Synergy as a new and sensitive marker of basal ganglia dysfunction: A study of asymptomatic welders

Force matching: motor effects that are not reported by the actor

We explored unintentional drifts of finger forces during force production and matching task. Based on earlier studies, we predicted that force matching with the other hand would reduce or stop the force drift in instructed fingers while uninstructed (enslaved) fingers remain unaffected. Twelve young, healthy, right-handed participants performed two types of tasks with both hands (task hand and match hand). The task hand produced constant force at 20% of MVC level with the Index and Ring fingers pressing in parallel on strain gauge force sensors. The Middle finger force wasn't instructed, and its enslaved force was recorded. Visual feedback on the total force by the instructed fingers was either present throughout the trial or only during the first 5 s (no-feedback condition). The other hand matched the perceived force level of the task hand starting at either 4, 8, or 15 s from the trial initiation. No feedback was ever provided for the match hand force. After the visual feedback was removed, the task hand showed a consistent drift to lower magnitudes of total force. Contrary to our prediction, over all conditions, force matching caused a brief acceleration of force drift in the task hand, which then reached a plateau. There was no effect of matching on drifts in enslaved finger force. We interpret the force drifts within the theory of control with spatial referent coordinates as consequences of drifts in the command (referent coordinate) to the antagonist muscles. This command is not adequately incorporated into force perception.

Whole-brain mapping of increased manganese levels in welders and its association with exposure and motor function

Although manganese (Mn) is a trace metal essential for humans, chronic exposure to Mn can cause accumulation of this metal ion in the brain leading to an increased risk of neurological and neurobehavioral health effects. This is a concern for welders exposed to Mn through welding fumes. While brain Mn accumulation in occupational settings has mostly been reported in the basal ganglia, several imaging studies also revealed elevated Mn in other brain areas. Since Mn functions as a magnetic resonance imaging (MRI) T1 contrast agent, we developed a whole-brain MRI approach to map in vivo Mn deposition differences in the brains of non-exposed factory controls and exposed welders. This is a cross-sectional analysis of 23 non-exposed factory controls and 36 exposed full-time welders from the same truck manufacturer. We collected high-resolution 3D MRIs of brain anatomy and R1 relaxation maps to identify regional differences using voxel-based quantification (VBQ) and statistical parametric mapping. Furthermore, we investigated the associations between excess Mn deposition and neuropsychological and motor test performance. Our results indicate that: (1) Using whole-brain MRI relaxometry methods we can generate excess Mn deposition maps in vivo, (2) excess Mn accumulation due to occupational exposure occurs beyond the basal ganglia in cortical areas associated with motor and cognitive functions, (3) Mn likely diffuses along white matter tracts in the brain, and (4) Mn deposition in specific brain regions is associated with exposure (cerebellum and frontal cortex) and motor metrics (cerebellum and hippocampus).

Two classes of action-stabilizing synergies reflecting spinal and supraspinal circuitry

We explored force-stabilizing synergies during accurate four-finger constant force production tasks in spaces of finger modes (commands to fingers computed to account for the finger interdependence) and of motor unit (MU) firing frequencies. The main specific hypothesis was that the multifinger synergies would disappear during unintentional force drifts without visual feedback on the force magnitude, whereas MU-based synergies would be robust to such drifts. Healthy participants performed four-finger accurate cyclical force production trials followed by trials of constant force production. Individual MUs were identified in the flexor digitorum superficialis (FDS) and extensor digitorum communis (EDC). Principal component analysis was applied to motor unit frequencies to identify robust MU groups (MU-modes) with parallel scaling of the firing frequencies in FDS, in EDC, and the combined MUs of FDS + EDC. The framework of the uncontrolled manifold hypothesis was used to quantify force-stabilizing synergies when visual feedback on the force magnitude was available and 15 s after turning the visual feedback off. Removing visual feedback led to a force drift toward lower magnitudes, accompanied by the disappearance of multifinger synergies. In contrast, MU-mode synergies were minimally affected by removing visual feedback off and continued to be robust for the FDS and for the EDC, while being absent for the (FDS + EDC) analysis. We interpret the findings within the theory of hierarchical control of action with spatial referent coordinates. The qualitatively different behavior of the multifinger and MU-mode-based synergies likely reflects the difference in the involved neural circuitry, supraspinal for the former and spinal for the latter.NEW & NOTEWORTHY Two types of synergies, in the space of commands to individual fingers and in the space of motor unit groups, show qualitatively different behaviors during accurate multifinger force-production tasks. After removing visual feedback, finger force synergies disappear, whereas motor unit-based synergies persist. These results point at different neural circuitry involved in these two basic classes of synergies: supraspinal for multieffector synergies, and spinal for motor unit-based synergies.

Multi-muscle synergies in preparation for gait initiation in Parkinson's disease

OBJECTIVE

We investigated changes in indices of muscle synergies prior to gait initiation and the effects of gaze shift in patients with Parkinson's disease (PD). A long-term objective of the study is to develop a method for quantitative assessment of gait-initiation problems in PD.

METHODS

PD patients without clinical signs of postural instability and two control groups (age-matched and young) performed a gait initiation task in a self-paced manner, with and without a quick prior gaze shift produced by turning the head. Muscle groups with parallel scaling of activation levels (muscle modes) were identified as factors in the muscle activation space. Synergy index stabilizing center of pressure trajectory in the anterior-posterior and medio-lateral directions (indices of stability) was quantified in the muscle mode space. A drop in the synergy index in preparation to gait initiation (anticipatory synergy adjustment, ASA) was quantified.

RESULTS

Compared to the control groups, PD patients showed significantly smaller synergy indices and ASA for both directions of the center of pressure shift. Both PD and age-matched controls, but not younger controls, showed detrimental effects of the prior gaze shift on the ASA indices.

CONCLUSIONS

PD patients without clinically significant posture or gait disorders show impaired stability of the center of pressure and its diminished adjustment during gait initiation.

SIGNIFICANCE

The indices of stability and ASA may be useful to monitor pre-clinical gait disorders, and lower ASA may be relevant to emergence of freezing of gait in PD.

Early-Life Critical Windows of Susceptibility to Manganese Exposure and Sex-Specific Changes in Brain Connectivity in Late Adolescence

Background

Early-life environmental exposures during critical windows (CWs) of development can impact life course health. Exposure to neuroactive metals such as manganese (Mn) during prenatal and early postnatal CWs may disrupt typical brain development, leading to persistent behavioral changes. Males and females may be differentially vulnerable to Mn, presenting distinctive CWs to Mn exposure.

Methods

We used magnetic resonance imaging to investigate sex-specific associations between early-life Mn uptake and intrinsic functional connectivity in adolescence. A total of 71 participants (15-23 years old; 53% female) from the Public Health Impact of Manganese Exposure study completed a resting-state functional magnetic resonance imaging scan. We estimated dentine Mn concentrations at prenatal, postnatal, and early childhood periods using laser ablation-inductively coupled plasma-mass spectrometry. We performed seed-based correlation analyses to investigate the moderating effect of sex on the associations between Mn and intrinsic functional connectivity adjusting for age and socioeconomic status.

Results

We identified significant sex-specific associations between dentine Mn at all time points and intrinsic functional connectivity in brain regions involved in cognitive and motor function: 1) prenatal: dorsal striatum, occipital/frontal lobes, and middle frontal gyrus; 2) postnatal: right putamen and cerebellum; and 3) early childhood: putamen and occipital, frontal, and temporal lobes. Network associations differed depending on exposure timing, suggesting that different brain networks may present distinctive CWs to Mn.

Conclusions

These findings suggest that the developing brain is vulnerable to Mn exposure, with effects lasting through late adolescence, and that females and males are not equally vulnerable to these effects. Future studies should investigate cognitive and motor outcomes related to these associations.

Effects of postural instability on the coordination between posture and arm reaching

Reaching from standing requires adjustments of hand movement and posture, which are assured by redundant kinematic degrees of freedom. However, the increased demand for postural adjustments may interfere with the stability of reaching. The objective of this study was to investigate the effect of postural instability on the use of kinematic redundancy to stabilize the finger and center-of-mass trajectories during reaching from standing in healthy adults. Sixteen healthy young adults performed reaching movements from standing with and without postural instability induced by small base-of-support. The three-dimensional positions of 48 markers were recorded at 100 Hz. The uncontrolled manifold (UCM) analysis was performed separately with the finger and center-of-mass positions being the performance variables, and joint angles being the elemental variables. ΔV, the normalized difference between the variance in joint angle that does not affect task performance (V_UCM) and the variance that does affect task performance (V_ORT), was calculated separately for finger (ΔV_EP) and center-of-mass (ΔV_COM) positions, and was compared between stable and unstable base-of-support conditions. ΔV_EP decreased after movement onset and reached its minimum value at around 30-50% of the normalized movement time, and increased until movement offset, while ΔV_COM remained stable. At 60%-100% normalized movement time, ΔV_EP was significantly reduced in the unstable base-of-support, compared to the stable base-of-support condition. ΔV_COM remained similar between the two conditions. At movement offset, ΔV_EP was significantly reduced in the unstable base-of-support, compared to the stable base-of-support condition, and was associated with a substantial increase in V_ORT. Postural instability might reduce the ability to use kinematic redundancy to stabilize the reaching movement. The central nervous system may prioritize the maintenance of postural stability over focal movement when postural stability is challenged.

Synergic control in asymptomatic welders during multi-finger force exertion and load releasing while standing

Motor synergies, i.e., neural mechanisms that organize multiple motor elements to ensure stability of actions, are affected by several neurological condition. Asymptomatic welders showed impaired synergy controlling the stability of multi-finger action compared to non-welders and this impairment was associated with microstructural damage in the globus pallidus. We further explored the effect of welding-related metal exposure on multi-finger synergy and extended our investigation to posture-stabilizing synergy during a standing task. Occupational, MRI, and performance-stabilizing synergies during multi-finger accurate force production and load releasing while standing were obtained from 29 welders and 19 age- and sex-matched controls. R2* and R1 relaxation rate values were used to estimate brain iron and manganese content, respectively, and diffusion tensor imaging was used to reflect brain microstructural integrity. Associations of brain MRI (caudate, putamen, globus pallidus, and red nucleus), and motor synergy were explored by group status. The results revealed that welders had higher R2* values in the caudate (p = 0.03), putamen (p = 0.01), and red nucleus (p = 0.08, trend) than controls. No group effect was revealed on multi-finger synergy index during steady-state phase of action (ΔVZss). Compared to controls, welders exhibited lower ΔVZss (-0.106 ± 0.084 vs. 0.160 ± 0.092, p = 0.04) and variance that did not affect the performance variable (VUCM, 0.022 ± 0.003 vs. 0.038 ± 0.007, p = 0.03) in the load releasing, postural task. The postural synergy index, ΔVZss, was associated negatively with higher R2* in the red nucleus in welders (r = -0.44, p = 0.03), but not in controls. These results suggest that the synergy index in the load releasing during a standing task may reflect welding-related neurotoxicity in workers with chronic metals exposure. This finding may have important clinical and occupational health implications.

Magnetic resonance imaging T1 indices of the brain as biomarkers of inhaled manganese exposure

Excessive exposure to manganese (Mn) is linked to its accumulation in the brain and adverse neurological effects. Paramagnetic properties of Mn allow the use of magnetic resonance imaging (MRI) techniques to identify it in biological tissues. A critical review was conducted to evaluate whether MRI techniques could be used as a diagnostic tool to detect brain Mn accumulation as a quantitative biomarker of inhaled exposure. A comprehensive search was conducted in MEDLINE, EMBASE, and PubMed to identify potentially relevant studies published prior to 9 May 2022. Two reviewers independently screened identified references using a two-stage process. Of the 6452 unique references identified, 36 articles were retained for data abstraction. Eligible studies used T1-weighted MRI techniques and reported direct or indirect T1 measures to characterize Mn accumulation in the brain. Findings demonstrate that, in subjects exposed to high levels of Mn, deposition in the brain is widespread, accumulating both within and outside the basal ganglia. Available evidence indicates that T1 MRI techniques can be used to distinguish Mn-exposed individuals from unexposed. Additionally, T1 MRI may be useful for semi-quantitative evaluation of inhaled Mn exposure, particularly when interpreted along with other exposure indices. T1 MRI measures appear to have a nonlinear relationship to Mn exposure duration, with R1 signal only increasing after critical thresholds. The strength of the association varied depending on the regions of interest imaged and the method of exposure measurement. Overall, available evidence suggests potential for future clinical and risk assessment applications of MRI as a diagnostic tool.

Effects of hand muscle function and dominance on intra-muscle synergies

The goal of the study was to explore the effects of hand dominance and muscle function (prime mover vs. supporting muscle) on recently discovered intra-muscle synergies as potential windows into their neural origin. Healthy right-handed subjects performed accurate cyclical force production tasks while pressing with the middle phalanges and distal phalanges of the fingers of the dominant and non-dominant hand. Surface electromyography was used to identify individual motor unit action potentials in two muscles, flexor digitorum superficialis (FDS) and extensor digitorum communis (EDC). Stable motor unit groups (MU-modes) were defined in each muscle and in both muscles together. The composition of the MU-modes allowed linking them to the reciprocal and co-activation command. Force-stabilizing synergies were quantified in each hand and during force production at both sites using the framework of the uncontrolled manifold hypothesis. Force-stabilizing synergies were seen in the spaces of MU-modes from FDS and EDC separately, but not of MU-modes defined for both muscles together. Synergy indices were similar for both hands and both sites of force application. In contrast, force-stabilizing synergies in the space of finger forces were present in the non-dominant hand and absent in the dominant hand. The data suggest existence of distributed mechanisms of synergic control. Finger force synergies are likely to reflect functioning of subcortical loops involving the basal ganglia and cerebellum, while MU-mode synergies are likely to reflect spinal circuitry. Studies of both force-based and motor-unit-based synergies may be clinically valuable for distinguishing effects of spinal and supraspinal disorders.

PI3K/Akt Signaling Pathway Ameliorates Oxidative Stress-Induced Apoptosis upon Manganese Exposure in PC12 Cells

Manganese (Mn)-induced neurotoxicity has aroused public concerns for many years, but its precise mechanism is still poorly understood. Herein, we report the impacts of the phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) signaling pathway in mediating neurological effects induced by manganese sulfate (MnSO₄) exposure in PC12 cells. In this study, cells were treated with MnSO₄ for 24 h in the absence or presence of LY294002 (a special inhibitor of PI3K). We investigated cell viability and apoptosis signals, as well as levels of superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), catalase (CAT), and malondialdehyde (MDA). The mRNA levels of B-cell lymphoma 2 (Bcl-2), Bcl-2-associated X protein (Bax), and Caspase-3 were also quantified through real-time quantitative PCR (RT-qPCR); protein levels of serine/threonine protein kinase (Akt) and forkhead box O3A (Foxo3a) were determined by western blot. Increasing of MnSO₄ doses led to decreased SOD, GSH-Px, and CAT activities, while the level of MDA was upregulated. Moreover, cell apoptosis was significantly increased, as the mRNA of Bcl-2 and Caspase-3 was significantly decreased, while Bax mRNA was increased. Phosphorylated Akt (p-Akt) and Foxo3a (p-Foxo3a) were upregulated in a dose-dependent manner. In addition, LY294002 pretreatment reduced the activity of SOD, GSH-Px, and CAT but elevated MDA levels. Meanwhile, LY294002 pretreatment also increased cell apoptosis given the upregulated Bax and Caspase-3 mRNAs and decreased Bcl-2 mRNA. In summary, the PI3K/Akt signaling pathway can be activated by MnSO₄ exposure and mediate MnSO₄-induced neurotoxicity.

Intramuscle Synergies: Their Place in the Neural Control Hierarchy

We accept a definition of synergy introduced by Nikolai Bernstein and develop it for various actions, from those involving the whole body to those involving a single muscle. Furthermore, we use two major theoretical developments in the field of motor control—the idea of hierarchical control with spatial referent coordinates and the uncontrolled manifold hypothesis—to discuss recent studies of synergies within spaces of individual motor units (MUs) recorded within a single muscle. During the accurate finger force production tasks, MUs within hand extrinsic muscles form robust groups, with parallel scaling of the firing frequencies. The loading factors at individual MUs within each of the two main groups link them to the reciprocal and coactivation commands. Furthermore, groups are recruited in a task-specific way with gains that covary to stabilize muscle force. Such force-stabilizing synergies are seen in MUs recorded in the agonist and antagonist muscles but not in the spaces of MUs combined over the two muscles. These observations reflect inherent trade-offs between synergies at different levels of a control hierarchy. MU-based synergies do not show effects of hand dominance, whereas such effects are seen in multifinger synergies. Involuntary, reflex-based, force changes are stabilized by intramuscle synergies but not by multifinger synergies. These observations suggest that multifinger (multimuscle synergies) are based primarily on supraspinal circuitry, whereas intramuscle synergies reflect spinal circuitry. Studies of intra- and multimuscle synergies promise a powerful tool for exploring changes in spinal and supraspinal circuitry across patient populations.

Recent Advances in the Neural Control of Movements: Lessons for Functional Recovery

We review the current views on the control and coordination of movements following the traditions set by Nikolai Bernstein. In particular, we focus on the theory of neural control of effectors - from motor units to individual muscles, to joints, limbs, and to the whole body - with spatial referent coordinates organized into a hierarchy with multiple few-to-many mappings. Further, we discuss synergies ensuring stability of natural human movements within the uncontrolled manifold hypothesis. Synergies are organized within the neural control hierarchy based on the principle of motor abundance. Movement disorders are discussed as consequences of an inability to use the whole range of changes in referent coordinates (as in spasticity) and an inability to ensure controlled stability of salient variables as reflected in indices of multi-element synergies and their adjustments in preparation to actions (as in brain disorders, including Parkinson's disease, multiple-system atrophy, and stroke). At the end of the review, we discuss possible implications of this theoretical approach to peripheral disorders and their rehabilitations using, as an example, osteoarthritis. In particular, "joint stiffening" is viewed as a maladaptive strategy, which can compromise stability of salient variables during walking.

One more time about motor (and non-motor) synergies

We revisit the concept of synergy based on the recently translated classical book by Nikolai Bernstein (On the construction of movements, Medgiz, Moscow 1947; Latash, Bernstein's Construction of Movements, Routledge, Abingdon 2020b) and progress in understanding the physics and neurophysiology of biological action. Two aspects of synergies are described: organizing elements into stable groups (modes) and ensuring dynamical stability of salient performance variables. The ability of the central nervous system to attenuate synergies in preparation for a quick action-anticipatory synergy adjustments-is emphasized. Recent studies have demonstrated synergies at the level of hypothetical control variables associated with spatial referent coordinates for effectors. Overall, the concept of synergies fits naturally the hierarchical scheme of control with referent coordinates with an important role played by back-coupling loops within the central nervous system and from peripheral sensory endings. Further, we review studies showing non-trivial changes in synergies with development, aging, fatigue, practice, and a variety of neurological disorders. Two aspects of impaired synergic control-impaired stability and impaired agility-are introduced. The recent generalization of the concept of synergies for non-motor domains, including perception, is discussed. We end the review with a list of unresolved and troubling issues.

Stability of Action and Kinesthetic Perception in Parkinson's Disease

We present a review of action and perception stability within the theoretical framework based on the idea of control with spatial referent coordinates for the effectors at a number of hierarchical levels. Stability of salient variables is ensured by synergies, neurophysiological structures that act in multi-dimensional spaces of elemental variables and limit variance to the uncontrolled manifold during action and iso-perceptual manifold during perception. Patients with Parkinson’s disease show impaired synergic control reflected in poor stability (low synergy indices) and poor agility (low indices of anticipatory synergy adjustments prior to planned quick actions). They also show impaired perception across modalities, including kinesthetic perception. We suggest that poor stability at the level of referent coordinates can be the dominant factor leading to poor stability of percepts.

Association of exposure to manganese and fine motor skills in welders - Results from the WELDOX II study

The aim of this study was to evaluate the effect of exposure to manganese (Mn) on fine motor functions. A total of 48 welders and 30 unexposed workers as controls completed questionnaires, underwent blood examinations, and a motor test battery. The shift exposure of welders to respirable Mn was measured with personal samplers. For all subjects accumulations of Mn in the brain were assessed with T1-weighted magnetic resonance imaging. Welders showed normal motor functions on the Movement Disorder Society-Sponsored Revision of the Unified Parkinson Disease Rating Scale part III. Furthermore welders performed excellent on a steadiness test, showing better results than controls. However, welders were slightly slower than controls in motor tests. There was no association between fine motor test results and the relaxation rates R1 in globus pallidus and substantia nigra as MRI-based biomarkers to quantify Mn deposition in the brain.

Movement Quality: A Novel Biomarker Based on Principles of Neuroscience

A major problem in neurorehabilitation is the lack of objective outcomes to measure movement quality. Movement quality features, such as coordination and stability, are essential for everyday motor actions. These features allow reacting to continuously changing environment or to resist external perturbations. Neurological disorders affect movement quality, leading to functionally impaired movements. Recent findings suggest that the central nervous system organizes motor elements (eg, muscles, joints, fingers) into task-specific ensembles to stabilize motor tasks performance. A method to quantify this feature has been previously developed based on the uncontrolled manifold (UCM) hypothesis. UCM quantifies movement quality in a spatial-temporal domain using intertrial analysis of covariation between motor elements. In this point-of-view article, we first describe major obstacles (eg, the need for group analysis) that interfere with UCM application in clinical settings. Then, we propose a process of quantifying movement quality for a single individual with a novel use of bootstrapping simulations and UCM analysis. Finally, we reanalyze previously published data from individuals with neurological disorders performing a wide range of motor tasks, that is, multi-digit pressing and postural balance tasks. Our method allows one to assess motor quality impairments in a single individual and to detect clinically important motor behavior changes. Our solution may be incorporated into a clinical setting to assess sensorimotor impairments, evaluate the effects of specific neurological treatments, or track movement quality recovery over time. We also recommended the proposed solution to be used jointly with a typical statistical analysis of UCM parameters in cohort studies.

Manganese Exposure and Neurologic Outcomes in Adult Populations

A review of published articles examining the effects of manganese exposure to workers and community residents shows adverse neurologic outcomes. Innovative biomarkers, including those from neuroimaging, were incorporated into many of these studies to assess both manganese exposure and neurologic outcomes. A variety of health effects were evaluated, including cognitive and motor impairments. Studies of community participants residing near manganese point sources show variability in outcomes, reflecting the complexities of exposure measurement, individual absorption, and assessment of neurologic effects. The aging population provides insight into the impacts of chronic exposure in younger populations.

Laws of nature that define biological action and perception

We describe a physical approach to biological functions, with the emphasis on the motor and sensory functions. The approach assumes the existence of biology-specific laws of nature uniting salient physical variables and parameters. In contrast to movements in inanimate nature, actions are produced by changes in parameters of the corresponding laws of nature. For movements, parameters are associated with spatial referent coordinates (RCs) for the effectors. Stability of motor actions is ensured by the abundant mapping of RCs across hierarchical control levels. The sensory function is viewed as based on an interaction of efferent and afferent signals leading to an iso-perceptual manifold where percepts of salient sensory variables are stable. This approach offers novel interpretations for a variety of known neurophysiological and behavioral phenomena and makes a number of novel testable predictions. In particular, we discuss novel interpretations for the well-known phenomena of agonist-antagonist co-activation and vibration-induced illusions of both position and force. We also interpret results of several new experiments with unintentional force changes and with analysis of accuracy of perception of variables produced by elements of multi-element systems. Recently, this approach has been expanded to interpret motor disorders including spasticity and consequences of subcortical disorders (such as Parkinson's disease). We suggest that the approach can be developed for cognitive functions.

Greater Sensorimotor Deficits and Abnormally Lower Globus Pallidus Fractional Anisotropy in HIV+ Women than in HIV+ Men

AIMS

Cognitive impairment may be greater in HIV-positive (HIV+) women than in HIV+ men. Whether sex-specific differences exist in brain microstructure of HIV+ individuals is unknown and was evaluated.

METHOD

39 HIV+ (21 men, 18 women) and 45 seronegative (SN, 20 men, 25 women) participants were assessed with brain diffusion tensor imaging and cognitive assessments (7 neuropsychological domains). Fractional anisotropy (FA) and mean diffusivity (MD) were measured with an automated atlas in selected brain regions. Group comparisons were assessed with linear mixed effects models, with sub-regions and hemisphere (left/right) as repeated factors for each region.

RESULTS

HIV+ women, but not HIV+ men, were slower than sex-matched SN controls on sensorimotor function (Dominant-hand: interaction-p = 0.007; Non-dominant hand: interaction-p = 0.039). Similarly, only HIV+ women had lower FA in the globus pallidus (GP, interaction-p = 0.011). Additionally, regardless of sex, the HIV+ group had poorer Fluency, Speed, and Attention than SN-controls (p = 0.006-0.008), as well as lower FA and higher MD in multiple brain regions (p = <0.001-0.044). Across all participants, performance on Attention was predicted by uncinate-FA (p < 0.001, r = 0.5) and corpus callosum (CC)-FA (p = 0.038, r = 0.23), while the Speed of Information Processing was predicted by CC-FA (p = 0.009, r = 0.3). Furthermore, faster sensorimotor function correlated with higher CC-FA and uncinate-FA in men but not in women (Sex*DTI-interaction-p = 0.03-0.06).

CONCLUSIONS

The relatively poorer sensorimotor function and abnormally lower GP_FA, suggesting lesser neuronal integrity, in HIV+ women demonstrate sex-specific effects from HIV-infection on these measures. These findings may be related to the greater immune activation and neuroinflammation in HIV+ women compared to HIV+ men. Graphical Abstract.

Stability of reaching during standing in stroke

Reaching from standing requires simultaneous adjustments of focal and postural task elements. We investigated the ability of people with stroke to stabilize the endpoint trajectory while maintaining balance during standing reaches. Nineteen stroke and 11 age-equivalent healthy subjects reached toward a target (n = 30 trials) located beyond arm length from standing. Endpoint and center-of-mass (COM) trajectories were analyzed using the uncontrolled manifold (UCM) approach, with segment angles as elemental variables. A synergy index (SI) represented the normalized difference between segment angle combinations, leading to endpoint or COM trajectory stabilization (V_UCM) and lack of stabilization (in an orthogonal space; V_ORT). A higher SI reflects greater stability. In both groups, the endpoint SI (SI_END) decreased in parallel with endpoint velocity and returned close to baseline at the end of the movement. The range of SI_END was significantly greater in stroke (median: 0.87; QR:0.54) compared with healthy subjects (median: 0.58; QR: 0.33; P = 0.009). In both groups, the lowest SI_END occurred at the endpoint peak velocity, whereas the minimal SI_END of the stroke group (median: 0.51; QR:0.41) was lower than the healthy group (median: 0.25; QR: 0.50; P = 0.033). The COM SI (SI_COM) remained stable in both groups (~0.8). The maintenance of a high SI_COM despite a large reduction of SI_END in stroke subjects suggests that kinematic redundancy was effectively used to stabilize the COM position, but less so for endpoint position stabilization. Both focal and postural task elements should be considered when analyzing whole body reaching deficits in patients with stroke.NEW & NOTEWORTHY Reaching from standing requires simultaneous adjustments of endpoint and center-of-mass (COM) positions. We used uncontrolled manifold analysis to investigate the impact of stroke on the ability to use kinematic redundancy in this task. Our results showed that COM position was stabilized, whereas endpoint trajectory was more variable in stroke than healthy subjects. Enhancing the capacity to meet multiple task goals may be beneficial for motor recovery after stroke.

Respirator usage protects brain white matter from welding fume exposure: A pilot magnetic resonance imaging study of welders

Welding fume exposure has been associated with structural brain changes and a wide variety of clinical and sub-clinical outcomes including cognitive, behavioral and motor abnormalities. Respirator use has been shown to decrease exposure to welding fumes; however, the associations between respirator use and health outcomes, particularly neurologic health, have been understudied. In this preliminary study, we used diffusion tensor imaging (DTI) to investigate the effectiveness of respirator use in protecting workers' white matter (WM) from the harmful effects related to welding fume exposure. Fractional anisotropy (FA), a common DTI measurement of water diffusion properties, was used as a marker of WM microstructure integrity. We hypothesized that FA in brain regions involved in motor and neurocognitive functions would differ between welders reporting respirator use compared to those not using a respirator. We enrolled a pilot cohort of 19 welders from labor unions in the New York City area. All welders completed questionnaires to assess welding history and occupational health. All completed a DTI acquisition on a 3 T Siemens scanner. Partial least squares discriminant analysis (PLS-DA), a bioinformatic analytical strategy, was used to model the divergence of WM microstructures in 48 regions defined by the ICBM-DTI-81 atlas between respirator users compared to non-users. This yielded an effective discrimination of respirator users from non-users, with the uncinate fasciculus, the cerebellar peduncle and the superior longitudinal fasciculus contributing most to the discrimination of these groups. These white matter tracts are involved in widespread motor and cognitive functions. To our knowledge, this study is the first to suggest a protective effect of respirator on WM microstructure, indicating that the lack of respirator may present unsafe working conditions for welders. These preliminary findings may inform a larger, longitudinal intervention study that would be more appropriate to investigate the potential protective effect of respirator usage on brain white matter in welders.

Synergic control of action in levodopa-naïve Parkinson's disease patients: I. Multi-finger interaction and coordination

We explored the origin of the impaired control of action stability in Parkinson's disease (PD) by testing levodopa-naïve PD patients to disambiguate effects of PD from possible effects of long-term exposure to levodopa. Thirteen levodopa-naïve PD patients and 13 controls performed single- and multi-finger force production tasks, including producing a self-paced quick force pulse into a target. A subgroup of patients (n = 10) was re-tested about 1 h after the first dose of levodopa. Compared to controls, PD patients showed lower maximal forces and synergy indices stabilizing total force (reflecting the higher inter-trial variance component affecting total force). In addition, PD patients showed a trend toward shorter anticipatory synergy adjustments (a drop in the synergy index in preparation to a quick action) and larger non-motor equivalent finger force deviations. Lower maximal force, higher unintentional force production (enslaving) and higher inter-trial variance indices occurred in PD patients after one dosage of levodopa. We conclude that impairment in synergies is present in levodopa-naïve patients, mainly in indices reflecting stability (synergy index), but not agility (anticipatory synergy adjustments). A single dose of levodopa, however, did not improve synergy indices, as it did in PD patients on chronic anti-PD medication, suggesting a different mechanism of action. The results suggest that indices of force-stabilizing synergies may be used as an early behavioral sign of PD, although it may not be sensitive to acute drug effects in drug-naïve patients.

Recommendation for the minimum number of steps to analyze when performing the uncontrolled manifold analysis on walking data

The uncontrolled manifold (UCM) analysis quantifies the extent to which co-variation among a set of variables facilitates consistent performance by partitioning variance in those variables into two components then calculating their normalized difference (i.e., the synergy index). Although UCM-derived measures are thought to depend on the number of data points analyzed, the minimum number needed to reasonably approximate true values of these measures is unknown. For each of two performance variables related to mechanical stability of gait, we evaluated changes in UCM-derived measures when increasing the number of analyzed points, here steps. Fourteen older adults walked on a treadmill while motion capture tracked movement. For each subject, n steps (where n = 2-99) were randomly sampled from the first 100, then used to calculate UCM-derived variables. For each subject, variables were expressed as a percent of the subject-specific value with n = 100 and averaged across 50 simulations. For each n, 95% confidence intervals (CIs) were calculated from group data. The minimum number of steps to "reasonably approximate" a variables was defined as the value of n for which the lower CI was >90% of the value with n = 100. Regardless of performance variable, reasonable approximations of the synergy index were attained with n = 16 steps, whereas n = 50 steps were needed for each of the variance components However, the differences between using 16 steps and 50 steps were small. Collecting 15-20 steps is recommended for a reasonable approximation of the synergy indices considered herein, particularly when data collection is constrained to a limited number of steps.

Quantitative analysis of multi-element synergy stabilizing performance: comparison of three methods with respect to their use in clinical studies

A number of analyses associated with the uncontrolled manifold (UCM) hypothesis have been used recently to investigate stability of actions across populations. We explored whether some of those methods have an advantage for clinical studies because they require fewer trials to achieve consistent findings. We compared the number of trials needed for the analysis of inter-trial variance, analysis of motor equivalence, and analysis in the space of referent coordinates. Young healthy adults performed four-finger accurate force production tasks under visual feedback with the right (dominant) and left hand over three days. Three methods [analytical (M1), experimental (M2), and cumulative mean (M3) methods] were used to define the minimal number of trials required to reach certain statistical criteria. Two of these methods, M1 and M2, showed qualitatively similar results. Fewer trials (M1: 5-13, M2: 4-10) were needed for analysis of motor equivalence compared to inter-trial variance analysis (M1: 14-24, M2: 10-14). The third method (M3) showed no major differences among the outcome variables. The index of synergy in the inter-trial variance analysis required a very small number of trials (M1, M2: 2-4). Variables related to referent coordinates required only a few trials (under 3), whereas the synergy index in this analysis required the largest number of trials (M1: 24-34, M2: 12-16). This is the first study to quantify the number of trials needed for UCM-based methods of assessing motor coordination broadly used in clinical studies. Clinical studies can take advantage of specific recommendations based on the current data regarding the number of trials needed for each analysis thus allowing minimizing the test session duration without compromising data reliability.

Individual preferences in motor coordination seen across the two hands: relations to movement stability and optimality

The framework of the uncontrolled manifold (UCM) hypothesis was used to explore variables related to stability of task performance in the two hands of young healthy individuals. Fourteen young adults performed four-finger accurate constant force production tasks interrupted by a voluntary quick force pulse production and by an externally imposed displacement of all fingers. Three groups of variables were used to quantify stability of steady force production: (1) indices of the inter-trial variance were computed within the UCM and orthogonal to the UCM; (2) indices of motor equivalence were computed between steady-state intervals separated by the force pulse and by the finger-lifting episode; and (3) referent coordinate and apparent stiffness were computed using the data during the ascending phase of the finger-lifting episode. In another task, the subjects performed accurate constant force production with visual feedback removal after the 8th second, and the drop in the total force after the removal was computed. There were differences between the right and left hand in some outcome variables such as variance within the UCM, and the timing of anticipatory synergy adjustments prior to the force pulse, consistent with the dynamic dominance hypothesis. There were significant correlations between the two hands for indices that were unrelated to accuracy of performance: variance within the UCM, index of motor equivalence, referent coordinate, apparent stiffness, and the drop of total force after visual feedback removal. We interpret these findings within the concept of stability-optimality trade-off. In particular, we conclude that individual subjects select particular, person-specific solutions within the spectrum allowed by the explicit task constraints, and this choice is consistent between the two hands. We conclude with a hypothesis that selecting specific solutions within the stability-optimality trade-off may represent an individual's personal preference consistent between the two hands.

Systemic effects of deep brain stimulation on synergic control in Parkinson's disease

OBJECTIVE

We explored effects of deep brain stimulation (DBS) in patients with Parkinson's disease (PD) on the synergic control of fingers in a multi-finger force production task and of muscles in a task involving vertical posture.

METHODS

The finger task involved the four fingers of a hand producing accurate total force followed by a targeted quick force pulse. The postural task involved releasing a load from extended arms. The analysis of synergies was performed within the framework of the uncontrolled manifold hypothesis.

RESULTS

DBS led to no significant changes in indices of stability during steady-state phases. In contrast, DBS improved indices of agility, quantified as anticipatory synergy adjustments that reduced stability of salient performance variables in preparation to their quick change. There were moderate-to-strong correlations between indices of both stability and agility measured in the multi-finger force production and multi-muscle whole-body action.

CONCLUSIONS

Our results point at systemic changes in synergic control in PD. They show that DBS is effective in improving only one components of synergic control related to agility in performance being relatively ineffective for the stability component.

SIGNIFICANCE

The results show systemic brain mechanisms of synergies and suggest differential effects of DBS on indices of stability and agility.

Long-term effects of mild traumatic brain injuries to oculomotor tracking performances and reaction times to simple environmental stimuli

Understanding the long-term effects of concussive events remains a challenge for the development of modern medical practices and the prevention of recurrent traumas. In this study, we utilized indices of oculomotor performance and the ability to react to simple environmental stimuli to assess the long-term motor effects of traumatic brain injury in its mildest form (mTBI). We performed analysis of eye movement accuracy, investigated the presence of abnormal eye movements, and quantified time to react to simple environmental stimuli on long-term mTBI survivors. Results indicated the presence of impairments to basic neural functions used to explore and respond to environmental demands long after the occurrence of mTBIs. Specifically, the result revealed the presence of abnormal saccadic eye movements while performing horizontal smooth pursuit, diminished accuracy of primary saccadic horizontal eye movement, and a widespread slower reaction to both visual and auditory stimuli. The methodology used in this study indicated to be potentially useful in aiding future investigations of neural circuitry impaired by mTBI and provide indices of recovery in future clinical trials testing mTBI-related clinical interventions.

Abundant Degrees of Freedom Are Not a Problem

The problem of motor redundancy has been one of the fundamental, albeit elusive, problems in motor control. Traditionally, it has been viewed as a computational problem for the brain, solved with either optimization methods or by introducing additional constraints to motor tasks. This review suggests that the problem was wrongly formulated, and that the abundant degrees of freedom are not to be eliminated but used to ensure dynamic stability of motor performance, which is vital given the unpredictable intrinsic states and external forces. The idea of synergies as mechanisms ensuring action stability is introduced based on the uncontrolled manifold hypothesis and the theory of control with spatial referent coordinates. The importance of controlled stability is illustrated with the phenomena of anticipatory synergy adjustments. This approach is productive for both basic and applied fields as illustrated, in particular, by changes in motor synergies with neurological disorder and exercise.

Synergies and Motor Equivalence in Voluntary Sway Tasks: The Effects of Visual and Mechanical Constraints

The authors used two analyses developed within the framework of the uncontrolled manifold hypothesis to quantify multimuscle synergies during voluntary body sway: analysis of intertrial variance and analysis of motor equivalence with respect to the center of pressure (COP) trajectory. Participants performed voluntary sway tasks in the anteroposterior direction at 0.33 and 0.66 Hz. Muscle groups were identified in the space of muscle activations and used as elemental variables in the synergy analyses. Changing mechanical and vision feedback-based constraints led to significant changes in indices of sway performance such as COP deviations in the uninstructed, mediolateral direction and indices of spontaneous postural sway. In contrast, there were no significant effects on synergy indices. These findings show that the neural control of performance and of its stability may involve different control variables and neurophysiological structures. There were strong correlations between the indices of motor equivalence and those computed using the intercycle variance analysis. This result is potentially important for studies of patients with movement disorders who may be unable to perform multiple trials (cycles) at any given task, making analysis of motor equivalence of single trials a viable alternative to explore changes in stability of actions.

Motor equivalence and structure of variance: multi-muscle postural synergies in Parkinson's disease

We explored posture-stabilizing multi-muscle synergies with two methods of analysis of multi-element, abundant systems: (1) Analysis of inter-cycle variance; and (2) Analysis of motor equivalence, both quantified within the framework of the uncontrolled manifold (UCM) hypothesis. Data collected in two earlier studies of patients with Parkinson's disease (PD) were re-analyzed. One study compared synergies in the space of muscle modes (muscle groups with parallel scaling of activation) during tasks performed by early-stage PD patients and controls. The other study explored the effects of dopaminergic medication on multi-muscle-mode synergies. Inter-cycle variance and absolute magnitude of the center of pressure displacement across consecutive cycles were quantified during voluntary whole-body sway within the UCM and orthogonal to the UCM space. The patients showed smaller indices of variance within the UCM and motor equivalence compared to controls. The indices were also smaller in the off-drug compared to on-drug condition. There were strong across-subject correlations between the inter-cycle variance within/orthogonal to the UCM and motor equivalent/non-motor equivalent displacements. This study has shown that, at least for cyclical tasks, analysis of variance and analysis of motor equivalence lead to metrics of stability that correlate with each other and show similar effects of disease and medication. These results show, for the first time, intimate links between indices of variance and motor equivalence. They suggest that analysis of motor equivalence, which requires only a handful of trials, could be used broadly in the field of motor disorders to analyze problems with action stability.

Welding-related brain and functional changes in welders with chronic and low-level exposure

Although an essential nutrient, manganese (Mn) can be toxic at high doses. There is, however, uncertainty regarding the effects of chronic low-level Mn-exposure. This review provides an overview of Mn-related brain and functional changes based on studies of a cohort of asymptomatic welders who had lower Mn-exposure than in most previous work. In welders with low-level Mn-exposure, we found: 1) Mn may accumulate in the brain in a non-linear fashion: MRI R1 (1/T1) signals significantly increased only after a critical level of exposure was reached (e.g., ≥300 welding hours in the past 90days prior to MRI). Moreover, R1 may be a more sensitive marker to capture short-term dynamic changes in Mn accumulation than the pallidal index [T1-weighted intensity ratio of the globus pallidus vs. frontal white matter], a traditional marker for Mn accumulation; 2) Chronic Mn-exposure may lead to microstructural changes as indicated by lower diffusion tensor fractional anisotropy values in the basal ganglia (BG), especially when welding years exceeded more than 30 years; 3) Mn-related subtle motor dysfunctions can be captured sensitively by synergy metrics (indices for movement stability), whereas traditional fine motor tasks failed to detect any significant differences; and 4) Iron (Fe) also may play a role in welding-related neurotoxicity, especially at low-level Mn-exposure, evidenced by higher R2* values (an estimate for brain Fe accumulation) in the BG. Moreover, higher R2* values were associated with lower phonemic fluency performance. These findings may guide future studies and the development of occupation- and public health-related polices involving Mn-exposure.

Motor synergies research in physical therapy: advantages of the uncontrolled manifold approach

ABSTRACT Movement is central to physical therapy identity and practice. Advances in the science of movement control, motor learning and development are thus inextricably tied to professional development and clinical activity. This paper aims to describe a prominent approach to motor control with potential to greatly advance the understanding of movement dysfunction: the uncontrolled manifold (UCM). An argument is formulated for incorporating this method of data analysis in rehabilitation research. It is a narrative review of the relevant literature. Research in physical therapy could greatly benefit from investigating synergies with the theory and methods of UCM. Research should seek connections between functioning in daily life tasks and the assembling of synergies to stabilize different performance variables, the UCM variability measures, the synergy strength indexes, and the anticipatory synergy adjustments. Changes in these synergy variables should also be quantified after rehabilitation interventions. UCM can offer one solid science-based approach to inform clinical decisions on whether synergies have to be broken, rebalanced, created, or reinforced in patients with movement dysfunction.

Association of neurobehavioral performance with R2* in the caudate nucleus of asymptomatic welders

OBJECTIVES

Welding fumes contain several metals including manganese (Mn) and iron (Fe) that may affect the nervous system. Previous studies of potential welding-related neurotoxicity have focused primarily on Mn exposure. The current study examined neurobehavioral and brain imaging changes in asymptomatic welders and their associations with both Mn and Fe exposure measurements.

METHODS

Data were obtained from subjects with (n=46) and without (controls; n=31) a history of welding exposure. Occupational questionnaires estimated recent (HrsW; welding hours and E₉₀; cumulative exposure, past 90days) and lifetime (YrsW; total welding years and ELT; cumulative exposure, lifetime) exposure. Brain MRI pallidal index (PI), R1 (1/T1), and R2* (1/T2*) were measured to estimate Mn and Fe concentrations in the basal ganglia [caudate nucleus (CN), putamen, and globus pallidus], amygdala, and hippocampus. Comprehensive neuropsychological tests were conducted to examine behavioral differences between welders and controls. Correlation analyses were conducted between neuropsychological tests and those exposure measurements that showed significant group differences.

RESULTS

Compared to controls, welders had significantly higher R2* in the CN and lower performance on the Phonemic Fluency test. Correlation analyses revealed that welders' Phonemic Fluency scores were inversely associated with R2* in the CN, but not with the PI or R1 in any brain region of interest studied.

DISCUSSION

The results showed that neurobehavioral performance for the asymptomatic welders in our study was worse than individuals who had not welded, and suggest the differences may be associated with higher Fe accumulation in the CN.