Contact-evoked changes in EMG activity during human grasp

Classifications of Dynamic EMG in Hand Gesture and Unsupervised Grasp Motion Segmentation

The electromyography (EMG) signals have been widely utilized in human-robot interaction for extracting user hand/arm motion instructions. A major challenge of the online interaction with robots is the reliable EMG recognition from real-time data. However, previous studies mainly focused on using steady-state EMG signals with a small number of grasp patterns to implement classification algorithms, which is insufficient to generate robust control regarding the dynamic muscular activity variation in practice. Introducing more EMG variability during training and validation could implement a better dynamic-motion detection, but only limited research focused on such grasp-movement identification, and all of those assessments on the non-static EMG classification require supervised ground-truth label of the movement status. In this study, we propose a framework for classifying EMG signals generated from continuous grasp movements with variations on dynamic arm/hand postures, using an unsupervised motion status segmentation method. We collected data from large gesture vocabularies with multiple dynamic motion phases to encode the transitions from one intent to another based on common sequences of the grasp movements. Two classifiers were constructed for identifying the motion-phase label and grasptype label, where the dynamic motion phases were segmented and labeled in an unsupervised manner. The proposed framework was evaluated in real-time with the accuracy variation over time presented, which was shown to be efficient due to the high degree of freedom of the EMG data.

Continuous supplementary tactile feedback can be applied (and then removed) to enhance precision manipulation

Background

Human sensorimotor control of dexterous manipulation relies on afferent sensory signals. Explicit tactile feedback is generally not available to prosthetic hand users, who have to rely on incidental information sources to partly close the control loop, resulting in suboptimal performance and manipulation difficulty. Recent studies on non-invasive supplementary sensory feedback indicated that time-discrete vibrational feedback delivered upon relevant mechanical events outperforms continuous tactile feedback. However, we hypothesize that continuous tactile feedback can be more effective in non-routine manipulation tasks (i.e., tasks where the grip force is modified reactively in response to the sensory feedback due to the unpredictable behavior of the manipulated object, such as picking and holding a virtual fragile object) if delivered to highly sensitive areas. We further hypothesize that this continuous tactile feedback is not necessary during all the duration of the manipulation task, since adaptation occurs.

Methods

We investigated the effectiveness of continuous tactile feedback in precision manipulation, together with a new sensory feedback policy, where the continuous tactile feedback is gradually removed when the grasp reaches a steady state (namely, transient tactile feedback). We carried out an experiment in a virtual-reality setting with custom tactile feedback devices, which can apply continuous pressure and vibrations, attached to the thumb and index finger. We enrolled 24 healthy participants and instructed them to pick and hold a fragile virtual cube without breaking it. We compared their manipulation performance when using four different sensory feedback methods, i.e., no tactile feedback, discrete vibrations, continuous tactile feedback, and transient tactile feedback. The latter consisted of gradually removing the continuous feedback in the static phase of the grasp.

Results

Continuous tactile feedback leads to a significantly larger number of successful trials than discrete vibrational cues and no feedback conditions, yet the gradual removal of the continuous feedback yields to comparable outcomes. Moreover, the participants preferred the continuous stimuli over the vibrational cues and the removal in the static phase did not significantly impact their appreciation of the continuous tactile feedback.

Conclusions

These results advocate for the use of continuous supplementary tactile feedback for fine manipulation control and indicate that it can seamlessly be removed in the static phase of the grasp, possibly due to the mechanism of sensory adaptation. This encourages the development of energy-efficient supplementary feedback devices for prosthetic and telemanipulation applications, where encumbrance and power consumption are burdensome constraints.

Classification of Anticipatory Signals for Grasp and Release from Surface Electromyography

Surface electromyography (sEMG) is a technique for recording natural muscle activation signals, which can serve as control inputs for exoskeletons and prosthetic devices. Previous experiments have incorporated these signals using both classical and pattern-recognition control methods in order to actuate such devices. We used the results of an experiment incorporating grasp and release actions with object contact to develop an intent-recognition system based on Gaussian mixture models (GMM) and continuous-emission hidden Markov models (HMM) of sEMG data. We tested this system with data collected from 16 individuals using a forearm band with distributed sEMG sensors. The data contain trials with shifted band alignments to assess robustness to sensor placement. This study evaluated and found that pattern-recognition-based methods could classify transient anticipatory sEMG signals in the presence of shifted sensor placement and object contact. With the best-performing classifier, the effect of label lengths in the training data was also examined. A mean classification accuracy of 75.96% was achieved through a unigram HMM method with five mixture components. Classification accuracy on different sub-movements was found to be limited by the length of the shortest sub-movement, which means that shorter sub-movements within dynamic sequences require larger training sets to be classified correctly. This classification of user intent is a potential control mechanism for a dynamic grasping task involving user contact with external objects and noise. Further work is required to test its performance as part of an exoskeleton controller, which involves contact with actuated external surfaces.

Genetically identified spinal interneurons integrating tactile afferents for motor control

Our movements are shaped by our perception of the world as communicated by our senses. Perception of sensory information has been largely attributed to cortical activity. However, a prior level of sensory processing occurs in the spinal cord. Indeed, sensory inputs directly project to many spinal circuits, some of which communicate with motor circuits within the spinal cord. Therefore, the processing of sensory information for the purpose of ensuring proper movements is distributed between spinal and supraspinal circuits. The mechanisms underlying the integration of sensory information for motor control at the level of the spinal cord have yet to be fully described. Recent research has led to the characterization of spinal neuron populations that share common molecular identities. Identification of molecular markers that define specific populations of spinal neurons is a prerequisite to the application of genetic techniques devised to both delineate the function of these spinal neurons and their connectivity. This strategy has been used in the study of spinal neurons that receive tactile inputs from sensory neurons innervating the skin. As a result, the circuits that include these spinal neurons have been revealed to play important roles in specific aspects of motor function. We describe these genetically identified spinal neurons that integrate tactile information and the contribution of these studies to our understanding of how tactile information shapes motor output. Furthermore, we describe future opportunities that these circuits present for shedding light on the neural mechanisms of tactile processing.

Direct mapping rather than motor prediction subserves modulation of corticospinal excitability during observation of actions in real time

Motor facilitation refers to the specific increment in corticospinal excitability (CSE) elicited by the observation of actions performed by others. To date, the precise nature of the mechanism at the basis of this phenomenon is unknown. One possibility is that motor facilitation is driven by a predictive process reminiscent of the role of forward models in motor control. Alternatively, motor facilitation may result from a model-free mechanism by which the basic elements of the observed action are directly mapped onto their cortical representations. Our study was designed to discern these alternatives. To this aim, we recorded the time course of CSE for the first dorsal interosseous (FDI) and the abductor digiti minimi (ADM) during observation of three grasping actions in real time, two of which strongly diverged in kinematics from their natural (invariant) form. Although artificially slow movements used in most action observation studies might enhance the observer's discrimination performance, the use of videos in real time is crucial to maintain the time course of CSE within the physiological range of daily actions. CSE was measured at 4 time points within a 240-ms window that best captured the kinematic divergence from the invariant form. Our results show that CSE of the FDI, not the ADM, closely follows the functional role of the muscle despite the mismatch between the natural and the divergent kinematics. We propose that motor facilitation during observation of actions performed in real time reflects the model-free coding of perceived movement following a direct mapping mechanism.

Comparing functional EMG characteristics between zero-order and first-order interface dynamics

The optimal control scheme for powered prostheses can be determined using simulation experiments, for which an accurate model of prosthesis control is essential. This paper focuses on electromyographic (EMG) control signal characteristics across two different control schemes. We constructed a functional EMG model comprising three EMG signal characteristics-standard deviation, kurtosis, and median power frequency-using data collected under realistic conditions for prosthesis control (closed-loop, dynamic, anisometric contractions). We examined how the model changed when subjects used zero-order or first-order control. Control order had a statistically significant effect on EMG characteristics, but the effect size was small and generally did not exceed inter-subject variability. Therefore, we suggest that this functional EMG model remains valid across different control schemes.

Cutaneous sensory feedback plays a critical role in agonist-antagonist co-activation

The purpose of this study was to investigate the role of cutaneous feedback in the agonist-antagonist co-activation mechanism during maximum voluntary force (MVF) production by the fingers. Seventeen healthy male subjects (age: 23.8 ± 1.0 years) were asked to press with maximal effort at their fingertips. Finger forces at the fingertips and muscle activities of the flexor digitorum superficialis (FDS, agonist) and extensor digitorum communis (EDC, antagonist) were recorded using force sensors and electromyography, respectively. There were two experimental conditions: with and without administration of a ring block to the fingers (i.e., anesthesia and normal conditions, or AC and NC, respectively). The ring block was used to deprive cutaneous feedback. Consistent with previous studies, finger MVF decreased significantly in AC compared with NC. Moreover, the force production of non-task fingers significantly increased in AC. Muscle activity of the EDC was significantly lower in AC than in NC; no significant changes in the FDS muscle were observed. The findings of this study show that cutaneous feedback not only increases MVF and force accuracy, but facilitates agonist-antagonist co-activation by increasing antagonist muscle activation. The results of this study imply that cutaneous feedback is linked to both primary and adjacent motor neurons.

Force Regulation of Ankle Extensor Muscle Activity in Freely Walking Cats

To gain insight into the relative importance of force feedback to ongoing ankle extensor activity during walking in the conscious cat, we isolated the medial gastrocnemius muscle (MG) by denervating the other ankle extensors and measured the magnitude of its activity at different muscle lengths, velocities, and forces accomplished by having the animals walk up and down a sloped pegway. Mathematical models of proprioceptor dynamics predicted afferent activity and revealed that the changes in muscle activity under our experimental conditions were strongly correlated with Ib activity and not consistently associated with changes in Ia or group II activity. This allowed us to determine the gains within the force feedback pathway using a simple model of the neuromuscular system and the measured relationship between MG activity and force. Loop gain increased with muscle length due to the intrinsic force–length property of muscle. The gain of the pathway that converts muscle force to motoneuron depolarization was independent of length. To better test for a causal relationship between modulation of force feedback and changes in muscle activity, a second set of experiments was performed in which the MG muscle was perturbed during ground contact of the hind foot by dropping or lifting the peg underfoot. Collectively, these investigations support a causal role for force feedback and indicate that about 30% of the total muscle activity is due to force feedback during level walking. Force feedback's role increases during upslope walking and decreases during downslope walking, providing a simple mechanism for compensating for changes in terrain.

Role of the primary motor and sensory cortex in precision grasping: a transcranial magnetic stimulation study

Human precision grip requires precise scaling of the grip force to match the weight and frictional conditions of the object. The ability to produce an accurately scaled grip force prior to lifting an object is thought to be the result of an internal feedforward model. However, relatively little is known about the roles of various brain regions in the control of such precision grip-lift synergies. Here we investigate the role of the primary motor (M1) and sensory (S1) cortices during a grip-lift task using inhibitory transcranial magnetic theta-burst stimulation (TBS). Fifteen healthy individuals received 40 s of either (i) M1 TBS, (ii) S1 TBS or (iii) sham stimulation. Following a 5-min rest, subjects lifted a manipulandum five times using a precision grip or completed a simple reaction time task. Following S1 stimulation, the duration of the pre-load phase was significantly longer than following sham stimulation. Following M1 stimulation, the temporal relationship between changes in grip and load force was altered, with changes in grip force coming to lag behind changes in load force. This result contrasts with that seen in the sham condition where changes in grip force preceded changes in load force. No significant difference was observed in the simple reaction task following either M1 or S1 stimulation. These results further quantify the contribution of the M1 to anticipatory grip-force scaling. In addition, they provide the first evidence for the contribution of S1 to object manipulation, suggesting that sensory information is not necessary for optimal functioning of anticipatory control.

Is the brain able to capture a new temporal relationship between a motor action and its consequence?

This study investigated how the brain can learn new temporal relationships between the predicted and actual consequences of a descending motor command. We chose a long and a short delay to assess cognitive and sensorimotor adaptation, respectively. Cognitive adaptation, in contrast to sensorimotor adaptation, would use higher cognitive mechanisms requiring attention and working memory associated with temporal processing. A horizontal cable, attached at pelvis level, was used to suspend a load that tended to move participants backwards. In experiment 1, the participants (n = 10) controlled unloading of the cable, which induced a forward destabilization. Three blocks of trials were performed. The first (PRE) and third (POST) blocks contained 30 trials each. For these conditions, the mechanical consequences of cable unloading were directly transmitted to the participants. The second block (DELAY) contained 60 trials. In this condition, a constant 600-ms delay was introduced between the motor action and the mechanical consequences of unloading. To determine whether the duration of the delay or the number of trials could explain the absence of prediction, we tested eight different participants (experiment 2). These participants first performed 15 imposed unloading trials (IMPOSED). In this condition, the experimenter controlled the onset of the unloading. Then, the participants were exposed to 120 self-triggered trials; the mechanical consequences of cable unloading were directly transmitted to the participants. Finally, four participants were exposed to a delay of 300 ms, whereas the other four were exposed to a delay of 600 ms. All the participants performed 120 delayed trials. Results of the first experiment revealed that when the participants controlled the timing of unloading and there was no delay, they activated their gastrocnemius (GM) muscle before unloading (120 ms and 126 ms for PRE and POST condition, respectively). When the unloading occurred 600 ms later, however, participants did not anticipate the unloading; their GM muscle onset followed the unloading. Across trials, however, they adapted their balance strategy as they decreased the activity of their tibialis anterior, and GM muscle onset occurred earlier than during imposed unloading. Nonetheless, overall participants succeeded in decreasing their peak center of mass velocity and center of mass and center of pressure ranges. Results from the second experiment showed that GM muscle onset occurred earlier than during imposed unloading for both delayed unloadings. Moreover, data for the shorter delay (i.e., 300 ms) showed that participants adapted their balance strategy response; their GM muscle onset occurred before the delayed unloading. Although, increasing the number of 600 ms delayed unloading (i.e., 60 to 120 trials) did not allow the brain to anticipate the unloading. In summary, the results of the present study suggest that the brain could adjust to some extent its balance control strategy when the temporal relationship between the motor action and its consequence was delayed. Nonetheless, cognitive adaptation would be limited compared to sensorimotor adaptation.

Activity performance and upper extremity function in cervical spinal cord injury patients according to the Klein-Bell ADL Scale

STUDY DESIGN

Cross-sectional study.

OBJECTIVES

(1) To examine whether the Klein-Bell ADL Scale (K-B Scale) discriminates cervical spinal cord injury (SCI) patients in daily activities and to explore its applicability in this group of patients. (2) To examine the association between basic ADL and upper extremity function. (3) To investigate if grip ability can be discerned in the scale.

SETTING

Spinal Cord Injury Unit, Sahlgrenska University Hospital, Göteborg, Sweden.

METHODS

Fifty-five patients with cervical SCI with no prior reconstructive hand surgery were included in the study. Analyses of the patient's independence were made according to the K-B Scale. Three additional analyses were carried out, the first examined whether the use of assistive devices and house and car adaptations influenced independence. The last two used different approaches to investigate whether arm and grip function could be detected in the K-B scale.

RESULTS

Raw score in the K-B Scale can discriminate for independence in daily activities but the scale's weight scheme does not function for cervical SCI patients. Assistive devices and car and house adaptations can compensate for dependence in daily activities. Lack of grip function decreases the patient's ability to become independent. Diagnosis-related activities cannot be assessed in all items.

CONCLUSION

The K-B Scale's raw score was useful assessing daily activities in cervical SCI patients. Its reliability in conjunction with arm and grip function in patients with cervical SCI has yet to be proven.

Influence of tactile afferents on the coordination of muscles during a simulated precision grip

The mechanisms by which the nervous system coordinates multiple muscles for the control of finger movements are not well understood. One possibility is that groups of muscles may be enlisted into synergies by last-order inputs that project across multiple motor nuclei. In this study we investigated the role that tactile input might play in coupling together the activities of motor units in two muscles involved in generating the precision grip. Cross-correlation analysis was used to assess the degree of synchrony in the discharge times of pairs of motor units recorded from index-finger and thumb flexor muscles while human subjects performed an isometric task that mimicked a precision grip. The magnitude of synchrony is thought to reflect the extent to which divergent last order inputs provide common synaptic input across motor neurons. Synchrony was evaluated under two simulated-gripping conditions: gripping with normal tactile input and gripping when tactile input from the digit pads was eliminated by applying flexion forces to fittings glued to the finger nails. Synchrony between motor units of index finger flexor and thumb flexor muscles, while substantial, was not significantly different across the two tactile-input conditions. These findings suggest that tactile input is not required to activate the divergent last-order inputs that couple together the activities of the index finger and thumb flexor muscles during the precision grip.

The impact of hand dominance and ulnar and median nerve impairment on strength and basic daily activities

The authors sought to determine the impact of hand dominance and its relation to function among ulnar alone and combined ulnar and median nerve lesions in leprosy patients. The study sample consisted of 62 persons affected with leprosy (mean age 36.1 years) who were either suffering from ulnar nerve lesions alone (34 patients) or combined ulnar and median lesions (28 patients). The data from the earlier work of this institution with normal hands in the adjacent rural uninvolved population were taken as controls. Grip, pulp-to-pulp, lateral, and three-jaw-chuck pinch strengths were measured using a hydraulic hand dynamometer and a pinch gauge. Basic daily activities were assessed using the battery developed at Karigiri. Means, standard errors, correlations, and t-tests were used in the analyses. The overall difference between grip and pinch strength measurements in both dominant and nondominant hands was 40% less than those in normal hands if the ulnar nerve alone was involved. If impairment of this nerve was combined with that of the median nerve, the overall difference in strength measurements increased to 51%. The hand strength of the dominant side was statistically significant between single and two nerves for pulp-to-pulp pinch (p = 0.019). No other strength tests produced statistically significantly results related to hand dominance (grip strength, lateral, chuck pinch) with either one or two nerve involvement. To observe differences in basic daily activities, scores were compared to Karigiri norms. The bilateral basic daily activities score was impaired by 45% compared with norms with only ulnar nerve involvement and by 59% with ulnar and median nerve involvement. The different pinch strengths related to basic daily activities was not significantly affected in nondominant hands, whether it was ulnar alone or combined ulnar and median nerve lesions. In this population diagnosed with leprosy, ulnar nerve impairment at the level of the elbow reduced the grip and pinch strengths and performance in basic daily activities by 40% and 45%, respectively. If combined with median nerve lesion at wrist level, the strengths and daily activity losses increased to 50% and 60%, respectively. There is no effect on grip strength either in ulnar or ulnar and median nerve injuries, irrespective of whether dominant or nondominant hands were involved. Different pinch strengths related to basic daily activities were significantly affected only in the dominant hand with involvement of these nerves.

The Effect of Low Force Chiropractic Adjustments for 4 Weeks on Body Surface Electromagnetic Field

OBJECTIVE

To study the effects of 4 weeks of low-force chiropractic adjustments on body surface electromagnetic fields (EMFs).

METHOD

Thirty-five chiropractic students randomly assigned into control (17 subjects) and experimental groups (28 subjects). A triaxial fluxgate magnetometer was used for EMF detection. The subjects' body surface EMF was determined in the prone position before and after the chiropractic adjustment. A Toftness low-force chiropractic adjustment was applied to the cervical, thoracic, lumbar, and sacral areas as determined by the practitioner. Heart rate variability analysis was recorded once a week to determine autonomic nervous system activity in both the control and experimental groups.

RESULTS

The EMF on the subjects' body surface decreased after chiropractic adjustment at the cervical, thoracic, lumbar, and sacral regions in all 6 visits during the 4-week treatment period. The EMF showed a downtrend over the 4-week period after the low-force adjustment. The same changes were not observed in the control group. The chiropractic adjustment group had a slight decrease in heart rate over the 4-week treatment period, and no significant change was observed in the control group. Heart rate variability analysis did not show consistent changes before and after the low-force adjustments during the treatment period.

CONCLUSION

Low-force chiropractic adjustment in the cervical and thoracic areas resulted in a consistent reduction of the body surface EMF after 4 weeks of active treatment. No statistically significant differences were found in the heart rate and heart rate variability in the 4-week study.

Human tactile perception as a standard for artificial tactile sensing—a review

In this paper, we examine the most important features of human skin tactile properties with special emphasis on the characteristics which are vital in the design of artificial systems. Contrary to the visual and auditory senses, the touch signal is not a well-defined quantity. As a result, the researchers of this field are still dealing with the basics of collecting the most relevant data. Following this, mimicking the sense of touch by producing artificial tactile skin is a challenging process. Although the sense of touch is widely distributed all over the human body, the tactile perception in the human hand is of great importance in terms of surgical and medical robotics applications. In this study, the role of various mechanoreceptors in the human hand, such as, RA, SA I, SA II, and PC units are discussed in relation to the stimuli like force, position, softness, and surface texture. Taking human hand as a suitable tactile model, the necessary engineering features of an artificial tactile sensor, such as, spatial and temporal resolutions, force sensitivity, and linearity, are being reviewed. In this work, we also report on the current and possible future applications of tactile sensors in various surgical procedures.

Patterns of muscle activity underlying object-specific grasp by the macaque monkey

During object grasp, a coordinated activation of distal muscles is required to shape the hand in relation to the physical properties of the object. Despite the fundamental importance of the grasping action, little is known of the muscular activation patterns that allow objects of different sizes and shapes to be grasped. In a study of two adult macaque monkeys, we investigated whether we could distinguish between EMG activation patterns associated with grasp of 12 differently shaped objects, chosen to evoke a wide range of grasping postures. Each object was mounted on a horizontal shuttle held by a weak spring (load force 1-2 N). Objects were located in separate sectors of a "carousel," and inter-trial rotation of the carousel allowed sequential presentation of the objects in pseudorandom order. EMG activity from 10 to 12 digit, hand, and arm muscles was recorded using chronically implanted electrodes. We show that the grasp of different objects was characterized by complex but distinctive patterns of EMG activation. Cluster analysis shows that these object-related EMG patterns were specific and consistent enough to identify the object unequivocally from the EMG recordings alone. EMG-based object identification required a minimum of six EMGs from simultaneously recorded muscles. EMG patterns were consistent across recording sessions in a given monkey but showed some differences between animals. These results identify the specific patterns of activity required to achieve distinct hand postures for grasping, and they open the way to our understanding of how these patterns are generated by the central motor network.

The effects of digital anesthesia on force control using a precision grip

A total of 20 right-handed subjects were asked to perform a grasp-lift-and-hold task using a precision grip. The grasped object was a one-degree-of-freedom manipuladum consisting of a vertically mounted linear motor capable of generating resistive forces to simulate a range of object weights. In the initial study, seven subjects (6 women, 1 man; ages 24-56 yr) were first asked to lift and hold the object stationary for 4 s. The object presented a metal tab with two different surface textures and offered one of four resistive forces (0.5, 1.0, 1.5, and 2.0 N). The lifts were performed both with and without visual feedback. Next, the subjects were asked to perform the same grasping sequence again after ring block anesthesia of the thumb and index finger with mepivacaine. The objective was to determine the degree to which an internal model obtained through prior familiarity might compensate for the loss of cutaneous sensation. In agreement with previous studies, it was found that all subjects applied significantly greater grip force after digital anesthesia, and the coordination between grip and load forces was disrupted. It appears from these data, that the internal model alone is insufficient to completely compensate for the loss of cutaneous sensation. Moreover, the results suggest that the internal model must have either continuous tonic excitation from cutaneous receptors or at least frequent intermittent reiteration to function optimally. A subsequent study performed with 10 additional subjects (9 women, 1 man; ages 24-49 yr) indicated that with unimpaired cutaneous feedback, the grasping and lifting forces were applied together with negligible forces and torques in other directions. In contrast, after digital anesthesia, significant additional linear and torsional forces appeared, particularly in the horizontal and frontal planes. These torques were thought to arise partially from the application of excessive grip force and partially from a misalignment of the two grasping fingers. These torques were further increased by an imbalance in the pressure exerted by the two opposing fingers. Vision of the grasping hand did not significantly correct the finger misalignment after digital anesthesia. Taken together, these results suggest that mechanoreceptors in the fingertips signal the source and direction of pressure applied to the skin. The nervous system uses this information to adjust the fingers and direct the pinch forces optimally for grasping and object manipulation.

Importance of cutaneous feedback in maintaining a secure grip during manipulation of hand-held objects

Previous research has shown that grip and load forces are modulated simultaneously during manipulation of a hand-held object. This close temporal coupling suggested that both forces are controlled by an internal model within the CNS that predicts the changes in tangential force on the fingers. The objective of the present study was to examine how the internal model would compensate for the loss of cutaneous sensation through local anesthesia of the index and thumb. Ten healthy adult subjects (5 men and 5 women aged 20-57 yr) were asked to grasp, lift, and hold stationary, a 250 g object for 20 s. Next, the subjects were asked to perform vertical oscillatory movements over a distance of 20 cm at a rate of 1.0 Hz for 30 s. Eleven trials were performed with intact sensation, and 11 trials after a local ring-block anesthesia of the index and thumb with bupivacain (5 mg/ml). During static holding, loss of cutaneous sensation produced a significant increase in the safety margin. However, the grip force declined significantly over the 20-s static hold period. During oscillatory arm movements, grip and load forces were continuously modulated together in a predictive manner as suggested by Flanagan and Wing. Again, the grip force declined over the 30-s movement, and 7/10 subjects dropped the object at least once. With intact sensation, the object was never dropped; but with the fingers anesthetized, it was dropped on 36% of the trials, and a significant slip occurred on a further 12%. The mean correlation between the grip and load forces for all subjects deteriorated from 0.71 with intact sensation to 0.48 after digital anesthesia. However, a cross-correlation calculated between the grip and load forces indicated that the phase lag was approximately zero both with and without digital anesthesia. Taken together, the data from the present study suggest that cutaneous afferents are required for setting and maintaining the background level of the grip force in addition to their phasic slip-detection function and their role in adapting the grip force/load force ratio to the friction on initial contact with an object. Finally, at a more theoretical level, they correct and maintain an internal model of the physical properties of hand-held objects.

Visual and tactile information about object-curvature control fingertip forces and grasp kinematics in human dexterous manipulation

Most objects that we manipulate have curved surfaces. We have analyzed how subjects during a prototypical manipulatory task use visual and tactile sensory information for adapting fingertip actions to changes in object curvature. Subjects grasped an elongated object at one end using a precision grip and lifted it while instructed to keep it level. The principal load of the grasp was tangential torque due to the location of the center of mass of the object in relation to the horizontal grip axis joining the centers of the opposing grasp surfaces. The curvature strongly influenced the grip forces required to prevent rotational slips. Likewise the curvature influenced the rotational yield of the grasp that developed under the tangential torque load due to the viscoelastic properties of the fingertip pulps. Subjects scaled the grip forces parametrically with object curvature for grasp stability. Moreover in a curvature-dependent manner, subjects twisted the grasp around the grip axis by a radial flexion of the wrist to keep the desired object orientation despite the rotational yield. To adapt these fingertip actions to object curvature, subjects could use both vision and tactile sensibility integrated with predictive control. During combined blindfolding and digital anesthesia, however, the motor output failed to predict the consequences of the prevailing curvature. Subjects used vision to identify the curvature for efficient feedforward retrieval of grip force requirements before executing the motor commands. Digital anesthesia caused little impairment of grip force control when subjects had vision available, but the adaptation of the twist became delayed. Visual cues about the form of the grasp surface obtained before contact was used to scale the grip force, whereas the scaling of the twist depended on visual cues related to object movement. Thus subjects apparently relied on different visuomotor mechanisms for adaptation of grip force and grasp kinematics. In contrast, blindfolded subjects used tactile cues about the prevailing curvature obtained after contact with the object for feedforward adaptation of both grip force and twist. We conclude that humans use both vision and tactile sensibility for feedforward parametric adaptation of grip forces and grasp kinematics to object curvature. Normal control of the twist action, however, requires digital afferent input, and different visuomotor mechanisms support the control of the grasp twist and the grip force. This differential use of vision may have a bearing to the two-stream model of human visual processing.

Impaired proactive and reactive grip force control in chronic hemiparetic patients

OBJECTIVES

The aim of the study was to test manipulative capacities of hemiparetic patients with partial recovery in a drawer task. The main objective was to assess adjustments of grip force in the face of load perturbations.

METHODS

The task was to pull and to hold the drawer manipulandum during predictable or unpredictable perturbations with short (90 ms) load pulses (factor set).

RESULTS

The following novel observations were made. (1) Load pulses elicited, at a latency of about 70 ms, a transient grip force response and a corresponding phasic EMG response. These reactive adjustments were larger during holding than during pulling (factor task). In patients, the reactive grip force adjustments and the EMG response in the grip muscles were reduced. (2) The above deficit was set-dependent. (3) With regular perturbations, grip force was scaled already before perturbation onset. This proactive adjustment was greatly reduced in the patient group. (4) Coordination between grip force and pull force before onset of the perturbation was also disturbed in the patients who generated less grip force per unit pull force than control subjects.

CONCLUSIONS

It is concluded that the patients had difficulties in adapting proactively and reactively to external load disturbances, in addition to their hand weakness.

Ankle restraint modifies motility at E12 in chick embryos

The chick's relationship to its environment changes dramatically over 21 days of embryonic development. At early ages embryos are buoyant; their posture and movements are relatively unconstrained. As embryos grow and fluid level in ovo decreases, movements are increasingly constrained by gravitational forces and reactive forces due to body contact with the shell wall. The issue of how age-related changes in the constraints on movement in ovo may affect embryonic motility is addressed in this paper. Our long-term goal is to determine whether experience imposed by these conditions contributes to development of posthatching motor behaviors. Because previous work indicated that parameters of motility can be modified by a reduction in buoyancy at embryonic day (E) 9, we sought to determine whether a restraint localized to a single joint could also alter either the episodic distribution of activity or the spatiotemporal patterns of limb movement at either E9 or E12. Thus a restraint was applied to the right ankle of embryos prepared for kinematic recordings. Video and kinematic analyses indicated that the restraint had minimal effect at E9, but significantly modified several motility parameters in both the wing and leg at E12. Ankle restraint decreased episode duration. Restraint also decreased most joint excursion parameters, including excursion range, cycles per sequence, and excursion velocity. Restraint increased cycle period duration and signal frequency content under 1.0 Hz. Parameters of intralimb and interlimb coordination exhibited small mixed effects. Results provide support for the hypothesis that environmental conditions contribute to features of embryonic motility. Further, significant modifications of wing excursions in ankle restrained embryos suggest that sensory feedback arising from mechanical perturbations of leg movements may entrain rostral spinal circuits for preservation of interlimb coordination at E12. Potential mechanisms and implications are discussed.