Sensory-motor coordination during grasping and manipulative actions

Impact of tactile function on upper limb motor function in children with Developmental Coordination Disorder

This study investigated the presence of, and relationship between tactile dysfunction and upper limb motor function in children with Developmental Coordination Disorder (DCD) compared to typical developing (TD) children. Participants were 36 children aged 6-12 years. Presence of DCD (n=20) or TD (n=16) was confirmed using the Movement Assessment Battery for Children, second edition. All children participated in a comprehensive assessment of tactile registration (Semmes Weinstein Monofilaments); tactile spatial perception (Single Point Localisation (SPL) and two-point discrimination (2PD)); haptic perception (Stereognosis); speed of simple everyday manual tasks (Jebsen-Taylor Test of Hand Function (JTTHF)); and handwriting speed and accuracy (Evaluation Tool of Children's Handwriting (ETCH)). Compared to TD children, children with DCD demonstrated poorer localisation of touch in the non-dominant hand (p=0.04), slower speed of alphabet writing (p<0.05) and less legible handwriting (p<0.01), but no difference in speed of simple everyday manual tasks (JTTHF: p>0.05). Regression analysis showed that spatial tactile perception (SPL) predicted handwriting legibility (ETCH: r=0.11) and speed of functional tasks (JTTHF: r=0.33). These results suggest that tactile function, specifically single point localisation, should be a primary tactile assessment employed to determine reasons for upper limb motor difficulties experienced by children with DCD.

Effects of Different Tactile Feedback on Myoelectric Closed-Loop Control for Grasping Based on Electrotactile Stimulation

Closed-loop control is important for amputees to manipulate myoelectric prostheses intuitively and dexterously. Tactile feedback can help amputees improve myoelectric control performance for grasping objects. To investigate the effects of different tactile feedback, we performed experiments on six amputees and six able-bodied subjects via electrotactile stimulation. Using a virtual environment, six kinds of objects with different weights and stiffnesses were used for grasping tasks. Five feedback conditions (no feedback, pressure feedback, slip feedback, pressure + slip feedback, and vision feedback) were considered. Nine evaluation indexes and three control objectives (rapidity, economy, and stability) were proposed. Under the five feedback conditions, our study investigated four issues: 1) three types of grasping-related failures; 2) four types of grasping-related time measures; 3) average grasping force; 4) standard deviation of the grasping force. Results indicate that: 1) slip feedback is better than pressure feedback; 2) pressure + slip feedback can improve grasping rapidity; 3) slip feedback significantly contributes to grasping economy and stability; and 4) pressure + slip feedback can perform as well as vision feedback.

Proximal arm kinematics affect grip force-load force coordination

During object manipulation, grip force is coordinated with load force, which is primarily determined by object kinematics. Proximal arm kinematics may affect grip force control, as proximal segment motion could affect control of distal hand muscles via biomechanical and/or neural pathways. The aim of this study was to investigate the impact of proximal kinematics on grip force modulation during object manipulation. Fifteen subjects performed three vertical lifting tasks that involved distinct proximal kinematics (elbow/shoulder), but resulted in similar end-point (hand) trajectories. While temporal coordination of grip and load forces remained similar across the tasks, proximal kinematics significantly affected the grip force-to-load force ratio (P = 0.042), intrinsic finger muscle activation (P = 0.045), and flexor-extensor ratio (P < 0.001). Biomechanical coupling between extrinsic hand muscles and the elbow joint cannot fully explain the observed changes, as task-related changes in intrinsic hand muscle activation were greater than in extrinsic hand muscles. Rather, between-task variation in grip force (highest during task 3) appears to contrast to that in shoulder joint velocity/acceleration (lowest during task 3). These results suggest that complex neural coupling between the distal and proximal upper extremity musculature may affect grip force control during movements, also indicated by task-related changes in intermuscular coherence of muscle pairs, including intrinsic finger muscles. Furthermore, examination of the fingertip force showed that the human motor system may attempt to reduce variability in task-relevant motor output (grip force-to-load force ratio), while allowing larger fluctuations in output less relevant to task goal (shear force-to-grip force ratio).

Force Variability during Dexterous Manipulation in Individuals with Mild to Moderate Parkinson's Disease

Parkinson’s disease (PD) is a progressive neurodegenerative disease affecting about 1–2% of the population over the age of 65. Individuals with PD experience gradual deterioration of dexterous manipulation for activities of daily living; however, current clinical evaluations are mostly subjective and do not quantify changes in dynamic control of fingertip force that is critical for manual dexterity. Thus, there is a need to develop clinical measures to quantify those changes with aging and disease progression. We investigated the dynamic control of fingertip forces in both hands of 20 individuals with PD (69.0 ± 7.4 years) using the Strength–Dexterity test. The test requires low forces (<3 N) to compress a compliant and slender spring prone to buckling. A maximal level of sustained compression is informative of the greatest instability the person can control, and thus is indicative of the integrity of the neuromuscular system for dexterous manipulation. Miniature sensors recorded fingertip force (F) during maximal sustained compressions. The force variability during sustained compression was quantified in two frequency bands: low (<4 Hz, F_LF) and high (4–12 Hz, F_HF). F_LF characterizes variability in voluntary fluctuations, while F_HF characterizes variability in involuntary fluctuations including tremor. The more-affected hand exhibited significantly lower F and lower F_LF than those in the less-affected hand. The more-affected hand showed significant negative correlations between F_LF and the Unified Parkinson’s Disease Rating Scale motor scores for both total and hand-only, suggesting that greater force variability in the voluntary range was associated with less clinical motor impairment. We conclude the nature of force variability in the voluntary range during this dynamic and dexterous task may be a biomarker of greater motor capability/flexibility/adaptability in PD. This approach may provide a more quantitative clinical assessment of changes of sensorimotor control in individuals with PD.

An experimental evaluation of a new designed apparatus (NDA) for the rapid measurement of impaired motor function in rats

BACKGROUND

Assessment of the ability of rat to balance by rotarod apparatus (ROTA) is frequently used as a measure of impaired motor system function. Most of these methods have some disadvantages, such as failing to sense motor coordination rather than endurance and as the sensitivity of the method is low, more animals are needed to obtain statistically significant results.

NEW METHOD

We have designed and tested a new designed apparatus (NDA) to measure motor system function in rats. Our system consists of a glass box containing 4 beams which placed with 1cm distance between them, two electrical motors for rotating the beams, and a camera to record the movements of the rats. The RPM of the beams is adjustable digitally between 0 and 50 rounds per minute.

RESULTS

We evaluated experimentally the capability of the NDA for the rapid measurement of impaired motor function in rats. Also we demonstrated that the sensitivity of the NDA increases by faster rotation speeds and may be more sensitive than ROTA for evaluating of impaired motor system function.

COMPARISON WITH EXISTING METHODS

Compared to a previous version of this task, our NDA provides a more efficient method to test rodents for studies of motor system function after impaired motor nervous system.

CONCLUSIONS

In summary, our NDA will allow high efficient monitoring of rat motor system function and may be more sensitive than ROTA for evaluating of impaired motor system function in rats.

Decoding tactile afferent activity to obtain an estimate of instantaneous force and torque applied to the fingerpad

Dexterous manipulation is not possible without sensory information about object properties and manipulative forces. Fundamental neuroscience has been unable to demonstrate how information about multiple stimulus parameters may be continuously extracted, concurrently, from a population of tactile afferents. This is the first study to demonstrate this, using spike trains recorded from tactile afferents innervating the monkey fingerpad. A multiple-regression model, requiring no a priori knowledge of stimulus-onset times or stimulus combination, was developed to obtain continuous estimates of instantaneous force and torque. The stimuli consisted of a normal-force ramp (to a plateau of 1.8, 2.2, or 2.5 N), on top of which -3.5, -2.0, 0, +2.0, or +3.5 mNm torque was applied about the normal to the skin surface. The model inputs were sliding windows of binned spike counts recorded from each afferent. Models were trained and tested by 15-fold cross-validation to estimate instantaneous normal force and torque over the entire stimulation period. With the use of the spike trains from 58 slow-adapting type I and 25 fast-adapting type I afferents, the instantaneous normal force and torque could be estimated with small error. This study demonstrated that instantaneous force and torque parameters could be reliably extracted from a small number of tactile afferent responses in a real-time fashion with stimulus combinations that the model had not been exposed to during training. Analysis of the model weights may reveal how interactions between stimulus parameters could be disentangled for complex population responses and could be used to test neurophysiologically relevant hypotheses about encoding mechanisms.

Impacts of elevated glycaemic haemoglobin and disease duration on the sensorimotor control of hands in diabetes patients

BACKGROUND

To understand the impacts of disease chronicity and hyperglycaemia on sensorimotor control of hands of diabetic patients, this study investigated the differences in hand sensation, strength and motor control by applying the pinch-holding-up activity test for patients with diabetes mellitus (DM) with different levels of glycaemic control and disease chronicity.

METHODS

One hundred and fifty-nine patients with clinically defined DM were included. Semmes-Weinstein monofilament, static two-point discrimination and moving two-point discrimination, maximal pinch strength precision pinch performance tests and nerve conduction studies (NCS) of the subjects were carried out. Forty-seven (29.6%) patients were in the HbA(1c) < 7% category, and 112 (70.4%) patients were in the >7% group. There were 87 (54.7%) patients with the disease duration <10 years, and 72 (45.3%) patients with disease duration ≧10 years.

RESULTS

The severity of hyperglycaemia significantly impacts the results for Semmes-Weinstein monofilament, precision pinch force control, sensory and motor NCS tests (p < 0.05). In addition, the chronicity of disease influences the motor control of precision pinch performance and the amplitude of motor NCS (p < 0.05) for the diabetes patients.

CONCLUSIONS

The evidence suggests that disease chronicity and hyperglycaemia have impacts on sensorimotor control in the hands of DM patients. In addition, the efficiency of prehensile forces of hand-to-object interactions in the pinch-holding-up activity test could be significant for identifying hand function, as well as pathologic changes in median nerve function, for patients with DM.

Moving a hand-held object: Reconstruction of referent coordinate and apparent stiffness trajectories

This study used the framework of the referent configuration hypothesis and slow changes in the external conditions during vertical oscillation of a hand-held object to infer the characteristics of hypothetical control variables. The study had two main objectives: (1) to show that hypothetical control variables, namely, referent coordinates and apparent stiffness of vertical hand position and grip force can be measured in an experiment; and (2) to establish relation(s) between these control variables that yield the classic grip-force-load-force coupling. Healthy subjects gripped a handle and performed vertical oscillations between visual targets at one of five metronome-prescribed frequencies. A HapticMaster robot was used to induce slow changes in the vertical force applied to the handle, while the size of the handle was changed slowly leading to changes in the grip aperture. The subjects were instructed not to react to possible changes in the external forces. A linear, second-order model was used to reconstruct the referent coordinate and apparent stiffness values for each phase of the vertical oscillation cycle using across-cycle regressions. The reconstructed time profiles of the referent coordinates and apparent stiffness showed consistent trends across subjects and movement frequencies. To validate the method, these values were used to predict the vertical force and the grip force applied to the handle for movement cycles that were not utilized in the reconstruction process. Analysis of the coupling between the four variables, two referent coordinates and two apparent stiffness values, revealed a single strong constraint reflecting the coupling between the grip force and vertical force. We view these data as providing experimental support for the idea of controlling natural, multi-muscle actions with shifts in a low-dimensional set of referent coordinates.

Sequential activation of premotor, primary somatosensory and primary motor areas in humans during cued finger movements

OBJECTIVE

Human voluntary movements are a final product of complex interactions between multiple sensory, cognitive and motor areas of central nervous system. The objective was to investigate temporal sequence of activation of premotor (PM), primary motor (M1) and somatosensory (S1) areas during cued finger movements.

METHODS

Electrocorticography (ECoG) was used to measure activation timing in human PM, S1, and M1 neurons in preparation for finger movements in 5 subjects with subdural grids for seizure localization. Cortical activation was determined by the onset of high gamma (HG) oscillation (70-150Hz). The three cortical regions were mapped anatomically using a common brain atlas and confirmed independently with direct electrical cortical stimulation, somatosensory evoked potentials and detection of HG response to tactile stimulation. Subjects were given visual cues to flex each finger or pinch the thumb and index finger. Movements were captured with a dataglove and time-locked with ECoG. A windowed covariance metric was used to identify the rising slope of HG power between two electrodes and compute time lag. Statistical constraints were applied to the time estimates to combat the noise. Rank sum testing was used to verify the sequential activation of cortical regions across 5 subjects.

RESULTS

In all 5 subjects, HG activation in PM preceded S1 by an average of 53±13ms (P=0.03), PM preceded M1 by 180±40ms (P=0.001) and S1 activation preceded M1 by 136±40ms (P=0.04).

CONCLUSIONS

Sequential HG activation of PM, S1 and M1 regions in preparation for movements is reported. Activity in S1 prior to any overt body movements supports the notion that these neurons may encode sensory information in anticipation of movements, i.e., an efference copy. Our analysis suggests that S1 modulation likely originates from PM.

SIGNIFICANCE

First electrophysiological evidence of efference copy in humans.

Innovative evaluation of dexterity in pediatrics

STUDY DESIGN

Review paper.

INTRODUCTION

Hand dexterity is multifaceted and essential to the performance of daily tasks. Timed performance and precision demands are the most common features of quantitative dexterity testing. Measurement concepts such as rate of completion, in-hand manipulation and dynamic force control of instabilities are being integrated into assessment tools for the pediatric population.

PURPOSE

To review measurement concepts inherent in pediatric dexterity testing and introduce concepts that are infrequently measured or novel as exemplified with two assessment tools.

METHODS

Measurement concepts included in common assessment tools are introduced first. We then describe seldom measured and novel concepts embedded in two instruments; the Functional Dexterity Test (FDT) and the Strength-Dexterity (SD) Test.

DISCUSSION

The inclusion of novel yet informative tools and measurement concepts in our assessments could aid our understanding of atypical dexterity, and potentially contribute to the design of targeted therapy programs.

Retention and interference of learned dexterous manipulation: interaction between multiple sensorimotor processes

An object can be used in multiple contexts, each requiring different hand actions. How the central nervous system builds and maintains memory of such dexterous manipulations remains unclear. We conducted experiments in which human subjects had to learn and recall manipulations performed in two contexts, A and B. Both contexts involved lifting the same L-shaped object whose geometry cued its asymmetrical mass distribution. Correct performance required producing a torque on the vertical handle at object lift onset to prevent it from tilting. The torque direction depended on the context, i.e., object orientation, which was changed by 180° object rotation about a vertical axis. With an A1B1A2 context switching paradigm, subjects learned A1 in the first block of eight trials as indicated by a torque approaching the required one. However, subjects made large errors in anticipating the required torque when switching to B1 immediately after A1 (negative transfer), as well as when they had to recall A1 when switching to A2 after learning B through another block of eight lifts (retrieval interference). Classic sensorimotor learning theories attribute such interferences to multi-rate, multi-state error-driven updates of internal models. However, by systematically changing the interblock break duration and within-block number of trials, our results suggest an alternative explanation underlying interference and retention of dexterous manipulation. Specifically, we identified and quantified through a novel computational model the nonlinear interaction between two sensorimotor mechanisms: a short-lived, context-independent, use-dependent sensorimotor memory and a context-sensitive, error-based learning process.

Analysis of grasping strategies and function in hemiparetic patients using an instrumented object

This paper validates a novel instrumented object, the iBox, dedicated to the analysis of grasping and manipulation. This instrumented box can be grasped and manipulated, is fitted with an Inertial Measurement Unit (IMU) and can sense the force applied on each side and transmits measured force, acceleration and orientation data wirelessly in real time. The iBox also provides simple access to data for analysing human motor control features such as the coordination between grasping and lifting forces and complex manipulation patterns. A set of grasping and manipulation experiments was conducted with 6 hemiparetic patients and 5 healthy control subjects. Measures made of the forces, kinematics and dynamics are developed, which can be used to analyse grasping and contribute to assessment in patients. Quantitative measurements provided by the iBox reveal numerous characteristics of the grasping strategies and function in patients: variations in the completion time, changes in the force distribution on the object and grasping force levels, difficulties to adjust the level of applied forces to the task and to maintain it, along with movement smoothness decrease and pathological tremor.

Motor learning of cue-dependent pull-force changes during an isometric precision grip task

The "raspberry task" represents a precision grip task that requires continuous adjustment of grip and pull forces. During this task subjects grip a specialized grip rod and have to increase the pull force linearly while the rod is locked. The aim of this study was to determine whether an associated, initially neutral cue is able to evoke pull-force changes in the raspberry task. A standard delay paradigm was used to study cued pull-force changes during an ongoing movement resulting in unloading. Pull force and EMG activity of hand and arm muscles were recorded from 13 healthy, young subjects. The cue was associated with a complex change in motor behavior. In this task, cued force changes take place more rapidly than in protective reflex systems (in median after the second presentation of the cueing stimulus). A cued force change was detectable in two-thirds of paired trials. Although the force change is produced by a decrease of the EMG activity in several grip- and pull-force-producing muscles, the most significant effect in the majority of the subjects was an increase of the activity of the flexor carpi ulnaris muscle which antagonises corresponding pull-force-producing muscles. Cued force changes require adequately and precisely controlled activation of the muscle groups involved in the movement.

Differences of the medial lemniscus and spinothalamic tract according to the cortical termination areas: A diffusion tensor tractography study

We investigated differences of the medial lemniscus and its thalamocortical pathway (ML), and the spinothalamic tract and its thalamocortical pathway (STT) according to the cortical termination areas. We found that the ML and STT terminated in the motor cortex and the somatosensory cortex. The ML may be closely related to the motor cortex for motor planning and execution, while the STT may be closely related to the cerebral cortex for somatosensory function and motor execution.

Learned manipulation at unconstrained contacts does not transfer across hands

Recent studies about sensorimotor control of the human hand have focused on how dexterous manipulation is learned and generalized. Here we address this question by testing the extent to which learned manipulation can be transferred when the contralateral hand is used and/or object orientation is reversed. We asked subjects to use a precision grip to lift a grip device with an asymmetrical mass distribution while minimizing object roll during lifting by generating a compensatory torque. Subjects were allowed to grasp anywhere on the object’s vertical surfaces, and were therefore able to modulate both digit positions and forces. After every block of eight trials performed in one manipulation context (i.e., using the right hand and at a given object orientation), subjects had to lift the same object in the second context for one trial (transfer trial). Context changes were made by asking subjects to switch the hand used to lift the object and/or rotate the object 180° about a vertical axis. Therefore, three transfer conditions, hand switch (HS), object rotation (OR), and both hand switch and object rotation (HS+OR), were tested and compared with hand matched control groups who did not experience context changes. We found that subjects in all transfer conditions adapted digit positions across multiple transfer trials similar to the learning of control groups, regardless of different changes of contexts. Moreover, subjects in both HS and HS+OR group also adapted digit forces similar to the control group, suggesting independent learning of the left hand. In contrast, the OR group showed significant negative transfer of the compensatory torque due to an inability to adapt digit forces. Our results indicate that internal representations of dexterous manipulation tasks may be primarily built through the hand used for learning and cannot be transferred across hands.

It's too quick to blame myself-the effects of fast and slow rates of change on credit assignment during object lifting

Although there have been substantial research efforts examining the effect of various rates of change in reaching movements, there has been little to no research devoted to this issue during object manipulation tasks. In force-field and visuomotor adaptation studies, two parallel processes have been identified: first, a fast process that adapts and de-adapts quickly is thought to enable the actor to deal with potentially transient perturbations. Second, a slower, but longer lasting process adapts if these initial perturbations persist over time. In a largely separate body of research, the role of credit assignment has been examined in terms of allotting the cause of errors to changes in the body vs. changes in the outside world. Of course, these two processes are usually linked within the real world, with short lasting perturbations most often being linked to external causes and longer lasting perturbations being linked to internal causes. Here, we demonstrate that the increases in load forces associated with a gradual increase in object weight during a natural object lifting task are transferred when lifting a novel object, whereas a sudden increase in object weight is not. We speculate that gradual rates of change in the weight of the object being lifted are attributed to the self, whereas fast rates of change are more likely to be attributed to the external environment. This study extends our knowledge of the multiple timescales involved in motor learning to a more natural object manipulation scenario, while concurrently providing support for the hypothesis that the multiple time scales involved in motor learning are tuned for different learning contexts.

Safety margins in older adults increase with improved control of a dynamic object

Older adults face decreasing motor capabilities due to pervasive neuromuscular degradations. As a consequence, errors in movement control increase. Thus, older individuals should maintain larger safety margins than younger adults. While this has been shown for object manipulation tasks, several reports on whole-body activities, such as posture and locomotion, demonstrate age-related reductions in safety margins. This is despite increased costs for control errors, such as a fall. We posit that this paradox could be explained by the dynamic challenge presented by the body or also an external object, and that age-related reductions in safety margins are in part due to a decreased ability to control dynamics. To test this conjecture we used a virtual ball-in-cup task that had challenging dynamics, yet afforded an explicit rendering of the physics and safety margin. The hypotheses were: (1) When manipulating an object with challenging dynamics, older adults have smaller safety margins than younger adults. (2) Older adults increase their safety margins with practice. Nine young and 10 healthy older adults practiced moving the virtual ball-in-cup to a target location in exactly 2 s. The accuracy and precision of the timing error quantified skill, and the ball energy relative to an escape threshold quantified the safety margin. Compared to the young adults, older adults had increased timing errors, greater variability, and decreased safety margins. With practice, both young and older adults improved their ability to control the object with decreased timing errors and variability, and increased their safety margins. These results suggest that safety margins are related to the ability to control dynamics, and may explain why in tasks with simple dynamics older adults use adequate safety margins, but in more complex tasks, safety margins may be inadequate. Further, the results indicate that task-specific training may improve safety margins in older adults.

A method to study precision grip control in viscoelastic force fields using a robotic gripper

Instrumented objects and multipurpose haptic displays have commonly been used to investigate sensorimotor control of grasping and manipulation. A major limitation of these devices, however, is the extent to which the experimenter can vary the interaction dynamics to fully probe sensorimotor control mechanisms. We propose a novel method to study precision grip control using a grounded robotic gripper with two moving, mechanically coupled finger pads instrumented with force sensors. The device is capable of stably rendering virtual mechanical properties with a wide dynamic range of achievable impedances. Eight viscoelastic force fields with different combinations of stiffness and damping parameters were implemented, and tested on eight healthy subjects performing 30 consecutive repetitions of a grasp, hold, and release task with time and position constraints. Rates of thumb and finger force were found to be highly correlated (r>0.9) during grasping, revealing that, despite the mechanical coupling of the two finger pads, subjects performed grasping movements in a physiological fashion. Subjects quickly adapted to the virtual dynamics (within seven trials), but, depending on the presented force field condition, used different control strategies to correctly perform the task. The proof of principle presented in this paper underscores the potential of such a one-degree-of-freedom robotic gripper to study neural control of grasping, and to provide novel insights on sensorimotor control mechanisms.

Lithium enhances survival and regrowth of spinal motoneurons after ventral root avulsion

Background

During the clinical treatment of the brachial plexus root avulsion (BPRA), reimplantation surgery can not completely repair the motor function of the hand because the axonal growth velocity of the spinal motoneurons (MNs) is too slow to re-innervate the intrinsic hand muscles before muscle atrophy. Here, we investigated whether lithium can enhance the regenerative capacity of the spinal MNs in a rat model of BPRA.

Results

The avulsion and immediate reimplantation of the C7 and C8 ventral roots were performed and followed with daily intraperitoneal administration of a therapeutic concentrationof LiCl. After a 20 week long-term rehabilitation, the motor function recovery of the injured forepaw was studied by a grasping test. The survival and regeneration of MNs were checked by choline acetyltransferase (ChAT) immunofluorescence and by Fluoro-Gold (FG) retrograde labeling through the median and ulnar nerves of the ventral horn MNs. The number and diameter of the nerve fibers in the median nerve were assessed by toluidine blue staining. Our results showed that lithium plus reimplantation therapy resulted in a significantly higher grasping strength of the digits of the injured forepaw. Lithium plus reimplantation allowed 45.1% ± 8.11% of ChAT-positive MNs to survive the injury and increased the number and diameter of nerve fibers in the median nerve. The number of FG-labeled regenerative MNs was significantly elevated in all of the reimplantation animals. Our present data proved that lithium can enhance the regenerative capacity of spinal MNs.

Conclusions

These results suggest that immediate administration of lithium could be used to assist reimplantation surgery in repairing BPRA injuries in clinical treatment.

Concurrent motor and cognitive function in multiple sclerosis: a motor overflow and motor stability study

OBJECTIVE AND BACKGROUND

The interplay between motor and cognitive functions during performance of concurrent tasks is not fully understood but is known to vary depending on task characteristics and across clinical populations. Our controlled study examined how a concurrent digit span task affected a motor stability and motor overflow task in patients with multiple sclerosis (MS).

METHOD

We asked 22 patients with MS and 22 matched controls to exert force on a transducer using 1 index finger at a time. We measured their motor stability (accuracy of voluntary force production) and motor overflow (involuntary force produced by the opposite, inactive finger). During half of the trials, the participants concurrently performed a digit span task.

RESULTS

Overall, the patients with MS had more motor overflow and less motor stability than the controls; these measures correlated with the patients' disease severity. Adding the concurrent task affected motor stability; this relationship varied with the required level of exerted force. Motor overflow was lower during trials with the concurrent task. The concurrent task affected patients and controls similarly for both motor stability and overflow.

CONCLUSIONS

This study demonstrates preserved motor function in a concurrent-task paradigm in patients with MS, and sheds further light on the relationship between attention and motor function in both the patients and controls. This research may help to inform rehabilitation for everyday life situations in which patients routinely perform cognitive and motor tasks simultaneously.

Impact of distal median neuropathy on handwriting performance for patients with carpal tunnel syndrome in office and administrative support occupations

PURPOSE

This study investigates the handwriting performance of patients with carpal tunnel syndrome (CTS) and healthy controls in office and administrative support occupations, adopting both biomechanical and functional perspectives. This work also explores how surgical intervention altered the performance of the CTS patients.

METHODS

Fourteen CTS patients and 14 control subjects were recruited to complete a self-reported survey and participate in sensory tests, hand strength, dexterity and handwriting tasks using a custom force acquisition pen along with motion capture technology. Based on the results of these, the sensory measurements, along with functional and biomechanical parameters, were used to determine the differences between the groups and also reveal any improvements that occurred in the CTS group after surgical intervention.

RESULTS

The CTS patients showed significantly poorer hand sensibility and dexterity than the controls, as well as excessive force exertion of the digits and pen tip, and less efficient force adjustment ability during handwriting. After surgery and sensory recovery, the hand dexterity and pen tip force of the CTS patients improved significantly. The force adjustment abilities of the digits also increased, but these changes were not statistically significant.

CONCLUSIONS

This study provides the objective measurements and novel apparatus that can be used to determine impairments in the handwriting abilities of office or administrative workers with CTS. The results can also help clinicians or patients to better understand the sensory-related deficits in sensorimotor control of the hand related to CTS, and thus develop and implement more suitable training or adaptive protocols.

Scaling and coordination deficits during dynamic object manipulation in Parkinson's disease

The ability to reach for and dynamically manipulate objects in a dexterous fashion requires scaling and coordination of arm, hand, and fingertip forces during reach and grasp components of this behavior. The neural substrates underlying dynamic object manipulation are not well understood. Insight into the role of basal ganglia-thalamocortical circuits in object manipulation can come from the study of patients with Parkinson's disease (PD). We hypothesized that scaling and coordination aspects of motor control are differentially affected by this disorder. We asked 20 PD patients and 23 age-matched control subjects to reach for, grasp, and lift virtual objects along prescribed paths. The movements were subdivided into two types, intensive (scaling) and coordinative, by detecting their underlying self-similarity. PD patients off medication were significantly impaired relative to control subjects for both aspects of movement. Intensive deficits, reduced peak speed and aperture, were seen during the reach. Coordinative deficits were observed during the reach, namely, the relative position along the trajectory at which peak speed and aperture were achieved, and during the lift, when objects tilted with respect to the gravitational axis. These results suggest that basal ganglia-thalamocortical circuits may play an important role in fine motor coordination. Dopaminergic therapy significantly improved intensive but not coordinative aspects of movements. These findings are consistent with a framework in which tonic levels of dopamine in the dorsal striatum encode the energetic cost of a movement, thereby improving intensive or scaling aspects of movement. However, repletion of brain dopamine levels does not restore finely coordinated movement.

How the impact of median neuropathy on sensorimotor control capability of hands for diabetes: an achievable assessment from functional perspectives

To comprehend the sensorimotor control ability in diabetic hands, this study investigated the sensation, motor function and precision pinch performances derived from a pinch-holding-up activity (PHUA) test of the hands of diabetic patients and healthy subjects. The precision, sensitivity and specificity of the PHUA test in the measurements of diabetic patients were also analyzed. We hypothesized that the diabetic hands would have impacts on the sensorimotor functions of the hand performances under functionally quantitative measurements. One hundred and fifty-nine patients with clinically defined diabetes mellitus (DM) and 95 age- and gender-matched healthy controls were included. Semmes-Weinstein monofilament (SWM), static and moving two-point discrimination (S2PD and M2PD), maximal pinch strength and precision pinch performance tests were conducted to evaluate the sensation, motor and sensorimotor status of the recruited hands. The results showed that there were significant differences (all p<0.05) in SWM, S2PD, M2PD and maximum pinch strength between the DM and control groups. A higher force ratio in the DM patients than in the controls (p<0.001) revealed a poor ability of pinch force adjustment in the DM patients. The percentage of maximal pinch strength was also significantly different (p<0.001) between the DM and control groups. The sensitivity, specificity and area under the receiver operating characteristic curve were 0.85, 0.51, and 0.724, respectively, for the PHUA test. Statistically significant degradations in sensory and motor functions and sensorimotor control ability were observed in the hands of the diabetic patients. The PHUA test could be feasibly used as a clinical tool to determine the sensorimotor function of the hands of diabetic patients from a functional perspective.

Assisted movement with proprioceptive stimulation reduces impairment and restores function in incomplete spinal cord injury

OBJECTIVE

To test whether treatment with assisted movement with enhanced sensation (AMES) using vibration to the antagonist muscle would reduce impairments and restore upper limb function in people with incomplete tetraplegia.

DESIGN

Prospective, pre-post study.

SETTING

Laboratory and rehabilitation hospital.

PARTICIPANTS

We recruited 15 arms from 10 individuals (8 men; mean age, 40.5 y; mean years postspinal cord injury [SCI], 3) with chronic, incomplete tetraplegia.

INTERVENTION

Two or three 20-minute sessions per week over 9 to 13 weeks (25 sessions total) on the AMES device, which combines repeated movement with targeted vibration to the antagonist muscle.

MAIN OUTCOME MEASURES

Strength and active motion tests on the AMES device; International Standards for the Neurological Classification of SCI (ISNCSCI) motor and sensory examinations; Modified Ashworth Scale (MAS); grasp and release test (GRT); Van Lieshout Test (VLT); and Capabilities of Upper Extremity questionnaire (CUE).

RESULTS

The AMES strength test scores improved significantly in metacarpophalangeal flexion (P=.024) and extension (P=.007) and wrist flexion (P=.001) and extension (P<.000). The AMES active motion scores improved in the hand (P=.001) and wrist (P=.001). The MAS and ISNCSCI scores remained unchanged, whereas the GRT scores increased (P=.025). Post hoc analysis showed a trend from pre- to posttreatment (P=.068) and a significant change from pretreatment to 3-month follow-up (P=.046). There was no significant change in the VLT (P=.951) or the CUE (P=.164). Five of the 10 participants reported a return of sensation to the digits after the first, second, or third treatment session.

CONCLUSIONS

People with chronic, incomplete tetraplegia may experience improvements in impairments and function after treatment on a device combining assisted movement and proprioceptive stimulation. Further investigation is warranted.

Usefulness of the combined motor evoked and somatosensory evoked potentials for the predictive index of functional recovery after primary pontine hemorrhage

Objective

To investigate the predictive index of functional recovery after primary pontine hemorrhage (PPH) using the combined motor evoked potential (MEP) and somatosensory evoked potential (SEP) in comparison to the hematoma volume and transverse diameter measured with computerized tomography.

Methods

Patients (n=14) with PPH were divided into good- and poor-outcome groups according to the modified Rankin Score (mRS). We evaluated clinical manifestations, radiological characteristics, and the combined MEP and SEP responses. The summed MEP and SEP (EP sum) was compared to the hematoma volume and transverse diameter predictive index of global disability, gait ability, and trunk stability in sitting posture.

Results

All measures of functional status and radiological parameters of the good-outcome group were significantly better than those of the poor-outcome group. The EP sum showed the highest value for the mRS and functional ambulatory category, and transverse diameter showed the highest value for "sitting-unsupported" of Berg Balance Scale.

Conclusion

The combined MEP and SEP is a reliable and useful tool for functional recovery after PPH.

Corticospinal excitability underlying digit force planning for grasping in humans

Control of digit forces for grasping relies on sensorimotor memory gained from prior experience with the same or similar objects and on online sensory feedback. However, little is known about neural mechanisms underlying digit force planning. We addressed this question by quantifying the temporal evolution of corticospinal excitability (CSE) using single-pulse transcranial magnetic stimulation (TMS) during two reach-to-grasp tasks. These tasks differed in terms of the magnitude of force exerted on the same points on the object to isolate digit force planning from reach and grasp planning. We also addressed the role of intracortical circuitry within primary motor cortex (M1) by quantifying the balance between short intracortical inhibition and facilitation using paired-pulse TMS on the same tasks. Eighteen right-handed subjects were visually cued to plan digit placement at predetermined locations on the object and subsequently to exert either negligible force ("low-force" task, LF) or 10% of their maximum pinch force ("high-force" task, HF) on the object. We found that the HF task elicited significantly smaller CSE than the LF task, but only when the TMS pulse coincided with the signal to initiate the reach. This force planning-related CSE modulation was specific to the muscles involved in the performance of both tasks. Interestingly, digit force planning did not result in modulation of M1 intracortical inhibitory and facilitatory circuitry. Our findings suggest that planning of digit forces reflected by CSE modulation starts well before object contact and appears to be driven by inputs from frontoparietal areas other than M1.

Virtual grasping: closed-loop force control using electrotactile feedback

Closing the control loop by providing somatosensory feedback to the user of a prosthesis is a well-known, long standing challenge in the field of prosthetics. Various approaches have been investigated for feedback restoration, ranging from direct neural stimulation to noninvasive sensory substitution methods. Although there are many studies presenting closed-loop systems, only a few of them objectively evaluated the closed-loop performance, mostly using vibrotactile stimulation. Importantly, the conclusions about the utility of the feedback were partly contradictory. The goal of the current study was to systematically investigate the capability of human subjects to control grasping force in closed loop using electrotactile feedback. We have developed a realistic experimental setup for virtual grasping, which operated in real time, included a set of real life objects, as well as a graphical and dynamical model of the prosthesis. We have used the setup to test 10 healthy, able bodied subjects to investigate the role of training, feedback and feedforward control, robustness of the closed loop, and the ability of the human subjects to generalize the control to previously “unseen” objects. Overall, the outcomes of this study are very optimistic with regard to the benefits of feedback and reveal various, practically relevant, aspects of closed-loop control.

The role of left supplementary motor area in grip force scaling

Skilled tool use and object manipulation critically relies on the ability to scale anticipatorily the grip force (GF) in relation to object dynamics. This predictive behaviour entails that the nervous system is able to store, and then select, the appropriate internal representation of common object dynamics, allowing GF to be applied in parallel with the arm motor commands. Although psychophysical studies have provided strong evidence supporting the existence of internal representations of object dynamics, known as “internal models”, their neural correlates are still debated. Because functional neuroimaging studies have repeatedly designated the supplementary motor area (SMA) as a possible candidate involved in internal model implementation, we used repetitive transcranial magnetic stimulation (rTMS) to interfere with the normal functioning of left or right SMA in healthy participants performing a grip-lift task with either hand. TMS applied over the left, but not right, SMA yielded an increase in both GF and GF rate, irrespective of the hand used to perform the task, and only when TMS was delivered 130–180 ms before the fingers contacted the object. We also found that both left and right SMA rTMS led to a decrease in preload phase durations for contralateral hand movements. The present study suggests that left SMA is a crucial node in the network processing the internal representation of object dynamics although further experiments are required to rule out that TMS does not affect the GF gain. The present finding also further substantiates the left hemisphere dominance in scaling GF.

Regional cerebral blood flow alterations in obstructive sleep apnea

Obstructive sleep apnea (OSA) is a condition characterized by upper airway muscle atonia with continued diaphragmatic efforts, resulting in repeated airway obstructions, periods of intermittent hypoxia, large thoracic pressure changes, and substantial shifts in arterial pressure with breathing cessation and resumption. The hypoxic exposure and hemodynamic changes likely induce the structural and functional deficits found in multiple brain areas, as shown by magnetic resonance imaging (MRI) procedures. Altered cerebral blood flow (CBF) may contribute to these localized deficits; thus, we examined regional CBF, using arterial spin labeling procedures, in 11 OSA (age, 49.1±12.2 years; 7 male) and 16 control subjects (42.3±10.2 years; 6 male) with a 3.0-Tesla MRI scanner. CBF maps were calculated, normalized to a common space, and regional CBF values across the brain quantified. Lowered CBF values emerged near multiple bilateral brain sites in OSA, including the corticospinal tracts, superior cerebellar peduncles, and pontocerebellar fibers. Lateralized, decreased CBF appeared near the left inferior cerebellar peduncles, left tapetum, left dorsal fornix/stria terminalis, right medial lemniscus, right red nucleus, right midbrain, and midline pons. Regional CBF values in OSA are significantly reduced in major sensory and motor fiber systems and motor regulatory sites, especially in structures mediating motor coordination; those reductions are often lateralized. The asymmetric CBF declines in motor regulatory areas may contribute to loss of coordination between upper airway and diaphragmatic musculature, and lead to further damage in the syndrome.

Haptic-motor transformations for the control of finger position

Dexterous manipulation relies on modulation of digit forces as a function of digit placement. However, little is known about the sense of position of the vertical distance between finger pads relative to each other. We quantified subjects' ability to match perceived vertical distance between the thumb and index finger pads (d_y) of the right hand (“reference” hand) using the same or opposite hand (“test” hand) after a 10-second delay without vision of the hands. The reference hand digits were passively placed non-collinearly so that the thumb was higher or lower than the index finger (d_y = 30 or –30 mm, respectively) or collinearly (d_y = 0 mm). Subjects reproduced reference hand d_y by using a congruent or inverse test hand posture while exerting negligible digit forces onto a handle. We hypothesized that matching error (reference hand d_y minus test hand d_y) would be greater (a) for collinear than non-collinear d_ys, (b) when reference and test hand postures were not congruent, and (c) when subjects reproduced d_y using the opposite hand. Our results confirmed our hypotheses. Under-estimation errors were produced when the postures of reference and test hand were not congruent, and when test hand was the opposite hand. These findings indicate that perceived finger pad distance is reproduced less accurately (1) with the opposite than the same hand and (2) when higher-level processing of the somatosensory feedback is required for non-congruent hand postures. We propose that erroneous sensing of finger pad distance, if not compensated for during contact and onset of manipulation, might lead to manipulation performance errors as digit forces have to be modulated to perceived digit placement.

Brain activity during motor imagery of an action with an object: a functional magnetic resonance imaging study

We utilized functional magnetic resonance imaging to investigate the brain regions activated during motor imagery of an action with an object both with and without passively holding the object. Participants performed the following tasks: (1) 'Imagery with Ball' condition: subjects imagined squeezing a foam ball (7cm diameter) while holding the ball, (2) 'Imagery' condition: subjects imagined squeezing a ball without holding the ball, and (3) 'Ball' condition: subjects held the ball without motor imagery. Regions activated by the 'Imagery with Ball' condition were located in the left dorsolateral prefrontal cortex (DLPFC), supplemental motor areas (SMA), inferior parietal lobule (IPL), superior parietal lobule (SPL), insula, cerebellum and basal ganglia. A direct comparison revealed that the right DLPFC and the right IPL showed a higher level of activation during the 'Imagery with Ball' than during the 'Imagery'+'Ball' conditions. Our studies suggested that the right front-parietal networks were involved in the motor imagery of an action with an object.

Cross recurrence quantification analysis of precision grip following peripheral median nerve block

Background

Precision grip by the thumb and index finger is vulnerable to sensorimotor deficits. Traditional biomechanical parameters offer limited insight into the dynamical coordination between digits during precision grip. In this study, the thumb and index finger were viewed as “coupled systems”, and a cross recurrence quantification analysis (CRQA) was used to examine the changes of interdigit dynamics and synchronization caused by peripheral median nerve block.

Methods

Seven subjects performed a precision grip by holding an instrumented handle before and after median nerve block at the wrist. The forces and the torques at each digit-handle interface were recorded with two six-component transducers. For CRQA, the percentage of recurrence rate (%RR), percentage of determinism (%DET), longest diagonal line (Lmax) and percentage of laminarity (%LAM) were computed for the force, torque and center of pressure (COP) signals. Phase synchronization of the thumb and index finger was examined based on the τ-recurrence rate. Paired t-tests and Wilcoxon signed-rank tests were used for statistical comparisons. The twin-surrogate hypothesis test was used to examine phase synchronization.

Results

Nerve block led to significant increases (p < 0.05) for %DET, Lmax and %LAM in all components of force, torque, and COP. Only the normal force met the conditions of phase synchronization for all successfully completed pre- and post-block grasping trials. The probability of synchronization with larger time lags (τ > 0.1 s) increased after nerve block. The percentage of trials that the thumb led the index finger increased from 52% (pre-block) to 86% (post-block).

Conclusions

Nerve block caused more deterministic structures in force, torque and COP when the thumb interacted with the index finger. A compensatory mechanism may be responsible for this change. Phase synchronization between the opposite normal forces exerted by the thumb and index finger would be an essential dynamical principle for a precision grip. The nerve block resulted in an increased interdigit phase delay and increased probability that the thumb leads the index finger. The CRQA provides an effective tool to examine interdigit coordination during precision grip and has the potential for clinical evaluation of hand dysfunction.

Setup of a novel biofeedback prototype for sensorimotor control of the hand and preliminary application in patients with peripheral nerve injuries

BACKGROUND

Biofeedback training is widely used for rehabilitative intervention in patients with central or peripheral nervous impairment to train correct movement patterns; however, no biofeedback apparatus is currently available to correct pinch force ratios for patients with sensory deficiencies.

DESIGN

A cross-sectional and longitudinal design was used in an observational measurement study for establishing a prototype and to determine the effects of biofeedback intervention, respectively.

OBJECTIVE

This study aimed to develop a computerized evaluation and re-education biofeedback (CERB) prototype for application in clinical settings.

METHODS

A CERB prototype was developed integrating pinch apparatus hardware, a biofeedback user-controlled interface, and a data processing/analysis interface to detect momentary pinch performances in 79 people with normal hand sensation. Nine patients with hand sensory impairments were recruited to investigate the effects of training hand function with the CERB prototype.

RESULTS

Hand dominance, pinch pattern, and age significantly affected the peak pinch force and force ratio for lifting a 480-g object with a steel surface. In the case of the 79 volunteers with normal hand sensation, hand dominance affected the time lag between peak pinch force and maximum load; however, it was unaffected by pinch pattern or age. Training with the CERB prototype produced significant improvements in force ratio and better performance in the pin insertion subtests, although the results for both 2-point discriminative and Semmes-Weinstein monofilament tests did not change significantly.

LIMITATIONS

The intervention findings are preliminary.

CONCLUSIONS

This study developed a conjunct system suited for evaluating and restoring sensorimotor function for patients with impaired hand sensibility. The results from the participants with normal hand sensation could serve as a reference database for comparison with patients with nerve injuries.

Contribution of the cerebellum to the coupling of grip force and pull force during an isometric precision grip task

This study addresses the influence of the cerebellum on the performance of an isometric precision grip task. For the task, in which the process of "picking a raspberry" is simulated, grip force and pull force had to be increased linearly for a duration of 1-5 s (pull phase) to accomplish the task skillfully. The performance of 11 patients suffering from degenerative cerebellar disease was analyzed and compared with the performance of 11 age- and sex-matched healthy control subjects. Patients with cerebellar disease showed systematic deviations of the pull force slope from a linear trend, dividing the pull phase into two intervals. After an initial sharp and brief increase of pull force (first interval), patients maintained the achieved pull force level almost constant without further increase (second interval). Although controls showed changes in the pull force slope also, they increased pull force during the whole pull phase. Coupling of grip force and pull force was analyzed using stochastic frontier analysis. This technique allows covariation of grip force and the resulting pull force to be analyzed depending on the variation of the grip force. In the patients, grip force and pull force were coupled efficiently only in the first interval. During the second interval, grip force was often exaggerated compared with pull force. In conclusion, patients with cerebellar diseases have difficulties in producing smooth isometric movements and in coupling grip force and pull force efficiently.

Effects of repetitive lifting on kinematics: inadequate anticipatory control or adaptive changes?

In the present study, the effects of repetition on the kinematics in discrete lifting were studied in 10 subjects who lifted a barbell weighing 10% of body mass at a determined speed and along a determined trajectory 630 times during about 40 min. Three-dimensional (3-D) kinematics of the feet, lower and upper legs, pelvis, and trunk were recorded in the first 3 and the final 3 lifting movements of each set of 70 lifts. Over time, trunk extension velocity in the initial 250 ms of the lifting movement decreased, reaching negative (increasing flexion) values in most subjects. In contrast, hip extension velocity increased. Those changes resulted in an increased phase lag between hip and trunk extension. Also, over time, subjects started the lifting movement with their legs more extended and their trunks further flexed. Finally, the motion of the trunk around its longitudinal axis (twisting) increased. The increase in phase lag between hip and trunk extension is interpreted as a consequence of fatigue-more specifically, as the result of a decreased rate of force development of the back muscles. The change in initial posture more likely is an adaptation that functions to retard further fatigue development.

An involuntary stereotypical grasp tendency pervades voluntary dynamic multifinger manipulation

We used a novel apparatus with three hinged finger pads to characterize collaborative multifinger interactions during dynamic manipulation requiring individuated control of fingertip motions and forces. Subjects placed the thumb, index, and middle fingertips on each hinged finger pad and held it-unsupported-with constant total grasp force while voluntarily oscillating the thumb's pad. This task combines the need to 1) hold the object against gravity while 2) dynamically reconfiguring the grasp. Fingertip force variability in this combined motion and force task exhibited strong synchrony among normal (i.e., grasp) forces. Mechanical analysis and simulation show that such synchronous variability is unnecessary and cannot be explained solely by signal-dependent noise. Surprisingly, such variability also pervaded control tasks requiring different individuated fingertip motions and forces, but not tasks without finger individuation such as static grasp. These results critically extend notions of finger force variability by exposing and quantifying a pervasive challenge to dynamic multifinger manipulation: the need for the neural controller to carefully and continuously overlay individuated finger actions over mechanically unnecessary synchronous interactions. This is compatible with-and may explain-the phenomenology of strong coupling of hand muscles when this delicate balance is not yet developed, as in early childhood, or when disrupted, as in brain injury. We conclude that the control of healthy multifinger dynamic manipulation has barely enough neuromechanical degrees of freedom to meet the multiple demands of ecological tasks and critically depends on the continuous inhibition of synchronous grasp tendencies, which we speculate may be of vestigial evolutionary origin.

Material evidence: interaction of well-learned priors and sensorimotor memory when lifting objects

Skilled object lifting requires the prediction of object weight. When lifting new objects, such prediction is based on well-learned size-weight and material-density correlations, or priors. However, if the prediction is erroneous, people quickly learn the weight of the particular object and can use this knowledge, referred to as sensorimotor memory, when lifting the object again. In the present study, we explored how sensorimotor memory, gained when lifting a given object, interacts with well-learned material-density priors when predicting the weight of a larger but otherwise similar-looking object. Different groups of participants 1st lifted 1 of 4 small objects 10 times. These included a pair of wood-filled objects and a pair of brass-filled objects where 1 of each pair was covered in a wood veneer and the other was covered in a brass veneer. All groups then lifted a larger, brass-filled object with the same covering as the small object they had lifted. For each lift, we determined the initial peak rate of change of vertical load-force rate and the load-phase duration, which provide estimates of predicted object weight. Analysis of the 10th lift of the small cube revealed no effects of surface material, indicating participants learned the appropriate forces required to lift the small cube regardless of object appearance. However, both surface material and core material of the small cube affected the 1st lift of the large block. We conclude that sensorimotor memory related to object density can contribute to weight prediction when lifting novel objects but also that long-term priors related to material properties can influence the prediction.

Grasping multiple sclerosis: do quantitative motor assessments provide a link between structure and function?

Motor disability in MS is commonly assessed by the Expanded Disability Status Scale (EDSS). Categorical rating scales are limited by subjective error and inter-rater variability. Therefore, objective and quantitative measures of motor disability may be useful to supplement the EDSS in the setting of clinical trials. It was previously shown that grip-force-variability (GFV) is increased in MS. We hypothesized that GFV may be an objective measure of motor disability in MS. To investigate whether the increase in GFV in MS is correlated to the clinical disability as assessed by the EDSS and to microstructural changes in the brain as assessed by diffusion tensor imaging, GFV was recorded in a grasping and lifting task in 27 MS patients and 23 controls using a grip-device equipped with a force transducer. The EDSS was assessed by neurologists experienced in MS. Patients underwent diffusion tensor imaging at 3T to assess the fractional anisotropy (FA) of the cerebral white matter as a measure of microstructural brain integrity. GFV was increased in MS and correlated to changes in the FA of white matter in the vicinity of the somatosensory and visual cortex. GFV also correlated with the EDSS. GFV may be a useful objective measure of motor dysfunction in MS linked to disability and structural changes in the brain. Our data suggests that GFV should be further explored as an objective measure of motor dysfunction in MS. It could supplement the EDSS, e.g., in proof of concept studies.

On-line coordination in complex goal-directed movements: a matter of interactions between several loops

Motor flexibility is the ability to rapidly modify behavior when unexpected perturbations occur. In goal directed movements, this process may be involved during the motor execution itself, by using on-line motor corrections, or off-line, on a trial-by-trial basis. A consensus has emerged to describe and unify these two dependant processes within the framework of the internal models theory in which the cerebellum is involved in error processing. However, this general framework may be incomplete to describe on-line motor corrections when complex motor coordination is involved in the task. In particular, interaction torques existing between different effectors limit the independence between different controllers that could be considered to control various body parts. In addition, recent findings suggest that different (sub)-cortical loops may be involved during orienting responses to visual stimuli but also during on-line motor corrections following visual perturbations. The way these different loops with different dynamics interact but achieve the same motor goal is an important problem in motor control. The simplest organization may be sequential, as in the well-known stretch reflex. This implies that during on-line corrections, the nervous system may be involved in a distributed fashion and that motor plans and synergies depend both on anatomical and temporal constraints. More particularly, motor plans and synergies may be stored and may differ according to the (sub)-cortical loops involved during the whole on-line correction process. Finally, questions concerning the independence (or not) of these loops remain unanswered. The case of strict independence would mean that between the various corrective loops, (i) error processing and (ii) motor plans/synergies would be different. By contrast, in a situation of dependency, it would probably mean that interactions would link lower (and faster) to upper (and longer) loops by informing these latter of the motor corrections sent by the former, similarly to an efference copy.

Effects of carpal tunnel syndrome on adaptation of multi-digit forces to object texture

OBJECTIVE

The ability to adapt digit forces to object properties requires both anticipatory and feedback-driven control mechanisms which can be disrupted in individuals with a compromised sensorimotor system. Carpal tunnel syndrome (CTS) is a median nerve compression neuropathy affecting sensory and motor function in a subset of digits in the hand. Our objective was to examine how CTS patients coordinate anticipatory and feedback-driven control for multi-digit grip force adaptation.

METHODS

We asked CTS patients and healthy controls to grasp, lift, and hold an object with different textures.

RESULTS

CTS patients effectively adapted their digit forces to changes in object texture, but produced excessive grip forces. CTS patients also produced larger peak force rate profiles with fewer modulations of normal force prior to lift onset than did controls and continued to increase grip force throughout the lift whereas forces were set at lift onset for the controls.

CONCLUSIONS

These findings suggest that CTS patients use less online sensory feedback for fine-tuning their grip forces, relying more on anticipatory control than do healthy controls.

SIGNIFICANCE

These characteristics in force adaptation in CTS patients indicate impaired sensorimotor control which leads to excessive grip forces with the potential to further exacerbate their median nerve compression.

Termination differences in the primary sensorimotor cortex between the medial lemniscus and spinothalamic pathways in the human brain

The medial lemniscus (ML) and its thalamocortical pathway is responsible for proprioception, in contrast, the spinothalamic tract (ST) and its thalamocortical pathway is the neural tract for pain and body temperature. Therefore, the ML pathway plays a crucial role in skillful movements and may be more linked to motor function than the ST pathway. We investigated the differences in the distribution of the primary motor cortex (M1) and the primary somatosensory cortex (S1) between the ML and ST pathways. Adults (mean age: 40.4 years, range: 21-61 years) were recruited for this study. The seed masks for the ML and ST pathways were given on the color map of the medulla according to the known anatomy and waypoint masks were placed on the ventro-postero-lateral nucleus of the thalamus. The volume of ML pathway did not show any difference between the M1 (10.94) and S1 (13.02) (p>0.05). By contrast, the mean voxel number of the ST pathway in the M1 (18.25) and S1 (27.38) showed significant difference between the M1 and S1 (p<0.05). As for relative voxel number percentage of the M1 compared to the S1, the ML pathway (84%) was significantly higher than ST pathway (67%) (p<0.05). We found that more neural fibers of the ML pathway were terminated in the M1 relative to the S1 compared to the SLP, and this may be linked to the inherent execution of movements of the M1.

Medial lemniscus lesion in pediatric hemiplegic patients without corticospinal tract and posterior thalamic radiation lesion

OBJECTIVES

Using diffusion tensor imaging (DTI), we investigated the state of medial lemniscus (ML), corticospinal tract (CST), and posterior thalamic radiation (PTR), which were expected as probable reasons for clinical hemiplegia in pediatric patients, especially those who showed impaired fine motor control and proprioception, but no definite motor weakness or spasticity.

METHODS

We recruited 13 hemiplegic patients and 8 age-matched healthy control subjects. Fractional anisotropy (FA) and apparent diffusion coefficient (ADC) for the bilateral ML, CST, and PTR were calculated and compared between the affected hemisphere of the patient (AP), the unaffected hemisphere of the patient (UP), and the mean value of the bilateral hemispheres in control subjects (MC).

RESULTS

FA and ADC values for the CST and PTR did not differ significantly between the AP, UP, and MC subgroups (p > 0.05). However, the FA value for the ML in AP showed a significant decrease, compared with that in UP (p = 0.012) and MC (p = 0.047). DTT for the CST and PTR showed preserved integrity and ML in the UP also had continuity to the cortex; however, ML in AP showed disruption.

CONCLUSIONS

Using DTI, we demonstrated that the ML lesion might be related to clinical hemiplegia in pediatric patients.

Grip force control in individuals with hand osteoarthritis

Individuals with hand osteoarthritis (OA) experience pain and stiffness that could result in significant limitations in the performance of everyday activities involving upper extremities. The purpose of the study was to evaluate grip force control in individuals with hand OA during the lifting of an object. The study design used was a case-control study. Ten females with hand OA and a group of age-matched females performed two functional tasks: lifting an instrumented object vertically while the load was suddenly changed and lifting and placing the object on a shelf. Load Force Peak, Grip force at liftoff, Grip Force Peak, Time Lag, and Latency were measured and analyzed. Individuals with hand OA were able to modulate the magnitude and temporal parameters of grip force; however, they applied higher grip forces (at liftoff [p=0.002]; Grip Force Peak [p=0.039]) and demonstrated a longer Latency (p=0.015) to manipulate the instrumented object when compared with the control subjects. The Load Force Peak and Time Lag were not significantly different between the two groups. Detailed information about how individuals with hand OA perform prehension activities of daily living will help to better understand the limitations of grip force control in these individuals.

Production of finely graded forces in humans: effects of simulated weightlessness by water immersion

We have shown before that subjects exposed to a changed gravitoinertial environment produce exaggerated manual forces. From the observed pattern of findings, we argued that initial forces were exaggerated because of abnormal vestibular activity and peak forces because of degraded proprioceptive feedback. If so, only peak but not initial forces should be affected by water immersion, an environment that influences proprioceptive feedback but not vestibular activity. The present study was undertaken to scrutinize this prediction. Twelve subjects sat in a chair once immersed in water and once on dry land, while producing pre-trained isometric forces with a joystick. In a control experiment, subjects performed a four-choice reaction-time task. During the joystick task, produced initial forces were comparable in water and on land, while peak (+24%) and end forces (+22%) were significantly higher in water, as was their reaction time (+6%). During the control task, reaction time was comparable in water and on land. Our findings corroborate the above notion that initial forces increase when the vestibular system is stimulated (gravitoinertial change, visual field motion, but not water immersion), while peak forces increase when proprioceptive feedback is degraded (probably all three scenarios) and are not corrected until response end. Our findings further confirm the absence of cognitive slowing in simple-choice reaction tasks under shallow-water immersion conditions.

Asymmetry in grasp force matching and sense of effort

While asymmetries in upper limb force matching have been observed, the mechanisms underlying asymmetry in the sense of effort have not been conceptualized. The aim of this study was to investigate asymmetries in the perception and reproduction of grasp force. Forty-two young adults, 22 right-handed (RH) and 20 left-handed (LH), were, respectively, divided into three groups according to differences between their right and left-hand strength (left stronger than right, right stronger than left and right & left equivalent). A reference force, representing 20% of the maximal voluntary contraction (MVC) produced by the right or left hand, was matched with same hand (Ipsilateral Remembered--IR) or opposite (Contralateral Remembered--CR) hand. The matching relative error was 92% (for RH) and 46% (for LH) greater in the CR than IR condition. Asymmetries in matching were significant for RH participants only in the CR condition and were dependent on right/left differences in hand strength as shown by the constant error (CE). For this RH population, right-hand overshoot of the left-hand reference and left-hand undershoot of the right-hand reference were significant when the right hand was stronger than the left. Asymmetry remained significant when CE was normalized (%MVC). Asymmetry was reduced when the strength of each hand was equivalent or when the left hand was stronger than the right. These findings suggest that effort perception is asymmetric in RH but not in LH individuals. The hand x strength interaction indicates that asymmetry in force matching is a consequence of both a difference in the respective cortical representations and motor components, which confer a different "gain" (input-output relationship) to each system. The similarity with position sense asymmetry suggests that the gain concept may be generalized to describe some functional/performance differences between the two hand/hemisphere systems. The more symmetrical performance of the LH than RH group underlines that context specific influence of handedness, hemisphere dominance and hemispheric interactions modulate performance symmetries/asymmetries.