The effects of 72 h of dynamic ankle immobilization on neural excitability and lower extremity kinematics.

BACKGROUND: Ankle injuries can foster maladaptive changes in nervous system function that predisposes patients to subsequent injury. Patients are often placed in a dynamic boot immobilizer (BI) following injury; however, little is known about the effects of this treatment on neuromechanical function. RESEARCH QUESTION: We aimed to determine the effect of 72 h of BI-use on neural excitability and lower extremity joint motion in a healthy cohort. METHODS: Twelve uninjured individuals (20.8 ± 1.4 yrs, 1.7 ± 0.1 m, 75.2 ± 9.9 kg) participated in this crossover study. Neural excitability and lower extremity kinematics were assessed before and after 72 h of BI or compression sock (CS) use. Neural excitability was assessed via the Hoffmann (H) reflex and transcranial magnetic stimulation of the motor cortex by measuring muscle activation at the tibialis anterior, peroneus longus, and soleus of the immobilized extremity. Three-dimensional lower extremity joint angles were assessed while participants walked on a treadmill. Repeated-measures analyses of variance detected changes in neural excitability and peak joint angles across time-points and testing conditions, while statistical parametric mapping (SPM) was implemented to determine continuous joint angle changes (α = 0.05). RESULTS: Pre-BI to post-BI, HMax:MMax ratio (F = 6.496; p = 0.031) significantly decreased. The BI did not alter resting motor threshold (F = 0.601; p = 0.468), or motor evoked potential amplitudes (F > 2.82; p > 0.608). Significant changes in peak knee and hip angles in the frontal and transverse planes were observed (p < 0.05), with no changes at the ankle. SPM analyses revealed significant hip and knee changes in range of motion (p < 0.05). SIGNIFICANCE: Decreased measures of reflex but not corticospinal excitability suggest that BI-use for 72 h unloaded the joint enough to generate peripheral changes, but not the CNS, as has been described in casting models. Further, kinematic changes were observed in proximal lower extremity joints, likely due to swing-phase adaptations while wearing the BI.

Short and long-term effects of gait retraining using real-time biofeedback to reduce knee hyperextension pattern in young women.

The use of real-time biofeedback has been shown to enable individuals to make changes to their gait patterns. It remains unknown whether the short-term improvements reported in previous studies are retained in the longer term. In this study, the paradigm used to investigate the short and long-term effects of real-time biofeedback was modifying knee range of motion during gait to prevent knee hyperextension in women. The purpose of this study was to investigate the short-term (1-month follow up) and long-term (8-month follow up) effects of a gait retraining program using real-time biofeedback to correct knee hyperextension in young women. Seventeen healthy women, ages 18-35 years, with asymptomatic knee hyperextension underwent a three-week (6 sessions) treadmill gait retraining program. Real-time feedback of kinematic data (Visual 3D) was provided during treadmill training. Knee extension range of motion was monitored during overground gait evaluations and training sessions. Gait evaluations were performed pretraining, posttraining (2days after), and 1-month, and 8-month after the last training session. This study showed significant reduction in knee hyperextension patterns immediately following training (mean±SD, 10.9°±4°), and at 1-month (7.5°±5°) and 8-month (6.3°±3.5°) follow ups. There was an increase in knee extension between posttraining and 1-month follow up (3.4°±5°). Reduction in knee hyperextension range of motion was retained at 8-month follow up evaluation. The present study shows the effects of real-time biofeedback in facilitating the acquisition and retention of proficiency in reducing knee hyperextension gait patterns, documenting that the retention is sustained for up to 8 months.

Recent History of Effector Use Modulates Practice-Dependent Changes in Corticospinal Excitability but Not Motor Learning.

BACKGROUND: The theory of homeostatic metaplasticity has significant implications for human motor cortical plasticity and motor learning. Previous work has shown that the extent of recent effector use before exogenously-induced plasticity can affect the direction, magnitude and variability of aftereffects. However, the impact of recent effector use on motor learning and practice-dependent plasticity is not known. HYPOTHESIS: We hypothesized that reducing effector use for 8 hours via hand/wrist immobilization would facilitate practice-dependent changes in corticospinal excitability and TMS-evoked thumb movement kinematics, while also promoting 24-hour retention of a ballistic motor skill. METHODS: Subjects participated in a crossover study involving two conditions. During the immobilization condition, subjects wore a splint that restricted motion of the left hand and thumb for 8 hours. While wearing the splint, subjects were instructed to avoid using their left hand as much as possible. During the control condition, subjects did not wear a splint at any time nor were they instructed to avoid hand use. After either an 8 hour period of immobilization or normal hand use, we collected MEP and TMS-evoked thumb movement recruitment curves, and subjects practiced a ballistic motor skill involving rapid thumb extension. After motor practice, MEP and TMS-evoked thumb movement recruitment curves were re-tested. Retention of the motor skill was tested 30 minutes and 24 hours after motor practice. RESULTS: Reduced effector use did not impact pre-practice corticospinal excitability but did facilitate practice-dependent changes in corticospinal excitability, and this enhancement was specific to the trained muscle. In contrast, reducing effector use did not affect practice-dependent changes in TMS-evoked thumb movements nor did it promote acquisition or retention of the skill. Finally, we detected some associations between pre-practice excitability levels, plasticity effects and learning effects, but these did not reach our adjusted criterion for significance. CONCLUSION: Experimentally enhancing practice-dependent changes in corticospinal excitability is not sufficient to promote learning or memory of a ballistic motor skill.

No Enhancement of 24-Hour Visuomotor Skill Retention by Post-Practice Caffeine Administration.

Caffeine is widely consumed throughout the world and appears to indirectly facilitate learning and memory through effects on attention and motivation. Animal work indicates that post-training caffeine administration augments inhibitory avoidance memory, spatial memory, and object memory. In humans, post-training caffeine administration enhances the ability to discern between familiar images and new, similar images. However, the effect of post-training caffeine administration on motor memory has not been examined. Therefore, we tested two groups of low caffeine consumers (average weekly consumption ≤500 mg) in a double-blind, placebo-controlled study involving acquisition of a continuous isometric visuomotor tracking skill. On Day 1, subjects completed 5 blocks (150 repetitions) of training on the continuous isometric visuomotor skill and subsequently ingested either 200 mg of caffeine or placebo. On day 2, subjects completed an additional 5 blocks of training. Day 1 mean performance and performance variability were both similar between groups, suggesting that both groups acquired the motor skill similarly. For mean performance on Day 2, patterns of re-learning, mean performance learning magnitudes, mean performance learning rates, and mean performance retention magnitudes were all similar between groups. For performance variability on Day 2, there was a small trend towards increased variability in the caffeine group during re-learning, but performance variability learning magnitudes and performance variability retention magnitudes did not differ between groups. Because motor skill acquisition can also be conceptualized as a reduction in performance variability, these results suggest that there may be a small negative effect of post-practice caffeine administration on memory of a newly-learned visuomotor skill. Overall, we found no evidence to suggest that post-training caffeine administration enhances 24-hour retention of a newly-learned continuous visuomotor skill, and these results support the notion that memory-enhancing effects of post-training caffeine ingestion may be task-specific.

Hand motor control: maturing an immature science.

In the target article Mark Latash has argued that there is but a single bona-fide theory for hand motor control (referent configuration theory). If this is true, and research is often phenomenological, then we must admit that the science of hand motor control is immature. While describing observations under varying conditions is a crucial (but early) stage of the science of any field, it is also true that the key to maturing any science is to vigorously subject extant theories and budding laws to critical experimentation. If competing theories are absent at the present time is it time for scientists to focus their efforts on maturing the science of hand motor control through critical testing of this long-standing theory (and related collections of knowledge such as the uncontrolled manifold)?

Effects of transcranial direct current stimulation on the control of finger force during dexterous manipulation in healthy older adults.

The contribution of poor finger force control to age-related decline in manual dexterity is above and beyond ubiquitous behavioral slowing. Altered control of the finger forces can impart unwanted torque on the object affecting its orientation, thus impairing manual performance. Anodal transcranial direct current stimulation (tDCS) over primary motor cortex (M1) has been shown to improve the performance speed on manual tasks in older adults. However, the effects of anodal tDCS over M1 on the finger force control during object manipulation in older adults remain to be fully explored. Here we determined the effects of anodal tDCS over M1 on the control of grip force in older adults while they manipulated an object with an uncertain mechanical property. Eight healthy older adults were instructed to grip and lift an object whose contact surfaces were unexpectedly made more or less slippery across trials using acetate and sandpaper surfaces, respectively. Subjects performed this task before and after receiving anodal or sham tDCS over M1 on two separate sessions using a cross-over design. We found that older adults used significantly lower grip force following anodal tDCS compared to sham tDCS. Friction measured at the finger-object interface remained invariant after anodal and sham tDCS. These findings suggest that anodal tDCS over M1 improved the control of grip force during object manipulation in healthy older adults. Although the cortical networks for representing objects and manipulative actions are complex, the reduction in grip force following anodal tDCS over M1 might be due to a cortical excitation yielding improved processing of object-specific sensory information and its integration with the motor commands for production of manipulative forces. Our findings indicate that tDCS has a potential to improve the control of finger force during dexterous manipulation in older adults.

Effects of transcranial direct current stimulation in combination with motor practice on dexterous grasping and manipulation in healthy older adults.

Abstract Transcranial anodal stimulation (tDCS) over primary motor cortex (M1) improves dexterous manipulation in healthy older adults. However, the beneficial effects of anodal tDCS in combination with motor practice on natural and clinically relevant functional manual tasks, and the associated changes in the digit contact forces are not known. To this end, we studied the effects of 20 min of tDCS applied over M1 for the dominant hand combined with motor practice (MP) in a sham-controlled crossover study. We monitored the forces applied to an object that healthy elderly individuals grasped and manipulated, and their performances on the Grooved Pegboard Test and the Key-slot task. Practice improved performance on the Pegboard test, and anodal tDCS + MP improved retention of this performance gain when tested 35 min later, whereas similar performance gains degraded in the sham group after 35 min. Interestingly, grip force variability on an isometric precision grip task performed with visual feedback of precision force increased following anodal tDCS + MP, but not sham tDCS + MP. This finding suggests that anodal tDCS over M1 might alter the descending drive to spinal motor neurons involved in the performance of isometric precision grip task under visual feedback leading to increased fluctuations in the grip force exerted on the object. Our results demonstrate that anodal stimulation in combination with motor practice helps older adults to retain their improved performance on a functionally relevant manual task in healthy older adults.

Transfer of learning between hands to handle a novel object in old age.

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.

Handling objects in old age: forces and moments acting on the object.

We measured the external moments and digit-tip force directions acting on a freely moveable object while it was grasped and manipulated by old (OA) and young (YA) adults. Participants performed a grasp and lift task and a precision orientation (key-slot) task with a precision (thumb-finger) grip. During the grasp-lift task the OA group misaligned their thumb and finger contacts and produced greater grip force, greater external moments on the object around its roll axis, and oriented force vectors differently compared with the YA group. During the key-slot task, the OA group was more variable in digit-tip force directions and performed the key-slot task more slowly. With practice the OA group aligned their digits, reduced their grip force, and minimized external moments on the object, clearly demonstrating that the nervous system monitored and actively manipulated one or more variables related to object tilt. This was true even for the grip-lift task, a task for which no instructions regarding object orientation were given and which could tolerate modest amounts of object tilt without interfering with task goals. Although the OA group performed the key-slot task faster with experience, they remained slower than the YA group. We conclude that with old age comes a reduced ability to control the forces and moments applied to objects during precision grasp and manipulation. This may contribute to the ubiquitous slowing and deteriorating manual dexterity in healthy aging.

Limited persistence of the sensorimotor memory when transferred across prehension tasks.

A recent study has shown that the sensorimotor memory for the fingertip forces used to grasp and lift an object can be shared across two prehension tasks. However, the persistence (or decay) of these memory resources is not known. Reports of within-task sensorimotor memory indicate persistence of lifting forces, with evidence for reduced persistence of grip forces. Here we investigated the temporal dynamics of the transfer of memory related to vertical lifting forces across prehension tasks. Young adult participants in two separate experimental groups first held the object placed on their palm and 'hefted' it (moved it up and down) followed by lifting the object using a precision grip (thumb-finger) with the dominant hand after a delay of 10 s, or 20 min. The Control group lifted the object with the dominant hand using a precision grip and then did so again 20 min later. The Control group used higher load force rates (LFR) for their first lift compared to subsequent lifts, both before and after the 20-min delay. This suggests that the Control group initially overestimated the weight of the object, corrected their LFRs, and then was able to retain this corrected force scaling after the 20-min delay. The Experimental 10-second delay group accurately scaled their LFRs upon their first lift, indicating that they obtained an accurate memory for LFR scaling during hefting, and transferred it to the lift task. In contrast, the Experimental 20-minute delay group was unable to scale their LFRs upon their first lift, as indicated by high LFRs that were no different than those of the Control group. Thus, the memory related to the production of LFR remained stable over 20 min when obtained from the same task, while that obtained from a different task decayed completely within 20 min. This decay may indicate weakened sensorimotor memories related to prehension forces due to its dependence not only on the memory for object mechanical properties, but also on sensory signals generated during the prehension act, along with strong visual prior estimates of a size-weight relationship.

Slowing of dexterous manipulation in old age: force and kinematic findings from the 'nut-and-rod' task.

The task of sliding a nut from a rod has been used to study manual slowing in old age (Smith et al. in Neurology 53:1458-1461, 1999; Neurobiol Aging 26:883-890, 2005). In this experiment, we sought to determine if the age-related slowing in this task occurs with losses of motor precision, as indicated by the forces exerted on the rod. The forces exerted by the nut on the rod were monitored along with the kinematics of the hand in old and young adults while they attempted to lift a nut from three vertically oriented rods of different shape (straight, single curve, double curve). Old adults performed the task 64% slower than young adults for the straight rod, 100% slower for the single-curve rod, and 80% slower for the double-curve rod. Old adults did not differ from the young adults in the amount of force exerted against the rods in the horizontal plane, or in the steadiness of these forces, but exerted greater force impulses in the vertical direction over the course of a trial (359% straight, 236% single curve, 214% double curve) and much more force in the vertical direction (255% straight, 267% single curve, 159% double curve). Old adults also performed the task with 35% greater average roll of the hand into pronation. We suspect that old adults tilted the nut, even for the straight rod, dragging it against the rod to create the elevated vertical forces. These observations support previous speculation that old adults do not control the external moments applied to grasped objects as well as young adults.

Lifting a familiar object: visual size analysis, not memory for object weight, scales lift force.

The brain can accurately predict the forces needed to efficiently manipulate familiar objects in relation to mechanical properties such as weight. These predictions involve memory or some type of central representation, but visual analysis of size also yields accurate predictions of the needed fingertip forces. This raises the issue of which process (weight memory or visual size analysis) is used during everyday life when handling familiar objects. Our aim was to determine if subjects use a sensorimotor memory of weight, or a visual size analysis, to predictively set their vertical lift force when lifting a recently handled object. Two groups of subjects lifted an opaque brown bottle filled with water (470 g) during the first experimental session, and then rested for 15 min in a different room. Both groups were told that they would lift the same bottle in their next session. However, the experimental group returned to lift a slightly smaller bottle filled with water (360 g) that otherwise was identical in appearance to the first bottle. The control group returned to lift the same bottle from the first session, which was only partially filled with water so that it also weighed 360 g. At the end of the second session subjects were asked if they observed any changes between sessions, but no subject indicated awareness of a specific change. An acceleration ratio was computed by dividing the peak vertical acceleration during the first lift of the second session by the average peak acceleration of the last five lifts during the first session. This ratio was >1 for the control subjects 1.30 (SEM 0.08), indicating that they scaled their lift force for the first lift of the second session based on a memory of the (heavier) bottle from the first session. In contrast, the acceleration ratio was 0.94 (0.10) for the experimental group (P < 0.011). We conclude that the experimental group processed visual cues concerning the size of the bottle. These findings raise the possibility that even with familiar objects we predict fingertip forces using an on-line visual analysis of size (along with memory of density), rather than accessing memory related to object weight.

Failure to disrupt the 'sensorimotor' memory for lifting objects with a precision grip.

When repetitively lifting an object with mechanical properties that vary from lift-to-lift, the fingertip forces for gripping and lifting are influenced strongly by the previous lift, revealing a 'sensorimotor' memory. Two recent reports indicate that the sensorimotor memory for grip force is easily disrupted by an unrelated task like a strong pinch or vibration, even when the lift was performed with the hand contralateral to the vibration or preceding pinch. These findings indicate that this memory may reflect sensory input or muscle contraction levels, rather than object properties or the specific task of gripping and lifting. Here we report that the predictive scaling of lift force was not disrupted by conditioning tasks that featured exerting a vertical isometric force with the upper extremity. When subjects lifted a 2 N object repetitively the peak lift force rate was 26.4 N/s. The lift force rate increased to 36.1 N/s when the 2 N object was lifted (regardless of hand) after lifting the 8 N object with the right hand, which reveals the expected 'sensorimotor' memory. The lift force rate did not increase (24.8 vs. 26.4 N/s for the control condition) when a bout of isometric exertion (9.8 N) in the vertical direction with the distal right forearm preceded lifts of the 2 N object. This finding was confirmed with another isometric task designed to more closely mimic lifting an object with a precision grip. This difference in the sensitivity of grip versus lift force to a preceding isometric contraction indicates that separate sensorimotor memories contribute to the predictive scaling of the commands for gripping and lifting an object.

Age-related directional bias of fingertip force.

We studied the direction of the three-dimensional fingertip force vector in young and old adults during a simple pressing task with the index finger. Ten young and ten old subjects pressed against a force plate with their index finger and maintained target forces that ranged from 2.5 to 15 N. Subjects viewed a display of the force normal to the force plate; forces tangential to the force plate were not displayed. Young adults produced fingertip forces that were nearly perpendicular to the plate at all target forces while old adults produced fingertip forces that deviated in the proximal direction of the horizontal plane, and ulnar direction of the vertical plane. The fingertip force deviated from the onset of pressing in both groups, but in young subjects the force aligned to perpendicular within 200 ms. Additional study is required to determine if these biased force vector directions contribute to clumsiness and slowing that are characteristic of fine manipulation in old age.

Can internal models of objects be utilized for different prehension tasks?

We examined if object information obtained during one prehension task is used to produce fingertip forces for handling the same object in a different prehension task. Our observations address the task specificity of the internal models presumed to issue commands for grasping and transporting objects. Two groups participated in a 2-day experiment in which they lifted a novel object (230 g; 1.2 g/cm3). On Day One, the high force group (HFG) lifted the object by applying 10 N of grip force prior to applying vertical lift force. This disrupted the usual coordination of grip and lift forces and represented a higher grip force than necessary. The self-selected force group (SSFG) lifted the object on Day One with no instructions regarding their grip or lift forces. They first generated grip forces of 5.8 N, which decreased to 2.6 N by the 10th lift. Four hours later, they lifted the same object in the manner of the HFG. On Day Two, both groups lifted the same object "naturally and comfortably" with the opposite hand. The SSFG began Day Two using a grip force of 2.5 N, consistent with the acquisition of an accurate object representation during Day One. The HFG began Day Two using accurately scaled lift forces, but produced grip forces that virtually replicated those of the SSFG on Day One. We concur with recent suggestions that separate, independently adapted internal models produce grip and lift commands. The object representation that scaled lift force was not available to scale grip force. Furthermore, the concept of a general-purpose object representation that is available across prehension tasks was not supported.

Distributing vertical forces between the digits during gripping and lifting: the effects of rotating the hand versus rotating the object.

During pinch grip we partition the vertical tangential forces at the digits according to the friction at the grip surfaces, and the mass distribution of the object. However, we cannot predictively partition the vertical forces to adjust to new frictional conditions after viewing a 180-deg rotation of an object with different textures at each grip surface. Hence, the processes that lead to predictive force partitioning may not access object representations, thereby suggesting that these processes are digit-specific. If this is true, then we should fail to predictively partition our fingertip forces when we rotate our hand. We tested this prediction by comparing the effects of object rotation with hand rotation for repeated lifts of an object that had one slippery grip surface and one rough grip surface. Subjects did not predictively redistribute the vertical tangential forces upon grasping the rotated object. Following object rotation, the vertical tangential force trajectories during the first 100 ms after contact indicated that 12/15 subjects failed to anticipate the reversed digit-friction relationships. All subjects appropriately partitioned the vertical tangential forces between the digits by the second lift after object rotation, confirming previous reports that sensory signals update the memory associated with lifting the object. In contrast, after hand rotation, 13/15 subjects anticipated the new digit-friction relationships and upon grasping the object immediately generated a steep rise in the vertical force trajectory at the rough surface. They also delayed the initial rise in vertical tangential force at the digit encountering the low-friction surface by approximately 65 ms. Thus, anticipatory partitioning of vertical fingertip forces is not strictly digit-specific. Internally driven motor plans can access the relevant memories or internal models for predictively partitioning the vertical tangential forces. It is not clear if this process involves rotating internal representations of fingertip force directly, or if the forces are derived after internally rotating a representation of the object. In contrast to the robust effects of vision on reach kinematics, or on wrist and finger configuration, visual signals about object rotation and orientation apparently do not influence vertical tangential fingertip forces.

Sensorimotor memory for fingertip forces: evidence for a task-independent motor memory.

When repetitively lifting an object with randomly varying mechanical properties, the fingertip forces reflect the previous lift. We examined the specificity of this "sensorimotor memory" by observing the effects of an isolated pinch on the subsequent lift of a known object. In this case, the pinch force was unrelated to the fingertip forces necessary to grip the object efficiently. The peak grip force used to lift the test object (4 N weight) depended on the preceding task. Compared with repetitively lifting the 4 N test object, the peak grip force was 2 N greater when a lift of the same object was preceded by a lift in which a hidden mass was attached to the object to increase the weight to 8 N. This 2 N increase in grip force also occurred when subjects lifted the 4 N test object after pinching a force transducer with a force of 8 N. Thus, similar grip forces were stored in sensorimotor memory for both tasks, and reflected subjects' use of 7.9 +/- 1.1 N to lift the 8 N object. Similar effects occurred when the preceding pinch or lift was performed with the opposite hand. The peak lift force was unaffected by the isolated pinch, suggesting that a generalized increase in fingertip and limb forces did not occur. We conclude that the sensorimotor memory is not specific for lifting an object. It is doubtful that this particular memory stores the physical properties of objects or reflects a forward internal model for predictively controlling fingertip forces.

The effects of graded compression of the median nerve in the carpal canal on grip force.

The relationship between tactile hypoesthesia and precision grip force was examined using compression of the median nerve in healthy adults. Hypoesthesia was graded by varying the pressure that an external clamp exerted over the carpal canal. Electrical stimulation of the median nerve in the forearm evoked a compound sensory nerve action potential (SNAP) that we recorded from the digital nerves of the index finger. Clamp pressure was varied to achieve SNAPs that were 75%, 50%, and 25% of precompression amplitude (100%). Grip force and tactile sensibility (Semmes-Weinstein filaments, cotton wisps, sharp/dull) did not change in parallel with reductions of the SNAP. Subjects reported paresthesias at the thumb and index finger at 75% SNAP. Tactile pressure thresholds increased to the clinical range of 'diminished light touch', but subjects detected cotton wisps stroked along the finger. At 75% SNAP grip force did not change compared to 100% SNAP. Simple prehension can proceed efficiently despite these modest reductions in tactile signals. At 50% SNAP the digits remained sensate, but were reported to feel "thick, like cardboard". No subject could detect cotton wisps and tactile thresholds increased by one filament. Sharp/dull distinctions remained. Grip force increased by 55% compared to grip force at SNAPs of 100% and 75%. There were no changes in skin slipperiness, so the increased grip force represented elevated 'safety margin' (grip force exceeding that needed to prevent slip). At 25% SNAP subjects described the skin innervated by the median nerve as feeling "numb", but grip force increased little compared to 50% SNAP. Grip force continued to reflect changes in grip surface friction, and mechanical transients from setting the object on the table triggered coordinated reductions in grip force. We suspect that the loss of information from SA I and FA I, but not FA II, tactile afferents provoked the increased grip force.

Old age impairs the use of arbitrary visual cues for predictive control of fingertip forces during grasp.

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.