effects and time x group interactions.

Conclusions

Our findings provide evidence that individuals with stroke might have difficulty in performing visuo-spatial movements and acquiring motor skills with the ipsilateral upper limb. Improvement of ipsilesional limb function increases self-care activity in daily life. Therefore, we recommend that clinicians adopt remedial strategies for ipsilesional limbs.

Neglect-Like Effects on Drawing Symmetry Induced by Adaptation to a Laterally Asymmetric Visuomotor Delay

In daily interactions, our sensorimotor system accounts for spatial and temporal discrepancies between the senses. Functional lateralization between hemispheres causes differences in attention and in the control of action across the left and right workspaces. In addition, differences in transmission delays between modalities affect movement control and internal representations. Studies on motor impairments such as hemispatial neglect syndrome suggested a link between lateral spatial biases and temporal processing. To understand this link, we computationally modeled and experimentally validated the effect of laterally asymmetric delay in visual feedback on motor learning and its transfer to the control of drawing movements without visual feedback. In the behavioral experiments, we asked healthy participants to perform lateral reaching movements while adapting to delayed visual feedback in either left, right, or both workspaces. We found that the adaptation transferred to blind drawing and caused movement elongation, which is consistent with a state representation of the delay. However, the pattern of the spatial effect varied between conditions: whereas adaptation to delay in only the left workspace or in the whole workspace caused selective leftward elongation, adaptation to delay in only the right workspace caused drawing elongation in both directions. We simulated arm movements according to different models of perceptual and motor spatial asymmetry in the representation of delay and found that the best model that accounts for our results combines both perceptual and motor asymmetry between the hemispheres. These results provide direct evidence for an asymmetrical processing of delayed visual feedback that is associated with both perceptual and motor biases that are similar to those observed in hemispatial neglect syndrome.

Movement imagery as a predictor of online control in typically developing children

The ability to mentally represent actions is suggested to play a role in the online control of movement in healthy adults. Children's movement imagery ability and online control have been shown to develop at similar nonlinear rates. The current study investigated the relationship between movement imagery and online control in children by comparing implicit and explicit movement imagery measures with the ability to make online trajectory corrections. Imagery ability was a significant predictor of children's online control of movement once general reaching efficiency was controlled for. These findings extend the proposed relationship between movement imagery and online control.

Online Control of Prehension Predicts Performance on a Standardized Motor Assessment Test in 8- to 12-Year-Old Children

Goal-directed hand movements are guided by sensory information and may be adjusted 'online,' during the movement. If the target of a movement unexpectedly changes position, trajectory corrections can be initiated in as little as 100 ms in adults. This rapid visual online control is impaired in children with developmental coordination disorder (DCD), and potentially in other neurodevelopmental conditions. We investigated the visual control of hand movements in children in a 'center-out' double-step reaching and grasping task, and examined how parameters of this visuomotor control co-vary with performance on standardized motor tests often used with typically and atypically developing children. Two groups of children aged 8-12 years were asked to reach and grasp an illuminated central ball on a vertically oriented board. On a proportion of trials, and at movement onset, the illumination switched unpredictably to one of four other balls in a center-out configuration (left, right, up, or down). When the target moved, all but one of the children were able to correct their movements before reaching the initial target, at least on some trials, but the latencies to initiate these corrections were longer than those typically reported in the adult literature, ranging from 211 to 581 ms. These later corrections may be due to less developed motor skills in children, or to the increased cognitive and biomechanical complexity of switching movements in four directions. In the first group (n = 187), reaching and grasping parameters significantly predicted standardized movement scores on the MABC-2, most strongly for the aiming and catching component. In the second group (n = 85), these same parameters did not significantly predict scores on the DCDQ'07 parent questionnaire. Our reaching and grasping task provides a sensitive and continuous measure of movement skill that predicts scores on standardized movement tasks used to screen for DCD.

Coupling of online control and inhibitory systems in children with atypical motor development: A growth curve modelling study

INTRODUCTION

Previous research indicates that children with Developmental Coordination Disorder (DCD) show deficits performing online corrections, an issue exacerbated by adding inhibitory constraints; however, cross-sectional data suggests that these deficits may reduce with age. Using a longitudinal design, the aim of the study presented here was to model the coupling that occurs between inhibitory systems and (predictive) online control in typically developing children (TDC) and in those with Developmental Coordination Disorder (DCD) over an extended period of time, using a framework of interactive specialization. We predicted that TDC would show a non-linear growth pattern, consistent with re-organisation in the coupling during the middle childhood period, while DCD would display a developmental lag.

METHOD

A group of 196 children (111 girls and 85 boys) aged between 6 and 12years participated in the study. Children were classified as DCD according to research criteria. Using a cohort sequential design, both TDC and DCD groups were divided into age cohorts. Predictive (online) control was defined operationally by performance on a Double-Jump Reaching Task (DJRT), which was assessed at 6-month intervals over two years (5 time points in total). Inhibitory control was examined using an anti-jump condition of the DJRT paradigm whereby children were instructed to touch a target location in the hemispace opposite a cued location.

RESULTS

For the TDC group, model comparison using growth curve analysis revealed that a quadratic trend was the most appropriate fit with evidence of rapid improvement in anti-reach performance up until middle childhood (around 8-9years of age), followed by a more gradual rate of improvement into late childhood and early adolescence. This pattern was evident on both chronometric and kinematic measures. In contrast, for children with DCD, a linear function provided the best to fit on the key metrics, with a slower rate of improvement than controls.

CONCLUSION

We conclude that children with DCD require a more extended period of development to effectively couple online motor control and executive systems when completing anti-reach movements, whereas TDC show rapid improvement in early and middle childhood. These group differences in growth curves are likely to reflect a maturational lag in the development of motor-cognitive networks in children with DCD.

Does hemispheric lateralization influence therapeutic effects of transcranial direct current stimulation?

This study investigated the effect of transcranial direct current stimulation (tDCS) polarity depending on lateralized function of task property in normal individuals performing visuomotor and simple repetitive tasks. Thirty healthy participants with no neurological disorders were recruited to participate in this study. Participants were randomly allocated into active or control condition. For the active condition, tDCS intensity was 2 mA with stimulation applied for 15 minutes to the right hemisphere (tDCS condition). For the sham control, electrodes were placed in the same position, but the stimulator was turned off after 30 seconds (sham condition). The tapping and tracking task tests were performed before and after for both conditions. Univariate analysis revealed significant difference only in the tracking task. For direct comparison of both tasks within each group, the tracking task had significantly higher Z score than the tapping task in the tDCS group (P < 0.05). Thus, our study indicates that stimulation of the right hemisphere using tDCS can effectively improve visuomotor (tracking) task over simple repetitive (tapping) task.

Reduced motor imagery efficiency is associated with online control difficulties in children with probable developmental coordination disorder

Recent evidence indicates that the ability to correct reaching movements in response to unexpected target changes (i.e., online control) is reduced in children with developmental coordination disorder (DCD). Recent computational modeling of human reaching suggests that these inefficiencies may result from difficulties generating and/or monitoring internal representations of movement. This study was the first to test this putative relationship empirically. We did so by investigating the degree to which the capacity to correct reaching mid-flight could be predicted by motor imagery (MI) proficiency in a sample of children with probable DCD (pDCD). Thirty-four children aged 8 to 12 years (17 children with pDCD and 17 age-matched controls) completed the hand rotation task, a well-validated measure of MI, and a double-step reaching task (DSRT), a protocol commonly adopted to infer one's capacity for correcting reaching online. As per previous research, children with pDCD demonstrated inefficiencies in their ability to generate internal action representations and correct their reaching online, demonstrated by inefficient hand rotation performance and slower correction to the reach trajectory following unexpected target perturbation during the DSRT compared to age-matched controls. Critically, hierarchical moderating regression demonstrated that even after general reaching ability was controlled for, MI efficiency was a significant predictor of reaching correction efficiency, a relationship that was constant across groups. Ours is the first study to provide direct pilot evidence in support of the view that a decreased capacity for online control of reaching typical of DCD may be associated with inefficiencies generating and/or using internal representations of action.

Rapid On-Line Control to Reaching Is Preserved in Children With Congenital Spastic Hemiplegia: Evidence From Double-Step Reaching Performance

This study aimed to investigate the integrity of on-line control of reaching in congenital spastic hemiplegia in light of disparate evidence. Twelve children with and without spastic hemiplegia (11-17 years old) completed a double-step reaching task requiring them to reach and touch a target that remained stationary for most trials (viz nonjump trial) but unexpectedly displaced laterally at movement onset for a minority of trials (20%: known as jump trials). Although children with spastic hemiplegia were generally slower than age-matched controls, they could account for target perturbation at age-appropriate levels shown by a lack of interaction effect on movement time and nonsignificant group difference for time to reach trajectory correction on jump trials. Our data suggest that at a group level, on-line control of reaching may be age-appropriate in spastic hemiplegia. However, our data also highlight the need to experimentally acknowledge the considerable heterogeneity of the spastic hemiplegia population when investigating motor cognition.

Coupling online control and inhibitory systems in children with Developmental Coordination Disorder: Goal-directed reaching

For children with Developmental Coordination Disorder (DCD), the real-time coupling between frontal executive function and online motor control has not been explored despite reported deficits in each domain. The aim of the present study was to investigate how children with DCD enlist online control under task constraints that compel the need for inhibitory control. A total of 129 school children were sampled from mainstream primary schools. Forty-two children who met research criteria for DCD were compared with 87 typically developing controls on a modified double-jump reaching task. Children within each skill group were divided into three age bands: younger (6-7 years), mid-aged (8-9), and older (10-12). Online control was compared between groups as a function of trial type (non-jump, jump, anti-jump). Overall, results showed that while movement times were similar between skill groups under simple task constraints (non-jump), on perturbation (or jump) trials the DCD group were significantly slower than controls and corrected trajectories later. Critically, the DCD group was further disadvantaged by anti-jump trials where inhibitory control was required; however, this effect reduced with age. While coupling online control and executive systems is not well developed in younger and mid-aged children, there is evidence of age-appropriate coupling in older children. Longitudinal data are needed to clarify this intriguing finding. The theoretical and applied implications of these results are discussed.

The left side of motor resonance

Motor resonance is defined as the internal activation of an observer's motor system, specifically attuned to the perceived movement. In social contexts, however, different patterns of observed and executed muscular activation are frequently required. This is the case, for instance, of seeing a key offered with a precision grip and received by opening the hand. Novel evidence suggests that compatibility effects in motor resonance can be altered by social response preparation. What is not known is how handedness modulates this effect. The present study aimed at determining how a left- and a right-handed actor grasping an object and then asking for a complementary response influences corticospinal activation in left- and right-handers instructed to observe the scene. Transcranial magnetic stimulation (TMS)-induced motor evoked potentials (MEPs) were thus recorded from the dominant hands of left- and right-handers. Interestingly, requests posed by the right-handed actor induced a motor activation in the participants' respective dominant hands, suggesting that left-handers tend to mirror right-handers with their most efficient hand. Whereas requests posed by the left-handed actor activated the anatomically corresponding muscles (i.e., left hand) in all the participants, right-handers included. Motor resonance effects classically reported in the literature were confirmed when observing simple grasping actions performed by the right-handed actor. These findings indicate that handedness influences both congruent motor resonance and complementary motor preparation to observed actions.

Intermanual transfer and proprioceptive recalibration following training with translated visual feedback of the hand

Reaching with visual feedback that is misaligned with respect to the actual hand's location leads to changes in reach trajectories (i.e., visuomotor adaptation). Previous studies have also demonstrated that when training to reach with misaligned visual feedback of the hand, the opposite hand also partially adapts, providing evidence of intermanual transfer. Moreover, our laboratory has shown that visuomotor adaptation to a misaligned hand cursor, either translated or rotated relative to the hand, also leads to changes in felt hand position (what we call proprioceptive recalibration), such that subjects' estimate of felt hand position relative to both visual and non-visual reference markers (e.g., body midline) shifts in the direction of the visuomotor distortion. In the present study, we first determined the extent that motor adaptation to a translated cursor leads to transfer to the opposite hand, and whether this transfer differs across the dominant and non-dominant hands. Second, we looked to establish whether changes in hand proprioception that occur with the trained hand following adaptation also transfer to the untrained hand. We found intermanual motor transfer to the left untrained (non-dominant) hand after subjects trained their right (dominant) hand to reach with translated visual feedback of their hand. Motor transfer from the left trained to the right untrained hand was not observed. Despite finding changes in felt hand position in both trained hands, we did not find similar evidence of proprioceptive recalibration in the right or left untrained hands. Taken together, our results suggest that unlike visuomotor adaptation, proprioceptive recalibration does not transfer between hands and is specific only to the arm exposed to the distortion.

Does implicit motor imagery ability predict reaching correction efficiency? A test of recent models of human motor control

Neurocomputational models of reaching indicate that efficient purposive correction of movement midflight (e.g., online control) depends on one's ability to generate and monitor an accurate internal (neural) movement representation. In the first study to test this empirically, the authors investigated the relationship between healthy young adults' implicit motor imagery performance and their capacity to correct their reaching trajectory. As expected, after controlling for general reaching speed, hierarchical regression demonstrated that imagery ability was a significant predictor of hand correction speed; that is, faster and more accurate imagery performance associated with faster corrections to reaching following target displacement at movement onset. They argue that these findings provide preliminary support for the view that a link exists between an individual's ability to represent movement mentally and correct movement online efficiently.

Tonal cues modulate line bisection performance: preliminary evidence for a new rehabilitation prospect?

The effect of the presentation of two different auditory pitches (high and low) on manual line-bisection performance was studied to investigate the relationship between space and magnitude representations underlying motor acts. Participants were asked to mark the midpoint of a given line with a pen while they were listening a pitch via headphones. In healthy participants, the effect of the presentation order (blocked or alternative way) of auditory stimuli was tested (Experiment 1). The results showed no biasing effect of pitch in blocked-order presentation, whereas the alternative presentation modulated the line-bisection. Lower pitch produced leftward or downward bisection biases whereas higher pitch produced rightward or upward biases, suggesting that visuomotor processing can be spatially modulated by irrelevant auditory cues. In Experiment 2, the effect of such alternative stimulations in line bisection in right brain damaged patients with a unilateral neglect and without a neglect was tested. Similar biasing effects caused by auditory cues were observed although the white noise presentation also affected the patient's performance. Additionally, the effect of pitch difference was larger for the neglect patient than for the no-neglect patient as well as for healthy participants. The neglect patient's bisection performance gradually improved during the experiment and was maintained even after 1 week. It is therefore, concluded that auditory cues, characterized by both the pitch difference and the dynamic alternation, influence spatial representations. The larger biasing effect seen in the neglect patient compared to the no-neglect patient and healthy participants suggests that auditory cues could modulate the direction of the attentional bias that is characteristic of neglect patients. Thus, the alternative presentation of auditory cues could be used as rehabilitation for neglect patients. The space-pitch associations are discussed in terms of a generalized magnitude system.

Terminal, but not concurrent prism exposure produces perceptual aftereffects in healthy young adults

A short period of prism adaptation (PA) has been shown to reduce spatial neglect symptoms. Recent evidence suggests that the positive effects of PA might be restricted to visually guided actions, with PA having little effect on perception. However, the majority of studies have adopted a concurrent exposure technique that fosters the development of a change in felt arm position (proprioceptive straight ahead, PSA). Few studies have used terminal exposure that promotes a change in the perceived visual direction (visual straight ahead, VSA). The positive effects of PA might appear to be primarily action based because studies have adopted an exposure technique that promotes a change in proprioception. Here, we compare the effects of the two exposure types on a perceptual and a manual line bisection task in healthy young adults. Before and after seven minutes of exposure to leftward displacing prisms we measured performance on two line bisection tasks (manual and perceptual) and perceived straight ahead (PSA and VSA). During the exposure period participants made pointing movements while the view of their pointing arm was either (i) restricted to the second half of the pointing movement (concurrent exposure) or (ii) restricted to the final part of the pointing movement (terminal exposure). In line with the previous research, concurrent exposure produced a large shift in PSA and a shift on the manual line bisection task. Interestingly, terminal exposure produced a large shift in VSA and a shift in performance on the perceptual line bisection task. Our results shed light on the underlying mechanisms of prism-induced neglect recovery and help to address an apparent discrepancy within the literature.

Prism adaptation therapy enhances rehabilitation of stroke patients with unilateral spatial neglect: a randomized, controlled trial

BACKGROUND

and objective. Unilateral spatial neglect (USN) can interfere with rehabilitation processes and lead to poor functional outcome. The purpose of this study was to determine whether prism adaptation (PA) therapy improves USN and functional outcomes in stroke patients in the subacute stage.

METHODS

. A multicenter, double-masked, randomized, controlled trial was conducted to evaluate the effects of a 2-week PA therapy on USN assessed with the Behavioral Inattention Test (BIT), the Catherine Bergego Scale (CBS), and activities of daily living (ADL) as evaluated with the Functional Independence Measure (FIM). A total of 38 USN patients with right-brain damage were divided into prism (n = 20) and control (n = 18) groups. Patients were divided into mild and severe USN groups according to BIT behavioral test (mild ≥ 55 and severe<55). The prism group performed repetitive pointing with prism glasses that induce rightward optical shift twice daily, 5 days per week, for 2 weeks, whereas the control group performed similar pointing training with neutral glasses.

RESULTS

. The FIM improved significantly more in the prism group. In mild USN patients, there was significantly greater improvement of BIT and FIM in the prism group.

CONCLUSIONS

. PA therapy can significantly improve ADL in patients with subacute stroke.

Online motor control in children with developmental coordination disorder: chronometric analysis of double-step reaching performance

BACKGROUND

Although there are a number of plausible accounts to explain movement clumsiness in children [or developmental coordination disorder (DCD)], the cause(s) of the disorder remain(s) an issue of debate. One aspect of motor control that is particularly important to the fluid expression of skill is rapid online control (ROC). Data on DCD have been conflicting. While some recent work using double-step reaching suggests no difficulty in online control, others suggest deficits (e.g. based on sequential pointing). To help resolve this debate, we suggest two things: use of recent neuro-computational models as a framework for investigating motor control in DCD, and more rigorous investigation of double-step reaching. Our working assumption here is that ROC is only viable through the seamless integration of predictive (or forward) models of movement and feedback-based mechanisms.

AIM

The aim of this chronometric study was to explore ROC in children with DCD using a double-step reaching paradigm. We predicted slower online adjustments in DCD based on the argument that these children manifest a core difficulty in predictive control.

METHODS

Participants were a group of 17 children with DCD and 27 typically developing children aged between 7 and 12 years. Visual targets were presented on a 17-inch LCD touch screen, inclined to an angle of 15° from horizontal. The children were instructed to press each target as it appeared as quickly and accurately as possible. For 80% of the trials, the central target location remained unchanged for the duration of the movement (non-jump trials), while for the remaining 20% of trials, the target jumped at movement onset to one of the two peripheral locations (jump trials). Reaction time (RT), movement time (MT) and reaching errors were recorded.

RESULTS

For both groups, RT did not vary according to trial condition, while children with DCD were slower to initiate movement. Further, the MT of children with DCD was prolonged to a far greater extent on jump trials relative to controls, with a large effect size. As well, children with DCD committed significantly more errors, notably a reduced ability to inhibit central responses on jump trials.

CONCLUSION

Our findings help reconcile some disparate findings in the literature using similar tasks. The pattern of performance in children with DCD suggests impairment in the ability to make rapid online adjustments that are based on a predictive (or internal) model of the action. These results pave the way for future kinematic investigation.

Prism adaptation does not change the rightward spatial preference bias found with ambiguous stimuli in unilateral neglect

Previous research has shown that prism adaptation (prism adaptation) can ameliorate several symptoms of spatial neglect after right-hemisphere damage. But the mechanisms behind this remain unclear. Recently we reported that prisms may increase leftward awareness for neglect in a task using chimeric visual objects, despite apparently not affecting awareness in a task using chimeric emotional faces (Sarri et al., 2006). Here we explored potential reasons for this apparent discrepancy in outcome, by testing further whether the lack of a prism effect on the chimeric face task task could be explained by: i) the specific category of stimuli used (faces as opposed to objects); ii) the affective nature of the stimuli; and/or iii) the particular task implemented, with the chimeric face task requiring forced-choice judgements of lateral 'preference' between pairs of identical, but left/right mirror-reversed chimeric face tasks (as opposed to identification for the chimeric object task). We replicated our previous pattern of no impact of prisms on the emotional chimeric face task here in a new series of patients, while also similarly finding no beneficial impact on another lateral 'preference' measure that used non-face non-emotional stimuli, namely greyscale gradients. By contrast, we found the usual beneficial impact of prism adaptation (prism adaptation) on some conventional measures of neglect, and improvements for at least some patients in a different face task, requiring explicit discrimination of the chimeric or non-chimeric nature of face stimuli. The new findings indicate that prism therapy does not alter spatial biases in neglect as revealed by 'lateral preference tasks' that have no right or wrong answer (requiring forced-choice judgements on left/right mirror-reversed stimuli), regardless of whether these employ face or non-face stimuli. But our data also show that prism therapy can beneficially modulate some aspects of visual awareness in spatial neglect not only for objects, but also for face stimuli, in some cases.

Impairment severity selectively affects the control of proximal and distal components of reaching movements in children with hemiplegic cerebral palsy

This study explored proximal-to-distal components during goal-directed reaching movements in children with mild or moderate hemiplegic cerebral palsy (HCP); [seven females, four males; mean age 8 y 6 mo; SD 27 mo], compared with age-matched, typically developing children (seven females, five males; mean age 8 y 3 mo [SD 25 mo]. Severity of HCP was assessed following the approach of Claeys et al. Optoelectronic registrations were made during unimanual reaching-to-grasp and reaching-to-hit movements with both the affected/non-preferred and unaffected/preferred side. Regardless of task, the children with HCP, particularly those with moderate impairment, displayed less optimal spatiotemporal organization of movements performed with the affected arm. Compared with the goal to hit, and increasingly with more severe impairment, children with HCP adapted to the goal to grasp by recruiting augmented shoulder movements when reaching with the affected side. A resulting impact on distal kinematics was found in shorter, straighter, and less segmented movement paths. Thus, depending on severity of hemispheric lesions and task complexity, unilateral brain injuries in HCP may selectively affect neural pathways underlying both proximal and distal arm movement control. Levels of both ipsi- and contralateral activation in relation to side and lesion severity should be considered in future studies on prehension movements in HCP.

Implications of prism adaptation asymmetry for unilateral visual neglect: theoretical note

Asymmetry in intermanual transfer of proprioceptive and visual prism adaptation is reviewed, which suggests asymmetric hemispheric representation of left and right space, directional connection from right to left visual hemispheres, and lateralization of limb motor control. Damage to the right visual hemisphere source of the directional connection could produce the general features of unilateral visual neglect.

Hemisphere Specific Impairments in Reach-to-Grasp Control After Stroke: Effects of Object Size

Background and objective. The authors investigated hemispheric specialization for the visuomotor transformation of grasp preshaping and the coordination between transport and grasp in individuals poststroke. Based on a bilateral model, the authors hypothesized that after unilateral stroke there would be hemisphere-specific deficits revealed by the ipsilesional limb. Methods. Right or left stroke and age- and limb-matched nondisabled participants performed rapid reach-to-grasp of 3 sized objects. The authors quantified grasp preshaping as the correlation between initial aperture velocity and peak aperture, and peak aperture and object diameter. A cross correlation analysis using transport velocity and aperture size was performed to quantify transport-grasp coordination. All statistical tests for hemisphere-specific deficits involved comparisons between each stroke group and the matched nondisabled group. Results. Overall, the right stroke group, but not left stroke group, demonstrated prolonged movement time. For grasp preshaping there was a higher correlation between initial aperture velocity and peak aperture for the right stroke group and a lower correlation between peak aperture and object diameter for the left stroke group. For transport-grasp coordination the correlation between transport velocity and aperture size was higher for the left stroke group and lower for the right stroke group, which also demonstrated a higher standard deviation of time lag. Conclusions. After left stroke, there was deficient scaling of grasp preshaping and stronger transport-grasp coordination. In contrast, after right stroke, grasp preshaping began earlier and transport-grasp coordination was weaker. Together, these hemisphere-specific deficits suggest a left hemisphere specialization for the visuomotor transformation of grasp preshaping and a right hemisphere specialization for transport-grasp coordination.

Effects of roll visual motion on online control of arm movement: reaching within a dynamic virtual environment

Reaching toward a visual target involves the transformation of visual information into appropriate motor commands. Complex movements often occur either while we are moving or when objects in the world move around us, thus changing the spatial relationship between our hand and the space in which we plan to reach. This study investigated whether rotation of a wide field-of-view immersive scene produced by a virtual environment affected online visuomotor control during a double-step reaching task. A total of 20 seated healthy subjects reached for a visual target that remained stationary in space or unpredictably shifted to a second position (either to the right or left of its initial position) with different inter-stimulus intervals. Eleven subjects completed two experiments which were similar except for the duration of the target's appearance. The final target was either visible throughout the entire trial or only for a period of 200 ms. Movements were performed under two visual field conditions: the virtual scene was matched to the subject's head motion or rolled about the line of sight counterclockwise at 130 degrees/s. Nine additional subjects completed a third experiment in which the direction of the rolling scene was manipulated (i.e., clockwise and counterclockwise). Our results showed that while all subjects were able to modify their hand trajectory in response to the target shift with both visual scenes, some of the double-step movements contained a pause prior to modifying trajectory direction. Furthermore, our findings indicated that both the timing and kinematic adjustments of the reach were affected by roll motion of the scene. Both planning and execution of the reach were affected by roll motion. Changes in proportion of trajectory types, and significantly longer pauses that occurred during the reach in the presence of roll motion suggest that background roll motion mainly interfered with the ability to update the visuomotor response to the target displacement. Furthermore, the reaching movement was affected differentially by the direction of roll motion. Subjects demonstrated a stronger effect of visual motion on movements taking place in the direction of visual roll (e.g., leftward movements during counterclockwise roll). Further investigation of the hand path revealed significant changes during roll motion for both the area and shape of the 95% tolerance ellipses that were constructed from the hand position following the main movement termination. These changes corresponded with a hand drift that would suggest that subjects were relying more on proprioceptive information to estimate the arm position in space during roll motion of the visual field. We conclude that both the spatial and temporal kinematics of the reach movement were affected by the motion of the visual field, suggesting interference with the ability to simultaneously process two consecutive stimuli.

Cortical activations in humans grasp-related areas depend on hand used and handedness

BACKGROUND

In non-human primates grasp-related sensorimotor transformations are accomplished in a circuit involving the anterior intraparietal sulcus (area AIP) and both the ventral and the dorsal sectors of the premotor cortex (vPMC and dPMC, respectively). Although a human homologue of such a circuit has been identified whether activity within this circuit varies depending on handedness has yet to be investigated.

METHODOLOGY/PRINCIPAL FINDINGS

We used functional magnetic resonance imaging (fMRI) to explicitly test how handedness modulates activity within human grasping-related brain areas. Right- and left-handers subjects were requested to reach towards and grasp an object with either the right or the left hand using a precision grip while scanned. A kinematic study was conducted with similar procedures as a behavioral counterpart for the fMRI experiment. Results from a factorial design revealed significant activity within the right dPMC, the right cerebellum and AIP bilaterally. The pattern of activity within these areas mirrored the results found for the behavioral study.

CONCLUSION/SIGNIFICANCE

Data are discussed in terms of an handedness-independent role for the right dPMC in monitoring hand shaping, the need for bilateral AIP activity for the performance of precision grip movements which varies depending on handedness and the involvement of the cerebellum in terms of its connections with AIP. These results provide the first compelling evidence of specific grasping related neural activity depending on handedness.

Coordination Between Reaching and Grasping in Patients With Hemiparesis and Healthy Subjects

OBJECTIVE

To investigate the coordination of reach-to-grasp components in hemiparetic and healthy subjects.

DESIGN

Split-plot repeated-measures design with 3 factors (group, object size, movement speed).

SETTING

Movement laboratory.

PARTICIPANTS

Twelve hemiparetic and 12 age-matched healthy subjects.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

We used motion analysis to collect information on the kinematic variables of movement duration, peak velocity, peak deceleration, and maximum aperture, and the time of peak velocity, peak deceleration, and maximum aperture expressed as a percentage of movement duration during 32 reaching movements by each subject. We examined the coordination between the 2 components in 2 ways. First, we investigated the correlation between time of hand opening and start of hand transport, and between time of maximum aperture and time of peak deceleration. Second, we compared movements at preferred and fast speeds (manipulation of transport component) and to 2 different-sized cups (manipulation of grasp component).

RESULTS

Both groups demonstrated a temporal coupling between grasp and transport components at the start of the reach and at the time of maximum aperture. Both groups increased the aperture of grasp for larger cups and increased the maximum grip aperture, and had a shorter deceleration phase for faster movements. The deceleration phase of the hemiparetic patients was longer than that of the healthy subjects, however, and the components were not as tightly coupled.

CONCLUSIONS

The hemiparetic patients, who had a moderate amount of functional recovery, were similar to healthy subjects in their ability to control reach-to-grasp components. Their performance was not as skilled, however.

Reaching within a dynamic virtual environment

BACKGROUND

Planning and execution of reaching requires a series of computational processes that involve localization of both the target and initial arm position, and the translation of this spatial information into appropriate motor commands that bring the hand to the target. We have investigated the effects of shifting the visual field on visuomotor control using a virtual visual environment in order to determine how changes in visuo-spatial relations alter motor planning during a reach.

METHODS

Five healthy subjects were seated in front of an immersive, stereo virtual scene while reaching for a visual target that remained stationary in space or unpredictably shifted to a second position (either to the right or left of the first target) with different inter-stimulus intervals. Motion of the scene either matched the motion of their head or was rotated counter clockwise at 130 deg/s in the roll plane.

RESULTS

Initial results suggested that both the temporal and spatial aspects of reaching were affected by a rolling visual field. Subjects were able to amend ongoing motion to match target position regardless of scene motion, but the presence of visual field motion produced significantly longer pauses during the reach movement when the target was shifted in space. In addition, terminal arm posture exhibited a drift in the direction opposite to the roll motion.

CONCLUSION

These findings suggest that roll motion of the visual field of view interfered with the ability to imultaneously process two consecutive stimuli. Observed changes in arm position following the termination of the reach suggest that subjects were compensating for a perceived change in their visual reference frame.

Cortical processing of visual motion in young infants

High-density EEG was used to investigate the cortical processing of a rotating visual pattern in 2-, 3-, and 5-month-old infants and in adults. Motion induced ERP in the parietal and the temporal-occipital border regions (OT) was elicited at all ages. The ERP was discernable in the 2-months-olds, significant and unilateral in the 3-month-olds and significantly bilateral in the 5-month-olds and adults. The motion induced ERP in the primary visual area was absent in the 2-month-olds and later than in the OT area for the 3-month-olds indicating that information to OT may be supplied by the V1 bypass at these ages. The results are in agreement with behavioural and psychophysical data in infants.

Simulation modifies prehension: evidence for a conjoined representation of the graspable features of an object and the action of grasping it

Movement formulas, engrams, kinesthetic images and internal models of the body in action are notions derived mostly from clinical observations of brain-damaged subjects. They also suggest that the prehensile geometry of an object is integrated in the neural circuits and includes the object's graspable characteristics as well as its semantic properties. In order to determine whether there is a conjoined representation of the graspable characteristics of an object in relation to the actual grasping, it is necessary to separate the graspable (low-level) from the semantic (high-level) properties of the object. Right-handed subjects were asked to grasp and lift a smooth 300-g cylinder with one hand, before and after judging the level of difficulty of a “grasping for pouring” action, involving a smaller cylinder and using the opposite hand. The results showed that simulated grasps with the right hand exert a direct influence on actual motor acts with the left hand. These observations add to the evidence that there is a conjoined representation of the graspable characteristics of the object and the biomechanical constraints of the arm.

Kinematic analysis of unimanual reaching and grasping movements in children with hemiplegic cerebral palsy

BACKGROUND

Kinematic investigations of prehension movements made by children with cerebral palsy are few and mainly focus on the reaching phase. To increase understandings of how both reaching and grasping movements are organized in these children, the aim was to investigate their spatiotemporal characteristics and additionally, how different kinematic parameters are related to the level of hand functions.

METHODS

Eleven children with hemiplegic cerebral palsy; categorized as "mild" and "moderate", in their age of 5.8-12 years and eleven typically developing, "control" children participated. Reaching and grasping made with both the affected/non-preferred and unaffected/preferred sides were recorded and several kinematic parameters investigated.

FINDINGS

In comparison to the control and the mild hemiplegic children, the moderate children exhibited more segmented reaches, longer reach- and grasp durations, and with no anticipatory shaping of the fingers when grasping with the affected hand. Their reaching path with the "unaffected" hand was also more segmented. The mild hemiplegic children showed ability for anticipatory hand closure and performed reaches with similar duration and trajectory as controls. The velocity at hand-object-contact and the quality of their grasping was however affected in comparison to the controls.

INTERPRETATION

Findings from the investigated kinematics provide evidence of how spatiotemporal organization of reaching and grasping are affected in children with hemiplegia and related to the level of impaired hand functions. The outcomes clearly show that if these children are treated as one homogeneous group much information about their prehension capacity described from the kinematics will be concealed. Thus, observations of importance for developing effective, individual interventions adjusted to different level of impairments.

Leg orientation as a clinical sign for pusher syndrome

Background

Effective control of (upright) body posture requires a proper representation of body orientation. Stroke patients with pusher syndrome were shown to suffer from severely disturbed perception of own body orientation. They experience their body as oriented 'upright' when actually tilted by nearly 20° to the ipsilesional side. Thus, it can be expected that postural control mechanisms are impaired accordingly in these patients. Our aim was to investigate pusher patients' spontaneous postural responses of the non-paretic leg and of the head during passive body tilt.

Methods

A sideways tilting motion was applied to the trunk of the subject in the roll plane. Stroke patients with pusher syndrome were compared to stroke patients not showing pushing behaviour, patients with acute unilateral vestibular loss, and non brain damaged subjects.

Results

Compared to all groups without pushing behaviour, the non-paretic leg of the pusher patients showed a constant ipsiversive tilt across the whole tilt range for an amount which was observed in the non-pusher subjects when they were tilted for about 15° into the ipsiversive direction.

Conclusion

The observation that patients with acute unilateral vestibular loss showed no alterations of leg posture indicates that disturbed vestibular afferences alone are not responsible for the disordered leg responses seen in pusher patients. Our results may suggest that in pusher patients a representation of body orientation is disturbed that drives both conscious perception of body orientation and spontaneous postural adjustment of the non-paretic leg in the roll plane. The investigation of the pusher patients' leg-to-trunk orientation thus could serve as an additional bedside tool to detect pusher syndrome in acute stroke patients.

Prism adaptation changes perceptual awareness for chimeric visual objects but not for chimeric faces in spatial neglect after right-hemisphere stroke

Prism adaptation can ameliorate some symptoms of left spatial neglect after right-hemisphere stroke. The mechanisms behind this remain unclear. Prism therapy may increase exploration towards the contralesional side, yet without improving perceptual awareness, as apparently for the left side of chimeric face stimuli (Ferber et al. 2003). However, other prism studies suggest that perceptual awareness might be improved (e.g., Maravita et al., 2003). We tested the impact of prism therapy on visual awareness for the left side of chimeric objects as well as chimeric faces, in three neglect patients. Prism therapy dramatically improved awareness for the identity of the left side of chimeric non-face objects, but had no effect on judging expressions for chimeric faces. The latter may thus be unique in showing no prism benefit.

Impaired grip force modulation in the ipsilesional hand after unilateral middle cerebral artery stroke

Understanding grasping control after stroke is important for relearning motor skills. The authors examined 10 individuals (5 males; 5 females; ages 32-86) with chronic unilateral middle cerebral artery (MCA) stroke (4 right lesions; 6 left lesions) when lifting a novel test object using skilled precision grip with their ipsilesional ("unaffected") hand compared to healthy controls (n = 14; 6 males; 8 females; ages 19-86). All subjects possessed normal range of motion, cutaneous sensation, and proprioception in the hand tested and had no apraxia or cognitive deficits. Subjects lifted the object 10 times at each object weight (260 g, 500 g, 780 g) using a moderately paced self-selected lifting speed. The normal horizontal ("grip") force and vertical tangential ("lift") force were separately measured at the thumb and index finger. Regardless of the object weight or stroke location, the stroke group generated greater grip forces at liftoff of the object (> or =39%; P < or = 0.05) and across the dynamic (P < or = 0.05) and static portions (P < or = 0.05) of the lifts compared to the healthy group. Peak lift forces were equivalent between groups, suggesting accurate load force information processing occurred. These results warrant further investigation of altered sensorimotor processing or compensatory biomechanical strategies that may lead to inaccurate grip force execution after strokes.