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1.
We have examined the interaction among individual finger forces in tasks that required the production of the total force by a subset of fingers in a particular direction in the flexion-extension plane. Nine subjects produced fingertip forces in a prescribed direction with a maximum voluntary contraction (MVC) effort and held the peak force for two seconds. Six finger combinations were tested, four single-finger tasks--Index (I), Middle (M), Ring (R) and Little (L)--one two-digit task (IM), and one four-digit task (IMRL). The subjects were asked to generate the finger forces in two directions, 0 degrees (perpendicular to the surface of the transducer) and 15 degrees toward the palm. In all task conditions, there were two experimental sessions, with and without visual feedback on the task force vector. The main findings were: 1. The target direction significantly affected the constant error (CE) but not the variable error (VE) while removal of the feedback resulted in an increase in VE. 2. The direction of the forces produced by fingers that were not explicitly required to produce force (enslaved fingers) depended on the target direction. 3. In multi-finger tasks, the individual fingers produced force in directions that could differ significantly from the target direction, while the resultant force pointed in the target direction. There was a negative co-variation among the deviations of the directions of the individual finger forces from the target direction. If a finger force vector deviated from the target, another finger force vector was likely to deviate in the opposite direction. We conclude that a multi-finger synergy is involved in the control of the finger force direction.  相似文献   

2.
Surface EMG was recorded from two intrinsic and two extrinsic muscles of the index finger during a two-dimensional isometric force task in the plane of flexion and extension. Subjects applied force isometrically at the fingertip in eight equally spaced directions, encompassing 360 degrees. Target forces spanned the range from 20% to 50% of maximum for each direction. The effect of varying the metacarpophalangeal (MCP) and interphalangeal (IP) joint angles was investigated. We found that when applying isometric force with the fingertip, the intrinsic muscles of the index finger behaved as a single unit whose region of activation overlapped that of the extrinsic flexor and extensor muscles. The activation region of the intrinsic muscles also spanned a range of force directions for which the extrinsic muscles were virtually inactive. The activation of all muscles, with the exception of the extrinsic extensor, was modified by changing the MCP and IP joint angles. Both IP flexion and MCP extension produced rotation of the resultant activity vector in the direction of MCP flexion. However, the relative rotation was much greater with IP flexion than MCP extension. The effect of IP flexion is linked to rotation of the force direction where joint torque switches from extension to flexion, while the effect of MCP extension is more likely related to changes in muscle length and MCP moment arm. Our results suggest that the primary role of intrinsic finger muscles is to precisely control the direction of fingertip force, while extrinsic muscles provide stability of the joints.  相似文献   

3.
This study examined grip force development in individuals with hemiparesis following unilateral stroke. Eleven patients with chronic stroke with severe hand impairment and five age-matched neurologically intact subjects grasped an instrumented object between the index finger and thumb while fingertip forces, digit posture, and muscle electromyographic activity were recorded. We tested a range of different grip conditions with varying grip sizes, object stability, and grip force level. We found that fingertip force direction in the paretic digits deviated from the direction normal to the grip surface by more than twice as much as for asymptomatic digits. Additionally, the paretic thumb had, on average, 18% greater deviation of grip force direction than the paretic index finger. This large deviation of finger force direction for the paretic digits was consistently observed regardless of grip size, grip force level, and object stability. Due to the large deviation of the force direction from the normal direction, the paretic digits slipped and moved more than 1 cm during 55% of all grasping trials. A regression analysis suggests that this altered grip force direction was associated with altered hand muscle activation patterns, but not with the posture at which the digit made contact with the object. Therapies to redirect the force direction at the digits may improve stroke survivors’ ability to stably grip an object.  相似文献   

4.
We used two methods, analytical inverse optimization (ANIO) and uncontrolled manifold (UCM) analysis of synergies, to explore age-related changes in finger coordination during accurate force and moment of force production tasks. The two methods address two aspects of the control of redundant systems: Finding an optimal solution (an optimal sharing pattern) and using variable solutions across trials (covarying finger forces) that are equally able to solve the task. Young and elderly subjects produced accurate combinations of total force and moment by pressing with the four fingers of the dominant hand on individual force sensors. In session-1, single trials covered a broad range of force–moment combinations. Principal component (PC) analysis showed that the first two PCs explained about 90% and 75% of finger force variance for the young and elderly groups, respectively. The magnitudes of the loading coefficients in the PCs suggested that the young subjects used mechanical advantage to produce moment while elderly subjects did not (confirmed by analysis of moments produced by individual digits). A co-contraction index was computed reflecting the magnitude of moment produced by fingers acting against the required direction of the total moment. This index was significantly higher in the young group. The ANIO approach yielded a quadratic cost function with linear terms. In the elderly group, the contribution of the forces produced by the middle and ring fingers to the cost function value was much smaller than in the young group. The angle between the plane of experimental observations and the plane of optimal solutions (D-angle), was very small (about 1.5°) in the young group and significantly larger (about 5°) in the elderly group. In session-2, four force–moment combinations were used with multiple trials at each. Covariation among finger forces (multifinger synergies) stabilizing total force, total moment, and both was seen in both groups with larger synergy indices in the young group. Multiple regression analysis has shown that, at higher force magnitudes, the synergy indices defined with the UCM method were significantly related to the percent of variance accounted by the first two PCs and to the D-angle computed using the ANIO method. We interpret the results as pointing at a transition with age from synergic control to element-based control (back-to-elements hypothesis). Optimization and analysis of synergies are complementary approaches that focus on two aspects of multidigit coordination, sharing and covariation, respectively.  相似文献   

5.
Accurate control of forces produced by the fingers is essential for performing object manipulation. This study examines the indices of finger interaction when accurate time profiles of force are produced in different directions, while using one of the fingers or all four fingers of the hand. We hypothesized that patterns of unintended force production among shear force components may involve features not observed in the earlier studies of vertical force production. In particular, we expected to see unintended forces generated by non-task fingers not in the direction of the instructed force but in the opposite direction as well as substantial force production in directions orthogonal to the instructed direction. We also tested a hypothesis that multi-finger synergies, quantified using the framework of the uncontrolled manifold hypothesis, will help reduce across-trials variance of both total force magnitude and direction. Young, healthy subjects were required to produce accurate ramps of force in five different directions by pressing on force sensors with the fingers of the right (dominant) hand. The index finger induced the smallest unintended forces in non-task fingers. The little finger showed the smallest unintended forces when it was a non-task finger. Task fingers showed substantial force production in directions orthogonal to the intended force direction. During four-finger tasks, individual force vectors typically pointed off the task direction, with these deviations nearly perfectly matched to produce a resultant force in the task direction. Multi-finger synergy indices reflected strong co-variation in the space of finger modes (commands to fingers) that reduced variability of the total force magnitude and direction across trials. The synergy indices increased in magnitude over the first 30% of the trial time and then stayed at a nearly constant level. The synergy index for stabilization of total force magnitude was higher for shear force components when compared to the downward pressing force component. The results suggest complex interactions between enslaving and synergic force adjustments, possibly reflecting the experience with everyday prehensile tasks. For the first time, the data document multi-finger synergies stabilizing both shear force magnitude and force vector direction. These synergies may play a major role in stabilizing the hand action during object manipulation.  相似文献   

6.
The aim of this study was to investigate the contralateral motor overflow in children during single-finger and multi-finger maximum force production tasks. Forty-five right handed children, 5–11 years of age produced maximum isometric pressing force in flexion or extension with single fingers or all four fingers of their right hand. The forces produced by individual fingers of the right and left hands were recorded and analyzed in four-dimensional finger force vector space. The results showed that increases in task (right) hand finger forces were linearly associated with non-task (left) hand finger forces. The ratio of the non-task hand finger force magnitude to the corresponding task hand finger force magnitude, termed motor overflow magnitude (MOM), was greater in extension than flexion. The index finger flexion task showed the smallest MOM values. The similarity between the directions of task hand and non-task hand finger force vectors in four-dimensional finger force vector space, termed motor overflow direction (MOD), was the greatest for index and smallest for little finger tasks. MOM of a four-finger task was greater than the sum of MOMs of single-finger tasks, and this phenomenon was termed motor overflow surplus. Contrary to previous studies, no single-finger or four-finger tasks showed significant changes of MOM or MOD with the age of children. We conclude that the contralateral motor overflow in children during finger maximum force production tasks is dependent upon the task fingers and the magnitude and direction of task finger forces.  相似文献   

7.
We investigated the effects of old age on the fingertip force responses that occurred when a grasped handle was pulled unexpectedly to increase the tangential load at the fingertip. These automatic responses, directed normal to the handle surface, help prevent slips between the handle and finger. Old adults (average age 78 years) responded with large peak fingertip forces compared to young adults (average age 30 years), even though the two subject groups showed similar skin slipperiness. For step-shaped loads the average response latency was the same for young and old subjects (about 80 ms). Thus, these automatic responses are not susceptible to the age-related central delays known for simple reaction-time tasks. For ramp-shaped loads the average response latency was inversely related to load rate. Response latency was 25 ms longer for the Old group versus the Young group for loads of 8 N/s, and this difference increased exponentially to a 110-ms difference for 2-N/s loads. A twofold difference in the tangential force required to evoke a response was predicted from linear regressions and can account for the latency difference (0.2 N vs 0.4 N threshold for young and old, respectively, r=0.93 for both groups). This theoretical elevation in load force threshold is consistent with degraded central information processing in old age, and the deterioration of cutaneous mechanoreceptors.  相似文献   

8.
The ability to match the voluntary isometric force output of the right and left index fingers when the contact surfaces differ in shape was examined. Before the experiment, subjects were trained to produce both a low force level (50±25 g) and a high force level (200±50 g) with the right and left index finger, separately. Following the training session, subjects were instructed to match the forces of both fingers simultaneously within the required range (either low or high) so that the forces were perceived to be identical. One of the index fingers pushed against a conical contact pad, while the other pushed against a flat contact pad. Midway through the experiment, the two contact pads were reversed. Subjects consistently produced less force with the finger pressing against the conical pad. This asymmetry could already be seen during the beginning of the ramp increase in force and continued throughout the trial, independent of the target force levels (low or high). These findings suggest tactile afferent information at the fingertip is important for determining the voluntary force exerted by the finger. It must be properly integrated with other peripheral information as well as with the central motor command, otherwise the perception of force is distorted. Furthermore, the perception of the force produced seemingly is dependent on the extent to which the skin of the fingertip is indented rather than the local pressure exerted at the skin.  相似文献   

9.
The goal of this experiment was to investigate the relationship between individual fingertip forces and the surface EMG of multi-digit muscles. The surface EMG of the hand extrinsic flexors (flexor digitorum profundis and superficialis) was recorded in eight subjects during multi-digit force production tasks. In one session, subjects pressed with all four fingers (IMRL, I=index, M=middle, R=ring, and L=little finger) with the total force ranging from 10% and 90% of their maximum force (MVC). Results showed a close linear relationship between an integrated EMG index and force. In another session, subjects produced constant total force of either 10% or 30% of their IMRL MVC, with different finger combinations such that the degree of involvement of each finger was manipulated (15 finger combinations were tested). The EMG level of the flexors depended greatly on the finger combination (P<0.001). Multi-variable regression made it possible to describe the flexor EMG as a linear function of individual fingertip forces. These results suggest that: (1) hand extrinsic flexors muscles are arranged in functional compartments serving individual fingers, and (2) each compartment has a force/EMG relationship that is close to being linear. Electronic Publication  相似文献   

10.
The organization of thumb and index finger forces in a pinch formation was investigated under conditions where kinetic constraints on interdigit force coupling were removed. Two visually guided isometric force tasks at submaximal levels were used to characterize the spatial and temporal aspects of interdigit force coupling. Task 1 provided an initial characterization of interdigit force coordination when the force relationship between the digits was not specified. Task 2 probed the extent to which a preferred coordination of the thumb and index finger could be decoupled, both temporally and with respect to force magnitude, by specifying the coordination between the digit forces. Digit forces were measured using a pinch apparatus that was instrumented to record the magnitude and direction of the thumb (F t) and index finger (F i) forces, independently. Two apparatus conditions allowed further examination of interdigit force coordination when the relationship between digit forces was mechanically constrained (pivot condition), and when the relationship between digit forces was not constrained, allowing the neuromotor system to select a preferred pattern of interdigit coordination (fixed condition). Sixteen right-handed adults exerted a pinch force against the apparatus to match a single-cycle sine wave that varied between 15 and 35% of each participant’s maximal voluntary pinch force. The target was presented with positive or negative target sense, to vary the order of force level and direction of force change across the trials. When the mechanical constraints allowed selection of a preferred coordination pattern, F t = F i was a robust result. In contrast, when the coordination between the digit forces was specified by the requirement to simultaneously produce and control independent thumb and index finger forces while acting on a stable object, subjects were able to produce forces that markedly deviated from the F t = F i coordination. The organization of pinch is characterized by a preferred, tight coupling of digit forces, which can be modified based on task demands.  相似文献   

11.
The present study was designed to examine the finger-pinch force control, digit force sharing and digit coupling relations of 13 young and 14 older adults. Subjects performed four isometric tri-digit finger-pinch force production conditions reflecting all combinations of mean force level (20 and 40% MVC) and target shape (constant and sinusoidal). Older adults had significantly reduced force control, as indicated by their greater levels of absolute and relative force variability and targeting error than young adults. The age-related loss of relative force control was more pronounced at low (20% MVC) than high (40% MVC) forces, and to a lesser extent, in sinusoidal than constant force conditions. Older adults had significantly greater peak and proportional power below 1.5 Hz than young adults, with this especially pronounced in constant force conditions. Digit force sharing results indicated that the index finger’s contribution to total force was increased and the middle finger’s contribution reduced in older than young adults. The results of the cross-correlation analyses revealed that older subjects had a significantly reduced level of coupling between the middle finger and the target force, thumb force and EMG signals, with longer time lags in comparison to young adults. These differences in force sharing and middle finger force coupling were more pronounced in sinusoidal than constant force conditions. Overall, these results suggest that the older adults’ reduced force control reflected age-related differences in the sharing and coupling of the finger forces. The results also highlighted that tasks of this nature display a degree of task-dependency, with these overall differences in digit force output and coupling not consistently observed across all force conditions.  相似文献   

12.
Old age impairs the ability to form new associations for declarative memory, but the ability to acquire and retain procedural memories remains relatively intact. Thus, it is unclear whether old age affects the ability to learn the visuomotor associations needed to set efficient fingertip forces for handling familiar objects. We studied the ability for human subjects to use visual cues (color) about the mechanical properties (texture or weight) of a grasped object to control fingertip forces during prehension. Old and young adults (mean age 77 years and 22 years, respectively) grasped and lifted an object that varied in texture at the gripped surfaces (experiment 1: sandpaper versus acetate surface materials) or weight (experiment 2: 200 g versus 400 g). The object was color-coded according to the mechanical property in the "visual cue" condition, and the mechanical property varied unpredictably across lifts in the "no visual cue" condition. In experiment 1 (texture), the young adults' grip force (force normal to the gripped surface) when the object lifted from the support surface was 24% smaller when the surfaces were color-coded. The old adults' grip force did not vary between the visual conditions despite their accurate reports of the grip surface colors prior to each lift. Comparable findings were obtained in experiment 2, when object weight was varied and peak grip force rate prior to object lift-off was measured. Furthermore old and young subjects alike used about 2 N of grip force when lifting the 200 g object in experiment 2. Therefore, the old adults' failure to adjust grip force when the color cue was present cannot be attributed to a general inability or unwillingness to use low grip force when handling objects. We conclude that old age affects the associative learning that links visual identification of an object with the fingertip forces for efficiently handling the object. In contrast, old and young subjects' grip force was influenced by the preceding lift, regardless of visual cues, which supports existing theories that multiple internal representations govern predictive control of fingertip forces during prehension.  相似文献   

13.
Postural sway during quiet stance is attenuated by actively maintained contact of the index finger with a stationary surface, even if the level of applied force (<1 N) cannot provide mechanical stabilization. In this situation, changes in force level at the fingertip lead changes in center of foot pressure by approximately 250 ms. These and related findings indicate that stimulation of the fingertip combined with proprioceptive information about the hand and arm can serve as an active sensor of body position relative to the point of contact. A geometric analysis of the relationship between hand and torso displacement during body sway led to the prediction that arm and hand proprioceptive and finger somatosensory information about body sway would be maximized with finger contact in the plane of body sway. Therefore, the most postural stabilization should be possible with such contact. To test this analysis, subjects touched a laterally versus anteriorly placed surface while in each of two stances: the heel-to-toe tandem Romberg stance that reduces medial-lateral stability and the heel-to-heel, toes-outward, knees-bent, "duck stance" that reduces fore-aft stability. Postural sway was always least with finger contact in the unstable plane: for the tandem stance, lateral fingertip contact was significantly more effective than frontal contact, and, for the duck stance, frontal contact was more effective than lateral fingertip contact. Force changes at the fingertip led changes in center of pressure of the feet by approximately 250 ms for both fingertip contact locations for both test stances. These results support the geometric analysis, which showed that 1) arm joint angles change by the largest amount when fingertip contact is maintained in the plane of greatest sway, and 2) the somatosensory cues at the fingertip provide both direction and amplitude information about sway when the finger is contacting a surface in the unstable plane.  相似文献   

14.
We addressed a hypothesis that changes in indices of finger interaction during maximal force production (MVC) tasks are accompanied by changed coordination of fingers in multi-finger accurate force production tasks. To modify relative involvement of extrinsic and intrinsic hand muscles, the subjects produced force by pressing either at their distal phalanges or at their proximal phalanges. As in earlier studies, in MVC trials, the elderly subjects showed a greater force decline when pressing at the proximal phalanges as compared to pressing at the distal phalanges. Two methods were applied to analyze finger coordination during the task of four-finger force production from zero to 30% of MVC over 5 s, at the level of finger forces (performance variables) and at the level of modes (control variables). Our previous observations of higher indices of variability during the ramp task in elderly subjects have been generalized to both sites of force application. An index of finger force covariation (the difference between the variance of the total force and the sum of the variances of individual finger forces) revealed small age related differences, which did not depend on the site of the force application. In contrast, analysis of covariation of force modes within the uncontrolled manifold (UCM) hypothesis showed much better stabilization of the time profile of the total force by young subjects. The UCM hypothesis was also used to test stabilization of the pronation/supination moment during the ramp task. Young subjects showed better moment stabilization than elderly. Age related differences in both force- and moment-stabilization effects were particularly strong during force application at the proximal phalanges. We conclude that the drop in MVC is accompanied in elderly subjects with worse coordination of control signals to fingers in multi-finger tasks. The UCM analysis was more powerful as compared to analysis of force variance profiles in revealing significant differences between the groups. This general result underscores the importance of efforts to analyze motor coordination using control rather than performance variables.  相似文献   

15.
Motor-unit synchronization, which is a measure of the near simultaneous discharge of action potentials by motor units, has the potential to influence spike-triggered average force and the steadiness of a low-force isometric contraction. The purpose of the study was to estimate the contribution of motor-unit synchronization to the larger spike-triggered average forces and the decreased steadiness exhibited by old adults. Eleven young (age 19-30 yr) and 14 old (age 63-81 yr) adults participated in the study. Motor-unit activity was recorded with two fine-wire intramuscular electrodes in the first dorsal interosseus muscle during isometric contractions that caused the index finger to exert an abduction force. In a separate session, steadiness measurements were obtained during constant-force isometric contractions at target forces of 2.5, 5, 7. 5, and 10% of the maximum voluntary contraction (MVC) force. Mean (+/-SD) motor-unit forces measured by spike-triggered averaging were larger in old (15.5 +/- 12.1 mN) compared with young (7.3 +/- 5.7 mN) adults, and the differences were more pronounced between young (8.7 +/- 6.4 mN) and old (19.9 +/- 12.2 mN) men. Furthermore, the old adults had a reduced ability to maintain a steady force during an isometric contraction, particularly at low target forces (2.5 and 5% MVC). Mean (+/-SD) motor-unit synchronization, expressed as the frequency of extra synchronous discharges above chance in the cross-correlogram, was similar in young [0.66 +/- 0.4 impulses/s (imp/s); range, 0.35-1.51 imp/s; 53 pairs) and old adults (0.72 +/- 0.5 imp/s; range, 0.27-1.38 imp/s; 56 pairs). The duration of synchronous peaks in the cross-correlogram was similar for each group (approximately 16 ms). These data suggest that motor-unit synchronization is not responsible for larger spike-triggered average forces in old adults and that motor-unit synchronization does not contribute to the decreased steadiness of low-force isometric contractions observed in old adults.  相似文献   

16.
Recent studies using bimanual force production have examined how factors influence redundancy in the nervous system. The present study examined effects of different movement durations on bimanual force control strategies. Ten healthy male participants produced periodic isometric forces such that the sum of two finger forces was a target cycling between 5 and 10 % of maximum voluntary contraction during five movement durations (500, 750, 1,000, 1,250, and 1,500 ms). Correlations between the two finger forces changed from positive to negative with an increase in duration. The polynomial regression analysis indicates that while the correlations between two finger forces were most negative at the target duration of 1,250 ms, they became more positive as the durations deviated from 1,250 ms. Similarly, while force variability was smallest at the target duration of 1,250 ms, it increased as the durations deviated from 1,250 ms. These findings suggested that while the duration of 1,250 ms might be a natural frequency of both fingers, bimanual force strategies changed from force error compensation to force coupling as the durations deviated from 1,250 ms. In addition, while the variance in the sum of two finger forces (the task-relevant variance) decreased with movement duration, the difference between both the finger forces (the task-irrelevant variance) did not change with the duration. Thus, a decrease in the task-relevant variance with movement duration resulted in the negative correlation between the two finger forces and the small force variability.  相似文献   

17.
Contact of the index finger with a stable surface greatly attenuates postural sway in blindfolded subjects. The time course of postural stabilization was measured after subjects made finger contact with a surface. Subjects (n=12) were tested standing in a heel-to-toe stance in 25 s duration trials. The subject stood with the index finger above but not contacting a laterally placed surface, and made finger contact when cued midway through the trial. Fingertip contact forces stabilized with a time constant of less than 0.5 s and postural stabilization occurred rapidly following fingertip contact. Sway amplitude of center of pressure of the feet decreased by half with a time constant of less than 1.6 s. The stereotypical pattern of force changes at the fingertip leading correlated changes in center of pressure by ~300 ms was evident within the first 0.5 s of finger contact. We conclude that the fingertip can serve as a sensory-motor probe that is stabilized nearly immediately on contact with a surface and that from the moment of contact the fingertip contributes sensory signals used to control sway.  相似文献   

18.
During ischemic nerve block of an extremity the cortical representations of muscles proximal to the block are known to expand, increasing the overlap of different muscle representations. Such reorganization mimics that seen in actual amputees. We investigated whether such changes degrade voluntary control of muscles proximal to the block. Nine subjects produced brief, isometric flexion force selectively with each fingertip before, during, and after ischemic block at the wrist. We recorded the isometric force exerted at the distal phalanx of each digit, along with electromyographic (EMG) activity from intrinsic and extrinsic finger muscles. Despite paralysis of the intrinsic hand muscles, and associated decrements in the flexion forces exerted by the thumb, index, and little fingers, the selectivity of voluntary finger flexion forces and of EMG activity in the extrinsic finger muscles that generated these forces remained unchanged. Our observations indicate that during ischemic nerve block reorganization does not eliminate or degrade motor representations of the temporarily deafferented and paralyzed fingers.  相似文献   

19.
The purpose of this study was to examine how contact forces normal to the skin surface and shear forces tangential to the skin surface are deployed during tactile exploration of a smooth surface in search of a tactile target. Six naive subjects participated in two experiments. In the first experiment, the subjects were asked to explore a series of unseen smooth plastic surfaces by using the index finger to search for either a raised or recessed target. The raised targets were squares with a height of 280 micro m above the background surface and that varied in side lengths from 0.2 mm to 8.0 mm. A second series of smooth plastic surfaces consisted of small recessed squares (side lengths: 2.0, 3.0, 4.0 and 8.0 mm) that were etched to a depth of 620 micro m. Although made of an identical material, the plastic substrate had a lower coefficient of friction against the skin because only the recessed square had been subjected to the electrolytic etching process. The surfaces were mounted on a six-axes force and torque sensor connected to a laboratory computer. From the three axes of linear force, the computer was able to calculate the instantaneous position of the index finger and the instantaneous tangential force throughout the exploratory period. When exploring for the raised squares, the subjects maintained a relatively constant, average normal force of about 0.49 N with an average exploration speed of 8.6 cm/s. In contrast, all subjects used a significantly higher average normal force (0.64 N) and slightly slower mean exploration speed (7.67 cm/s) when searching for the small recessed squares. This appeared to be an attempt to maximize the amount of skin penetrating the recessed squares to improve the probability of target detection. In a second experiment, subjects were requested to search for an identical set of raised squares but with the fingertip having been coated with sucrose to impede the scanning movement by increasing the friction. Overall, the subjects maintained the same constant normal force that they used on the uncoated surface. However, they increased the tangential force significantly. The similarity of the search strategy employed by all subjects supports the hypothesis that shear forces on the skin provide a significant stimulus to mechanoreceptors in the skin during tactile exploration. Taken together, these data suggest that, in active tactile exploration with the fingertip, the tangential finger speed, the normal contact force, and the tangential shear force are adjusted optimally depending on the surface friction and whether the target is a raised asperity or a recessed indentation.  相似文献   

20.
Transferring information about object weight between hands for use in scaling prehension forces likely depends on the integrity of the structures linking the two sides of the brain. It is unknown whether healthy older adults, who demonstrate a modest decline in this connectivity, transfer fingertip force scaling for object weight between hands. In the present study, healthy older and young adults performed two tasks: gripping and lifting an object, and a ballistic finger abduction movement. For the grip and lift task, participants practiced lifting a novel object using a precision pinch grip with the right hand (RH) and then did so again with the left hand (LH). For the ballistic task, participants were trained to maximally accelerate the right index finger by abducting it. On the grip and lift task, all participants appeared to overestimate the object weight during the 1st RH lift, followed by a progressive reduction on successive lifts. This adaptation was transferred to the LH in both groups on their first lift and remained stable over subsequent lifts. In contrast, the training-induced peak abduction acceleration on the ballistic task transferred poorly to the LH in older with considerably better transfer in young adults. We conclude that the memory representations scaling the lift force for the grip and lift task generalized to the untrained hand, while the greater acceleration that was acquired during practice of the ballistic task showed an incomplete transfer to the opposite hand. These differences may indicate task-dependent interhemispheric transfer of learning in old age.  相似文献   

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