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Benamati A, Ricotta JM, De SD, Latash ML. Three Levels of Neural Control Contributing to Performance-stabilizing Synergies in Multi-finger Tasks. Neuroscience 2024; 551:262-275. [PMID: 38838976 DOI: 10.1016/j.neuroscience.2024.05.044] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2023] [Revised: 05/28/2024] [Accepted: 05/30/2024] [Indexed: 06/07/2024]
Abstract
We tested a hypothesis on force-stabilizing synergies during four-finger accurate force production at three levels: (1) The level of the reciprocal and coactivation commands, estimated as the referent coordinate and apparent stiffness of all four fingers combined; (2) The level of individual finger forces; and (3) The level of firing of individual motor units (MU). Young, healthy participants performed accurate four-finger force production at a comfortable, non-fatiguing level under visual feedback on the total force magnitude. Mechanical reflections of the reciprocal and coactivation commands were estimated using small, smooth finger perturbations applied by the "inverse piano" device. Firing frequencies of motor units in the flexor digitorum superficialis (FDS) and extensor digitorum communis (EDC) were estimated using surface recording. Principal component analysis was used to identify robust MU groups (MU-modes) with parallel changes in the firing frequency. The framework of the uncontrolled manifold hypothesis was used to compute synergy indices in the spaces of referent coordinate and apparent stiffness, finger forces, and MU-mode magnitudes. Force-stabilizing synergies were seen at all three levels. They were present in the MU-mode spaces defined for MUs in FDS, in EDC, and pooled over both muscles. No effects of hand dominance were seen. The synergy indices defined at different levels of analysis showed no correlations across the participants. The findings are interpreted within the theory of control with spatial referent coordinates for the effectors. We conclude that force stabilization gets contributions from three levels of neural control, likely associated with cortical, subcortical, and spinal circuitry.
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Affiliation(s)
- Anna Benamati
- Department of Neurosciences, Biomedicine and Movement Sciences, University of Verona, Verona, Italy; Department of Kinesiology, The Pennsylvania State University, University Park, PA 16802, USA
| | - Joseph M Ricotta
- Department of Kinesiology, The Pennsylvania State University, University Park, PA 16802, USA
| | - Sayan D De
- Department of Kinesiology, The Pennsylvania State University, University Park, PA 16802, USA
| | - Mark L Latash
- Department of Kinesiology, The Pennsylvania State University, University Park, PA 16802, USA.
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2
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Klemm L, Kuehn E, Kalyani A, Schreiber S, Reichert C, Azañón E. Age-related differences in finger interdependence during complex hand movements. J Appl Physiol (1985) 2024; 137:181-193. [PMID: 38695353 DOI: 10.1152/japplphysiol.00606.2023] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/29/2023] [Revised: 04/17/2024] [Accepted: 04/25/2024] [Indexed: 07/14/2024] Open
Abstract
The well-known decrease in finger dexterity during healthy aging leads to a significant reduction in quality of life. Still, the exact patterns of altered finger kinematics of older adults in daily life are fairly unexplored. Finger interdependence is the unintentional comovement of fingers that are not intended to move, and it is known to vary across the lifespan. Nevertheless, the magnitude and direction of age-related differences in finger interdependence are ambiguous across studies and tasks and have not been explored in the context of daily life finger movements. We investigated five different free and daily-life-inspired finger movements of the right, dominant hand as well as a sequential finger tapping task of the thumb against the other fingers, in 17 younger (22-37 yr) and 17 older (62-80 yr) adults using an exoskeleton data glove for data recording. Using inferential statistics, we found that the unintentional comovement of fingers generally decreases with age in all performed daily-life-inspired movements. Finger tapping, however, showed a trend towards higher finger interdependence for older compared with younger adults. Using machine learning, we predicted the age group of a person from finger interdependence features of single movement trials significantly better than chance level for the daily-life-inspired movements, but not for finger tapping. Taken together, we show that for specific tasks, decreased finger interdependence (i.e., less comovement) could potentially act as a marker of human aging that specifically characterizes older adults' complex finger movements in daily life.NEW & NOTEWORTHY Kinematic finger movement data were analyzed with regard to age-related differences. Extensive analyses of complex and daily-life-inspired movements reveal that the direction of age effects is not uniform but task-dependent: Although older adults generally show more finger interdependence than younger adults in a simple finger tapping task, this effect is reversed for daily-life-inspired movement tasks. For these tasks, finger interdependence indices offer potential new markers to predict the age group of an individual using machine learning approaches.
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Affiliation(s)
- Lisa Klemm
- Department of Neurology, University Medical Center, Magdeburg, Germany
- Department of Behavioral Neurology, Leibniz Institute for Neurobiology, Magdeburg, Germany
| | - Esther Kuehn
- Institute for Cognitive Neurology and Dementia Research (IKND), Otto-von-Guericke University, Magdeburg, Germany
- Hertie Institute for Clinical Brain Research (HIH), Tübingen, Germany
- German Center for Neurodegenerative Diseases (DZNE), Tübingen, Germany
- Center for Behavioral Brain Sciences (CBBS), Magdeburg, Germany
- Center for Intervention and Research on Adaptive and Maladaptive Brain Circuits Underlying Mental Health (C-I-R-C), Jena-Magdeburg-Halle, Germany
| | - Avinash Kalyani
- Institute for Cognitive Neurology and Dementia Research (IKND), Otto-von-Guericke University, Magdeburg, Germany
| | - Stefanie Schreiber
- Department of Neurology, University Medical Center, Magdeburg, Germany
- Center for Behavioral Brain Sciences (CBBS), Magdeburg, Germany
- Center for Intervention and Research on Adaptive and Maladaptive Brain Circuits Underlying Mental Health (C-I-R-C), Jena-Magdeburg-Halle, Germany
| | - Christoph Reichert
- Department of Behavioral Neurology, Leibniz Institute for Neurobiology, Magdeburg, Germany
- Center for Behavioral Brain Sciences (CBBS), Magdeburg, Germany
- Forschungscampus STIMULATE, Magdeburg, Germany
| | - Elena Azañón
- Department of Neurology, University Medical Center, Magdeburg, Germany
- Department of Behavioral Neurology, Leibniz Institute for Neurobiology, Magdeburg, Germany
- Center for Behavioral Brain Sciences (CBBS), Magdeburg, Germany
- Center for Intervention and Research on Adaptive and Maladaptive Brain Circuits Underlying Mental Health (C-I-R-C), Jena-Magdeburg-Halle, Germany
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Pawłowski M, Ricotta JM, De SD, Latash ML. Force matching: motor effects that are not reported by the actor. Exp Brain Res 2024; 242:1439-1453. [PMID: 38652273 PMCID: PMC11108883 DOI: 10.1007/s00221-024-06829-4] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/06/2023] [Accepted: 04/02/2024] [Indexed: 04/25/2024]
Abstract
We explored unintentional drifts of finger forces during force production and matching task. Based on earlier studies, we predicted that force matching with the other hand would reduce or stop the force drift in instructed fingers while uninstructed (enslaved) fingers remain unaffected. Twelve young, healthy, right-handed participants performed two types of tasks with both hands (task hand and match hand). The task hand produced constant force at 20% of MVC level with the Index and Ring fingers pressing in parallel on strain gauge force sensors. The Middle finger force wasn't instructed, and its enslaved force was recorded. Visual feedback on the total force by the instructed fingers was either present throughout the trial or only during the first 5 s (no-feedback condition). The other hand matched the perceived force level of the task hand starting at either 4, 8, or 15 s from the trial initiation. No feedback was ever provided for the match hand force. After the visual feedback was removed, the task hand showed a consistent drift to lower magnitudes of total force. Contrary to our prediction, over all conditions, force matching caused a brief acceleration of force drift in the task hand, which then reached a plateau. There was no effect of matching on drifts in enslaved finger force. We interpret the force drifts within the theory of control with spatial referent coordinates as consequences of drifts in the command (referent coordinate) to the antagonist muscles. This command is not adequately incorporated into force perception.
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Affiliation(s)
- Michał Pawłowski
- Department of Kinesiology, The Pennsylvania State University, University Park, PA, 16802, USA.
- Institute of Sport Science, Department of Human Motor Behavior, Academy of Physical Education in Katowice, 72A Mikołowska St, Katowice, 40-065, Poland.
| | - Joseph M Ricotta
- Department of Kinesiology, The Pennsylvania State University, University Park, PA, 16802, USA
| | - Sayan D De
- Department of Kinesiology, The Pennsylvania State University, University Park, PA, 16802, USA
| | - Mark L Latash
- Department of Kinesiology, The Pennsylvania State University, University Park, PA, 16802, USA
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Abolins V, Ormanis J, Latash ML. Unintentional drifts in performance during one-hand and two-hand finger force production. Exp Brain Res 2023; 241:699-712. [PMID: 36690719 DOI: 10.1007/s00221-023-06559-z] [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: 10/05/2022] [Accepted: 01/16/2023] [Indexed: 01/25/2023]
Abstract
We explored the phenomena of force drifts and unintentional finger force production (enslaving), and their dependence on visual feedback. Predictions have been drawn based on the theory of control with spatial referent coordinates for condition with feedback on instructed (master) finger force, enslaved finger force, and total force for one-hand and two-hand tasks. Subjects produced force under the different feedback conditions without their knowledge. No feedback condition was also used for the one-hand tasks. Overall, feedback of master finger force led to an increase in the enslaved force, feedback on the slave finger force led to a drop in the master force, feedback on the total force led to balanced drifts in the master force down and enslaved force up, and under the no-feedback condition, master and total force drifted down with large variability in the enslaved force drifts. The patterns were the same in both hands in the two-hand tasks when feedback was provided on the forces of one hand only (without subject's knowledge). The index of enslaving always drifted toward higher values. We interpret the findings as reflecting three main factors: drifts in the referent coordinates toward actual finger coordinates, spread of cortical excitation over representations of the fingers, and robust sharing of referent coordinates between the two hands in bimanual tasks. The large consistent drifts in enslaving toward higher values have to be considered in studies of multi-finger synergies.
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Affiliation(s)
- Valters Abolins
- Cyber-Physical Systems Laboratory, Institute of Electronics and Computer Science, Dzerbenes Iela 14, Riga, 1006, Latvia.
| | - Juris Ormanis
- Cyber-Physical Systems Laboratory, Institute of Electronics and Computer Science, Dzerbenes Iela 14, Riga, 1006, Latvia
| | - Mark L Latash
- Department of Kinesiology, The Pennsylvania State University, University Park, PA, 16802, USA.
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Optimality, Stability, and Agility of Human Movement: New Optimality Criterion and Trade-Offs. Motor Control 2023; 27:123-159. [PMID: 35279021 DOI: 10.1123/mc.2021-0135] [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: 12/02/2021] [Revised: 01/20/2022] [Accepted: 02/05/2022] [Indexed: 12/31/2022]
Abstract
This review of movement stability, optimality, and agility is based on the theory of motor control with changes in spatial referent coordinates for the effectors, the principle of abundance, and the uncontrolled manifold hypothesis. A new optimality principle is suggested based on the concept of optimal sharing corresponding to a vector in the space of elemental variables locally orthogonal to the uncontrolled manifold. Motion along this direction is associated with minimal components along the relatively unstable directions within the uncontrolled manifold leading to a minimal motor equivalent motion. For well-practiced actions, this task-specific criterion is followed in spaces of referent coordinates. Consequences of the suggested framework include trade-offs among stability, optimality, and agility, unintentional changes in performance, hand dominance, finger specialization, individual traits in performance, and movement disorders in neurological patients.
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Khatibi A, Vahdat S, Lungu O, Finsterbusch J, Büchel C, Cohen-Adad J, Marchand-Pauvert V, Doyon J. Brain-spinal cord interaction in long-term motor sequence learning in human: An fMRI study. Neuroimage 2022; 253:119111. [PMID: 35331873 DOI: 10.1016/j.neuroimage.2022.119111] [Citation(s) in RCA: 14] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/16/2021] [Revised: 03/03/2022] [Accepted: 03/16/2022] [Indexed: 10/18/2022] Open
Abstract
The spinal cord is important for sensory guidance and execution of skilled movements. Yet its role in human motor learning is not well understood. Despite evidence revealing an active involvement of spinal circuits in the early phase of motor learning, whether long-term learning engages similar changes in spinal cord activation and functional connectivity remains unknown. Here, we investigated spinal-cerebral functional plasticity associated with learning of a specific sequence of visually-guided joystick movements (sequence task) over six days of training. On the first and last training days, we acquired high-resolution functional images of the brain and cervical cord simultaneously, while participants practiced the sequence or a random task while electromyography was recorded from wrist muscles. After six days of training, the subjects' motor performance improved in the sequence compared to the control condition. These behavioral changes were associated with decreased co-contractions and increased reciprocal activations between antagonist wrist muscles. Importantly, early learning was characterized by activation in the C8 level, whereas a more rostral activation in the C6-C7 was found during the later learning phase. Motor sequence learning was also supported by increased spinal cord functional connectivity with distinct brain networks, including the motor cortex, superior parietal lobule, and the cerebellum at the early stage, and the angular gyrus and cerebellum at a later stage of learning. Our results suggest that the early vs. late shift in spinal activation from caudal to rostral cervical segments synchronized with distinct brain networks, including parietal and cerebellar regions, is related to progressive changes reflecting the increasing fine control of wrist muscles during motor sequence learning.
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Affiliation(s)
- Ali Khatibi
- McConnell Brain Imaging Center, Montreal Neurological Institute, McGill University, Montreal, QC, Canada; Centre de recherche de l'Institut Universitaire de Gériatrie de Montréal, Montréal, QC, Canada; Centre of Precision Rehabilitation for Spinal Pain (CPR Spine), University of Birmingham, UK; Centre for Human Brain Health, University of Birmingham, UK.
| | - Shahabeddin Vahdat
- McConnell Brain Imaging Center, Montreal Neurological Institute, McGill University, Montreal, QC, Canada; Centre de recherche de l'Institut Universitaire de Gériatrie de Montréal, Montréal, QC, Canada; Department of Applied Physiology and Kinesiology, University of Florida, Gainesville, FL, USA
| | - Ovidiu Lungu
- McConnell Brain Imaging Center, Montreal Neurological Institute, McGill University, Montreal, QC, Canada; Centre de recherche de l'Institut Universitaire de Gériatrie de Montréal, Montréal, QC, Canada; Department of psychiatry and addictology, University of Montreal, Montreal, QC, Canada
| | - Jurgen Finsterbusch
- Department of Systems Neuroscience, University Medical Center Hamburg-Eppendorf, Germany
| | - Christian Büchel
- Department of Systems Neuroscience, University Medical Center Hamburg-Eppendorf, Germany
| | - Julien Cohen-Adad
- NeuroPoly Lab, Institute of Biomedical Engineering, Polytechnique Montreal, Montreal, QC, Canada; Functional Neuroimaging Unit, CRIUGM, University of Montreal, Montreal, QC, Canada; Mila Quebec AI Institute, Montreal, QC, Canada
| | | | - Julien Doyon
- McConnell Brain Imaging Center, Montreal Neurological Institute, McGill University, Montreal, QC, Canada; Centre de recherche de l'Institut Universitaire de Gériatrie de Montréal, Montréal, QC, Canada
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Abolins V, Latash ML. Unintentional force drifts across the human fingers: implications for the neural control of finger tasks. Exp Brain Res 2022; 240:751-761. [PMID: 35022805 DOI: 10.1007/s00221-021-06287-2] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/09/2021] [Accepted: 12/05/2021] [Indexed: 11/26/2022]
Abstract
We explored the unintentional force drift across the four fingers of the dominant hand during accurate force production in isometric conditions caused by turning the visual feedback on force off. Our hypotheses were that the Index finger would show smallest drifts and best ability to eliminate the drifts with knowledge of performance in previous trials. Young healthy subjects produced force at 20% of the maximal force level by one finger at a time. There was no significant difference among the fingers in the root mean square error of force during performance with visual feedback. Turning visual feedback off caused force drift to lower magnitudes. The magnitude of force drift was the largest during tasks performed by the Index finger. After each block of twelve trials, the subjects were given feedback on the drift magnitude in that block and used it to correct performance in future trials. There was a total of six blocks. The magnitude of drift correction between consecutive blocks correlated with the magnitude of drift in the earlier block only after the second and fourth blocks. The Index finger failed to improve its performance more than other fingers and demonstrated significant residual drifts to lower force magnitudes in the sixth block of trials. These findings falsified both our hypotheses. Taken together with earlier studies showing advantage of the Index finger across a variety of tasks that require quick and accurate changes in performance, our results suggest that effector specialization along the stability-agility continuum is not limited to the phenomenon of cortical arm/hand dominance but can also be seen across fingers of a hand controlled by the same hemisphere, possibly reflecting the differences in the finger role in prehensile tasks.
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Affiliation(s)
- Valters Abolins
- Cyber-Physical Systems Laboratory, Institute of Electronics and Computer Science, Dzerbenes iela 14, Riga, 1006, Latvia.
| | - Mark L Latash
- Department of Kinesiology, The Pennsylvania State University, University Park, PA, 16802, USA.
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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.
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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.
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9
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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.
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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.
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Abolins V, Stremoukhov A, Walter C, Latash ML. On the origin of finger enslaving: control with referent coordinates and effects of visual feedback. J Neurophysiol 2020; 124:1625-1636. [PMID: 32997555 DOI: 10.1152/jn.00322.2020] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023] Open
Abstract
When a person tries to press with a finger, other fingers of the hand produce force unintentionally. We explored this phenomenon of enslaving during unintentional force drifts in the course of continuous force production by pairs of fingers of a hand. Healthy subjects performed accurate force production tasks by finger pairs Index-Middle, Middle-Ring, and Ring-Little with continuous visual feedback on the combined force of the instructed (master) fingers or of the noninstructed (enslaved) fingers. The feedback scale was adjusted to ensure that the subjects did not know the difference between these two, randomly presented, conditions. Across all finger pairs, enslaved force showed a drift upward under feedback on the master finger force, and master force showed a drift downward under feedback on the enslaved finger force. The subjects were unaware of the force drifts, which could reach over 50% of the initial force magnitude over 15 s. Across all conditions, the index of enslaving increased by ∼50% over the trial duration. The initial moment of force magnitude in pronation-supination was not a consistent predictor of the force drift magnitude. These results falsify the hypothesis that the counter-directional force drifts reflected drifts in the moment of force. They suggest that during continuous force production, enslaving increases with time, possibly due to the spread of excitation over cortical finger representations or other mechanisms, such as increased synchronization of firing of α-motoneurons innervating different compartments of extrinsic flexors. These changes in enslaving, interpreted at the level of control with referent coordinates for the fingers, can contribute to a variety of phenomena, including unintentional force drifts.NEW & NOTEWORTHY We report a consistent slow increase in finger enslaving (force production by noninstructed fingers) when visual feedback was presented on the force produced by either two instructed fingers or two noninstructed fingers of the hand. In contrast, force drifts could be in opposite directions depending on the visual feedback. We interpret enslaving and its drifts at the level of control with referent coordinates for the involved muscles, possibly reflecting spread of cortical excitation.
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Affiliation(s)
- Valters Abolins
- Department of Kinesiology, The Pennsylvania State University, University Park, Pennsylvania.,Institute of Electronics and Computer Science, Riga, Latvia
| | - Alex Stremoukhov
- Department of Kinesiology, The Pennsylvania State University, University Park, Pennsylvania
| | - Caroline Walter
- Department of Kinesiology, The Pennsylvania State University, University Park, Pennsylvania
| | - Mark L Latash
- Department of Kinesiology, The Pennsylvania State University, University Park, Pennsylvania
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