1
|
Intra-muscle Synergies Stabilizing Reflex-mediated Force Changes. Neuroscience 2022; 505:59-77. [DOI: 10.1016/j.neuroscience.2022.10.009] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/14/2022] [Revised: 09/23/2022] [Accepted: 10/10/2022] [Indexed: 11/20/2022]
|
2
|
Intramuscle Synergies: Their Place in the Neural Control Hierarchy. Motor Control 2022; 27:402-441. [PMID: 36543175 DOI: 10.1123/mc.2022-0094] [Citation(s) in RCA: 7] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/29/2022] [Revised: 10/03/2022] [Accepted: 10/24/2022] [Indexed: 12/24/2022]
Abstract
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.
Collapse
|
3
|
Role of Post-Trial Visual Feedback on Unintentional Force Drift During Isometric Finger Force Production Tasks. Motor Control 2021; 26:1-14. [PMID: 34891126 DOI: 10.1123/mc.2020-0031] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/18/2020] [Revised: 08/30/2021] [Accepted: 09/05/2021] [Indexed: 11/18/2022]
Abstract
A reduction in fingertip forces during a visually occluded isometric task is called unintentional drift. In this study, unintentional drift was studied for two conditions, with and without "epilogue." We define epilogue as the posttrial visual feedback in which the outcome of the just-concluded trial is shown before the start of the next trial. For this study, 14 healthy participants were recruited and were instructed to produce fingertip forces to match a target line at 15% maximum voluntary contraction. The results showed a significant reduction in unintentional drift in the epilogue condition. This reduction is probably due to the difference in the shift in λ, the threshold of the tonic stretch reflex, the hypothetical control variable that the central controller can set.
Collapse
|
4
|
Abolins V, Latash ML. Unintentional Force Drifts as Consequences of Indirect Force Control with Spatial Referent Coordinates. Neuroscience 2021; 481:156-165. [PMID: 34774968 DOI: 10.1016/j.neuroscience.2021.11.006] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/03/2021] [Revised: 10/16/2021] [Accepted: 11/02/2021] [Indexed: 11/29/2022]
Abstract
We explored the phenomenon of unintentional force drifts in the absence of visual feedback. Based on the idea of direct force control with internal models and on the idea of indirect force control with referent coordinates to the involved muscle groups, contrasting predictions were drawn for changes in the drift magnitude when acting against external spring loads. Fifteen young subjects performed typical accurate force production tasks by pressing with the Index finger at 20% of maximal voluntary contraction (MVC) in isometric conditions and while acting against one of the three external springs with different stiffness. The visual feedback on the force was turned off after 5 s. At the end of each 20-s trial, the subjects relaxed and then tried to reproduce the final force level. The force drifts were significantly smaller in the spring conditions, particularly when acting against more compliant springs. The subjects were unaware of the force drifts and, during force matching, produced forces close to the initial force magnitude, which were not different across the conditions. There was a trend toward larger drifts during performance by the dominant hand. We view these observations as strong arguments in favor of the theory of control with spatial referent coordinates. In particular, force drifts were likely consequences of drifts of referent coordinates to both agonist and antagonist muscles. The lack of drift effects on both perception-to-report and perception-to-act fit the scheme of kinesthetic perception based on the interaction of efferent (referent coordinate) and afferent processes.
Collapse
Affiliation(s)
- Valters Abolins
- Cyber-Physical Systems Laboratory, Institute of Electronics and Computer Science, Riga LV-1006, Latvia.
| | - Mark L Latash
- Department of Kinesiology, The Pennsylvania State University, University Park, PA 16802, USA.
| |
Collapse
|
5
|
The Nature of Finger Enslaving: New Results and Their Implications. Motor Control 2021; 25:680-703. [PMID: 34530403 DOI: 10.1123/mc.2021-0044] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/24/2021] [Revised: 07/01/2021] [Accepted: 07/25/2021] [Indexed: 11/18/2022]
Abstract
We present a review on the phenomenon of unintentional finger action seen when other fingers of the hand act intentionally. This phenomenon (enslaving) has been viewed as a consequence of both peripheral (e.g., connective tissue links and multifinger muscles) and neural (e.g., projections of corticospinal pathways) factors. Recent studies have shown relatively large and fast drifts in enslaving toward higher magnitudes, which are not perceived by subjects. These and other results emphasize the defining role of neural factors in enslaving. We analyze enslaving within the framework of the theory of motor control with spatial referent coordinates. This analysis suggests that unintentional finger force changes result from drifts of referent coordinates, possibly reflecting the spread of cortical excitation.
Collapse
|
6
|
Abstract
Physical approach to biological movement is based on the idea of control with referent spatial coordinates for effectors, from the whole body to single muscles. Within this framework, neural control signals induce changes in parameters of corresponding biology-specific laws of nature, and motor performance emerges as a result of interaction with the external force field. This approach is naturally compatible with the principle of abundance and the uncontrolled manifold hypothesis, which offer the framework for analysis of movement stability. The presence of two basic commands, reciprocal and co-activation, makes even single-effector tasks abundant and allows stabilizing their performance at the control level. Kinesthetic perception can be viewed as the process of estimating afferent signals within a reference system provided by the efferent process. Percepts are reflections of stable iso-perceptual manifolds in the combined afferent-efferent multi-dimensional space. This approach offers new, logical and based on laws of nature, interpretations for such phenomena as muscle co-activation, unintentional drifts in performance, and vibration-induced kinesthetic illusions. It also allows predicting new phenomena such as counter-intuitive effects of muscle co-activation of force production and perception, vibration-induced force illusions, performance drifts at two different speeds, and high variability in matching the contribution of individual elements in multi-element tasks. This approach can be developed for various subfields of movement studies including studies of athletics, movement disorders, and movement rehabilitation.
Collapse
|
7
|
Abstract
A number of notions in the fields of motor control and kinesthetic perception have been used without clear definitions. In this review, we consider definitions for efference copy, percept, and sense of effort based on recent studies within the physical approach, which assumes that the neural control of movement is based on principles of parametric control and involves defining time-varying profiles of spatial referent coordinates for the effectors. The apparent redundancy in both motor and perceptual processes is reconsidered based on the principle of abundance. Abundance of efferent and afferent signals is viewed as the means of stabilizing both salient action characteristics and salient percepts formalized as stable manifolds in high-dimensional spaces of relevant elemental variables. This theoretical scheme has led recently to a number of novel predictions and findings. These include, in particular, lower accuracy in perception of variables produced by elements involved in a multielement task compared with the same elements in single-element tasks, dissociation between motor and perceptual effects of muscle coactivation, force illusions induced by muscle vibration, and errors in perception of unintentional drifts in performance. Taken together, these results suggest that participation of efferent signals in perception frequently involves distorted copies of actual neural commands, particularly those to antagonist muscles. Sense of effort is associated with such distorted efferent signals. Distortions in efference copy happen spontaneously and can also be caused by changes in sensory signals, e.g., those produced by muscle vibration.
Collapse
Affiliation(s)
- Mark L Latash
- Department of Kinesiology, The Pennsylvania State University, University Park, Pennsylvania
| |
Collapse
|
8
|
Perturbation-induced fast drifts in finger enslaving. Exp Brain Res 2021; 239:891-902. [PMID: 33423068 DOI: 10.1007/s00221-020-06027-y] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/31/2020] [Accepted: 12/24/2020] [Indexed: 10/22/2022]
Abstract
We explored changes in finger forces and in an index of unintentional finger force production (enslaving) under a variety of visual feedback conditions and positional finger perturbations. In particular, we tested a hypothesis that enslaving would show a consistent increase with time at characteristic times of about 1-2 s. Young healthy subjects performed accurate force production tasks under visual feedback on the total force of the instructed fingers (index and ring) or enslaved fingers (middle and little). Finger feedback was covertly alternated between master and enslaved fingers in a random fashion. The feedback could be presented over the first 5 s of the trial only or over the whole trial duration (21 s). After 5 s, the fingers were lifted by 1 cm, and after 15 s, the fingers were lowered to the initial position. The force of the instructed fingers drifted toward lower magnitudes in all conditions except the one with continuous feedback on that force. The force of enslaved fingers showed variable behavior across conditions. In all conditions, the index of enslaving showed a consistent increase with the time constant varying between 1 and 3 s. We interpret the results as pointing at the spread of excitation to enslaved fingers (possibly, in the cortical M1 areas). The relatively fast changes in enslaving under positional finger perturbations suggest that quick changes of the input into M1 from pre-M1 areas can accelerate the hypothesized spread of cortical excitation.
Collapse
|
9
|
What do people match when they try to match force? Analysis at the level of hypothetical control variables. Exp Brain Res 2020; 238:1885-1901. [PMID: 32537705 DOI: 10.1007/s00221-020-05850-7] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/01/2020] [Accepted: 06/09/2020] [Indexed: 01/06/2023]
Abstract
We used the theory of control with spatial referent coordinates (RC) to explore how young, healthy persons modify finger pressing force and match forces between the two hands. Three specific hypotheses were tested related to patterns of RC and apparent stiffness (defined as the slope of force-coordinate relation) used in the presence of visual feedback on the force and in its absence. The subjects used the right hand to produce accurate force under visual feedback; further the force could be increased or decreased, intentionally or unintentionally (induced by controlled lifting or lowering of the fingertips). The left hand was used to match force without visual feedback before and after the force change; the match hand consistently underestimated the actual force change in the task hand. The "inverse piano" device was used to compute RC and apparent stiffness. We found very high coefficients of determination for the inter-trial hyperbolic regressions between RC and apparent stiffness in the presence of visual feedback; the coefficients of determination dropped significantly without visual feedback. There were consistent preferred sharing patterns in the space of RC and apparent stiffness between the task and match hands across subjects. In contrast, there was much less consistency between the task and match hands in the magnitudes of RC and apparent stiffness observed in individual trials. Compared to the task hand, the match hand showed consistently lower magnitudes of apparent stiffness and, correspondingly, larger absolute magnitudes of RC. Involuntary force changes produced by lifting and lowering the force sensors led to significantly lower force changes compared to what could be expected based on the computed values of apparent stiffness and sensor movement amplitude. The results confirm the importance of visual feedback for stabilization of force in the space of hypothetical control variables. They suggest the existence of personal traits reflected in preferred ranges of RC and apparent stiffness across the two hands. They also show that subjects react to external perturbations, even when instructed "not to interfere": Such perturbations cause unintentional and unperceived drifts in both RC and apparent stiffness.
Collapse
|
10
|
Synergic control of action in levodopa-naïve Parkinson's disease patients: I. Multi-finger interaction and coordination. Exp Brain Res 2019; 238:229-245. [PMID: 31838566 DOI: 10.1007/s00221-019-05709-6] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/23/2019] [Accepted: 12/09/2019] [Indexed: 01/25/2023]
Abstract
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.
Collapse
|
11
|
Yamagata M, Popow M, Latash ML. Beyond rambling and trembling: effects of visual feedback on slow postural drift. Exp Brain Res 2019; 237:865-871. [PMID: 30635703 DOI: 10.1007/s00221-019-05470-w] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2018] [Accepted: 01/04/2019] [Indexed: 11/24/2022]
Abstract
We explored one of the unusual predictions of the concept of back-coupling within the theoretical scheme of the control of posture and movement with setting referent coordinates for the effectors. This concept implies slow drifts of referent coordinates toward actual coordinates leading to unintentional drift in performance. During standing, such slow drifts may lead to a protective step or even a fall and, therefore, corrections are expected leading to body sway at frequencies under 0.1 Hz. Young healthy subjects stood on the force platform quietly for 60 s under two single-task conditions, with eyes open and closed, and two double-task conditions, matching an irrelevant muscle activation signal to a target (MATCH) and performing a subtraction task. The latter was performed with eyes open and closed. The rambling-trembling decomposition was applied to the displacements of the center of pressure in the anterior-posterior direction. Spectral analysis was used to quantify power within typical ranges for Tr and Rm, as well as for a slow Rm component (under 0.1 Hz) addressed as Drift. Closing eyes led to a significant increase in Rm and Tr, but no effects on Drift. Drift increased significantly in the MATCH task with no changes in Rm and a drop in Tr. No effects of the subtraction task were seen on Drift. Overall, our findings suggest that unintentional slow drift of referent body orientation towards the actual body orientation leads to Drift, a specific example of back-coupling reflected in postural sway. This observation can be also seen as an example of physiological minimization of activity of motoneurons. Natural visual feedback is used to avoid the COP drift and/or correct it quickly and effectively; this ability is compromised when vision is used for an unrelated task.
Collapse
Affiliation(s)
- Momoko Yamagata
- Department of Kinesiology, The Pennsylvania State University, Rec.Hall-268N, University Park, PA, 16802, USA.,Department of Human Health Science, Graduate School of Medicine, Kyoto University, Kyoto, Japan
| | - Marta Popow
- Department of Kinesiology, The Pennsylvania State University, Rec.Hall-268N, University Park, PA, 16802, USA
| | - Mark L Latash
- Department of Kinesiology, The Pennsylvania State University, Rec.Hall-268N, University Park, PA, 16802, USA.
| |
Collapse
|
12
|
Tillman M, Ambike S. Expectation of movement generates contrasting changes in multifinger synergies in young and older adults. Exp Brain Res 2018; 236:2765-2780. [PMID: 30022260 DOI: 10.1007/s00221-018-5333-0] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/02/2018] [Accepted: 07/14/2018] [Indexed: 12/28/2022]
Abstract
Anticipatory synergy adjustment (ASA) is a feed-forward control mechanism that describes a continuous decrease in the stability of the current motor state beginning about 150 ms prior to a state transition. Recently, we described an associated phenomenon in which the system stability was reduced solely in response to a cue that generates an expectation of a state change, independent of whether the state change actually occurs. Both phenomena are of the same kind (stability reduction), but evoked by distinct antecedent conditions. Since, logically, cuing for movement must occur before the initiation of that movement, we named this new phenomenon 'Stage-1 ASA' and rechristened the well-established version 'Stage-2 ASA'. Here, we used a four-finger, isometric force production task to explore (1) the effect of healthy aging on Stage-1 ASA, and (2) if Stage-1 ASA resulted in a more rapid state change. Young and older adult participants produced 10% of their maximal force when they did not expect to produce any change in the force, and when they expected to change their force in an unknown direction and at an unknown time. In the latter condition, the 10% constant-force phase was followed by a choice reaction time task, in which the participants rapidly changed their force to follow a moving target presented on a computer monitor. Both young and older adults displayed equivalent amount of Stage-1 ASA. This was driven by a 42% reduction in finger-force variability in young adults. In contrast, it was driven by a 38% increase in finger-force variability in older adults. We speculate that the reduction in finger force variability assists the young adults in rapid state changes via two mechanisms: (1) the finger forces occupy a restricted set of states that are optimal for quick state transitions, and (2) lower variability during steady state translates into lower self-motion during state transition. Self-motion is the covariation between finger forces that fails to change the total force. The older adults are unable to adopt this strategy, and the increase in finger-force variability arises from (1) the adoption of an alternative strategy of destabilizing the attractor associated with the current state to facilitate state transitions and (2) the inability to coordinate multiple finger forces. Finally, older adults displayed longer reaction times than young adults, but a clear relation between Stage-1 ASA and consequent behavioral benefit in terms of reduced reaction time remained elusive.
Collapse
Affiliation(s)
- Mitchell Tillman
- Department of Health and Kinesiology, Purdue University, 800 West Stadium Ave, West Lafayette, IN, 47907, USA
| | - Satyajit Ambike
- Department of Health and Kinesiology, Purdue University, 800 West Stadium Ave, West Lafayette, IN, 47907, USA.
| |
Collapse
|
13
|
Reschechtko S, Latash ML. Stability of hand force production. II. Ascending and descending synergies. J Neurophysiol 2018; 120:1045-1060. [PMID: 29873618 DOI: 10.1152/jn.00045.2018] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
We combined the theory of neural control of movement with referent coordinates and the uncontrolled manifold hypothesis to investigate multifinger coordination. We tested hypotheses related to stabilization of performance by covarying control variables, translated into apparent stiffness and referent coordinate, at different levels of an assumed hierarchy of control. Subjects produced an accurate combination of total force and total moment of force with the four fingers under visual feedback on both variables and after feedback was partly or completely removed. The "inverse piano" device was used to estimate control variables. We observed strong synergies in the space of hypothetical control variables that stabilized total force and moment of force, as well as weaker synergies stabilizing individual finger forces; whereas the former were attenuated by alteration of visual feedback, the latter were much less affected. In addition, we investigated the organization of "ascending synergies" stabilizing task-level control variables by covaried adjustments of finger-level control variables. We observed intertrial covariation of individual fingers' referent coordinates that stabilized hand-level referent coordinate, but we observed no such covariation for apparent stiffness. The observations suggest the existence of both descending and ascending synergies in a hierarchical control system. They confirm a trade-off between synergies at different levels of control and corroborate the hypothesis on specialization of different fingers for the control of force and moment. The results provide strong evidence for the importance of central back-coupling loops in ensuring stability of action. NEW & NOTEWORTHY We expand analysis of action in the space of hypothetical control variables to hierarchically organized multieffector systems. We also introduce the novel concept of ascending synergies, which reflect covariation of control variables to individual effectors (fingers) that stabilize task-specific control variables at a hierarchically higher, task-specific level (hand).
Collapse
Affiliation(s)
- Sasha Reschechtko
- Department of Kinesiology, The Pennsylvania State University , University Park, Pennsylvania
| | - Mark L Latash
- Department of Kinesiology, The Pennsylvania State University , University Park, Pennsylvania
| |
Collapse
|
14
|
Abstract
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.
Collapse
|
15
|
Tillman M, Ambike S. Cue-induced changes in the stability of finger force-production tasks revealed by the uncontrolled manifold analysis. J Neurophysiol 2018; 119:21-32. [PMID: 28931612 DOI: 10.1152/jn.00519.2017] [Citation(s) in RCA: 18] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
Abstract
A motor system configured to maximize the stability of its current state cannot dexterously transition between states. Yet, we routinely resolve the stability-dexterity conflict and rapidly change our current behavior without allowing it to become unstable before the desired transition. The phenomenon called anticipatory synergy adjustment (ASA) partly describes how the central nervous system handles this conflict. ASA is a continuous decrease in the stability of the current motor state beginning 150–400 ms before a rapid state transition accomplished using redundant sets of motor inputs (more input variables than task-specific output variables). So far, ASAs have been observed only when the timing of the upcoming transition is known. We utilized a multifinger, isometric force-production task to demonstrate that compared with a condition where no state transition is expected, the stability of the current state is lower by ~12% when a participant is cued to make a transition, even when the nature and timing of that transition are unknown. This result (stage 1 ASA) is distinct from its traditional version (stage 2 ASA), and it describes early destabilization that occurs solely in response to the expectation to move. Stage 2 ASA occurs later, only if the timing of the transition is known sufficiently in advance. Stage 1 ASA lasts much longer (~1.5 s) and may scale in response to the perceived difficulty of the upcoming task. Therefore, this work reveals a much refined view of the processes that underlie the resolution of the stability-dexterity conflict. NEW & NOTEWORTHY We compared the stability of multifinger, isometric force-production tasks for trials in which force changes of unknown direction and timing were expected with trials in which there was no expectation of any force change. Mere expectation of a change caused the stability of the current motor state to drop. This novel result provides a much refined view of the processes that facilitate dexterous switching between motor states.
Collapse
Affiliation(s)
- Mitchell Tillman
- Department of Health and Kinesiology, Purdue University, West Lafayette, Indiana
| | - Satyajit Ambike
- Department of Health and Kinesiology, Purdue University, West Lafayette, Indiana
| |
Collapse
|
16
|
Abstract
Cyclic isometric finger-force patterns established using visual feedback show systematic drifts when the feedback is removed. Force changes at multiple time scales and in opposite directions have been reported. For further characterization of these drifts, healthy subjects produced isometric, cyclic finger force with and without visual feedback at various initial amplitudes and frequencies. We hypothesized that on feedback removal, the amplitude will be attracted toward a preferred value that is frequency dependent. We found that the amplitude always increased after feedback removal. The magnitude of the amplitude increase changed with initial frequency, but it was invariant over the explored range of initial amplitudes. Thus, the existence of a preferred amplitude of force oscillations was not supported. We interpret these results within the referent configuration and the referent configuration back-coupling hypotheses. These data will inform a mathematical model of finger-force drifts. However, currently, they raise more questions than they answer, and a coherent account of finger-force drifts remains a challenge.
Collapse
|
17
|
Reschechtko S, Hasanbarani F, Akulin VM, Latash ML. Unintentional force changes in cyclical tasks performed by an abundant system: Empirical observations and a dynamical model. Neuroscience 2017; 350:94-109. [PMID: 28344070 DOI: 10.1016/j.neuroscience.2017.03.022] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/20/2016] [Revised: 03/09/2017] [Accepted: 03/14/2017] [Indexed: 11/18/2022]
Abstract
The study explored unintentional force changes elicited by removing visual feedback during cyclical, two-finger isometric force production tasks. Subjects performed two types of tasks at 1Hz, paced by an auditory metronome. One - Force task - required cyclical changes in total force while maintaining the sharing, defined as relative contribution of a finger to total force. The other task - Share task - required cyclical changes in sharing while keeping total force unchanged. Each trial started under full visual feedback on both force and sharing; subsequently, feedback on the variable that was instructed to stay constant was frozen, and finally feedback on the other variable was also removed. In both tasks, turning off visual feedback on total force elicited a drop in the mid-point of the force cycle and an increase in the peak-to-peak force amplitude. Turning off visual feedback on sharing led to a drift of mean share toward 50:50 across both tasks. Without visual feedback there was consistent deviation of the two force time series from the in-phase pattern (typical of the Force task) and from the out-of-phase pattern (typical of the Share task). This finding is in contrast to most earlier studies that demonstrated only two stable patterns, in-phase and out-of-phase. We interpret the results as consequences of drifts of parameters in a dynamical system leading in particular to drifts in the referent finger coordinates toward their actual coordinates. The relative phase desynchronization is caused by the right-left differences in the hypothesized drift processes, consistent with the dynamic dominance hypothesis.
Collapse
Affiliation(s)
| | - Fariba Hasanbarani
- Pennsylvania State University, University Park, PA 16802, USA; University of Tehran, Tehran, Iran
| | - Vladimir M Akulin
- Laboratoire Aimé Cotton, 91405 Orsay, France; Laboratoire Jean-Victor Poncelet, CNRS, Moscow 119002, Russia; Institute for Problems of Information Transmission, Moscow 127994, Russia
| | - Mark L Latash
- Pennsylvania State University, University Park, PA 16802, USA; Moscow Institute of Physics and Technology, Dolgoprudny 141700, Russia.
| |
Collapse
|
18
|
Solnik S, Qiao M, Latash ML. Effects of visual feedback and memory on unintentional drifts in performance during finger-pressing tasks. Exp Brain Res 2017; 235:1149-1162. [PMID: 28168396 DOI: 10.1007/s00221-017-4878-7] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/22/2016] [Accepted: 01/07/2017] [Indexed: 11/25/2022]
Abstract
This study tested two hypotheses on the nature of unintentional force drifts elicited by removing visual feedback during accurate force production tasks. The role of working memory (memory hypothesis) was explored in tasks with continuous force production, intermittent force production, and rest intervals over the same time interval. The assumption of unintentional drifts in referent coordinate for the fingertips was tested using manipulations of visual feedback: young healthy subjects performed accurate steady-state force production tasks by pressing with the two index fingers on individual force sensors with visual feedback on the total force, sharing ratio, both, or none. Predictions based on the memory hypothesis have been falsified. In particular, we observed consistent force drifts to lower force values during continuous force production trials only. No force drift or drifts to higher forces were observed during intermittent force production trials and following rest intervals. The hypotheses based on the idea of drifts in referent finger coordinates have been confirmed. In particular, we observed superposition of two drift processes: a drift of total force to lower magnitudes and a drift of the sharing ratio to 50:50. When visual feedback on total force only was provided, the two-finger forces showed drifts in opposite directions. We interpret the findings as evidence for the control of motor actions with changes in referent coordinates for participating effectors. Unintentional drifts in performance are viewed as natural relaxation processes in the involved systems; their typical time reflects stability in the direction of the drift. The magnitude of the drift was higher in the right (dominant) hand, which is consistent with the dynamic dominance hypothesis.
Collapse
Affiliation(s)
- Stanislaw Solnik
- Department of Kinesiology, The Pennsylvania State University, Rec.Hall-268N, University Park, PA, 16802, USA
- Department of Physical Therapy, University of North Georgia, Dahlonega, USA
- University School of Physical Education, Wroclaw, Poland
| | - Mu Qiao
- Department of Kinesiology, The Pennsylvania State University, Rec.Hall-268N, University Park, PA, 16802, USA
- Joint NC State/UNC Department of Biomedical Engineering, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27514, USA
| | - Mark L Latash
- Department of Kinesiology, The Pennsylvania State University, Rec.Hall-268N, University Park, PA, 16802, USA.
| |
Collapse
|
19
|
Parsa B, Terekhov A, Zatsiorsky VM, Latash ML. Optimality and stability of intentional and unintentional actions: I. Origins of drifts in performance. Exp Brain Res 2017; 235:481-496. [PMID: 27785549 PMCID: PMC5274564 DOI: 10.1007/s00221-016-4809-z] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/29/2016] [Accepted: 10/19/2016] [Indexed: 10/20/2022]
Abstract
We address the nature of unintentional changes in performance in two papers. This first paper tested a hypothesis that unintentional changes in performance variables during continuous tasks without visual feedback are due to two processes. First, there is a drift of the referent coordinate for the salient performance variable toward the actual coordinate of the effector. Second, there is a drift toward minimum of a cost function. We tested this hypothesis in four-finger isometric pressing tasks that required the accurate production of a combination of total moment and total force with natural and modified finger involvement. Subjects performed accurate force-moment production tasks under visual feedback, and then visual feedback was removed for some or all of the salient variables. Analytical inverse optimization was used to compute a cost function. Without visual feedback, both force and moment drifted slowly toward lower absolute magnitudes. Over 15 s, the force drop could reach 20% of its initial magnitude while moment drop could reach 30% of its initial magnitude. Individual finger forces could show drifts toward both higher and lower forces. The cost function estimated using the analytical inverse optimization reduced its value as a consequence of the drift. We interpret the results within the framework of hierarchical control with referent spatial coordinates for salient variables at each level of the hierarchy combined with synergic control of salient variables. The force drift is discussed as a natural relaxation process toward states with lower potential energy in the physical (physiological) system involved in the task.
Collapse
Affiliation(s)
- Behnoosh Parsa
- Department of Kinesiology, The Pennsylvania State University, Rec.Hall-268 N, University Park, PA, 16802, USA
| | - Alexander Terekhov
- Laboratory of Psychology of Perception, University of Paris Descartes, Paris, France
| | - Vladimir M Zatsiorsky
- Department of Kinesiology, The Pennsylvania State University, Rec.Hall-268 N, University Park, PA, 16802, USA
| | - Mark L Latash
- Department of Kinesiology, The Pennsylvania State University, Rec.Hall-268 N, University Park, PA, 16802, USA.
- Moscow Institute of Physics and Technology, Dolgoprudny, Russia.
| |
Collapse
|
20
|
Ambike S, Mattos D, Zatsiorsky VM, Latash ML. Unsteady steady-states: central causes of unintentional force drift. Exp Brain Res 2016; 234:3597-3611. [PMID: 27540726 DOI: 10.1007/s00221-016-4757-7] [Citation(s) in RCA: 42] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/04/2016] [Accepted: 08/13/2016] [Indexed: 10/21/2022]
Abstract
We applied the theory of synergies to analyze the processes that lead to unintentional decline in isometric fingertip force when visual feedback of the produced force is removed. We tracked the changes in hypothetical control variables involved in single fingertip force production based on the equilibrium-point hypothesis, namely the fingertip referent coordinate (R FT) and its apparent stiffness (C FT). The system's state is defined by a point in the {R FT; C FT} space. We tested the hypothesis that, after visual feedback removal, this point (1) moves along directions leading to drop in the output fingertip force, and (2) has even greater motion along directions that leaves the force unchanged. Subjects produced a prescribed fingertip force using visual feedback and attempted to maintain this force for 15 s after the feedback was removed. We used the "inverse piano" apparatus to apply small and smooth positional perturbations to fingers at various times after visual feedback removal. The time courses of R FT and C FT showed that force drop was mostly due to a drift in R FT toward the actual fingertip position. Three analysis techniques, namely hyperbolic regression, surrogate data analysis, and computation of motor-equivalent and non-motor-equivalent motions, suggested strong covariation in R FT and C FT stabilizing the force magnitude. Finally, the changes in the two hypothetical control variables {R FT; C FT} relative to their average trends also displayed covariation. On the whole, the findings suggest that unintentional force drop is associated with (a) a slow drift of the referent coordinate that pulls the system toward a low-energy state and (b) a faster synergic motion of R FT and C FT that tends to stabilize the output fingertip force about the slowly drifting equilibrium point.
Collapse
Affiliation(s)
- Satyajit Ambike
- Department of Health and Kinesiology, Purdue University, 800 West Stadium Ave, West Lafayette, IN, 47907, USA.
| | - Daniela Mattos
- Program in Occupational Therapy, Washington University School of Medicine, Saint Louis, MO, USA
| | - Vladimir M Zatsiorsky
- Department of Kinesiology, The Pennsylvania State University, University Park, PA, USA
| | - Mark L Latash
- Department of Kinesiology, The Pennsylvania State University, University Park, PA, USA
| |
Collapse
|
21
|
Parsa B, O'Shea DJ, Zatsiorsky VM, Latash ML. On the nature of unintentional action: a study of force/moment drifts during multifinger tasks. J Neurophysiol 2016; 116:698-708. [PMID: 27193319 DOI: 10.1152/jn.00180.2016] [Citation(s) in RCA: 35] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/29/2016] [Accepted: 05/18/2016] [Indexed: 11/22/2022] Open
Abstract
We explored the origins of unintentional changes in performance during accurate force production in isometric conditions seen after turning visual feedback off. The idea of control with referent spatial coordinates suggests that these phenomena could result from drifts of the referent coordinate for the effector. Subjects performed accurate force/moment production tasks by pressing with the fingers of a hand on force sensors. Turning the visual feedback off resulted in slow drifts of both total force and total moment to lower magnitudes of these variables; these drifts were more pronounced in the right hand of the right-handed subjects. Drifts in individual finger forces could be in different direction; in particular, fingers that produced moments of force against the required total moment showed an increase in their forces. The force/moment drift was associated with a drop in the index of synergy stabilizing performance under visual feedback. The drifts in directions that changed performance (non-motor equivalent) and in directions that did not (motor equivalent) were of about the same magnitude. The results suggest that control with referent coordinates is associated with drifts of those referent coordinates toward the corresponding actual coordinates of the hand, a reflection of the natural tendency of physical systems to move toward a minimum of potential energy. The interaction between drifts of the hand referent coordinate and referent orientation leads to counterdirectional drifts in individual finger forces. The results also demonstrate that the sensory information used to create multifinger synergies is necessary for their presence over the task duration.
Collapse
Affiliation(s)
- Behnoosh Parsa
- Department of Kinesiology, The Pennsylvania State University, University Park, Pennsylvania
| | - Daniel J O'Shea
- Department of Kinesiology, The Pennsylvania State University, University Park, Pennsylvania
| | - Vladimir M Zatsiorsky
- Department of Kinesiology, The Pennsylvania State University, University Park, Pennsylvania
| | - Mark L Latash
- Department of Kinesiology, The Pennsylvania State University, University Park, Pennsylvania
| |
Collapse
|
22
|
Jo HJ, Maenza C, Good DC, Huang X, Park J, Sainburg RL, Latash ML. Effects of unilateral stroke on multi-finger synergies and their feed-forward adjustments. Neuroscience 2016; 319:194-205. [PMID: 26828408 DOI: 10.1016/j.neuroscience.2016.01.054] [Citation(s) in RCA: 36] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/15/2015] [Revised: 01/21/2016] [Accepted: 01/24/2016] [Indexed: 11/17/2022]
Abstract
We explored the changes in multi-finger synergies in patients after a single cortical stroke with mild motor impairments. We hypothesized that both synergy indices and anticipatory synergy adjustments prior to the initiation of a self-paced quick action would be diminished in the patients compared to age-matched controls. The patients with history of cortical stroke, and age-matched controls (n=12 in each group) performed one-finger and multi-finger accurate force production tasks involving both steady-state and quick force pulse production. Finger interdependence (enslaving) and multi-finger synergies stabilizing total force were quantified. The stroke patients showed lower maximal finger forces, in particular in the contralesional hand, which also showed increased enslaving indices. Multi-finger synergies during steady-state force production were, however, unchanged after stroke. In contrast, a drop in the synergy index prior to the force pulse generation was significantly delayed in the stroke patients. Our results show that mild cortical stroke leads to no significant changes in multifinger synergies, but there is impairment in feed-forward adjustments of the synergies prior to a quick action, a drop in the maximal force production, and an increase in enslaving. We conclude that studies of synergies reveal two aspects of synergic control differentially affected by cortical stroke.
Collapse
Affiliation(s)
- H J Jo
- Department of Kinesiology, The Pennsylvania State University, University Park, PA 16802, USA
| | - C Maenza
- Department of Kinesiology, The Pennsylvania State University, University Park, PA 16802, USA; Department of Neurology, Pennsylvania State University-Milton S. Hershey Medical Center, Hershey, PA 17033, USA
| | - D C Good
- Department of Neurology, Pennsylvania State University-Milton S. Hershey Medical Center, Hershey, PA 17033, USA
| | - X Huang
- Department of Kinesiology, The Pennsylvania State University, University Park, PA 16802, USA; Department of Neurology, Pennsylvania State University-Milton S. Hershey Medical Center, Hershey, PA 17033, USA; Department of Pharmacology, Pennsylvania State University-Milton S. Hershey Medical Center, Hershey, PA 17033, USA; Department of Radiology, Pennsylvania State University-Milton S. Hershey Medical Center, Hershey, PA 17033, USA; Department of Neurosurgery, Pennsylvania State University-Milton S. Hershey Medical Center, Hershey, PA 17033, USA
| | - J Park
- Department of Physical Education, Seoul National University, Seoul, South Korea
| | - R L Sainburg
- Department of Kinesiology, The Pennsylvania State University, University Park, PA 16802, USA; Department of Neurology, Pennsylvania State University-Milton S. Hershey Medical Center, Hershey, PA 17033, USA
| | - M L Latash
- Department of Kinesiology, The Pennsylvania State University, University Park, PA 16802, USA.
| |
Collapse
|
23
|
Latash ML. Fifty Years of Physics of Living Systems. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2016; 957:81-103. [PMID: 28035561 DOI: 10.1007/978-3-319-47313-0_5] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
The equilibrium-point hypothesis and its more recent version, the referent configuration hypothesis, represent the physical approach to the neural control of action. This hypothesis can be naturally combined with the idea of hierarchical control of movements and of synergic organization of the abundant systems involved in all actions. Any action starts with defining trajectories of a few referent coordinates for a handful of salient task-specific variables. Further, referent coordinates at hierarchically lower levels emerge down to thresholds of the tonic stretch reflex for the participating muscles. Stability of performance with respect to salient variables is reflected in the structure of inter-trial variance and phenomena of motor equivalence. Three lines of recent research within this framework are reviewed. First, synergic adjustments of the referent coordinate and apparent stiffness have been demonstrated during finger force production supporting the main idea of control with referent coordinates. Second, the notion of unintentional voluntary movements has been introduced reflecting unintentional drifts in referent coordinates. Two types of unintentional movements have been observed with different characteristic times. Third, this framework has been applied to studies of impaired movements in neurological patients. Overall, the physical approach searching for laws of nature underlying biological movement has been highly stimulating and productive.
Collapse
Affiliation(s)
- Mark L Latash
- Department of Kinesiology, The Pennsylvania State University, Rec. Hall-267, University Park, Pennsylvania, 16802, USA.
| |
Collapse
|
24
|
Slomka K, Juras G, Sobota G, Furmanek M, Rzepko M, Latash ML. Intra-Personal and Inter-Personal Kinetic Synergies During Jumping. J Hum Kinet 2015; 49:75-88. [PMID: 26839608 PMCID: PMC4723184 DOI: 10.1515/hukin-2015-0110] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 12/01/2015] [Indexed: 11/16/2022] Open
Abstract
We explored synergies between two legs and two subjects during preparation for a long jump into a target. Synergies were expected during one-person jumping. No such synergies were expected between two persons jumping in parallel without additional contact, while synergies were expected to emerge with haptic contact and become stronger with strong mechanical contact. Subjects performed jumps either alone (each foot standing on a separate force platform) or in dyads (parallel to each other, each person standing on a separate force platform) without any contact, with haptic contact, and with strong coupling. Strong negative correlations between pairs of force variables (strong synergies) were seen in the vertical force in one-person jumps and weaker synergies in two-person jumps with the strong contact. For other force variables, only weak synergies were present in one-person jumps and no negative correlations between pairs of force variable for two-person jumps. Pairs of moment variables from the two force platforms at steady state showed positive correlations, which were strong in one-person jumps and weaker, but still significant, in two-person jumps with the haptic and strong contact. Anticipatory synergy adjustments prior to action initiation were observed in one-person trials only. We interpret the different results for the force and moment variables at steady state as reflections of postural sway.
Collapse
Affiliation(s)
- Kajetan Slomka
- Department of Human Motor Behavior, The Jerzy Kukuczka Academy of Physical Education, Katowice, Poland
| | - Grzegorz Juras
- Department of Human Motor Behavior, The Jerzy Kukuczka Academy of Physical Education, Katowice, Poland
| | - Grzegorz Sobota
- Department of Biomechanics, The Jerzy Kukuczka Academy of Physical Education, Katowice, Poland
| | - Mariusz Furmanek
- Department of Human Motor Behavior, The Jerzy Kukuczka Academy of Physical Education, Katowice, Poland
| | - Marian Rzepko
- Department of Physical Education, University of Rzeszow, Rzeszow, Poland
| | - Mark L Latash
- Department of Kinesiology, The Pennsylvania State University, State College, Pennsylvania, USA
| |
Collapse
|
25
|
Unintentional movements induced by sequential transient perturbations in a multi-joint positional task. Hum Mov Sci 2015; 46:1-9. [PMID: 26703914 DOI: 10.1016/j.humov.2015.12.002] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/02/2015] [Revised: 11/30/2015] [Accepted: 12/10/2015] [Indexed: 11/23/2022]
Abstract
We explored the phenomenon of unintentional movements of a multi-joint effector produced by multiple transient changes in the external force. The subjects performed a position-holding task against a constant bias force produced by a robot and were instructed not to intervene voluntarily with arm movements produced by changes in the robot force. The robot produced a smooth force increase leading to hand movement from the trunk, followed by a dwell time. Then, the force dropped to its initial value leading to hand movement toward the initial position but with an undershot. Such perturbation episodes were repeated four times in a row. The accumulated perturbation and undershoot distances kept increasing without saturation within the sequence of four perturbation episode. The limb apparent stiffness before dwell time increased over sequential perturbations while apparent stiffness after dwell time decreased. We interpret the results as consequences of a drift of the hand referent coordinate (RC) caused by a hypothesized RC-back-coupling mechanism and a coupled drift of the apparent stiffness. The results show that RC-back-coupling continues to lead to unintentional movements over repeated perturbations and is accompanied by a relatively slow re-setting process.
Collapse
|
26
|
Ambike S, Mattos D, Zatsiorsky VM, Latash ML. The nature of constant and cyclic force production: unintentional force-drift characteristics. Exp Brain Res 2015; 234:197-208. [PMID: 26419663 DOI: 10.1007/s00221-015-4453-z] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/09/2015] [Accepted: 09/18/2015] [Indexed: 11/30/2022]
Abstract
We explored unintentional changes in forces during performance of constant and cyclic force-production tasks (F-tasks) after visual feedback removal. Based on earlier studies, we expected all force parameters to drop exponentially with time. We also explored possible role of working memory in the force drop phenomena. Healthy subjects performed constant or cyclic isometric F-tasks with the index finger under visual feedback. The cyclic task was paced by a metronome. Removing visual feedback resulted in a consistent force drop in constant F-tasks and a qualitatively similar drift in the mean force in the cyclic F-task. Both were slow with characteristic times of about 10-20 s. In contrast, force amplitude in the cyclic F-task increased quickly (within 1-2 s). When the subjects were asked to stop producing force for 5 s after the visual feedback disappeared and then resume force production, no downward force drift was seen in constant F-tasks, while in cyclic F-tasks, the drift of the mean force was present and an exaggerated increase in force amplitude was also observed. We conclude that while working memory limitations may influence cyclic F-tasks, their role in determining the force drift in constant F-tasks is limited. The results of both experiments are interpreted within the referent configuration hypothesis supplemented with an idea of unintentional drift of referent coordinates (RC-back-coupling) induced by differences between the referent and actual body configurations.
Collapse
Affiliation(s)
- Satyajit Ambike
- Department of Health and Kinesiology, Purdue University, Lambert Fieldhouse, Room 110B, 800 West Stadium Avenue, West Lafayette, IN, 47907, USA. .,Department of Kinesiology, The Pennsylvania State University, University Park, PA, USA.
| | - Daniela Mattos
- Department of Kinesiology, The Pennsylvania State University, University Park, PA, USA.,Biomechanics and Movement Science Program, University of Delaware, Newark, DE, USA
| | - Vladimir M Zatsiorsky
- Department of Kinesiology, The Pennsylvania State University, University Park, PA, USA
| | - Mark L Latash
- Department of Kinesiology, The Pennsylvania State University, University Park, PA, USA
| |
Collapse
|
27
|
Zhou T, Zatsiorsky VM, Latash ML. Unintentional changes in the apparent stiffness of the multi-joint limb. Exp Brain Res 2015; 233:2989-3004. [PMID: 26169103 DOI: 10.1007/s00221-015-4369-7] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/15/2015] [Accepted: 06/26/2015] [Indexed: 10/23/2022]
Abstract
We explored the phenomenon of unintentional changes in the apparent stiffness of the human arm produced by transient changes in the external force. The subjects performed a positional task against a constant baseline force and were instructed not to react to changes in the force. A HapticMaster robot produced a smooth force increase (a perturbation) leading to a hand movement, followed by a dwell time. No visible hand drift was observed during the dwell time. After the robot force dropped to its initial baseline value, the hand moved toward the initial position but stopped short of it. Small perturbations were applied at different time intervals along different directions during the dwell time. Arm apparent stiffness distribution in a horizontal plane was approximated with an ellipse. The apparent stiffness magnitude along the main axis of the ellipse showed a non-monotonic increase with dwell time, while the apparent stiffness along the minor axis did not change significantly. We interpreted the early part of the changes in the apparent stiffness as due to peripheral muscle properties. The later part is interpreted as caused by a combination of two processes: a drift in the referent hand coordinate due to the hypothesized back-coupling between the referent and actual hand coordinates and an implicit instruction to keep the hand steady when no changes in robot-generated force took place. The data provide support for the idea of back-coupling between the referent and actual body configurations, which may be an important contributor to stability of motor actions.
Collapse
Affiliation(s)
- Tao Zhou
- Department of Kinesiology, The Pennsylvania State University, Rec.Hall-267, University Park, PA, 16802, USA
| | - Vladimir M Zatsiorsky
- Department of Kinesiology, The Pennsylvania State University, Rec.Hall-267, University Park, PA, 16802, USA
| | - Mark L Latash
- Department of Kinesiology, The Pennsylvania State University, Rec.Hall-267, University Park, PA, 16802, USA. .,Moscow Institute of Physics and Technology, Moscow, Russia.
| |
Collapse
|
28
|
Solnik S, Reschechtko S, Wu YH, Zatsiorsky VM, Latash ML. Force-stabilizing synergies in motor tasks involving two actors. Exp Brain Res 2015; 233:2935-49. [PMID: 26105756 DOI: 10.1007/s00221-015-4364-z] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/15/2015] [Accepted: 06/16/2015] [Indexed: 10/23/2022]
Abstract
We investigated the ability of two persons to produce force-stabilizing synergies in accurate multi-finger force production tasks under visual feedback on the total force only. The subjects produced a time profile of total force (the sum of two hand forces in one-person tasks and the sum of two subject forces in two-person tasks) consisting of a ramp-up, steady-state, and ramp-down segments; the steady-state segment was interrupted in the middle by a quick force pulse. Analyses of the structure of inter-trial finger force variance, motor equivalence, anticipatory synergy adjustments (ASAs), and the unintentional drift of the sharing pattern were performed. The two-person performance was characterized by a dramatically higher amount of inter-trial variance that did not affect total force, higher finger force deviations that did not affect total force (motor equivalent deviations), shorter ASAs, and larger drift of the sharing pattern. The rate of sharing pattern drift correlated with the initial disparity between the forces produced by the two persons (or two hands). The drift accelerated following the quick force pulse. Our observations show that sensory information on the task-specific performance variable is sufficient for the organization of performance-stabilizing synergies. They suggest, however, that two actors are less likely to follow a single optimization criterion as compared to a single performer. The presence of ASAs in the two-person condition might reflect fidgeting by one or both of the subjects. We discuss the characteristics of the drift in the sharing pattern as reflections of different characteristic times of motion within the subspaces that affect and do not affect salient performance variables.
Collapse
Affiliation(s)
- Stanislaw Solnik
- Department of Kinesiology, The Pennsylvania State University, Rec.Hall-267, University Park, PA, 16802, USA.,University School of Physical Education, Wroclaw, Poland
| | - Sasha Reschechtko
- Department of Kinesiology, The Pennsylvania State University, Rec.Hall-267, University Park, PA, 16802, USA
| | - Yen-Hsun Wu
- Department of Kinesiology, The Pennsylvania State University, Rec.Hall-267, University Park, PA, 16802, USA.,Rehabilitation Science Department, University of Hartford, Hartford, CT, USA
| | - Vladimir M Zatsiorsky
- Department of Kinesiology, The Pennsylvania State University, Rec.Hall-267, University Park, PA, 16802, USA
| | - Mark L Latash
- Department of Kinesiology, The Pennsylvania State University, Rec.Hall-267, University Park, PA, 16802, USA.
| |
Collapse
|
29
|
Zhou T, Zhang L, Latash ML. Intentional and unintentional multi-joint movements: their nature and structure of variance. Neuroscience 2015; 289:181-93. [PMID: 25596318 DOI: 10.1016/j.neuroscience.2014.12.079] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/24/2014] [Revised: 12/13/2014] [Accepted: 12/19/2014] [Indexed: 10/24/2022]
Abstract
We tested predictions of a hierarchical scheme on the control of natural movements with referent body configurations. Subjects occupied an initial hand position against a bias force generated by a HapticMaster robot. A smooth force perturbation was applied to the hand consisting of an increase in the bias force, keeping it at a new level for 5s, and decreasing it back to the bias value. When the force returned to the bias value, the arm stopped at a position different from the initial one interpreted as an involuntary movement. We then asked subjects to make voluntary movements to targets corresponding to the measured end-position of the unintentional movements. No target for hand orientation was used. The joint configuration variance was compared between intentional and unintentional movements within the framework of the uncontrolled manifold hypothesis. Our central hypothesis was that both unintentional and intentional movements would be characterized by structure of joint configuration variance reflecting task-specific stability of salient performance variables, such as hand position and orientation. The analysis confirmed that most variance at the final steady states was compatible with unchanged values of both hand position and orientation following both intentional and unintentional movements. We interpret unintentional movements as consequences of back-coupling between the actual and referent configurations at the task level. The results suggested that both intentional and unintentional movements resulted from shifts of the body referent configuration produced intentionally or as a result of the hypothesized back-coupling. Inter-trial variance signature reflects similar task-specific stability properties of the system following both types of movements, intentional and unintentional.
Collapse
Affiliation(s)
- T Zhou
- Department of Kinesiology, The Pennsylvania State University, University Park, PA 16802, USA
| | - L Zhang
- Department of Neurology, Ludwig-Maximilians-Universität, Munich, Germany
| | - M L Latash
- Department of Kinesiology, The Pennsylvania State University, University Park, PA 16802, USA; Moscow Institute of Physics and Technology, Russia.
| |
Collapse
|
30
|
Abstract
The authors explored the phenomenon of unintentional changes in the equilibrium state of a multijoint effector produced by transient changes in the external force. The subjects performed a position-holding task against a constant force produced by a robot and were instructed not to intervene voluntarily with movements produced by changes in the robot force. The robot produced a smooth force increase leading to a hand movement, followed by a dwell time. Then, the force dropped to its initial value leading to hand movement toward the initial position, but the hand stopped short of the initial position. The undershoot magnitude increased linearly with the peak hand displacement and exponentially with dwell time (time constant of about 1 s). For long dwell times, the hand stopped at about half the total distance to the initial position. The authors interpret the results as consequences of a drift of the referent hand coordinate. Our results provide support for back-coupling between the referent and actual body configurations during multijoint actions and produce the first quantitative analysis of this phenomenon. This mechanism can also explain the phenomena of slacking and force drop after turning visual feedback off during accurate force production task.
Collapse
Affiliation(s)
- Tao Zhou
- a Department of Kinesiology , The Pennsylvania State University , University Park
| | | | | |
Collapse
|
31
|
Reschechtko S, Zatsiorsky VM, Latash ML. Task-specific stability of multifinger steady-state action. J Mot Behav 2015; 47:365-77. [PMID: 25565327 DOI: 10.1080/00222895.2014.996281] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/24/2022]
Abstract
The 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.
Collapse
Affiliation(s)
- Sasha Reschechtko
- a Department of Kinesiology , The Pennsylvania State University , University Park
| | | | | |
Collapse
|
32
|
Processes underlying unintentional finger-force changes in the absence of visual feedback. Exp Brain Res 2014; 233:711-21. [DOI: 10.1007/s00221-014-4148-x] [Citation(s) in RCA: 43] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/29/2014] [Accepted: 11/11/2014] [Indexed: 10/24/2022]
|
33
|
Wu YH, Truglio TS, Zatsiorsky VM, Latash ML. Learning to Combine High Variability With High Precision: Lack of Transfer to a Different Task. J Mot Behav 2014; 47:153-65. [DOI: 10.1080/00222895.2014.961892] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/24/2022]
|
34
|
Mattos D, Schöner G, Zatsiorsky VM, Latash ML. Motor equivalence during multi-finger accurate force production. Exp Brain Res 2014; 233:487-502. [PMID: 25344311 DOI: 10.1007/s00221-014-4128-1] [Citation(s) in RCA: 30] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/04/2014] [Accepted: 10/12/2014] [Indexed: 12/22/2022]
Abstract
We explored stability of multi-finger cyclical accurate force production action by analysis of responses to small perturbations applied to one of the fingers and inter-cycle analysis of variance. Healthy subjects performed two versions of the cyclical task, with and without an explicit target. The "inverse piano" apparatus was used to lift/lower a finger by 1 cm over 0.5 s; the subjects were always instructed to perform the task as accurate as they could at all times. Deviations in the spaces of finger forces and modes (hypothetical commands to individual fingers) were quantified in directions that did not change total force (motor equivalent) and in directions that changed the total force (non-motor equivalent). Motor equivalent deviations started immediately with the perturbation and increased progressively with time. After a sequence of lifting-lowering perturbations leading to the initial conditions, motor equivalent deviations were dominating. These phenomena were less pronounced for analysis performed with respect to the total moment of force with respect to an axis parallel to the forearm/hand. Analysis of inter-cycle variance showed consistently higher variance in a subspace that did not change the total force as compared to the variance that affected total force. We interpret the results as reflections of task-specific stability of the redundant multi-finger system. Large motor equivalent deviations suggest that reactions of the neuromotor system to a perturbation involve large changes in neural commands that do not affect salient performance variables, even during actions with the purpose to correct those salient variables. Consistency of the analyses of motor equivalence and variance analysis provides additional support for the idea of task-specific stability ensured at a neural level.
Collapse
Affiliation(s)
- Daniela Mattos
- Biomechanics and Movement Science Program, University of Delaware, Newark, DE, USA
| | | | | | | |
Collapse
|
35
|
Reschechtko S, Zatsiorsky VM, Latash ML. Stability of multifinger action in different state spaces. J Neurophysiol 2014; 112:3209-18. [PMID: 25253478 DOI: 10.1152/jn.00395.2014] [Citation(s) in RCA: 32] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022] Open
Abstract
We investigated stability of action by a multifinger system with three methods: analysis of intertrial variance, application of transient perturbations, and analysis of the system's motion in different state spaces. The "inverse piano" device was used to apply transient (lifting-and-lowering) perturbations to individual fingers during single- and two-finger accurate force production tasks. In each trial, the perturbation was applied either to a finger explicitly involved in the task or one that was not. We hypothesized that, in one-finger tasks, task-specific stability would be observed in the redundant space of finger forces but not in the nonredundant space of finger modes (commands to explicitly involved fingers). In two-finger tasks, we expected that perturbations applied to a nontask finger would not contribute to task-specific stability in mode space. In contrast to our expectations, analyses in both force and mode spaces showed lower stability in directions that did not change total force output compared with directions that did cause changes in total force. In addition, the transient perturbations led to a significant increase in the enslaving index. We consider these results within a theoretical scheme of control with referent body configurations organized hierarchically, using multiple few-to-many mappings organized in a synergic way. The observed volatility of enslaving, greater equifinality of total force compared with elemental variables, and large magnitude of motor equivalent motion in both force and mode spaces provide support for the concept of task-specific stability of performance and the existence of multiple neural loops, which ensure this stability.
Collapse
Affiliation(s)
- Sasha Reschechtko
- Department of Kinesiology, The Pennsylvania State University, University Park, Pennsylvania
| | - Vladimir M Zatsiorsky
- Department of Kinesiology, The Pennsylvania State University, University Park, Pennsylvania
| | - Mark L Latash
- Department of Kinesiology, The Pennsylvania State University, University Park, Pennsylvania
| |
Collapse
|
36
|
Zhou T, Solnik S, Wu YH, Latash ML. Unintentional movements produced by back-coupling between the actual and referent body configurations: violations of equifinality in multi-joint positional tasks. Exp Brain Res 2014; 232:3847-59. [PMID: 25150552 DOI: 10.1007/s00221-014-4059-x] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/03/2014] [Accepted: 08/01/2014] [Indexed: 10/24/2022]
Abstract
We tested several predictions of a recent theory that combines the ideas of control with referent configurations, hierarchical control, and the uncontrolled manifold (UCM) hypothesis. In particular, we tested a hypothesis that unintentional changes in hand coordinate can happen following a long-lasting transient perturbation. The subjects grasped a handle with the right hand, occupied an initial position against a bias force produced by the HapticMaster robot, and then tried not to react to changes in the robot-produced force. Changes in the force were smooth and transient; they always ended with the same force as the bias force. The force-change amplitude and the time the force was kept at the new level (dwell time) varied across conditions. After the transient force change was over, the handle rested in a position that differed significantly from the initial position. The amplitude of this unintentional movement increased with the amplitude of transient force change and with the dwell time. In the new position, the across-trials joint configuration variance was mostly confined to a subspace compatible with the average handle coordinate and orientation (the UCMs for these variables). We view these results as the first experimental support for the hypothesis on back-coupling between the referent and actual body configurations during multi-joint actions. The results suggest that even under the instruction "not to react to transient force changes," the subjects may be unable to prevent unintentional drift of the referent configuration. The structure of joint configuration variance after such movements was similar to that in earlier reports on joint configuration variance after intentional movements. We conclude that the intentional and unintentional movements are products of a single neural system that can lead to intentional and unintentional shifts of the referent body configuration.
Collapse
Affiliation(s)
- Tao Zhou
- Department of Kinesiology, Rec.Hall-267, The Pennsylvania State University, University Park, PA, 16802, USA
| | | | | | | |
Collapse
|
37
|
Falaki A, Towhidkhah F, Zhou T, Latash ML. Task-specific stability in muscle activation space during unintentional movements. Exp Brain Res 2014; 232:3645-58. [PMID: 25092272 DOI: 10.1007/s00221-014-4048-0] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/25/2014] [Accepted: 07/17/2014] [Indexed: 10/24/2022]
Abstract
We used robot-generated perturbations applied during position-holding tasks to explore stability of induced unintentional movements in a multidimensional space of muscle activations. Healthy subjects held the handle of a robot against a constant bias force and were instructed not to interfere with hand movements produced by changes in the external force. Transient force changes were applied leading to handle displacement away from the initial position and then back toward the initial position. Intertrial variance in the space of muscle modes (eigenvectors in the muscle activations space) was quantified within two subspaces, corresponding to unchanged handle coordinate and to changes in the handle coordinate. Most variance was confined to the former subspace in each of the three phases of movement, the initial steady state, the intermediate position, and the final steady state. The same result was found when the changes in muscle activation were analyzed between the initial and final steady states. Changes in the dwell time between the perturbation force application and removal led to different final hand locations undershooting the initial position. The magnitude of the undershot scaled with the dwell time, while the structure of variance in the muscle activation space did not depend on the dwell time. We conclude that stability of the hand coordinate is ensured during both intentional and unintentional actions via similar mechanisms. Relative equifinality in the external space after transient perturbations may be associated with varying states in the redundant space of muscle activations. The results fit a hierarchical scheme for the control of voluntary movements with referent configurations and redundant mapping between the levels of the hierarchy.
Collapse
Affiliation(s)
- Ali Falaki
- Department of Kinesiology, Rec. Hall-267, The Pennsylvania State University, University Park, PA, 16802, USA
| | | | | | | |
Collapse
|
38
|
Latash ML. Motor control: on the way to physics of living systems. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2014; 826:1-16. [PMID: 25330881 DOI: 10.1007/978-1-4939-1338-1_1] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Affiliation(s)
- Mark L Latash
- Department of Kinesiology, The Pennsylvania State University, Rec. Hall-268N, 16802, University Park, PA, USA,
| |
Collapse
|