Random walk and drifts within the uncontrolled manifold during multi-finger force production tasks

In this study, we quantified changes in finger interdependence (enslaving), multi-finger synergies, and feedforward modulation of synergy properties (i.e., anticipatory synergy adjustment) during single- and multi-finger force production tasks in individuals with cerebral palsy (CP). Spastic diplegic CP and healthy control subjects performed sets of finger force production tasks by each of the hands, including maximal force production and submaximal quick pulse force production in an isometric condition. The framework of the uncontrolled manifold hypothesis was used to quantify the indices of multi-finger synergies and the anticipatory synergy adjustment (ASA). The CP group showed lower maximal forces and higher indices of finger interdependence (enslaving), while the indices of multi-finger synergies stabilizing total finger forces during stable force production were not different significantly compared to the controls. Further, the time of ASA for the CP group was not delayed. The CP group showed a significantly less drop in the synergy indices during the anticipatory and quick pulse phase compared to the control group, which was accompanied by larger co-contraction indices of the forearm muscles. These findings suggest that the function of assembling motor synergies for stable force production is not affected by CP, while the ability to modulate synergy properties may be impaired with CP partially due to spasticity. The spasticity presumably hampers the purposeful feedforward destabilization of the performance. The results suggest that quantification of multi-digit synergies may provide an alternative tool for quantitative assessment of impaired coordination in the CP individuals.

ABSTRACTThe authors explored task-specific stability during accurate multifinger force production tasks with different numbers of instructed fingers. Subjects performed steady-state isometric force production tasks and were instructed not to interfere voluntarily with transient lifting-and-lowering perturbations applied to the index finger. The main results were (a) intertrial variance in the space of finger modes at steady states was larger within the subspace that had no effect on the total force (the uncontrolled manifold [UCM]); (b) perturbations caused large deviations of finger modes within the UCM (motor equivalence); and (c) deviations caused by the perturbation showed larger variance within the UCM. No significant effects of the number of task fingers were noted in any of the 3 indicators. The results are discussed within the frameworks of the UCM and referent configuration hypotheses. The authors conclude, in particular, that all the tasks were effectively 4-finger tasks with different involvement of task and nontask fingers.

Performance monitoring beyond choice tasks: The time course of force execution monitoring investigated by event-related potentials and multivariate pattern analysis

Coupling phenomena during asynchronous submaximal two-hand, multi-finger force production tasks in humans

Essential tremor (ET) is one of the most common movement disorders and is marked by centrally generated oscillations that interfere with stable hand function. The contribution of spinal circuitry to impaired movement stability remains largely unexplored. In this study, we assessed intramuscle synergies as a potential biomarker of spinal deficits during accurate cyclic force production in individuals with ET and compared them with those in patients with Parkinson's disease (PD), age-matched controls (AMC), and young adult controls. Motor unit activity was extracted through surface EMG decomposition of flexor digitorum superficialis, and force-stabilizing synergies were quantified using the uncontrolled manifold framework. Across groups, multifinger synergies stabilizing total force were similar, with ET participants not different from AMC and PD. In contrast, intramuscle synergies were selectively disrupted in ET. The reduction of the synergy index in ET compared with all other groups was driven primarily by markedly lower variance that did not affect force magnitude, while variance affecting force output was comparable across groups. We conclude that ET shows reduced flexibility in motor unit recruitment and firing. These findings identify a previously unrecognized alteration at the level of spinal circuitry in ET during voluntary force control. Such impaired organization of motor units may contribute directly to difficulties maintaining stable hand action in daily tasks. Intramuscle synergy metrics thus represent a promising biomarker of spinal-level dysfunction in ET and a potential mechanistic target for future therapeutic interventions.NEW & NOTEWORTHY This is the first study exploring the recently described phenomenon of intramuscle synergies in neurological patients. Patients with essential tremor showed reduced indices of such synergies during cyclical finger force production compared with patients with Parkinson's disease and two groups of controls. This reduction reflected the reduced flexibility in the recruitment of motor unit groups. Studies of intramuscle synergies are promising to provide a sensitive biomarker of spinal circuitry.

Changes in corticospinal excitability during motor imagery by physical practice of a force production task: Effect of the rate of force development during practice

A core feature of joint action is task-sharing-the dynamic coordination of individual efforts to produce and stabilize a shared outcome. Yet, it remains unclear which features of task-sharing dynamics relate to stable performance. In this study, we address this gap by identifying behavioral markers that capture the organization and robustness of task-sharing in a joint force-production task. Thirty dyads performed a continuous force-production task in which each partner independently applied force to jointly match a target total force. Real-time visual feedback was provided on their combined output. The task was completed under low-force (10% maximum voluntary contraction) and high-force (30% maximum voluntary contraction) conditions to manipulate difficulty. Trials revealed qualitatively distinct patterns of force contribution, including cases in which a single partner carried most of the output, patterns in which partners contributed with distinct temporal roles, and fully synergistic task-sharing in which both partners continuously shaped total force fluctuations. We used within-trial uncontrolled manifold (UCM) analysis to characterize the dominant, synergistic mode along two dimensions. The first is the degree of variation in task-sharing patterns that leaves performance unchanged (VUCM). The second is the degree of irregularity-computed as sample entropy-of these patterns (SampEnUCM). Dyads with lower VUCM and lower SampEnUCM demonstrated greater performance stability. VUCM and SampEnUCM thus serve as indices of robust task-sharing patterns, offering behavioral access to the control mechanisms that enable stable joint performance, particularly under more challenging conditions.

The use of flexible arm muscle synergies to perform an isometric stabilization task

The purpose of this study was to determine the effect of visual feedback and verbal encouragement on concentric peak torque of the quadriceps and hamstrings muscles of males and females at an isokinetic velocity of 60 deg/s. All testing was performed on the Biodex B-2000 isokinetic dynamometer in three sessions, separated by 7 to 14 days. Fifteen males and 15 females (age = 25.4 ± 2.4 yrs, wt = 76.6 ± 16.5 kg, ht = 173.61 ± 9.5 cm) were tested under the following conditions: (a) visual feedback, (b) verbal encouragement, (c) combined visual feedback and verbal encouragement, and (d) no feedback (control). Testing order was counterbalanced to control for possible learning effects. Two separate 2 x 4 repeated measure analyses of variance (ANOVA) were used to examine the difference among test conditions. Examination of quadriceps force production revealed that subjects generated greater peak torque when visual feedback was provided than when verbal encouragement or no feedback were provided. Similarly, quadriceps force production was greater when combined visual feedback and verbal encouragement was provided than when verbal encouragement or no feedback were provided (p < 0.05). Examination of hamstrings force production revealed that subjects generated greater peak torque when combined visual feedback and verbal encouragement was provided than when verbal encouragement, and no feedback were provided. Additionally, hamstrings force production was greater when visual feedback was provided than when no feedback was provided (p < 0.05). These results indicate that the use of visual and combined visual and verbal feedback increased quadriceps and hamstrings force production when compared to the control condition where no feedback was provided.

This study involved an optimization, numerical analysis of a network for two-hand multi-finger force production, analogous in its structure to the double-representation mirror image (DoReMi) network suggested earlier based on neurophysiological data on cortical finger representations. The network accounts for phenomena of enslaving (unintended finger force production), force deficit (smaller force produced by a finger in multi-finger tasks as compared to its single-finger task), and bilateral deficit (smaller forces produced in two-hand tasks as compared to one-hand tasks). Matrices of connection weights were computed, and the results of optimization were compared to the experimental data on finger forces during one- and two-hand maximal force production (MVC) tasks. The network was able to reproduce the experimental data in two-hand experiments with high accuracy (average error was 1.2 N); it was also able to reproduce findings in one-hand multi-finger MVC tasks, which were not used during the optimization procedure, although with a somewhat higher error (2.8 N). Our analysis supports the feasibility of the DoReMi network. It suggests that within-a-hand force deficit and bilateral force deficit are phenomena of different origins whose effects add up. Is also supports a hypothesis that force deficit and enslaving have different neural origins.

This dissertation investigates the application of the Hurst exponent as a tool for understanding market efficiency, persistence, and structural changes in financial markets, particularly during periods of economic turbulence. The Hurst exponent, which quantifies long-term memory in time series, serves as an alternative to the Efficient Market Hypothesis (EMH), enabling the assessment of market behaviors beyond random walks. The study explores two core approaches: first, examining the Hurst exponent across major financial indices (Ibovespa, FTSE, S&P 500, and Dow Jones) during significant crises, including the Dot-Com Bubble, the Financial Crisis of 2008, and the COVID-19 crisis; and second, analyzing the Hurst exponent over 21-day intervals to correlate with short-term volatility and returns. Employing Rescaled Range (R/S) analysis as the primary methodological tool, this research demonstrates the ability of the Hurst exponent to capture persistent behaviors and trend-following tendencies within financial time series, which deviate from traditional EMH assumptions. The findings reveal that, during crisis periods, financial indices exhibit high Hurst values, indicating persistent market behavior and potential predictability amidst instability. Additionally, the analysis shows that the Hurst exponent correlates moderately with returns but less so with volatility over short-term intervals, suggesting its capacity as a trend indicator rather than a measure of immediate risk. This study contributes to the literature by highlighting the practical utility of the Hurst exponent in both crisis prediction and market analysis, supporting its role as a complementary tool alongside traditional financial metrics. Future research could further explore the predictive applications of the Hurst exponent across diverse asset classes and under varying economic conditions.

Článek shrnuje teoretický koncept "Hypotéza Efektivního Trhu" a představuje nový koncept "Fraktální Hypotéza Akciového Trhu". Podle této hypotézy výnosy z akcií na trhu vykonávají zkreslenou náhodnou procházku, tzv. Hurstův persistentní proces, který je charakterizován svou dlouhou pamětí. Při ověření tohoto konceptu na českých indexech PX 50 byla provedena (R/S) analýza a byl vypočten Hurstův exponent. Ukazuje se, že výnosy z akciového indexu PX 50 vykonávají persistentní Hurstův proces s hodnotou Hurstova exponentu rovnající se 0,662. Tato hodnota je zcela odlišná od hodnoty pro náhodnou procházku a odpovídá výsledkům dosaženým v dřívějších pracích.

Humans have the ability to identify external physical properties because skilled object manipulation requires knowledge, or internal models, of object dynamics relating to applied force or movement. The accuracy of identification depends on those of the input and output signal to the object : motor command from brain as the input and information from multiple sensory modalities as the output. In this paper, control strategies of isometric contraction by hand are investigated. Subjects are asked to generate constant levels of force under two conditions : force production tasks a) by self scaling without visual feedback and b) with visual feed back of reference force levels. The results show subjective assessments of force are larger than objective one, and it is indicated that the variability is composed of signal dependent noise and error of human intention. It is also demonstrated that visual feedback information has an important role of generating accurate reference force level.

Fractional Brownian Motions, Fractional Noises and Applications

Adaptive learning requires interpreting prediction errors in light of environmental dynamics. However, environments may not only differ in terms of when changes occur, but also how they occur: some nonstationary processes in nature exhibit slow drifts over time, while others show more abrupt changes. This raises the question as to whether humans can adapt their learning to reflect the generative structure of their environment. Here, we compared how humans learn in two canonical nonstationary environments: abrupt change points versus gradual random walks. Using a predictive inference task and a unified Bayesian framework, we show across two behavioral experiments that humans adapt their learning normatively between these two environments. Notably, identical prediction errors were interpreted differently across them. Large prediction errors triggered sharp increases in learning rate under change-point but not random-walk dynamics, where small and large errors were more equally weighted. This matched the predictions of a normative Bayesian model, which itself adopted the appropriate generative model for each environment. In addition, we found that humans could dissociate two latent variables that needed to be jointly inferred: the mean and variance (or stochasticity). This was demonstrated by explicit uncertainty reports that closely tracked the current variance, and only showed sustained changes following change points in variance but not mean. Together, these results show that humans adapt their learning strategy to both how and what aspects of the environment are changing. They establish a unified computational account of adaptive learning across different environmental dynamics.