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Joachim Hermsdorfer - One of the best experts on this subject based on the ideXlab platform.
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Grip Force Adjustments Reflect Prediction of Dynamic Consequences in Varying Gravitoinertial Fields
Frontiers in Physiology, 2018Co-Authors: Olivier White, Philippe Lefèvre, Jean-louis Thonnard, Joachim HermsdorferAbstract:Humans have a remarkable ability to adjust the way they manipulate tools through a genuine regulation of Grip Force according to the task. However, rapid changes in the dynamical context may challenge this skill, as shown in many experimental approaches. Most experiments adopt perturbation paradigms that affect only one sensory modality. We hypothesize that very fast adaptation can occur if coherent information from multiple sensory modalities is provided to the central nervous system. Here, we test whether participants can switch between different and never experienced dynamical environments induced by centrifugation of the body. Seven participants lifted an object four times in a row successively in 1, 1.5, 2, 2.5, 2, 1.5, and 1 g. We continuously measured Grip Force, load Force and the gravitoinertial acceleration that was aligned with body axis (perceived gravity). Participants adopted stereotyped grasping movements immediately upon entry in a new environment and needed only one trial to adapt Grip Forces to a stable performance in each new gravity environment. This result was underlined by good correlations between Grip and load Forces in the first trial. Participants predictively applied larger Grip Forces when they expected increasing gravity steps. They also decreased Grip Force when they expected decreasing gravity steps, but not as much as they could, indicating imperfect anticipation in that condition. The participants' performance could rather be explained by a combination of successful scaling of Grip Force according to gravity changes and a separate safety factor. The data suggest that in highly unfamiliar dynamic environments, Grip Force regulation is characterized by a combination of a successful anticipation of the experienced environmental condition, a safety factor reflecting strategic response to uncertainties about the environment and rapid feedback mechanisms to optimize performance under constant conditions.
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Grip Force adjustments reflect prediction of dynamic consequences in varying gravitoinertial fields
bioRxiv, 2017Co-Authors: Olivier White, Philippe Lefèvre, Jean-louis Thonnard, Joachim HermsdorferAbstract:One remarkable capacity when we grasp and manipulate tools relies on the ability to predict the Grip Force required to handle them in relation to their mechanical properties and the surrounding environment. However, rapid changes in the dynamical context may constitute a substantial challenge. Here, we test how participants can switch between different and never experienced dynamical environments induced by centrifugation of the body. Seven subjects lifted an object four times in a row successively in 1, 1.5, 2, 2.5, 2, 1.5 and 1g. We continuously measured Grip Force, load Force and the gravitoinertial acceleration that was aligned with body axis (perceived gravity). Participants adopted stereotyped grasping movements immediately upon entry in a new environment and needed only one trial to adapt Grip Forces to a stable performance in each new gravity environment. While participants predictively applied larger Grip Forces when they expected increasing gravity steps, they did not decrease Grip Force proportionally when they expected decreasing gravity steps, indicating imperfect anticipation in that condition. The subjects' performance could rather be explained by a combination of successful scaling of Grip Force according to gravity changes and a separate safety factor. The data suggest that in highly unfamiliar dynamic environments, Grip Force regulation is characterized by a combination of a successful anticipation of the experienced environmental condition, a safety factor reflecting strategic response to uncertainties about the environment and rapid feedback mechanisms to optimize performance under constant conditions.
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auditory Grip Force feedback in the treatment of writer s cramp
Journal of Hand Therapy, 2009Co-Authors: Barbara Baur, C Marquardt, W Furholzer, Joachim HermsdorferAbstract:Abstract Study Design Pre-post, single-group. Introduction Writer's cramp (WC) is a focal dystonia causing impairments in daily life. Behavioral treatment approaches have been shown to improve handwriting performance, though outcomes remain sub-optimal. Purpose of the Study To examine the effects of the handwriting training and auditory Grip Force feedback in seven patients with WC. Methods Handwriting performance was examined before and after treatment. Writing frequency, fluency, and pressure were recorded with a digitizing tablet and Grip Forces during handwriting were recorded. Subjective writing performance and pain were rated on visual analog scales. Results The treatment resulted in significant reductions in writing pressure and pain, while writing performance was significantly improved. Conclusions Patients in this study with WC, who exhibit Grip Force and pressure problems, benefit from feedback-supported handwriting training. Level of Evidence 4.
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Grip Force control of predictable external loads
Experimental Brain Research, 2008Co-Authors: Joachim Hermsdorfer, H BlankenfeldAbstract:The Grip Force used to grasp and hold an object is modulated synchronously and precisely with a self-produced load indicating predictive feedforward control. It is unclear whether an externally produced load can be anticipated with similar feedforward-timing and precision if it can be predicted, e.g., because it has a periodic time course. In the present study we tested eight healthy subjects during the compensation of an externally produced sinusoidal load with cycle duration 1.5 s and more than 700 repetitions during two successive sessions. Performance parameters characterizing the timing and precision of the Grip Force-load coupling were analyzed across the sessions and compared with a retention measurement on the following day and with an experimental condition when the same loads were self-produced. The time lag between the Grip Force and the load decreased from values greater than zero to values close to zero during the practice sessions indicating a change from a more reactive to a predictive response. In contrast, the precision and economy of the coupling showed no improvement. Performance on the second day was similar to initial performance, only some retention of feedforward timing was obvious. Precision and economy of Grip Force control during self-produced loading was clearly superior to external loading even after extended practice. Our findings confirm that periodic external loads are controlled by predictive feedforward mechanism after sufficient experience. However, performance was not stable and did not reach the level of self-generated loading. These results are interpreted as reflecting the significance of an efferent copy of the motor command in sensorimotor processing that may be associated with a distinct neuronal representation.
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preserved and impaired aspects of feed forward Grip Force control after chronic somatosensory deafferentation
Neurorehabilitation and Neural Repair, 2008Co-Authors: Joachim Hermsdorfer, Z Elias, J D Cole, Barbara M Quaney, Dennis NowakAbstract:BACKGROUND: . Although feed-forward mechanisms of Grip Force control are a prerequisite for skilled object manipulation, somatosensory feedback is essential to acquire, maintain, and adapt these mechanisms. OBJECTIVE: . Individuals with complete peripheral deafferentation provide the unique opportunity to study the function of the motor system deprived of somatosensory feedback. METHODS: . Two individuals (GL and IW) with complete chronic deafferentation of the trunk and limbs were tested during cyclic vertical movements of a handheld object. Such movements induce oscillating loads that are typically anticipated by parallel modulations of the Grip Force. Load magnitude was altered by varying either the movement frequency or object weight. RESULTS: . GL and IW employed excessive Grip Forces probably reflecting a compensatory mechanism. Despite this overall Force increase, both deafferented participants adjusted their Grip Force level according to the load magnitude, indicating preserved scaling of the background Grip Force to physical demands. The dynamic modulation of the Grip Force with the load Force was largely absent in GL, whereas in IW only slower movements were clearly affected. CONCLUSIONS: . The authors hypothesize that the deafferented patients may have utilized visual and vestibular cues and/or an efferent copy of the motor command of the arm movement to scale the Grip Force level. Severely impaired Grip Force-load coupling in GL suggests that sensory information is important for maintaining a precise internal model of dynamic Grip Force control. However, comparably better performance in IW argues for the possibility that alternative cues can be used to trigger a residual internal model.
Dennis A. Nowak - One of the best experts on this subject based on the ideXlab platform.
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on the role of the ventral premotor cortex and anterior intraparietal area for predictive and reactive scaling of Grip Force
Brain Research, 2008Co-Authors: Manuel Dafotakis, Roland Sparing, Simon B Eickhoff, Gereon R Fink, Dennis A. NowakAbstract:When lifting objects of different mass but identical visual appearance, we apply Grip Forces that match the expected mass of the object. Here we study the role of the primary motor cortex (M1), the ventral premotor cortex (PMv) and the anterior intraparietal area (AIP) for predictive and reactive scaling of Grip Forces. Participants performed a precision Grip between the index finger and thumb of the right hand to lift two different masses of identical visual appearance in random order. Neuronavigated single pulse transcranial magnetic stimulation (TMS) over (i) left M1, (ii) left PMv, (iii) left AIP and (iv) the vertex (for control) was applied at two time points of the grasping movement after an unexpected change in mass had occurred: (a) at the time of movement onset and (b) at the time of peak grasp aperture. TMS over the PMv, but not over the vertex, M1 or the AIP, interfered with the predictive scaling of Grip Forces according to the most recent lift when applied at the time of peak grasp aperture. In contrast, TMS over AIP, but not over the vertex, M1 or PMv, disrupted the reactive adjustment of Grip Force to the novel mass of the object at hand. The findings highlight the differential involvement of PMv in the predictive scaling of Grip Force and of AIP in the reactive online adjustment of Grip Force during object manipulation.
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Disturbances of Grip Force behaviour in focal hand dystonia: evidence for a generalised impairment of sensory-motor integration?
Journal of Neurology Neurosurgery and Psychiatry, 2005Co-Authors: Dennis A. Nowak, Karin Rosenkranz, Helge Topka, John C. RothwellAbstract:Background: Focal task specific dystonia occurs preferentially during performance of a specific task. There may be an inefficiently high Grip Force when doing manipulative tasks other than the trigger task, possibly reflecting a generalised impairment of sensory-motor integration. Objective: To examine how well subjects with writer’s cramp (n = 4) or musician’s cramp (n = 5) adapted their Grip Force when lifting a new object or catching a weight. Methods: Nine patients with focal hand dystonia and 10 controls were studied. Experiments addressed different motor behaviours: (A) lifting and holding an object; (B) adjusting Grip Force in anticipation of or in reaction to a change in load Force by catching a small weight dropped expectedly or unexpectedly into a hand held receptacle. Results: In (A), patients produced a Grip Force overshoot during the initial lifts; Force overflow was most pronounced in those with writer’s cramp. Patients and controls adjusted their Grip Force to object weight within one or two lifts, though patients settled to a steady Force level above normal. In (B), patients with focal hand dystonia and normal controls showed similar predictive Grip Force adjustments to expected changes in object load, suggesting that this aspect of sensory-motor integration was normal. Patients had a shorter latency of Grip Force response than controls after an unexpected load increase, reflecting either a greater level of preparatory motor activity or a disinhibited spinal reflex response. Conclusions: The overall increased Grip Force in patients with focal hand dystonia is likely to be a prelearned phenomenon rather than a primary disorder of sensory-motor integration.
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Grip Force behavior during object manipulation in neurological disorders toward an objective evaluation of manual performance deficits
Movement Disorders, 2005Co-Authors: Dennis A. Nowak, Joachim HermsdorferAbstract:The control of prehensile finger Forces is an essential feature of skilled manual performance. The basic aspects of healthy Grip Force behavior have been well documented. In healthy subjects, Grip Force is precisely adjusted to the mechanical object properties. Grip Force is always slightly higher than the minimum necessary to prevent the object from slipping. When we move a hand-held object, Grip Force is modulated in parallel with movements-induced load fluctuations without an obvious delay. The absence of a temporal delay between Grip and load Force profiles suggests that the central nervous system is able to predict the load variations before the intended manipulation and consequently regulates Grip Force in anticipation. Feedback from the grasping fingertips is used to adjust the level of applied fingertip Force efficiently to the actual loading requirements. Pathologic Grip Force control affects the efficiency of produced Force and the precision of the temporal coupling between Grip and load Force profiles. Here, we review the characteristics of pathologic Grip Force behavior in various neurological disorders. Detailed examination of Grip Force control is simple and well suited for the objective evaluation of impaired motor function of the hand and its rehabilitation.
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memory for fingertip Forces passive hand muscle vibration interferes with predictive Grip Force scaling
Experimental Brain Research, 2004Co-Authors: Karin Rosenkranz, Dennis A. Nowak, Joachim Hermsdorfer, John C. RothwellAbstract:When subjects repetitively lift an object, the Grip Force they select is influenced by the mechanical object properties of the preceding lift. Similar effects on Grip Force scaling are observed whether the subsequent lift is performed with the same hand or the hand contralateral to the preceding lift. Here we demonstrate that passive vibration of the hand muscles involved in the generation of Grip Force in the interval between two blocks of lifting trials interferes with predictive Grip Force scaling. Following ten trials in which subjects lifted an object with constant mechanical properties with the dominant hand, muscle vibration was given to the first interosseus and adductor pollicis muscles of the dominant hand during a 10-min rest period. Compared with the last lift preceding vibration, peak rates of Grip Force increase and peak Grip Forces were scaled too high during the first lift following vibration whether the lift was made with the dominant or non-dominant hand. Subjects scaled Grip Force accurately to the object properties within three lifts following vibration. If subjects rested for 10 min after the first ten trials and received no vibration, then there was no significant difference in the peak Grip Force or its rate of increase between the last lift preceding rest and the first lift following it. We suggest that vibration impairs the memory processes responsible for predictive Grip Force scaling. Our data are consistent with the recent suggestion that these memory processes are neither specific for a certain motor action nor do they reflect internal representations of mechanical object properties.
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how predictive is Grip Force control in the complete absence of somatosensory feedback
Brain, 2004Co-Authors: Dennis A. Nowak, Stefan Glasauer, Joachim HermsdorferAbstract:Grip Force control relies on accurate internal models of the dynamics of our motor system and the external objects we manipulate. Internal models are not fixed entities, but rather are trained and updated by sensory experience. Sensory feedback signals relevant object properties and mechanical events, e.g. at the skin-object interface, to modify motor commands and update internal representations automatically. Here we prove that intact sensory feedback is essential for predictive Grip Force regulation. The efficiency and precision of Grip Force adjustments to load fluctuations arising from vertical and horizontal point-to-point arm movements with a hand-held object were analysed in a chronically deafferented subject (G.L.) and three healthy control subjects. Point-to-point movements started and ended with the object being held stationary. G.L. and healthy controls produced similar accelerations of the grasped object and consequently similar load magnitudes during vertical and horizontal movements. Compared with healthy controls, G.L. employed inefficiently high Grip Forces when holding and moving the object, indicating inaccurate Force scaling to object weight and inertial loads. For healthy controls, the Grip Force profile was precisely timed to the movement-induced load fluctuations during vertical and horizontal movements. However, G.L.'s Grip Force profile was not processed to match differential loading requirements of movement direction. We conclude that predictive Grip Force control requires at least intermittent sensory feedback to signal the effectiveness of descending motor commands and to update internal models.
Alan M. Wing - One of the best experts on this subject based on the ideXlab platform.
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Grip Force Regulates Hand Impedance to Optimize Object Stability in High Impact Loads
Neuroscience, 2011Co-Authors: Olivier White, R M Bracewell, Jörn Diedrichsen, Jean-louis Thonnard, Alan M. Wing, Philippe LefèvreAbstract:Anticipatory Grip Force adjustments are a prime example of the predictive nature of motor control. An object held in precision Grip is stabilized by fine adjustments of the Grip Force against changes in tangential load Force arising from inertia during acceleration and deceleration. When an object is subject to sudden impact loads, prediction becomes critical as the time available for sensory feedback is very short. Here, we investigated the control of Grip Force when participants performed a targeted tapping task with a hand-held object. During the initial transport phase of the movement, load Force varied smoothly with acceleration. In contrast, in the collision, load Forces sharply increased to very large values. In the transport phase, Grip Force and load Force were coupled in phase, as expected. However, in the collision, Grip Force did not parallel load Force. Rather, it exhibited a stereotyped profile with maximum similar to 65 ms after peak load at contact. By using catch trials and a virtual environment, we demonstrate that this peak of Grip Force is pre-programmed. This observation is validated across experimental manipulations involving different target stiffness and directions of movement. We suggest that the central nervous system optimizes stability in object manipulation as in catching by regulating mechanical parameters including stiffness and damping through Grip Force. This study provides novel insights about how the brain coordinates Grip Force in manipulation involving an object interacting with the environment. (C) 2011 IBRO. Published by Elsevier Ltd. All rights reserved.
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the role of internal models in motion planning and control evidence from Grip Force adjustments during movements of hand held loads
The Journal of Neuroscience, 1997Co-Authors: Randall J Flanagan, Alan M. WingAbstract:We investigated the issue of whether or not the CNS makes use of an internal model of the motor apparatus in planning and controlling arm movements. In particular, we tested the ability of subjects to predict different hand-held loads by examining Grip Force adjustments used to stabilize the load in the hand during arm movements. Subjects grasped a manipulandum using a precision Grip with the tips of the thumb and index finger on either side. The Grip Force (normal to the contact surfaces) and the load Force (tangential to the surfaces) were measured, along with the trajectory of the hand. The manipulandum was attached to two servo-controlled linear motors used to create inertial and viscous loads as well as a composite load, including inertial, viscous, and elastic components. The form of the hand trajectory was independent of load for some subjects but varied systematically across load conditions in others. Nevertheless, under all load conditions and in all subjects, Grip Force was modulated in parallel with, and thus anticipated, fluctuations in load Force despite the marked variation in the form of the load function. This indicates that the CNS is able to predict the load Force and the kinematics of hand movement on which the load depends. We suggest this prediction is based on an internal model of the motor apparatus and external load and is used to determine the Grip Forces required to stabilize the load.
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effects of surface texture and Grip Force on the discrimination of hand held loads
Attention Perception & Psychophysics, 1997Co-Authors: Randall J Flanagan, Alan M. WingAbstract:In this paper, we report the results from two experiments in which subjects were required to discriminate horizontal load Forces applied to a manipulandum held with a precision Grip. The roughness (and hence friction) of the Grip surfaces and required Grip Force were manipulated. In the first experiment, subjects were instructed to judge the load while maintaining hand position and not letting the manipulandum slip. It was found that performance was influenced by surface texture; a given load was judged to be greater when the surface texture was smooth than when it was rough. This result is consistent with a previous study based on lifting objects and indicates that the effect of surface texture applies to loads in general and not just to gravitational loads (i.e., weight). To test whether the load acting on a smooth object is judged to be greater because the Grip Force required to prevent it from slipping is larger, a second experiment was carried out. Subjects used a visual feedback display to maintain the same Grip Force for smooth and rough manipulandum surfaces. In this case, there was no effect of surface texture on load perception. These results provide evidence that perceived load depends on the Grip Force used to resist the load. The implications of these results in terms of central and peripheral factors underlying load discrimination are considered.
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modulation of Grip Force with load Force during point to point arm movements
Experimental Brain Research, 1993Co-Authors: Randall J Flanagan, Alan M. WingAbstract:In this paper, we examine Grip Forces and load Forces during point-to-point arm movements with objects grasped with a precision Grip. We demonstrate that Grip Force is finely modulated with load Force. Variations in load Force arise from inertial Forces related to movement; Grip Force rises as the load Force increases and falls as load Force decreases. The same finding is observed in vertical and horizontal movements performed at various rates. In vertical movements, maximum Grip Force coincides in time with maximum load Force. The maxima occur early in upward and later in downward movements. In horizontal movements, where peaks in load Force are observed during both the acceleratory and deceleratory phases, Grip Force rises at the beginning of the movement and remains high until the end. The results suggest that when moving an object with the hand the programming of Grip Force is an integral part of the planning process.
Marc A Maier - One of the best experts on this subject based on the ideXlab platform.
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Affected and unaffected quantitative aspects of Grip Force control in hemiparetic patients after stroke.
Brain Research, 2012Co-Authors: Påvel G Lindberg, Nicolas Roche, Johanna Robertson, Agnès Roby-brami, Bernard Bussel, Marc A MaierAbstract:Adequate Grip Force modulation is critical to manual dexterity and often impaired in hemiparetic stroke patients. Previous studies in hemiparetic patients suggest that aspects of Grip Force control may be differently affected by the lesion. We developed a visuomotor power Grip Force-tracking task allowing quantification of tracking error, Force variability and release duration. We investigated Force control in 24 chronic stroke patients with varying severity of hemiparesis and in healthy control subjects. Force tracking was performed at 10, 20, and 30% maximal voluntary contraction (MVC). Control subjects were also tested at absolute Force levels similar to those of the patients. Patients tracking with their paretic hand at similar relative (%MVC) Grip Force levels showed increased error, Force variability and release duration, but surprisingly, there was no difference in tracking error or variability between patients and control subjects performing at similar absolute Force levels. Furthermore, patients improved their tracking performance across repeated blocks similar to control subjects. Release duration, however, was increased (also in the non-paretic hand), was Force-independent and did not correlate with MVC strength. Of the three performance measures, only release duration explained some of the variance in arm and hand function (Frenchay Arm Test score), independent of MVC strength. The findings show (i) that hemiparetic stroke patients preserve the ability to modulate (generate and maintain) power Grip Force within their limited Force range and (ii) that MVC Grip strength and duration of Grip release are differently affected and are two complementary predictors of arm function after stroke.
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white matter organization in cervical spinal cord relates differently to age and control of Grip Force in healthy subjects
The Journal of Neuroscience, 2010Co-Authors: Påvel G Lindberg, A Feydy, Marc A MaierAbstract:Diffusion tensor imaging (DTI) can be used to elucidate relations between CNS structure and function. We hypothesized that the degree of spinal white matter organization relates to the accuracy of control of Grip Force. Healthy subjects of different age were studied using DTI and visuomotor tracking of precision Grip Force. The latter is a prime component of manual dexterity. A regional analysis of spinal white matter [fractional anisotropy (FA)] across multiple cervical levels (C2-C3, C4-C5, and C6-C7) and in different regions of interest (left and right lateral or medial spinal cord) was performed. FA was highest at the C2-C3 level, higher on the right than the left side, and higher in the lateral than in the medial spinal cord (p < 0.001). FA of whole cervical spinal cord (C2-C7) was lower in subjects with high tracking error (r = -0.56, p = 0.004) and decreased with age (r = -0.63, p = 0.001). A multiple regression analysis revealed an independent contribution of each predictor (semipartial correlations: age, r = -0.55, p < 0.001; tracking error, r = -0.49, p = 0.003). The closest relation between FA and tracking error was found at the C6-C7 level in the lateral spinal cord, in which the corticospinal tract innervates spinal circuitry controlling hand and digit muscles. FA of the medial spinal cord correlated consistently with age across all cervical levels, whereas FA of the lateral spinal cord did not. The results suggest (1) a functionally relevant specialization of lateral spinal cord white matter and (2) an increased sensitivity to age-related decline in medial spinal cord white matter in healthy subjects.
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precision in isometric precision Grip Force is reduced in middle aged adults
Experimental Brain Research, 2009Co-Authors: Påvel G Lindberg, Marc A Maier, Chrystele Ody, A FeydyAbstract:We investigated age related changes in the control of precision Grip in 29 healthy adults spanning early adulthood to middle age (21–67 years). Subjects performed a visually guided, isometric precision Grip ramp-and-hold Force-tracking task. Target Force levels were 3, 6, and 9 N. Precision and performance of Force regulation was quantified. Larger errors were made during the ramp than during the hold phase. Age correlated positively with the amount of error at the lowest (3 N) Force level in both phases. Force onsets were systematically earlier in middle-aged subjects and the average slope of the Force during the ramp decreased with increasing age. The results show that precision during low Grip Force control decreases already during middle age and those subjects may modify their Force generation strategies to compensate for early and subtle degenerative changes in the motor system before decline in Grip strength is apparent.
Randall J Flanagan - One of the best experts on this subject based on the ideXlab platform.
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Grip Force when reaching with target uncertainty provides evidence for motor optimization over averaging
Scientific Reports, 2017Co-Authors: Joseph Y Nashed, Jonathan S Diamond, Jason P Gallivan, Daniel M Wolpert, Randall J FlanaganAbstract:When presented with competing potential reach targets and required to launch a movement before knowing which one will be cued as the target, people initially reach in the average target direction. Although this spatial averaging could arise from executing a weighted average of motor plans for the potential targets, it could also arise from planning a single, optimal movement. To test between these alternatives we used a task in which participants were required to reach to either a single target or towards two potential targets while grasping an object. A robotic device applied a lateral elastic load to the object requiring large Grip Forces for reaches to targets either side of midline and a minimal Grip Force for midline movements. As expected, in trials with two targets located either side of midline, participants initially reached straight ahead. Critically, on these trials the initial Grip Force was minimal, appropriate for the midline movement, and not the average of the large Grip Forces required for movements to the individual targets. These results indicate that under conditions of target uncertainty, people do not execute an average of planned actions but rather a single movement that optimizes motor costs.
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the role of internal models in motion planning and control evidence from Grip Force adjustments during movements of hand held loads
The Journal of Neuroscience, 1997Co-Authors: Randall J Flanagan, Alan M. WingAbstract:We investigated the issue of whether or not the CNS makes use of an internal model of the motor apparatus in planning and controlling arm movements. In particular, we tested the ability of subjects to predict different hand-held loads by examining Grip Force adjustments used to stabilize the load in the hand during arm movements. Subjects grasped a manipulandum using a precision Grip with the tips of the thumb and index finger on either side. The Grip Force (normal to the contact surfaces) and the load Force (tangential to the surfaces) were measured, along with the trajectory of the hand. The manipulandum was attached to two servo-controlled linear motors used to create inertial and viscous loads as well as a composite load, including inertial, viscous, and elastic components. The form of the hand trajectory was independent of load for some subjects but varied systematically across load conditions in others. Nevertheless, under all load conditions and in all subjects, Grip Force was modulated in parallel with, and thus anticipated, fluctuations in load Force despite the marked variation in the form of the load function. This indicates that the CNS is able to predict the load Force and the kinematics of hand movement on which the load depends. We suggest this prediction is based on an internal model of the motor apparatus and external load and is used to determine the Grip Forces required to stabilize the load.
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effects of surface texture and Grip Force on the discrimination of hand held loads
Attention Perception & Psychophysics, 1997Co-Authors: Randall J Flanagan, Alan M. WingAbstract:In this paper, we report the results from two experiments in which subjects were required to discriminate horizontal load Forces applied to a manipulandum held with a precision Grip. The roughness (and hence friction) of the Grip surfaces and required Grip Force were manipulated. In the first experiment, subjects were instructed to judge the load while maintaining hand position and not letting the manipulandum slip. It was found that performance was influenced by surface texture; a given load was judged to be greater when the surface texture was smooth than when it was rough. This result is consistent with a previous study based on lifting objects and indicates that the effect of surface texture applies to loads in general and not just to gravitational loads (i.e., weight). To test whether the load acting on a smooth object is judged to be greater because the Grip Force required to prevent it from slipping is larger, a second experiment was carried out. Subjects used a visual feedback display to maintain the same Grip Force for smooth and rough manipulandum surfaces. In this case, there was no effect of surface texture on load perception. These results provide evidence that perceived load depends on the Grip Force used to resist the load. The implications of these results in terms of central and peripheral factors underlying load discrimination are considered.
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modulation of Grip Force with load Force during point to point arm movements
Experimental Brain Research, 1993Co-Authors: Randall J Flanagan, Alan M. WingAbstract:In this paper, we examine Grip Forces and load Forces during point-to-point arm movements with objects grasped with a precision Grip. We demonstrate that Grip Force is finely modulated with load Force. Variations in load Force arise from inertial Forces related to movement; Grip Force rises as the load Force increases and falls as load Force decreases. The same finding is observed in vertical and horizontal movements performed at various rates. In vertical movements, maximum Grip Force coincides in time with maximum load Force. The maxima occur early in upward and later in downward movements. In horizontal movements, where peaks in load Force are observed during both the acceleratory and deceleratory phases, Grip Force rises at the beginning of the movement and remains high until the end. The results suggest that when moving an object with the hand the programming of Grip Force is an integral part of the planning process.
