Controlling instabilities in manipulation requires specific cortical-striatal-cerebellar networks

Task-dependent alteration of beta-band intermuscular coherence is associated with ipsilateral corticospinal tract excitability

Beta-band (15-30 Hz) synchronization between the EMG signals of active limb muscles can serve as a non-invasive assay of corticospinal tract integrity. Tasks engaging a single limb often primarily utilize one corticospinal pathway, although bilateral neural circuits can participate in goal-directed actions involving multi-muscle coordination and utilization of feedback. Suboptimal utilization of such circuits after CNS injury can result in unintended mirror movements and activation of pathological synergies. Accordingly, it is important to understand how the actions of one limb (e.g., a less-affected limb after strokes) influence the opposite corticospinal pathway for the rehabilitation target. Certain unimanual actions decrease the excitability of the "unengaged" corticospinal tract, presumably to prevent mirror movement, but there is no direct way to predict the extent to which this will occur. In this study, we tested the hypothesis that task-dependent changes in beta-band drives to muscles of one hand will inversely correlate with changes in the opposite corticospinal tract excitability. Ten participants completed spring pinching tasks known to induce differential 15-30 Hz drive to muscles. During compressions, transcranial magnetic stimulation single pulses to the ipsilateral M1 were delivered to generate motor-evoked potentials in the unengaged hand. The task-induced changes in ipsilateral corticospinal excitability were inversely correlated with associated changes in EMG-EMG coherence of the task hand. These results demonstrate a novel connection between intermuscular coherence and the excitability of the "unengaged" corticospinal tract and provide a springboard for further mechanistic studies of unimanual tasks of varying difficulty and their effects on neural pathways relevant to rehabilitation.

Impairments in grip and pinch force accuracy and steadiness in people with osteoarthritis of the hand: A case-control comparison

BACKGROUND

Symptomatic hand osteoarthritis (OA) is severely disabling condition. Limited evidence has focused on force control measures in this population.

OBJECTIVES

It was the aim of the present study to determine whether force matching accuracy and steadiness are impaired in people with hand OA. In addition, the relationship between force control measures (accuracy and steadiness) and measures of hand function and pain in people with symptomatic hand OA was explored.

DESIGN

Case-control study.

METHOD

Sixty-two participants with symptomatic hand OA and 26 healthy pain-free controls undertook an isometric grip and pinch force matching task at 50 % of their maximum voluntary contraction. Average pain hand pain was recorded. In addition, the Disability of the Arm Shoulder and Hand Questionnaire (DASH), and the Functional Index of Hand Osteoarthritis were collected.

RESULTS

Grip force-matching accuracy and steadiness were significantly impaired in the hand OA group compared to controls (P < 0.05). Pinch force-matching error was greater in people with hand OA (P < 0.05), however, pinch force steadiness was not different between groups. There was a learning effect in people with hand OA, with resolution of force matching impairments with task repetition. A small positive correlation was identified between grip force control and the DASH. No association was found between other measures of force control and self-reported measures of function or pain.

CONCLUSIONS

People with hand OA presented with greater impairments in measures of submaximal force control. These were correlated with self-reported hand function but not pain. Future studies may wish to examine whether objective measures of functional performance are related to force-matching error and steadiness.

Effects of Transcranial Direct Current Stimulation on Hand Dexterity in Multiple Sclerosis: A Design for a Randomized Controlled Trial

BACKGROUND

Cerebellar and motor tracts are frequently impaired in multiple sclerosis (MS). Altered hand dexterity constitutes a challenge in clinical practice, since medical treatment shows very limited benefits in this domain. Cerebellar control is made via several cerebellocortical pathways, of which the most studied one links the cerebellum to the contralateral motor cortex via the contralateral ventro-intermediate nucleus of the thalamus influencing the corticospinal outputs. Modulating the activity of the cerebellum or of the motor cortex could be of help.

METHOD

The main interest here is to evaluate the efficacy of transcranial direct current stimulation (tDCS), a noninvasive brain stimulation technique, in treating altered dexterity in MS. Forty-eight patients will be recruited in a randomized, double-blind, sham-controlled, and crossover study. They will randomly undergo one of the three interventions: anodal tDCS over the primary motor area, cathodal tDCS over the cerebellum, or sham. Each block consists of five consecutive daily sessions with direct current (2 mA), lasting 20 min each. The primary outcome will be the improvement in manual dexterity according to the change in the time required to complete the nine-hole pegboard task. Secondary outcomes will include fatigue, pain, spasticity, and mood. Patients' safety and satisfaction will be rated.

DISCUSSION

Due to its cost-effective, safe, and easy-to-use profile, motor or cerebellar tDCS may constitute a potential tool that might improve dexterity in MS patients and therefore ameliorate their quality of life.

Wireless Sensing of Lower Lip and Thumb-Index Finger 'Ramp-and-Hold' Isometric Force Dynamics in a Small Cohort of Unilateral MCA Stroke: Discussion of Preliminary Findings

Automated wireless sensing of force dynamics during a visuomotor control task was used to rapidly assess residual motor function during finger pinch (right and left hand) and lower lip compression in a cohort of seven adult males with chronic, unilateral middle cerebral artery (MCA) stroke with infarct confirmed by anatomic magnetic resonance imaging (MRI). A matched cohort of 25 neurotypical adult males served as controls. Dependent variables were extracted from digitized records of ‘ramp-and-hold’ isometric contractions to target levels (0.25, 0.5, 1, and 2 Newtons) presented in a randomized block design; and included force reaction time, peak force, and dF/dt_max associated with force recruitment, and end-point accuracy and variability metrics during the contraction hold-phase (mean, SD, criterion percentage ‘on-target’). Maximum voluntary contraction force (MVCF) was also assessed to establish the force operating range. Results based on linear mixed modeling (LMM, adjusted for age and handedness) revealed significant patterns of dissolution in fine force regulation among MCA stroke participants, especially for the contralesional thumb-index finger followed by the ipsilesional digits, and the lower lip. For example, the contralesional thumb-index finger manifest increased reaction time, and greater overshoot in peak force during recruitment compared to controls. Impaired force regulation among MCA stroke participants during the contraction hold-phase was associated with significant increases in force SD, and dramatic reduction in the ability to regulate force output within prescribed target force window (±5% of target). Impaired force regulation during contraction hold-phase was greatest in the contralesional hand muscle group, followed by significant dissolution in ipsilateral digits, with smaller effects found for lower lip. These changes in fine force dynamics were accompanied by large reductions in the MVCF with the LMM marginal means for contralesional and ipsilesional pinch forces at just 34.77% (15.93 N vs. 45.82 N) and 66.45% (27.23 N vs. 40.98 N) of control performance, respectively. Biomechanical measures of fine force and MVCF performance in adult stroke survivors provide valuable information on the profile of residual motor function which can help inform clinical treatment strategies and quantitatively monitor the efficacy of rehabilitation or neuroprotection strategies.

Effect of Sensory Deprivation on Maximal Force Abilities from Local to Non-local Digits

The present study investigates the effect of sensory deprivation of the index and middle finger on motor function of all digits during maximal voluntary force production tasks. A total of 27 subjects performed maximal isometric pressing tasks by using different instructed finger combinations. Subjects completed the same tasks in two visits: a control visit when they had normal sensory feedback in all fingers, and an anesthesia visit when digital nerve blocks were performed on their right index and middle fingers. We evaluated three aspects of motor adaptation on both local (anesthetized) and non-local (non-anesthetized) digits during maximal force production: (1) task-relevant and overall force magnitude, (2) force directional application, and (3) digital individuation and force sharing. Our results indicate that selective digital anesthesia resulted in decreased maximal force magnitude, changed direction of force production, and significant changes extended to non-local digits. The motor weakness and inefficiency revealed in the non-local digits implies that sensory information from each digit can be shared across the digits to assist motor execution within the same hand.

Predictability, force, and (anti)resonance in complex object control

Manipulation of complex objects as in tool use is ubiquitous and has given humans an evolutionary advantage. This study examined the strategies humans choose when manipulating an object with underactuated internal dynamics, such as a cup of coffee. The dynamics of the object renders the temporal evolution complex, possibly even chaotic, and difficult to predict. A cart-and-pendulum model, loosely mimicking coffee sloshing in a cup, was implemented in a virtual environment with a haptic interface. Participants rhythmically manipulated the virtual cup containing a rolling ball; they could choose the oscillation frequency, whereas the amplitude was prescribed. Three hypotheses were tested: 1) humans decrease interaction forces between hand and object; 2) humans increase the predictability of the object dynamics; and 3) humans exploit the resonances of the coupled object-hand system. Analysis revealed that humans chose either a high-frequency strategy with antiphase cup-and-ball movements or a low-frequency strategy with in-phase cup-and-ball movements. Counter to hypothesis 1, they did not decrease interaction force; instead, they increased the predictability of the interaction dynamics, quantified by mutual information, supporting hypothesis 2. To address hypothesis 3, frequency analysis of the coupled hand-object system revealed two resonance frequencies separated by an antiresonance frequency. The low-frequency strategy exploited one resonance, whereas the high-frequency strategy afforded more choice, consistent with the frequency response of the coupled system; both strategies avoided the antiresonance. Hence, humans did not prioritize small interaction forces but rather strategies that rendered interactions predictable. These findings highlight that physical interactions with complex objects pose control challenges not present in unconstrained movements. NEW & NOTEWORTHY Daily actions involve manipulation of complex nonrigid objects, which present a challenge since humans have no direct control of the whole object. We used a virtual-reality experiment and simulations of a cart-and-pendulum system coupled to hand movements with impedance to analyze the manipulation of this underactuated object. We showed that participants developed strategies that increased the predictability of the object behavior by exploiting the resonance structure of the object but did not minimize the hand-object interaction force.

Bilateral deep brain stimulation of the subthalamic nucleus increases pointing error during memory-guided sequential reaching

Deep brain stimulation of the subthalamic nucleus (STN DBS) significantly improves clinical motor symptoms, as well as intensive aspects of movement like velocity and amplitude in patients with Parkinson's disease (PD). However, the effects of bilateral STN DBS on integrative and coordinative aspects of motor control are equivocal. The aim of this study was to investigate the effects of bilateral STN DBS on integrative and coordinative aspects of movement using a memory-guided sequential reaching task. The primary outcomes were eye and finger velocity and end-point error. We expected that bilateral STN DBS would increase reaching velocity. More importantly, we hypothesized that bilateral STN DBS would increase eye and finger end-point error and this would not simply be the result of a speed accuracy trade-off. Ten patients with PD and bilaterally implanted subthalamic stimulators performed a memory-guided sequential reaching task under four stimulator conditions (DBS-OFF, DBS-LEFT, DBS-RIGHT, and DBS-BILATERAL) over 4 days. DBS-BILATERAL significantly increased eye velocity compared to DBS-OFF, DBS-LEFT, and DBS-RIGHT. It also increased finger velocity compared to DBS-OFF and DBS-RIGHT. DBS-BILATERAL did not change eye end-point error. The novel finding was that DBS-BILATERAL increased finger end-point error compared to DBS-OFF, DBS-LEFT, and DBS-RIGHT even after adjusting for differences in velocity. We conclude that bilateral STN DBS may facilitate basal ganglia-cortical networks that underlie intensive aspects of movement like velocity, but it may disrupt selective basal ganglia-cortical networks that underlie certain integrative and coordinative aspects of movement such as spatial accuracy.

On neuromechanical approaches for the study of biological and robotic grasp and manipulation

Biological and robotic grasp and manipulation are undeniably similar at the level of mechanical task performance. However, their underlying fundamental biological vs. engineering mechanisms are, by definition, dramatically different and can even be antithetical. Even our approach to each is diametrically opposite: inductive science for the study of biological systems vs. engineering synthesis for the design and construction of robotic systems. The past 20 years have seen several conceptual advances in both fields and the quest to unify them. Chief among them is the reluctant recognition that their underlying fundamental mechanisms may actually share limited common ground, while exhibiting many fundamental differences. This recognition is particularly liberating because it allows us to resolve and move beyond multiple paradoxes and contradictions that arose from the initial reasonable assumption of a large common ground. Here, we begin by introducing the perspective of neuromechanics, which emphasizes that real-world behavior emerges from the intimate interactions among the physical structure of the system, the mechanical requirements of a task, the feasible neural control actions to produce it, and the ability of the neuromuscular system to adapt through interactions with the environment. This allows us to articulate a succinct overview of a few salient conceptual paradoxes and contradictions regarding under-determined vs. over-determined mechanics, under- vs. over-actuated control, prescribed vs. emergent function, learning vs. implementation vs. adaptation, prescriptive vs. descriptive synergies, and optimal vs. habitual performance. We conclude by presenting open questions and suggesting directions for future research. We hope this frank and open-minded assessment of the state-of-the-art will encourage and guide these communities to continue to interact and make progress in these important areas at the interface of neuromechanics, neuroscience, rehabilitation and robotics.

Sex differences in leg dexterity are not present in elite athletes

We studied whether the time-varying forces that control unstable foot-ground interactions provide insight into the neural control of dynamic leg function. Twenty elite (10F, 26.4±3.5yrs) and 20 recreational (10F, 24.8±2.4yrs) athletes used an isolated leg to maximally compress a slender spring designed to buckle at low forces while seated. The foot forces during the compression at the edge of instability quantify the maximal sensorimotor ability to control dynamic foot-ground interactions. Using the nonlinear analysis technique of attractor reconstruction, we characterized the spatial (interquartile range IQR) and geometric (trajectory length TL, volume V, and sum of edge lengths SE) features of the dynamical behavior of those force time series. ANOVA confirmed the already published effect of sex, and a new effect of athletic ability, respectively, in TL (p=0.014 and p<0.001), IQR (p=0.008 and p<0.001), V (p=0.034 and p=0.002), and SE (p=0.033 and p<0.001). Further analysis revealed that, for recreational athletes, females exhibited weaker corrective actions and greater stochasticity than males as per their greater mean values of TL (p=0.003), IQR (p=0.018), V (p=0.017), and SE (p=0.025). Importantly, sex differences disappeared in elite athletes. These results provide an empirical link between sex, athletic ability, and nonlinear dynamical control. This is a first step in understanding the sensorimotor mechanisms for control of unstable foot-ground interactions. Given that females suffer a greater incidence of non-contact knee ligament injuries, these non-invasive and practical metrics of leg dexterity may be both indicators of athletic ability, and predictors of risk of injury.

Beta Band Corticomuscular Drive Reflects Muscle Coordination Strategies

During force production, hand muscle activity is known to be coherent with activity in primary motor cortex, specifically in the beta-band (15–30 Hz) frequency range. It is not clear, however, if this coherence reflects the control strategy selected by the nervous system for a given task, or if it instead reflects an intrinsic property of cortico-spinal communication. Here, we measured corticomuscular and intermuscular coherence between muscles of index finger and thumb while a two-finger pinch grip of identical net force was applied to objects which were either stable (allowing synergistic activation of finger muscles) or unstable (requiring individuated finger control). We found that beta-band corticomuscular coherence with the first dorsal interosseous (FDI) and abductor pollicis brevis (APB) muscles, as well as their beta-band coherence with each other, was significantly reduced when individuated control of the thumb and index finger was required. We interpret these findings to show that beta-band coherence is reflective of a synergistic control strategy in which the cortex binds task-related motor neurons into functional units.

Characterization of the disruption of neural control strategies for dynamic fingertip forces from attractor reconstruction

The Strength-Dexterity (SD) test measures the ability of the pulps of the thumb and index finger to compress a compliant and slender spring prone to buckling at low forces (<3N). We know that factors such as aging and neurodegenerative conditions bring deteriorating physiological changes (e.g., at the level of motor cortex, cerebellum, and basal ganglia), which lead to an overall loss of dexterous ability. However, little is known about how these changes reflect upon the dynamics of the underlying biological system. The spring-hand system exhibits nonlinear dynamical behavior and here we characterize the dynamical behavior of the phase portraits using attractor reconstruction. Thirty participants performed the SD test: 10 young adults, 10 older adults, and 10 older adults with Parkinson's disease (PD). We used delayed embedding of the applied force to reconstruct its attractor. We characterized the distribution of points of the phase portraits by their density (number of distant points and interquartile range) and geometric features (trajectory length and size). We find phase portraits from older adults exhibit more distant points (p = 0.028) than young adults and participants with PD have larger interquartile ranges (p = 0.001), trajectory lengths (p = 0.005), and size (p = 0.003) than their healthy counterparts. The increased size of the phase portraits with healthy aging suggests a change in the dynamical properties of the system, which may represent a weakening of the neural control strategy. In contrast, the distortion of the attractor in PD suggests a fundamental change in the underlying biological system, and disruption of the neural control strategy. This ability to detect differences in the biological mechanisms of dexterity in healthy and pathological aging provides a simple means to assess their disruption in neurodegenerative conditions and justifies further studies to understand the link with the physiological changes.

Unilateral Eccentric Contraction of the Plantarflexors Leads to Bilateral Alterations in Leg Dexterity

Eccentric contractions can affect musculotendon mechanical properties and disrupt muscle proprioception, but their behavioral consequences are poorly understood. We tested whether repeated eccentric contractions of plantarflexor muscles of one leg affected the dexterity of either leg. Twenty healthy male subjects (27.3 ± 4.0 yrs) compressed a compliant and slender spring prone to buckling with each isolated leg. The maximal instability they could control (i.e., the maximal average sustained compression force, or lower extremity dexterity force, LED_force) quantified the dexterity of each leg. We found that eccentric contractions did not affect LED_force, but reduced force variability (LED_SD). Surprisingly, LED_force increased in the non-exposed, contralateral leg. These effects were specific to exposure to eccentric contractions because an effort-matched exposure to walking did not affect leg dexterity. In the exposed leg, eccentric contractions (i) reduced voluntary error corrections during spring compressions (i.e., reduced 0.5–4 Hz power of LED_force); (ii) did not change spinal excitability (i.e., unaffected H-reflexes); and (iii) changed the structure of the neural drive to the α-motoneuron pool (i.e., reduced EMG power within the 4–8 Hz physiological tremor band). These results suggest that repeated eccentric contractions alter the feedback control for dexterity in the exposed leg by reducing muscle spindle sensitivity. Moreover, the unexpected improvement in LED_force in the non-exposed contralateral leg was likely a consequence of crossed-effects on its spinal and supraspinal feedback control. We discuss the implications of these bilateral effects of unilateral eccentric contractions, their effect on spinal and supraspinal control of dynamic foot-ground interactions, and their potential to facilitate rehabilitation from musculoskeletal and neuromotor impairments.

Strength, Multijoint Coordination, and Sensorimotor Processing Are Independent Contributors to Overall Balance Ability

For young adults, balance is essential for participation in physical activities but is often disrupted following lower extremity injury. Clinical outcome measures such as single limb balance (SLB), Y-balance (YBT), and the single limb hop and balance (SLHB) tests are commonly used to quantify balance ability following injury. Given the varying demands across tasks, it is likely that such outcome measures provide useful, although task-specific, information. But the extent to which they are independent and contribute to understanding the multiple contributors to balance is not clear. Therefore, the purpose of this study was to investigate the associations among these measures as they relate to the different contributors to balance. Thirty-seven recreationally active young adults completed measures including Vertical Jump, YBT, SLB, SLHB, and the new Lower Extremity Dexterity test. Principal components analysis revealed that these outcome measures could be thought of as quantifying the strength, multijoint coordination, and sensorimotor processing contributors to balance. Our results challenge the practice of using a single outcome measure to quantify the naturally multidimensional mechanisms for everyday functions such as balance. This multidimensional approach to, and interpretation of, multiple contributors to balance may lead to more effective, specialized training and rehabilitation regimens.

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.

Outcome measures for hand function naturally reveal three latent domains in older adults: strength, coordinated upper extremity function, and sensorimotor processing

Understanding the mapping between individual outcome measures and the latent functional domains of interest is critical to a quantitative evaluation and rehabilitation of hand function. We examined whether and how the associations among six hand-specific outcome measures reveal latent functional domains in elderly individuals. We asked 66 healthy older adult participants (38F, 28M, 66.1 ± 11.6 years, range: 45–88 years) and 33 older adults (65.8 ± 9.7 years, 44–81 years, 51 hands) diagnosed with osteoarthritis (OA) of the carpometacarpal (CMC) joint, to complete six functional assessments: hand strength (Grip, Key and Precision Pinch), Box and Block, Nine Hole Pegboard, and Strength-Dexterity tests. The first three principal components suffice to explain 86% of variance among the six outcome measures in healthy older adults, and 84% of variance in older adults with CMC OA. The composition of these dominant associations revealed three distinct latent functional domains: strength, coordinated upper extremity function, and sensorimotor processing. Furthermore, in participants with thumb CMC OA we found a blurring of the associations between the latent functional domains of strength and coordinated upper extremity function. This motivates future work to understand how the physiological effects of thumb CMC OA lead upper extremity coordination to become strongly associated with strength, while dynamic sensorimotor ability remains an independent functional domain. Thus, when assessing the level of hand function in our growing older adult populations, it is particularly important to acknowledge its multidimensional nature—and explicitly consider how each outcome measure maps to these three latent and fundamental domains of function. Moreover, this ability to distinguish among latent functional domains may facilitate the design of treatment modalities to target the rehabilitation of each of them.

Activity in the brain network for dynamic manipulation of unstable objects is robust to acute tactile nerve block: An fMRI study

OBJECTIVE

To study whether a temporary block of the tactile afferents from the fingers causes altered activity in the neural network for dexterous manipulation.

METHODS

Whole-brain functional Magnetic Resonance Imaging (fMRI) was conducted in 18 healthy subjects, while they compressed an unstable spring between the thumb and index finger of the right hand. Two sensory conditions--with and without tactile input from the fingers--were employed. In the latter condition the digital nerves were blocked by local anesthesia.

RESULTS

Compression of the unstable spring was associated with activity in an earlier described network for object manipulation. We found that this entire network remained active after a nerve block, and the activity was increased in the dorsal premotor cortex.

CONCLUSIONS

The neural network for dexterous manipulation is robust with only minor alterations after acute loss of tactile information from the fingers. There was no loss of activity, but, unexpectedly, an increased activity in some parts of the network.

SIGNIFICANCE

This study gives new insights to possible neural compensatory mechanisms that make fine motor control possible after acute disruption of tactile information in natural situations like cold weather or wearing surgical gloves.

Functional connectivity of the striatum in experts of stenography

INTRODUCTION

Stenography, or shorthand, is a unique set of skills that involves intensive training which is nearly life-long and orchestrating various brain functional modules, including auditory, linguistic, cognitive, mnemonic, and motor. Stenography provides cognitive neuroscientists with a unique opportunity to investigate the neural mechanisms underlying the neural plasticity that enables such a high degree of expertise. However, shorthand is quickly being replaced with voice recognition technology. We took this nearly final opportunity to scan the brains of the last alive shorthand experts of the Japanese language.

METHODS

Thirteen right-handed stenographers and fourteen right-handed controls participated in the functional magnetic resonance imaging (fMRI) study.

RESULTS

The fMRI data revealed plastic reorganization of the neural circuits around the putamen. The acquisition of expert skills was accompanied by structural and functional changes in the area. The posterior putamen is known as the execution center of acquired sensorimotor skills. Compared to nonexperts, the posterior putamen in stenographers had high covariation with the cerebellum and midbrain.The stenographers' brain developed different neural circuits from those of the nonexpert brain.

CONCLUSIONS

The current data illustrate the vigorous plasticity in the putamen and in its connectivity to other relevant areas in the expert brain. This is a case of vigorous neural plastic reorganization in response to massive overtraining, which is rare especially considering that it occurred in adulthood.

Long term functional outcomes after early childhood pollicization

STUDY DESIGN

Retrospective Cohort

INTRODUCTION

Important outcomes of polliciation to treat thumb hypoplasia/aplasia include strength, function, dexterity, and quality of life.

PURPOSE OF THE STUDY

To evaluate outcomes and examine predictors of outcome after early childhood pollicization.

METHODS

8 children (10 hands) were evaluated 3-15 years after surgery. Physical examination, questionnaires, grip and pinch strength, Box and Blocks, 9-hole pegboard, and strength-dexterity (S-D) tests were performed.

RESULTS

Pollicized hands had poor strength and performance on functional tests. Six of 10 pollicized hands had normal dexterity scores but less stability in maintaining a steady-state force. Predictors of poorer outcomes included older age at surgery, reduced metacarpophalangeal and interphalangeal range of motion, and radial absence.

DISCUSSION

Pollicization resulted in poor strength and overall function, but normal dexterity was often achieved using altered control strategies.

CONCLUSIONS

Most children should obtain adequate dexterity despite weakness after pollicization except older or severely involved children.

LEVEL OF EVIDENCE

IV.

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.

Quantitative assessment of dynamic control of fingertip forces after pollicization

Dexterity after finger pollicization (reconstruction to thumb) is critical to functional outcomes. While most tests of hand function evaluate a combination of strength, coordination, and motor control, the Strength-Dexterity (S-D) paradigm focuses on the dynamic control of fingertip forces. We evaluated 10 pollicized and 5 non-pollicized hands from 8 participants ages 4-17 years (2 female, 6 male; 10.6 ± 4.5 years). Participants partially compressed and held an instrumented spring prone to buckling between the thumb and first finger to quantify dynamic control over the direction and magnitude of fingertip forces. They also completed traditional functional tests including grip, lateral pinch, and tripod pinch strength, Box and Blocks, and 9-hole peg test. Six of 10 pollicized hands and all non-pollicized hands had S-D scores comparable to typically developing children. However, dynamical analysis showed that pollicized hands exhibit greater variability in compression force, indicating poorer corrective action. Almost all pollicized hands scored below the normal range for the traditional functional tests. The S-D test Z-scores correlated moderately with Z-scores from the other functional tests (r = 0.54-0.61; p = 0.02-0.04) but more weakly than amongst the other functional measures (r = 0.58-0.83; p = 0.0002-0.02), suggesting that the S-D test captures a different domain of function. A higher incidence of radial absence in the hands with poor S-D scores (3/4 vs. 0/6 in hands with normal S-D scores, p = 0.03) was the only clinical characteristic associated with S-D outcome. Overall, these results suggest that while most pollicized hands can control fingertip forces, the nature of that control is altered.

Interhemispheric cerebral blood flow balance during recovery of motor hand function after ischemic stroke--a longitudinal MRI study using arterial spin labeling perfusion

BACKGROUND

Unilateral ischemic stroke disrupts the well balanced interactions within bilateral cortical networks. Restitution of interhemispheric balance is thought to contribute to post-stroke recovery. Longitudinal measurements of cerebral blood flow (CBF) changes might act as surrogate marker for this process.

OBJECTIVE

To quantify longitudinal CBF changes using arterial spin labeling MRI (ASL) and interhemispheric balance within the cortical sensorimotor network and to assess their relationship with motor hand function recovery.

METHODS

Longitudinal CBF data were acquired in 23 patients at 3 and 9 months after cortical sensorimotor stroke and in 20 healthy controls using pulsed ASL. Recovery of grip force and manual dexterity was assessed with tasks requiring power and precision grips. Voxel-based analysis was performed to identify areas of significant CBF change. Region-of-interest analyses were used to quantify the interhemispheric balance across nodes of the cortical sensorimotor network.

RESULTS

Dexterity was more affected, and recovered at a slower pace than grip force. In patients with successful recovery of dexterous hand function, CBF decreased over time in the contralesional supplementary motor area, paralimbic anterior cingulate cortex and superior precuneus, and interhemispheric balance returned to healthy control levels. In contrast, patients with poor recovery presented with sustained hypoperfusion in the sensorimotor cortices encompassing the ischemic tissue, and CBF remained lateralized to the contralesional hemisphere.

CONCLUSIONS

Sustained perfusion imbalance within the cortical sensorimotor network, as measured with task-unrelated ASL, is associated with poor recovery of dexterous hand function after stroke. CBF at rest might be used to monitor recovery and gain prognostic information.

The handyman's brain: a neuroimaging meta-analysis describing the similarities and differences between grip type and pattern in humans

BACKGROUND

Handgrip is a ubiquitous human movement that was critical in our evolution. However, the differences in brain activity between grip type (i.e. power or precision) and pattern (i.e. dynamic or static) are not fully understood. In order to address this, we performed Activation Likelihood Estimation (ALE) analysis between grip type and grip pattern using functional magnetic resonance imaging (fMRI) data. ALE provides a probabilistic summary of the BOLD response in hundreds of subjects, which is often beyond the scope of a single fMRI experiment.

METHODS

We collected data from 28 functional magnetic resonance data sets, which included a total of 398 male and female subjects. Using ALE, we analyzed the BOLD response during power, precision, static and dynamic grip in a range of forces and age in right handed healthy individuals without physical impairment, cardiovascular or neurological dysfunction using a variety of grip tools, feedback and experimental training.

RESULTS

Power grip generates unique activation in the postcentral gyrus (areas 1 and 3b) and precision grip generates unique activation in the supplementary motor area (SMA, area 6) and precentral gyrus (area 4a). Dynamic handgrip generates unique activation in the precentral gyrus (area 4p) and SMA (area 6) and of particular interest, both dynamic and static grip share activation in the area 2 of the postcentral gyrus, an area implicated in the evolution of handgrip. According to effect size analysis, precision and dynamic grip generates stronger activity than power and static, respectively.

CONCLUSION

Our study demonstrates specific differences between grip type and pattern. However, there was a large degree of overlap in the pre and postcentral gyrus, SMA and areas of the frontal-parietal-cerebellar network, which indicates that other mechanisms are potentially involved in regulating handgrip. Further, our study provides empirically based regions of interest, which can be downloaded here within, that can be used to more effectively study power grip in a range of populations and conditions.

Kinematics of the Reach-to-Grasp Movement in Vascular Parkinsonism: A Comparison with Idiopathic Parkinson's Disease Patients

The performance of patients with vascular parkinsonism (VPD) on a reach-to-grasp task was compared with that of patients affected by idiopathic Parkinson’s disease (IPD) and age-matched control subjects. The aim of the study was to determine how patients with VPD and IPD compare at the level of the kinematic organization of prehensile actions. We examined how subjects concurrently executed the transport and grasp components of reach-to-grasp movements when grasping differently sized objects. When comparing both VPD and IPD groups to control subjects, all patients showed longer movement duration and smaller hand opening, reflecting bradykinesia and hypometria, respectively. Furthermore, for all patients, the onset of the manipulation component was delayed with respect to the onset of the transport component. However, for patients with VPD this delay was significantly smaller than that found for the IPD group. It is proposed that this reflects a deficit – which is moderate for VPD as compared to IPD patients – in the simultaneous (or sequential) implementation of different segments of a complex movement. Altogether these findings suggest that kinematic analysis of reach-to-grasp movement has the ability to provide potential instruments to characterize different forms of parkinsonism.

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.

Quantification of dexterity as the dynamical regulation of instabilities: comparisons across gender, age, and disease

Dexterous manipulation depends on using the fingertips to stabilize unstable objects. The Strength-Dexterity paradigm consists of asking subjects to compress a slender and compliant spring prone to buckling. The maximal level of compression [requiring low fingertip forces <300 grams force (gf)] quantifies the neural control capability to dynamically regulate fingertip force vectors and motions for a dynamic manipulation task. We found that finger dexterity is significantly affected by age (p = 0.017) and gender (p = 0.021) in 147 healthy individuals (66F, 81M, 20-88 years). We then measured finger dexterity in 42 hands of patients following treatment for osteoarthritis of the base of the thumb (CMC OA, 33F, 65.8 ± 9.7 years), and 31 hands from patients being treated for Parkinson's disease (PD, 6F, 10M, 67.68 ± 8.5 years). Importantly, we found no differences in finger compression force among patients or controls. However, we did find stronger age-related declines in performance in the patients with PD (slope -2.7 gf/year, p = 0.002) than in those with CMC OA (slope -1.4 gf/year, p = 0.015), than in controls (slope -0.86 gf/year). In addition, the temporal variability of forces during spring compression shows clearly different dynamics in the clinical populations compared to the controls (p < 0.001). Lastly, we compared dexterity across extremities. We found stronger age (p = 0.005) and gender (p = 0.002) effects of leg compression force in 188 healthy subjects who compressed a larger spring with the foot of an isolated leg (73F, 115M, 14-92 years). In 81 subjects who performed the tests with all four limbs separately, we found finger and leg compression force to be significantly correlated (females ρ = 0.529, p = 0.004; males ρ = 0.403, p = 0.003; 28F, 53M, 20-85 years), but surprisingly found no differences between dominant and non-dominant limbs. These results have important clinical implications, and suggest the existence - and compel the investigation - of systemic versus limb-specific mechanisms for dexterity.

Dexterous manipulation is poorer at older ages and is dissociated from decline of hand strength

BACKGROUND

The ability to dynamically control fingertip force vector magnitude and direction is critical for dexterous manipulation. We quantified the dynamic control of fingertip forces to examine how dexterous manipulation declines with age.

METHODS

The strength-dexterity (SD) test measures fingertip forces during compression of a slender spring prone to instability and buckling. The greatest sustained compression (designed to be under 3 N), and force dynamics therein, have been shown to be simple and quick measures of dynamic dexterous manipulation ability. We measured pinch strength and strength-dexterity test in a cross-sectional population of 98 people from 18 to 89 years of age.

RESULTS

Dexterous manipulation ability is poorer at older ages, beginning in middle age (p < .001), with greater decline past 65 years of age. Fingertip force dynamics during spring compression and stabilization show a deterioration of neuromuscular control with age. Importantly, this novel detection of decline in dynamic manipulation ability is not correlated with, and thus cannot be entirely explained by, the known decline in pinch strength. We also measured standardized tests of dexterity in participants older than 45, and discuss how the strength-dexterity test uniquely captures features of sensorimotor capabilities for dexterous manipulation in this adult population.

CONCLUSIONS

Starting in middle age, changes in the functional interactions among sensory, motor, and neural capabilities result in measurably poorer dynamic dexterous manipulation. This deterioration of neuromuscular control motivates and enables future studies to understand the physiological bases for this functional decline so critical to activities of daily living and quality of life.

Control of dynamic foot-ground interactions in male and female soccer athletes: females exhibit reduced dexterity and higher limb stiffness during landing

Controlling dynamic interactions between the lower limb and ground is important for skilled locomotion and may influence injury risk in athletes. It is well known that female athletes sustain anterior cruciate ligament (ACL) tears at higher rates than male athletes, and exhibit lower extremity biomechanics thought to increase injury risk during sport maneuvers. The purpose of this study was to examine whether lower extremity dexterity (LED)--the ability to dynamically control endpoint force magnitude and direction as quantified by compressing an unstable spring with the lower limb at submaximal forces--is a potential contributing factor to the "at-risk" movement behavior exhibited by female athletes. We tested this hypothesis by comparing LED-test performance and single-limb drop jump biomechanics between 14 female and 14 male high school soccer players. We found that female athletes exhibited reduced LED-test performance (p=0.001) and higher limb stiffness during landing (p=0.008) calculated on average within 51 ms of foot contact. Females also exhibited higher coactivation at the ankle (p=0.001) and knee (p=0.02) before landing. No sex differences in sagittal plane joint angles and center of mass velocity at foot contact were observed. Collectively, our results raise the possibility that the higher leg stiffness observed in females during landing is an anticipatory behavior due in part to reduced lower extremity dexterity. The reduced lower extremity dexterity and compensatory stiffening strategy may contribute to the heightened risk of ACL injury in this population.

Developmental improvements in dynamic control of fingertip forces last throughout childhood and into adolescence

While it is clear that the development of dexterous manipulation in children exhibits dramatic improvements over an extended period, it is difficult to separate musculoskeletal from neural contributors to these important functional gains. This is in part due to the inability of current methods to disambiguate improvements in hand strength from gains in finger dexterity (i.e., the dynamic control of fingertip force vectors at low magnitudes). We adapted our novel instrumentation to evaluate finger dexterity in 130 typically developing children between the ages of 4 and 16 yr. We find that finger dexterity continues to develop well into late adolescence and musculoskeletal growth and strength are poorly correlated with the improvements in dexterity. Importantly, because these behavioral results seem to mirror the known timelines of neuroanatomical development up to adolescence, we speculate that they reflect the functional benefits of such continual neural maturation. This novel perspective now enables the systematic study of the functional roles of specific neuroanatomical structures and their connectivity, maturity, and plasticity. Moreover, the temporal dynamics of the fingertip force vectors shows improvements in stability that provide a novel way to look at the maturation of finger control. From a clinical perspective, our results provide a practical means to chart functional development of dexterous manipulation in typically developing children and could be adapted for clinical use and for use in children with developmental disorders.

The lower extremity dexterity test as a measure of lower extremity dynamical capability

The capability of the lower extremity to dynamically interact with the ground is important for skilled locomotor performance. However, there is currently no test method designed to specifically quantify this sensorimotor ability, which we refer to as lower extremity dexterity. We describe a new method to quantify lower extremity dexterity, examine its reliability (n=10), and evaluate the extent to which it is associated with lower extremity strength and anthropometry in healthy young adults (n=38). The lower extremity dexterity test (LED-test)-an adaptation of the Strength-Dexterity test for the fingers-consists of using the isolated lower extremity to compress a slender spring prone to buckling at low forces. The goal of the LED-test is to sustain the highest compression force possible. Applying higher forces makes the spring increasingly unstable, thus achieving higher compression forces represents better ability to dynamically control instability at low force levels. As such, the LED-test provides a novel way to quantify the capability of the lower extremity to regulate dynamic and unstable foot-ground interactions at submaximal forces. LED-test performance ranged between 88.6 and 119.6N, test-retest reliability was excellent (ICC(2,3)=0.94), and the minimal detectable difference was 5.5N. Performance was not correlated with strength or height (r(2)≤0.053, p>0.05), and only weakly with body mass (r(2)=0.116, p=0.04). We propose that the unique lower extremity capability quantified by the LED-test could be informative of skilled locomotor performance and injury risk.

Dissociation of brain areas associated with force production and stabilization during manipulation of unstable objects

Multifinger dexterous manipulation of unstable or deformable objects requires control of both direction and magnitude of fingertip force vectors. Our aim was to study the neuroanatomical correlates of these two distinct control functions. Brain activity was measured using functional magnetic resonance imaging while 16 male subjects (age: 26-42, M = 32, SD ± 4 years) compressed four springs representing a 2 × 2 factorial design with two levels of force and instability requirements. Significant activations associated with higher instability were located bilaterally in the precentral gyri, the postcentral gyrus, and the cerebellum. In the main effect for high force, activity was found in areas located in the primary motor regions contralateral to the active hand and bilaterally in the cerebellum. An overlap in activation between the two main effects was found bilaterally in the cerebellum (lobule VI). This study not only confirms a recently described bilateral fronto-parieto-cerebellar network for manipulation of increasingly unstable objects, but critically extends our understanding by describing its differentiated modulation with both force magnitude and instability requirements. Our results, therefore, expose a previously unrecognized and context-sensitive system of brain regions that enable dexterous manipulation for different force magnitude and instability requirements of the task.