Arm-trunk coordination in the absence of proprioception

Adapted Training to Boost Upper Body Sensorimotor Control and Daily Living Functionality in Visually Impaired Baseball Players

Background and Objectives: Vision significantly contributes to postural control, balance, coordination, and body kinematics, thus deeply influencing everyday functionality. Sight-impaired subjects often show upper body anatomofunctional and kinetic chain alterations negatively impacting daily living efficiency and autonomy. The present study aimed to investigate and train, for the first time, upper body sensorimotor control in an Italian blind baseball team to boost global and segmental functionality while contemporarily prevent injuries. Materials and Methods: The whole team underwent a validated test battery using both quantitative traditional tools, such as goniometric active range of motion and muscular/functional tests, and an innovative biofeedback-based device, a Libra proprioceptive board. Consequently, a 6-week adapted training protocol was designed and leaded to improve sensorimotor control and, hence, counteract disability-related deficits and sport-specific overuse syndromes. Results: Statistically significant improvements were observed in all the investigated parameters. Noteworthy, an overall boost of global and segmental stability was detected through an orthostatic dynamic balance enhancement during the Y Balance test (p = 0.01) and trunk multiplanar control improvement on the Libra board (p = 0.01). Concurrently, the comparison of baseline vs. post-intervention outcomes revealed a consistent increase in upper body mobility (p < 0.05 for all the assessed districts), core recruitment (p = 0.01 for all the administered functional tests), and proprioceptive postural control (p = 0.01 for the Libra board validated test). Conclusions: Our findings suggest that a tailored sensorimotor training, conceived and led by an adapted physical activity kinesiologist, may effectively improve upper body functional prerequisites and global proprioceptive control, thus potentially promoting autonomy, quality of life, and physical activity/sport practice adherence in visually impaired individuals.

Concentrating to avoid falling: interaction between peripheral sensory and central attentional demands during a postural stability limit task in sedentary seniors

Evidence suggests falls and postural instabilities among seniors are attributed to a decline in both the processing of afferent signals (e.g., proprioceptive, vestibular) and attentional resources. We investigated the interaction between the non-visual and attentional demands of postural control in sedentary seniors. Old and young adults performed a postural stability limit task involving a maximal voluntary leaning movement with and without vision as well as a cognitive-attentional subtraction task. These tasks were performed alone (single-task) or simultaneously (dual-task) to vary the sensory-attentional demands. The functional limits of stability were quantified as the maximum center of pressure excursion during voluntary leaning. Seniors showed significantly smaller limits of postural stability compared to young adults in all sensory-attentional conditions. However, surprisingly, both groups of subjects reduced their stability limits by a similar amount when vision was removed. Furthermore, they similarly decreased their anterior-posterior stability limits when concurrently performing the postural and the cognitive-attentional tasks with vision. The overall average cognitive performance of young adults was higher than seniors and was only slightly affected during dual-tasking. In contrast, older adults markedly degraded their cognitive performance from the single- to the dual-task situations, especially when vision was unavailable. Thus, their dual-task costs were higher than those of young adults and increased in the eyes-closed condition, when postural control relied more heavily on non-visual sensory signals. Our findings provide the first evidence that as posture approaches its stability limits, sedentary seniors allot increasingly large cognitive attentional resources to process critical sensory inputs.

Immunofluorescence analysis of sensory nerve endings in the periarticular tissue of the human elbow joint

INTRODUCTION

To investigate the dynamic aspects of elbow stability, we aimed to analyze sensory nerve endings in the ligaments and the capsule of elbow joints.

MATERIALS AND METHODS

The capsule with its anterior (AJC) and posterior (PJC) parts, the radial collateral ligament (RCL), the annular ligament (AL), and the ulnar collateral ligament with its posterior (PUCL), transverse (TUCL) and anterior parts (AUCL) were dissected from eleven human cadaver elbow joints. Sensory nerve endings were analyzed in two levels per specimen as total cell amount/ cm² after immunofluorescence staining with low-affinity neurotrophin receptor p75, protein gene product 9.5, S-100 protein and 4',6-Diamidin-2-phenylindol, Carbonic anhydrase II and choline acetyltransferase on an Apotome microscope according to Freeman and Wyke's classification.

RESULTS

Free nerve endings were the predominant mechanoreceptor in all seven structures followed by Ruffini, unclassifiable, Golgi-like, and Pacini corpuscles (p ≤ 0.00001, respectively). Free nerve endings were observed significant more often in the AJC than in the RCL (p < 0.00002). A higher density of Ruffini endings than Golgi-like endings was observed in the PJC (p = 0.004). The RCL contained significant more Ruffini endings than Pacini corpuscles (p = 0.004). Carbonic anhydrase II was significantly more frequently positively immunoreactive than choline acetyltransferase in all sensory nerve endings (p < 0.05). Sensory nerve endings were significant more often epifascicular distributed in all structures (p < 0.006, respectively) except for the AJC, which had a pronounced equal distribution (p < 0.00005).

CONCLUSION

The high density of free nerve endings in the joint capsule indicates that it has pronounced nociceptive functions. Joint position sense is mainly detected by the RCL, AUCL, PUCL, and the PJC. Proprioceptive control of the elbow joint is mainly monitored by the joint capsule and the UCL, respectively. However, the extreme range of motion is primarily controlled by the RCL mediated by Golgi-like endings.

Age-related changes in upper limb coordination in a complex reaching task

When reaching to targets within arm's reach, intentional trunk motion must be neutralized by compensatory motion of the upper limb (UL). Advanced age has been associated with deterioration in the coordination of multi-joint UL movements. In the current study, we looked to determine if older adults also have difficulties modifying their UL movements (i.e., coordination between the shoulder and elbow joints), during a complex reaching task when trunk motion is manipulated. Two groups of healthy participants were recruited: 18 young (mean age = 24.28 ± 2.89 years old) and 18 older (mean age = 72.11 ± 2.39 years old) adults. Participants reached to a target with their eyes closed, while simultaneously moving the trunk forward. In 40% of trials, the trunk motion was unexpectedly blocked. Participants performed the task with both their dominant and non-dominant arms, and at a preferred and fast speed. All participants were able to coordinate motion at the elbow and shoulder joints in a similar manner and modify this coordination in accordance with motion at the trunk, regardless of the hand used or speed of movement. Specifically, in reaches that involved forward trunk motion (free-trunk trials), all participants demonstrated increased elbow flexion (i.e., less elbow extension) compared to blocked-trunk trials. In contrast, when trunk motion was blocked (blocked-trunk trials), all reaching movements were accompanied by increased shoulder horizontal adduction. While coordination of UL joints was similar across older and young adults, the extent of changes at the elbow and shoulder was smaller and less consistent in older adults compared to young participants, especially when trunk motion was involved. These results suggest that older adults can coordinate their UL movements based on task requirements, but that their performance is not as consistent as young adults.

Vestibular contributions to online reach execution are processed via mechanisms with knowledge about limb biomechanics

Studies of reach control with the body stationary have shown that proprioceptive and visual feedback signals contributing to rapid corrections during reaching are processed by neural circuits that incorporate knowledge about the physical properties of the limb (an internal model). However, among the most common spatial and mechanical perturbations to the limb are those caused by our body's own motion, suggesting that processing of vestibular signals for online reach control may reflect a similar level of sophistication. We investigated this hypothesis using galvanic vestibular stimulation (GVS) to selectively activate the vestibular sensors, simulating body rotation, as human subjects reached to remembered targets in different directions (forward, leftward, rightward). If vestibular signals contribute to purely kinematic/spatial corrections for body motion, GVS should evoke reach trajectory deviations of similar size in all directions. In contrast, biomechanical modeling predicts that if vestibular processing for online reach control takes into account knowledge of the physical properties of the limb and the forces applied on it by body motion, then GVS should evoke trajectory deviations that are significantly larger during forward and leftward reaches as compared with rightward reaches. When GVS was applied during reaching, the observed deviations were on average consistent with this prediction. In contrast, when GVS was instead applied before reaching, evoked deviations were similar across directions, as predicted for a purely spatial correction mechanism. These results suggest that vestibular signals, like proprioceptive and visual feedback, are processed for online reach control via sophisticated neural mechanisms that incorporate knowledge of limb biomechanics.NEW & NOTEWORTHY Studies examining proprioceptive and visual contributions to rapid corrections for externally applied mechanical and spatial perturbations during reaching have provided evidence for flexible processing of sensory feedback that accounts for musculoskeletal system dynamics. Notably, however, such perturbations commonly arise from our body's own motion. In line with this, we provide compelling evidence that, similar to proprioceptive and visual signals, vestibular signals are processed for online reach control via sophisticated mechanisms that incorporate knowledge of limb biomechanics.

Motor-Equivalent Intersegmental Coordination Is Impaired in Chronic Stroke

Background. Kinematic abundance permits using different movement patterns for task completion. Individuals poststroke may take advantage of abundance by using compensatory trunk displacement to overcome upper limb (UL) movement deficits. However, movement adaptation in tasks requiring specific intersegment coordination may remain limited. Objective. We tested movement adaptation in both arms of individuals with chronic stroke (n = 16) and nondominant arms of controls (n = 12) using 2 no-vision reaching tasks involving trunk movement (40 trials/arm). Methods. In the "stationary hand task" (SHT), subjects maintained the hand motionless over a target while leaning the trunk forward. In the "reaching hand task" (RHT), subjects reached to the target while leaning forward. For both tasks, trunk movement was unexpectedly blocked in 40% of trials to assess the influence of trunk movement on adaptive arm positioning or reaching. UL sensorimotor impairment, activity, and sitting balance were assessed in the stroke group. The primary outcome measure for SHT was gain (g), defined as the extent to which trunk displacement contributing to hand motion was offset by appropriate changes in UL movements (g = 1: complete compensation) and endpoint deviation for RHT. Results. Individuals poststroke had lower gains and greater endpoint deviation using the more-affected compared with less-affected UL and controls. Those with less sensorimotor impairment, greater activity levels, and better sitting balance had higher gains and smaller endpoint deviations. Lower gains were associated with diminished UL adaptability. Conclusions. Tests of condition-specific adaptability of interjoint coordination may be used to measure UL adaptability and changes in adaptability with treatment.

Effects of knee sleeves on coordination of lower-limb segments in healthy adults during level walking and one-leg hopping

The evaluation of multisegment coordination is important in gaining a better understanding of the gait and physical activities in humans. Therefore, this study aims to verify whether the use of knee sleeves affects the coordination of lower-limb segments during level walking and one-leg hopping. Eleven healthy male adults participated in this study. They were asked to walk 10 m on a level ground and perform one-leg hops with and without a knee sleeve. The segment angles and the response velocities of the thigh, shank, and foot were measured and calculated by using a motion analysis system. The phases between the segment angle and the velocity were then calculated. Moreover, the continuous relative phase (CRP) was calculated as the phase of the distal segment subtracted from the phase of the proximal segment and denoted as CRPTS (thigh–shank), CRPSF (shank–foot), and CRPTF (thigh–foot). The root mean square (RMS) values were used to evaluate the in-phase or out-of-phase states, while the standard deviation (SD) values were utilized to evaluate the variability in the stance and swing phases during level walking and in the preflight, flight, and landing phases during one-leg hopping. The walking velocity and the flight time improved when the knee sleeve was worn (p < 0.05). The segment angles of the thigh and shank also changed when the knee sleeve was worn during level walking and one-leg hopping. The RMS values of CRPTS and CRPSF in the stance phase and the RMS values of CRPSF in the preflight and landing phases changed (p < 0.05 in all cases). Moreover, the SD values of CRPTS in the landing phase and the SD values of CRPSF in the preflight and landing phases increased (p < 0.05 in all cases). These results indicated that wearing a knee sleeve caused changes in segment kinematics and coordination.

Effects of visual feedback and memory on unintentional drifts in performance during finger-pressing tasks

This study tested two hypotheses on the nature of unintentional force drifts elicited by removing visual feedback during accurate force production tasks. The role of working memory (memory hypothesis) was explored in tasks with continuous force production, intermittent force production, and rest intervals over the same time interval. The assumption of unintentional drifts in referent coordinate for the fingertips was tested using manipulations of visual feedback: young healthy subjects performed accurate steady-state force production tasks by pressing with the two index fingers on individual force sensors with visual feedback on the total force, sharing ratio, both, or none. Predictions based on the memory hypothesis have been falsified. In particular, we observed consistent force drifts to lower force values during continuous force production trials only. No force drift or drifts to higher forces were observed during intermittent force production trials and following rest intervals. The hypotheses based on the idea of drifts in referent finger coordinates have been confirmed. In particular, we observed superposition of two drift processes: a drift of total force to lower magnitudes and a drift of the sharing ratio to 50:50. When visual feedback on total force only was provided, the two-finger forces showed drifts in opposite directions. We interpret the findings as evidence for the control of motor actions with changes in referent coordinates for participating effectors. Unintentional drifts in performance are viewed as natural relaxation processes in the involved systems; their typical time reflects stability in the direction of the drift. The magnitude of the drift was higher in the right (dominant) hand, which is consistent with the dynamic dominance hypothesis.

Reaching with the sixth sense: Vestibular contributions to voluntary motor control in the human right parietal cortex

The vestibular system constitutes the silent sixth sense: It automatically triggers a variety of vital reflexes to maintain postural and visual stability. Beyond their role in reflexive behavior, vestibular afferents contribute to several perceptual and cognitive functions and also support voluntary control of movements by complementing the other senses to accomplish the movement goal. Investigations into the neural correlates of vestibular contribution to voluntary action in humans are challenging and have progressed far less than research on corresponding visual and proprioceptive involvement. Here, we demonstrate for the first time with event-related TMS that the posterior part of the right medial intraparietal sulcus processes vestibular signals during a goal-directed reaching task with the dominant right hand. This finding suggests a qualitative difference between the processing of vestibular vs. visual and proprioceptive signals for controlling voluntary movements, which are pre-dominantly processed in the left posterior parietal cortex. Furthermore, this study reveals a neural pathway for vestibular input that might be distinct from the processing for reflexive or cognitive functions, and opens a window into their investigation in humans.

Evidence for a reference frame transformation of vestibular signal contributions to voluntary reaching

To contribute appropriately to voluntary reaching during body motion, vestibular signals must be transformed from a head-centered to a body-centered reference frame. We quantitatively investigated the evidence for this transformation during online reach execution by using galvanic vestibular stimulation (GVS) to simulate rotation about a head-fixed, roughly naso-occipital axis as human subjects made planar reaching movements to a remembered location with their head in different orientations. If vestibular signals that contribute to reach execution have been transformed from a head-centered to a body-centered reference frame, the same stimulation should be interpreted as body tilt with the head upright but as vertical-axis rotation with the head inclined forward. Consequently, GVS should perturb reach trajectories in a head-orientation-dependent way. Consistent with this prediction, GVS applied during reach execution induced trajectory deviations that were significantly larger with the head forward compared with upright. Only with the head forward were trajectories consistently deviated in opposite directions for rightward versus leftward simulated rotation, as appropriate to compensate for body vertical-axis rotation. These results demonstrate that vestibular signals contributing to online reach execution have indeed been transformed from a head-centered to a body-centered reference frame. Reach deviation amplitudes were comparable to those predicted for ideal compensation for body rotation using a biomechanical limb model. Finally, by comparing the effects of application of GVS during reach execution versus prior to reach onset we also provide evidence that spatially transformed vestibular signals contribute to at least partially distinct compensation mechanisms for body motion during reach planning versus execution.

Arm-trunk coordination for beyond-the-reach movements in adults with stroke

BACKGROUND

By involving additional degrees of freedom, the nervous system may preserve hand trajectories when making pointing movements with or without trunk displacement. Previous studies indicate that the potential contribution of trunk movement to hand displacement for movements made within arm reach is neutralized by appropriate compensatory shoulder and elbow rotations. For beyond-the-reach movements, compensatory coordination is attenuated after the hand peak velocity, allowing trunk movement to contribute to hand displacement.

OBJECTIVE

To investigate if the timing and spatial coordination of arm and trunk movements during beyond-the-reach movements is preserved in stroke.

METHODS

Eleven healthy control subjects and 11 individuals with mild-to-moderate chronic unilateral hemiparesis participated. Arm and trunk kinematics during 60 target reaches to an ipsilaterally placed target were recorded. In 30% of randomly chosen trials, trunk movement was unexpectedly prevented (blocked-trunk trials) by an electromagnetic device, resulting in divergence of the hand trajectory from that in free-trunk trials. Hand trajectories and elbow-shoulder interjoint coordination were compared between trials.

RESULTS

In stroke participants, hand trajectory divergence occurred at a shorter movement extent and interjoint coordination patterns diverged at a relatively greater distance compared to controls. Thus, arm movements in stroke participants only partially compensated trunk displacement resulting in the trunk movement contributing to arm movement earlier and to a larger extent during reaching.

CONCLUSION

Individuals with mild-to-moderate stroke have deficits in timing and spatial coordination of arm and trunk movements during different parts of a reaching movement. This deficit may be targeted in therapy to improve upper limb function.

Control of wrist position and muscle relaxation by shifting spatial frames of reference for motoneuronal recruitment: possible involvement of corticospinal pathways

It has previously been established that muscles become active in response to deviations from a threshold (referent) position of the body or its segments, and that intentional motor actions result from central shifts in the referent position. We tested the hypothesis that corticospinal pathways are involved in threshold position control during intentional changes in the wrist position in humans. Subjects moved the wrist from an initial extended to a final flexed position (and vice versa). Passive wrist muscle forces were compensated with a torque motor such that wrist muscle activity was equalized at the two positions. It appeared that motoneuronal excitability tested by brief muscle stretches was also similar at these positions. Responses to mechanical perturbations before and after movement showed that the wrist threshold position was reset when voluntary changes in the joint angle were made. Although the excitability of motoneurons was similar at the two positions, the same transcranial magnetic stimulus (TMS) elicited a wrist extensor jerk in the extension position and a flexor jerk in the flexion position. Extensor motor-evoked potentials (MEPs) elicited by TMS at the wrist extension position were substantially bigger compared to those at the flexion position and vice versa for flexor MEPs. MEPs were substantially reduced when subjects fully relaxed wrist muscles and the wrist was held passively in each position. Results suggest that the corticospinal pathway, possibly with other descending pathways, participates in threshold position control, a process that pre-determines the spatial frame of reference in which the neuromuscular periphery is constrained to work. This control strategy would underlie not only intentional changes in the joint position, but also muscle relaxation. The notion that the motor cortex may control motor actions by shifting spatial frames of reference opens a new avenue in the analysis and understanding of brain function.

The kinematics of upper extremity reaching: a reliability study on people with and without shoulder impingement syndrome

Background

Tasks chosen to evaluate motor performance should reflect the movement deficits characteristic of the target population and present an appropriate challenge for the patients who would be evaluated. A reaching task that evaluates impairment characteristics of people with shoulder impingement syndrome (SIS) was developed to evaluate the motor performance of this population. The objectives of this study were to characterize the reproducibility of this reaching task in people with and without SIS and to evaluate the impact of the number of trials on reproducibility.

Methods

Thirty subjects with SIS and twenty healthy subjects participated in the first measurement session to evaluate intrasession reliability. Ten healthy subjects were retested within 2 to 7 days to assess intersession reliability. At each measurement session, upper extremity kinematic patterns were evaluated during a reaching task. Ten trials were recorded. Thereafter, the upper extremity position at the end of reaching and total joint excursion that occurred during reaching were calculated. Intraclass correlation coefficient (ICC) and minimal detectable change (MDC) were used to estimate intra and intersession reliability.

Results

Intrasession reliability for total joint excursion was good to very good when based on the first two trials (0.77<ICC<0.99), and very good when based on either the first or last five trials (ICC>0.92). As for end-reach position, intrasession reliability was very good when using either the first two, first five or last five trials (ICC>0.82). Globally, MDC were smaller for the last five trials. Intersession reliability of total joint excursion and position at the end of reaching was good to very good when using the mean of the first two or five trials (0.69<ICC<0.95), and very good when using the mean of the ten trials (ICC>0.82). For most joints, MDC were smaller when using all ten trials.

Conclusions

The reaching task proposed to evaluate the upper limb motor performance was found reliable in people with and without SIS. Furthermore, the minimal difference necessary to infer a meaningful change in motor performance was determined, indicating that relatively small changes in task performance can be interpreted as a change in motor performance.

Insights into the control of arm movement during body motion as revealed by EMG analyses

Recent studies have revealed that vestibulomotor transformations contribute to maintain the hand stationary in space during trunk rotation. Here we tested whether these vestibulomotor transformations have the same latencies and whether they are subject to similar cognitive control than the visuomotor transformations during manual tracking of a visual target. We recorded hand displacement and shoulder-muscle activity in two tasks: a stabilization task in which subjects stabilized their hand during passive 30 degrees body rotations, and a tracking task in which subjects tracked with their finger a visual target as it moved 30 degrees around them. The EMG response times recorded in the stabilization task (approximately 165 ms) were twice as short as those observed for the tracking task (approximately 350 ms). Tested with the same paradigm, a deafferented subject showed EMG response times that closely matched those recorded in healthy subjects, thus, suggesting a vestibular origin of the arm movements. Providing advance information about the direction of the required arm movement reduced the response times in the tracking task (by approximately 115 ms) but had no significant effect in the stabilization task. Generally, when providing false information about movement direction in the tracking task, an EMG burst first appeared in the muscle moving the arm in the direction opposite to the actual target motion (i.e., in accord with the precueing). This behavior was rarely observed in the stabilization task. These results show that the sensorimotor transformations that move the arm relative to the trunk have shorter latencies when they originate from vestibular inputs than from visual information and that vestibulomotor transformations are more resistant to cognitive processes than visuomotor transformations.

The effects of bilateral arm training on motor control and functional performance in chronic stroke: a randomized controlled study

BACKGROUND

Most studies of bilateral arm training (BAT) did not employ a randomized controlled trial design and involved very limited functional training tasks.

OBJECTIVE

Compare the effects of BAT with control intervention (CI) on motor control and motor performance of the upper extremity and also functional gains in patients with chronic stroke.

METHODS

. This 2-group randomized controlled trial with pretreatment and posttreatment measures enrolled 33 stroke patients (mean age = 53.85 years) 6 to 67 months after onset of a first stroke. They received either a BAT program concentrating on both upper extremities moving simultaneously in functional tasks by symmetric patterns or CI (control treatment) for 2 hours on weekdays for 3 weeks. Outcome measures included kinematic analyses assessing motor control strategies for unilateral and bimanual reaching and clinical measures involving the Fugl-Meyer Assessment (FMA) of motor-impairment severity and the Functional Independence Measure (FIM) and the Motor Activity Log (MAL) evaluating functional ability.

RESULTS

After treatment, the BAT group showed better temporal and spatial efficiency during unilateral and bilateral tasks and less online error correction only during the bilateral task than the control group. The BAT group showed a significantly greater improvement in the FMA than the control group but not in the FIM and MAL.

CONCLUSIONS

Relative to CI, BAT improved the spatiotemporal control of the affected arm in both bilateral and unilateral tasks, decreased online corrections to perform bilateral tasks, and reduced motor impairment. These findings support the use of BAT to improve motor control and motor function of the affected upper limb in stroke patients.

Effect on movement selection of an evolving sensory representation: a multiple controller model of skill acquisition

Change in behavior and neural activity in skill acquisition suggests that control is transferred from cortical planning areas (e.g., the prefrontal cortex, PFC) to the basal ganglia (BG). Planning has large computational and representational requirements but requires little experience with a task. The BG are thought to employ a simpler control scheme and reinforcement learning; these mechanisms rely on extensive experience. Many theoretical accounts of behavior in the face of uncertainty invoke planning mechanisms that explicitly take uncertainty into account. We suggest that the simpler mechanisms of the BG can also contribute to the development of behavior under such conditions. We focus on learning under conditions in which sensory information takes time to resolve, e.g., when a poorly perceived goal stimulus takes non-negligible time to identify. It may be advantageous to begin acting quickly under uncertainty--possibly via decisions that are suboptimal for the actual goal--rather than to wait for sensory information to fully resolve. We present a model of skill acquisition in which control is transferred, with experience, from a planning controller (denoted A), corresponding to the PFC, to a simpler controller (B), corresponding to the BG. We apply our model to a task in which a learning agent must execute a series of actions to achieve a goal (selected randomly at each trial from a small set). Over the course of a trial, the agent's goal representation evolves from representing all possible goals to only the selected goal. A is restricted to select movements only when goal representation is fully resolved. Model behavior is similar to that observed in humans accomplishing similar tasks. Thus, B can by itself account for the development of behavior under an evolving sensory representation, suggesting that the BG can contribute to learning and control under conditions of uncertainty.

Differential recruitment of anterior intraparietal sulcus and superior parietal lobule during visually guided grasping revealed by electrical neuroimaging

Dorsal parietal cortex is required for visually guided prehension. Transcranial magnetic stimulation to either the anterior intraparietal sulcus (aIPS) or superior parietal lobule (SPL) disrupts on-line adaptive adjustments of grasp when objects are perturbed. We used high-density electroencephalography during grasping to determine the relative timing of these two areas and to test whether the temporal contribution of each site would change when the task goal was perturbed. During object grasping with the right-hand, two distinct evoked responses were present over the 50-100 and 100-200 ms periods after movement onset. Distributed linear source estimation of these scalp potentials localized left lateralized sources, first in the aIPS and then the SPL. The duration of the response from the aIPS area was longer when there was an object perturbation. Initiation of a corrective movement coincided with activation in SPL. These data support a two-stage process: the integration of target goal and an emerging action plan within aIPS and subsequent on-line adjustments within SPL.

Movement planning with probabilistic target information

We examined how subjects plan speeded reaching movements when the precise target of the movement is not known at movement onset. Before each reach, subjects were given only a probability distribution on possible target positions. Only after completing part of the movement did the actual target appear. In separate experiments we varied the location of the mode and the scale of the prior distribution for possible targets. In both cases we found that subjects made use of prior probability information when planning reaches. We also devised two tests (Composite Benefit and Row Dominance tests) to determine whether subjects' performance met necessary conditions for optimality (defined as maximizing expected gain). We could not reject the hypothesis of optimality in the experiment where we varied the mode of the prior, but departures from optimality were found in response to changes in the scale of prior distributions.

Vestibular system may provide equivalent motor actions regardless of the number of body segments involved in the task

The vestibulospinal system likely plays an essential role in motor equivalence--the ability to reach the desired motor goal despite intentional or imposed changes in the number of body segments involved in the task. To test this hypothesis, we compared the ability of healthy subjects and patients with unilateral vestibular lesions (surgical acoustic neuroma resection 0.6 to 6.7 yr before the study) to maintain either the same hand position or the same trajectory of within arm reach movements while flexing the trunk, in the absence of vision. In randomly selected trials, the trunk motion was prevented by an electromagnetic device. Healthy subjects were able to preserve the hand position or trajectory by modifying the elbow and shoulder joint rotations in a condition-dependent way, at a minimal latency of about 60 ms after the trunk movement onset. In contrast, six of seven patients showed deficits in the compensatory angular modifications at least in one of two tasks so that 30-100% of the trunk displacement was not compensated and thus influenced the hand position or trajectory. Results suggest that vestibular influences evoked by the head motion during trunk flexion play a major role in maintaining the consistency of arm motor actions in external space despite changes in the number of body segments involved. Our findings also suggest that despite long-term plasticity in the vestibular system and related neural structures, unilateral vestibular lesion may reduce the capacity of the nervous system to achieve motor equivalence.

Coordination between postural and movement controls: effect of changes in body mass distribution on postural and focal component characteristics

Whole-body reaching movements are accomplished through a combination of anticipatory postural adjustments (APAs) and focal movements. Two different modes of central organization is usually proposed for this coordination: first, a single-process control, where the APAs and the focal movements would share a common command; second, where the APAs and the focal movements would be independently controlled through parallel commands (dual-process control). Here, we investigated which one of these modes of control could better explain the coordination between the trunk and the upper limb for standing subjects reaching for a target located beyond arm's length. This was done evaluating the effect of changing the APAs settings on the arm movement. The APAs modification was achieved by shifting the subject's centre of mass prior to the focal movement onset; this was done by adding an asymmetric load on either side of the head (a control condition with the load fixed centrally at the top of the head was also performed). As it changed the body mass distribution, the muscular torques and the orientation of the head inertia tensor, it is assumed that the addition of the asymmetric load led to a change in the APAs. Analyses indeed showed that both the initial head and trunk displacement towards the supporting side (during the unloading of the moving leg) were smaller when the load was fixed on the side of the supporting leg than when it was fixed on the side of the moving leg. However, changing the initial conditions, and therefore the APAs settings, had no significant effect on the path and kinematics of the focal hand movement. Therefore, subjects cancelled out the effect of the trunk motion on the hand-in-space motion through compensatory arm movements. These results support the dual-process control hypothesis for the postural and the focal components.

Position-dependent torque coupling and associated muscle activation in the hemiparetic upper extremity

Previous studies have demonstrated abnormal joint torque coupling and associated muscle coactivations of the upper extremity in individuals with unilateral stroke. We investigated the effect of upper limb configuration on the expression of the well-documented patterns of shoulder abduction/elbow flexion and shoulder adduction/elbow extension. Maximal isometric shoulder and elbow torques were measured in stroke subjects in four different arm configurations. Additionally, an isometric combined torque task was completed where subjects were required to maintain various levels of shoulder abduction/adduction torque while attempting to maximize elbow flexion or extension torque. The dominant abduction/elbow flexion pattern was insensitive to changes in limb configuration while the elbow extension component of the adduction/extension pattern changed to elbow flexion at smaller shoulder abduction angles. This effect was not present in control subjects without stroke. The reversal of the torque-coupling pattern could not be explained by mechanical factors such as muscle length changes or muscle strength imbalances across the elbow joint. Potential neural mechanisms underlying the sensitivity of the adduction/elbow extension pattern to different somatosensory input resultant from changes in limb configuration are discussed along with the implications for future research.

Postural sway at ground and bevel levels in subjects with spina bifida occulta

To assess whether the postural function is impaired by comparing the performances in upright standing at ground and bevel levels in adult subjects with spina bifida occulta (SBO). Eighty subjects with SBO (38 with minor type and 42 with major type) and 35 healthy control subjects participated in the study. All participants performed ten tests while standing upright on a platform at ground level (0 degrees, baseline) and on a beveled surface (with their feet in dorsiflexion and plantarflexion at 10 degrees and 20 degrees). Tests were done with their eyes open and closed. The postural sway was examined using a force platform (CATSYS, Danish) that records sway intensity and velocity. Sway intensity and sway velocity were universally associated with group, degree of bevel, open- or closed-eyes condition, and dorsiflexion or plantarflexion after adjusting for age and gender. With respect to sway intensity, the differences of minor or major SBO group were significantly decreased at different bevel degrees when compared with control groups, whereas the differences between minor and major SBO were significant differences at 10 degrees and 20 degrees. With respect to sway velocity, the differences of major SBO group were significantly decreased at different bevel degrees when compared with minor SBO and control groups, whereas the difference in minor SBO was only significant at 0 degrees when compared with control. Group differences (minor SBO vs. control, major SBO vs. control) showed a significant decrease in sway velocity when comparing at 10 degrees than at 0 degrees and at 20 degrees than at 0 degrees. In all subjects with SBO, the sway intensity/velocity values obtained with open eyes and with plantarflexion had lower values, when compared with values obtained with closed eyes and with dorsiflexion. This study supports the hypothesis that SBO impairs control of postural sway in both the resting upright and stressful postures. Our results imply that the larger the bone defect at the lumbosacral midline, the more the group differences in different stressful conditions. Both velocity and intensity were able to reflect the function of the postural sway from our results. This is the first report to add the bevel degree and foot position, as well as visual input as being the part of the study in investigating the postural sway.

The role of different submovement types during pointing to a target

The present study extends our previous findings in challenging the traditional interpretation of irregularities in the velocity profile of pointing movements as corrective submovements performed to improve accuracy of target achievement. The study is driven by a hypothesis that pointing includes at least two subtasks, accurate target achievement and motion termination, each of which can cause submovements (Dounskaia et al. Exp Brain Res 164:505-516, 2005). To investigate submovements associated with these subtasks, two tasks were performed in the experiment. Task 1 was used to examine the contribution of the two subtasks on submovement production by comparing submovements in discrete movements that include motion termination and in cyclic movements during which motion termination is not performed. Target size manipulations emphasized submovements related to the accuracy subtask. The results confirmed that both subtasks included in pointing cause submovements. Gross types of submovements (types 1 and 2) were associated with motion termination and fine submovements (type 3) with accuracy regulation. Task 2 further investigated sources of the accuracy-associated type 3 submovements by including only cyclic movements performed at two levels of frequency. Most (97.6%) of the submovements in task 2 were of type 3. Submovement incidence was strongly (inversely) associated with cyclic frequency, and it was independent of target size. This result questions the accuracy subtask as a primary source for type 3 submovements, and it raises the possibility that these submovements are an inherent property of low-speed movements. Together, results of the two tasks support our previous finding that gross submovements are not necessarily related to accuracy regulation. They also provide evidence that challenges the interpretation of fine submovements as corrections performed voluntarily to improve pointing accuracy. Alternative interpretations of accuracy regulation mechanisms, such as regulation of muscle stiffness and of the muscle co-contraction level are discussed in light of the present results.

Fusion of visuo-ocular and vestibular signals in arm motor control

Keeping the finger pointing at an Earth-fixed object during body displacements can be achieved if compensatory arm movements counteract the effect of the rotation on the hand's position in space. Here we investigated the fusion of signals that originated from systems having different neurophysiological properties (i.e., the visuo-oculomotor and vestibular systems) in the production of such compensatory arm movements. To this end, we analyzed the subjects' performance in three conditions that differed according to the information they provided about relative target-body motion. This information originated either from the vestibular or visuo-oculomotor system, or from a combination of the two. To highlight the integration of visuo-oculomotor and vestibular signals, we compared the arm response to motion frequencies presumed to allow or not to allow optimal vestibular and oculomotor responses. When they could be used in isolation, the ocular signals allowed long-latency but precise kinematics control of the arm movement, whereas vestibular signals allowed accurate motor response early in the rotation but their contribution declined as body rotation developed. Optimal performance was obtained throughout the whole movement and for all rotation frequencies when the visuo-oculomotor and vestibular signals could be used together. This increase in hand-tracking performance could not be explained by a unimodal model or an additive model of vestibular and ocular cues, even when using weighted signals. Rather, the results supported a functional model in which vestibular and visuo-oculomotor signals have different influences on the temporal and spatial aspects of hand movement compensating for body motion.

On the nature of the vestibular control of arm-reaching movements during whole-body rotations

Recent studies report efficient vestibular control of goal-directed arm movements during body motion. This contribution tested whether this control relies (a) on an updating process in which vestibular signals are used to update the perceived egocentric position of surrounding objects when body orientation changes, or (b) on a sensorimotor process, i.e. a transfer function between vestibular input and the arm motor output that preserves hand trajectory in space despite body rotation. Both processes were separately and specifically adapted. We then compared the respective influences of the adapted processes on the vestibular control of arm-reaching movements. The rationale was that if a given process underlies a given behavior, any adaptive modification of this process should give rise to observable modification of the behavior. The updating adaptation adapted the matching between vestibular input and perceived body displacement in the surrounding world. The sensorimotor adaptation adapted the matching between vestibular input and the arm motor output necessary to keep the hand fixed in space during body rotation. Only the sensorimotor adaptation significantly altered the vestibular control of arm-reaching movements. Our results therefore suggest that during passive self-motion, the vestibular control of arm-reaching movements essentially derives from a sensorimotor process by which arm motor output is modified on-line to preserve hand trajectory in space despite body displacement. In contrast, the updating process maintaining up-to-date the egocentric representation of visual space seems to contribute little to generating the required arm compensation during body rotations.

Deficits in adaptive upper limb control in response to trunk perturbations in Parkinson's disease

The ability of patients with Parkinson's disease (PD) to compensate for unexpected perturbations remains relatively unexplored. To address this issue PD subjects were required to compensate at the arm for an unexpected mechanical perturbation of the trunk while performing a trunk-assisted reach. Twelve healthy and nine PD subjects (off medication) performed trunk-assisted reaching movements without vision or knowledge of results to a remembered target in the ipsilateral (T1) or contralateral (T2) workspace. On 60% of the trials trunk motion was unrestrained (free condition). On the remaining 40% of randomly selected trials trunk motion was arrested at movement onset (blocked condition). If subjects appropriately changed arm joint angles to compensate for the trunk arrest, there should be spatial and temporal invariance in the hand trajectories and in the endpoint errors across conditions. The control group successfully changed their arm configuration in a context-dependent manner which resulted in invariant hand trajectory profiles across the free and blocked conditions. More so, they initiated these changes rapidly after the trunk perturbation (group mean 70 ms). Some PD subjects were unable to maintain invariant hand paths and movement errors across conditions. Their hand velocity profiles were also more variable relative to those of the healthy subjects in the blocked-trunk trials but not in the free-trunk trials. Furthermore, the latency of compensatory changes in arm joint angles in movements toward T1 was longer in the PD group (group mean 153 ms). Finally, PD subjects' arm and trunk were desynchronized at movement onset, confirming our previous findings and consistent with PD patients' known problems in the sequential or parallel generation of different movement components. The findings that individual PD subjects were unsuccessful or delayed in producing context-dependent responses at the arm to unexpected perturbations of the trunk suggests that the basal ganglia are important nodes in the organization of adaptive behavior.