Directional tuning of human forearm muscle afferents during voluntary wrist movements

Presynaptic gating of monkey proprioceptive signals for proper motor action

Our rich behavioural repertoire is supported by complicated synaptic connectivity in the central nervous system, which must be modulated to prevent behavioural control from being overwhelmed. For this modulation, presynaptic inhibition is an efficient mechanism because it can gate specific synaptic input without interfering with main circuit operations. Previously, we reported the task-dependent presynaptic inhibition of the cutaneous afferent input to the spinal cord in behaving monkeys. Here, we report presynaptic inhibition of the proprioceptive afferent input. We found that the input from shortened muscles is transiently facilitated, whereas that from lengthened muscles is persistently reduced. This presynaptic inhibition could be generated by cortical signals because it started before movement onset, and its size was correlated with the performance of stable motor output. Our findings demonstrate that presynaptic inhibition acts as a dynamic filter of proprioceptive signals, enabling the integration of task-relevant signals into spinal circuits.

Acute effects of muscle vibration on elbow joint position sense in healthy young men: A randomized trial

BACKGROUND

The study aimed to investigate and compare the acute effects of 2 local vibration frequencies (63 Hz vs 42 Hz frequencies) applied to the biceps brachii muscles on the elbow joint position sense (JPS) in healthy young men.

METHODS

A 3-arm parallel-group design with randomization of participants was used. Forty-five healthy young men aged 19 to 30 years were randomly assigned to 3 groups: to receive 63 Hz (n = 15) in experimental group 1 (EG1) or 42 Hz (n = 15) in experimental group 2 (EG2) or sham vibration in the control group (n = 15). Participants in the EG1 and EG2 received 5 bouts of 1-minute vibration exposure to the biceps brachii muscle, with a 1-minute rest between the bouts. In control group, sham vibration was applied with the same duration and interval as in EG1 and EG2. The active elbow joint position error was selected as an outcome measure to assess elbow JPS. The target angle and the angle reproduced were measured using a Universal Goniometer. The difference between the target and the reproduced angles was calculated to determine active elbow joint position error. Measurements were made before the vibration application and right after it.

RESULTS

No statistically significant difference was observed in the JPS of the elbow joint over time in any group (P = .625). No statistically significant differences were observed between the 33 groups in the JPS of the elbow (P = .075).

CONCLUSION

There was no acute effect of vibration of the biceps brachii muscle at 63 and 42 Hz on active elbow JPS in healthy adults.

A Biorealistic Computational Model Unfolds Human-Like Compliant Properties for Control of Hand Prosthesis

Objective: Human neuromuscular reflex control provides a biological model for a compliant hand prosthesis. Here we present a computational approach to understanding the emerging human-like compliance, force and position control, and stiffness adaptation in a prosthetic hand with a replica of human neuromuscular reflex. Methods: A virtual twin of prosthetic hand was constructed in the MuJoCo environment with a tendon-driven anthropomorphic hand structure. Biorealistic mathematic models of muscle, spindle, spiking-neurons and monosynaptic reflex were implemented in neuromorphic chips to drive the virtual hand for real-time control. Results: Simulation showed that the virtual hand acquired human-like ability to control fingertip position, force and stiffness for grasp, as well as the capacity to interact with soft objects by adaptively adjusting hand stiffness. Conclusion: The biorealistic neuromorphic reflex model restores human-like neuromuscular properties for hand prosthesis to interact with soft objects.

Tuning of motor outputs produced by spinal stimulation during voluntary control of torque directions in monkeys

Spinal stimulation is a promising method to restore motor function after impairment of descending pathways. While paresis, a weakness of voluntary movements driven by surviving descending pathways, can benefit from spinal stimulation, the effects of descending commands on motor outputs produced by spinal stimulation are unclear. Here, we show that descending commands amplify and shape the stimulus-induced muscle responses and torque outputs. During the wrist torque tracking task, spinal stimulation, at a current intensity in the range of balanced excitation and inhibition, over the cervical enlargement facilitated and/or suppressed activities of forelimb muscles. Magnitudes of these effects were dependent on directions of voluntarily produced torque and positively correlated with levels of voluntary muscle activity. Furthermore, the directions of evoked wrist torque corresponded to the directions of voluntarily produced torque. These results suggest that spinal stimulation is beneficial in cases of partial lesion of descending pathways by compensating for reduced descending commands through activation of excitatory and inhibitory synaptic connections to motoneurons.

Three-Dimensional Assessment of Upper Limb Proprioception via a Wearable Exoskeleton

Proprioception—the sense of body segment’s position and movement—plays a crucial role in human motor control, integrating the sensory information necessary for the correct execution of daily life activities. Despite scientific evidence recognizes that several neurological diseases hamper proprioceptive encoding with consequent inability to correctly perform movements, proprioceptive assessment in clinical settings is still limited to standard scales. Literature on physiology of upper limb’s proprioception is mainly focused on experimental approaches involving planar setups, while the present work provides a novel paradigm for assessing proprioception during single—and multi-joint matching tasks in a three-dimensional workspace. To such extent, a six-degrees of freedom exoskeleton, ALEx-RS (Arm Light Exoskeleton Rehab Station), was used to evaluate 18 healthy subjects’ abilities in matching proprioceptive targets during combined single and multi-joint arm’s movements: shoulder abduction/adduction, shoulder flexion/extension, and elbow flexion/extension. Results provided evidence that proprioceptive abilities depend on the number of joints simultaneously involved in the task and on their anatomical location, since muscle spindles work along their preferred direction, modulating the streaming of sensory information accordingly. These findings suggest solutions for clinical sensorimotor evaluation after neurological disease, where assessing proprioceptive deficits can improve the recovery path and complement the rehabilitation outcomes.

Cuneate nucleus: The somatosensory gateway to the brain

Much remains unknown about the transformation of proprioceptive afferent input from the periphery to the cortex. Until recently, the only recordings from neurons in the cuneate nucleus (CN) were from anesthetized animals. We are beginning to learn more about how the sense of proprioception is transformed as it propagates centrally. Recent recordings from microelectrode arrays chronically implanted in CN have revealed that CN neurons with muscle-like properties have a greater sensitivity to active reaching movements than to passive limb displacement, and we find that these neurons have receptive fields that resemble single muscles. In this review, we focus on the varied uses of proprioceptive input and the possible role of CN in processing this information.

Adaptation of reach action to a novel force-field is not predicted by acuity of dynamic proprioception in either older or younger adults

Healthy ageing involves degeneration of the neuromuscular system which impacts movement control and proprioception. Yet the relationship between these sensory and motor deficits in upper limb reaching has not been examined in detail. Recently, we reported that age-related proprioceptive deficits were unrelated to accuracy in rapid arm movements, but whether this applied in motor tasks more heavily dependent on proprioceptive feedback was not clear. To address this, we have tested groups of younger and older adults on a force-field adaptation task under either full or limited visual feedback conditions and examined how performance was related to dynamic proprioceptive acuity. Adaptive performance was similar between the age groups, regardless of visual feedback condition, although older adults showed increased after-effects. Physically inactive individuals made larger systematic (but not variable) proprioceptive errors, irrespective of age. However, dynamic proprioceptive acuity was unrelated to adaptation and there was no consistent evidence of proprioceptive recalibration with adaptation to the force-field for any group. Finally, in spite of clear age-dependent loss of spatial working memory capacity, we found no relationship between memory capacity and adaptive performance or proprioceptive acuity. Thus, non-clinical levels of deficit in dynamic proprioception, due to age or physical inactivity, do not affect force-field adaptation, even under conditions of limited visual feedback that might require greater proprioceptive control.

Age-Dependent Asymmetry of Wrist Position Sense Is Not Influenced by Stochastic Tactile Stimulation

Stochastic stimulation has been shown to improve movement, balance, the sense of touch, and may also improve position sense. This stimulation can be non-invasive and may be a simple technology to enhance proprioception. In this study, we investigated whether sub-threshold stochastic tactile stimulation of mechanoreceptors reduces age-related errors in wrist position estimation. Fifteen young (24.5±1.5y) and 23 elderly (71.7±7.3y) unimpaired, right-handed adults completed a wrist position gauge-matching experiment. In each trial, the participant's concealed wrist was moved to a target position between 10 and 30° of wrist flexion or extension by a robotic manipulandum. The participant then estimated the wrist's position on a virtual gauge. During half of the trials, sub-threshold stochastic tactile stimulation was applied to the wrist muscle tendon areas. Stochastic stimulation did not significantly influence wrist position sense. In the elderly group, estimation errors decreased non-significantly when stimulation was applied compared to the trials without stimulation [mean constant error reduction Δμ(θconof)=0.8° in flexion and Δμ(θconoe)=0.7° in extension direction, p = 0.95]. This effect was less pronounced in the young group [Δμ(θcony)=0.2° in flexion and in extension direction, p = 0.99]. These improvements did not yield a relevant effect size (Cohen's d < 0.1). Estimation errors increased with target angle magnitude in both movement directions. In young participants, estimation errors were non-symmetric, with estimations in flexion [μ(θconyf)=1.8°, σ(θconyf)=7.0°] being significantly more accurate than in extension [μ(θconye)=8.3°, σ(θconye)=9.3°, p < 0.01]. This asymmetry was not present in the elderly group, where estimations in flexion [μ(θconof)=7.5°, σ(θconof)=9.8°] were similar to extension [μ(θconoe)=7.7°, σ(θconoe)=9.3°]. Hence, young and elderly participants performed equally in extension direction, whereas wrist position sense in flexion direction deteriorated with age (p < 0.01). Though unimpaired elderly adults did not benefit from stochastic stimulation, it cannot be deduced that individuals with more severe impairments of their sensory system do not profit from this treatment. While the errors in estimating wrist position are symmetric in flexion and extension in elderly adults, young adults are more accurate when estimating wrist flexion, an effect that has not been described before.

Seeing Your Foot Move Changes Muscle Proprioceptive Feedback

Multisensory effects are found when the input from single senses combines, and this has been well researched in the brain. Presently, we examined in humans the potential impact of visuo-proprioceptive interactions at the peripheral level, using microneurography, and compared it with a similar behavioral task. We used a paradigm where participants had either proprioceptive information only (no vision) or combined visual and proprioceptive signals (vision). We moved the foot to measure changes in the sensitivity of single muscle afferents, which can be altered by the descending fusimotor drive. Visual information interacted with proprioceptive information, where we found that for the same passive movement, the response of muscle afferents increased when the proprioceptive channel was the only source of information, as compared with when visual cues were added, regardless of the attentional level. Behaviorally, when participants looked at their foot moving, they more accurately judged differences between movement amplitudes, than in the absence of visual cues. These results impact our understanding of multisensory interactions throughout the nervous system, where the information from different senses can modify the sensitivity of peripheral receptors. This has clinical implications, where future strategies may modulate such visual signals during sensorimotor rehabilitation.

Proprioceptive deficits in inactive older adults are not reflected in fast targeted reaching movements

During normal healthy ageing there is a decline in the ability to control simple movements, characterised by increased reaction times, movement durations and variability. There is also growing evidence of age-related proprioceptive loss which may contribute to these impairments. However, this relationship has not been studied in detail for the upper limb. We recruited 20 younger adults (YAs) and 31 older adults (OAs) who each performed 2 tasks on a 2D robotic manipulandum. The first assessed dynamic proprioceptive acuity using active, multi-joint movements constrained by the robot to a pre-defined path. Participants made perceptual judgements of the lateral position of the unseen arm. The second task required fast, accurate and discrete movements to the same targets in the absence of visual feedback of the hand, and without robotic intervention. We predicted that the variable proprioceptive error (uncertainty range) assessed in Task 1 would be increased in physically inactive OAs and would predict increased movement variability in Task 2. Instead we found that physically inactive OAs had larger systematic proprioceptive errors (bias) than YAs (t[33] = 2.8, p = 0.009), and neither proprioceptive uncertainty nor bias was related to motor performance in either age group (all regression model R2 ≤ 0.06). We suggest that previously reported estimates of proprioceptive decline with ageing may be exaggerated by task demands and that the extent of these deficits is unrelated to control of discrete, rapid movement. The relationship between dynamic proprioceptive acuity and movement control in other tasks with greater emphasis on online feedback is still unclear and warrants further investigation.

Firing properties of muscle spindles supplying the intrinsic foot muscles of humans in unloaded and freestanding conditions

We recently developed an approach for recording from muscle spindles in the intrinsic muscles of the foot in freestanding humans by inserting a tungsten microelectrode into the posterior tibial nerve behind the medial malleolus of the ankle. Here we characterize the behavior of muscle spindles in the small muscles of the foot in 1) seated subjects with the leg horizontal and the foot naturally plantarflexed and 2) standing subjects. In the first study, recordings were made from 26 muscle spindle afferents located within flexor digiti minimi brevis ( n = 4), abductor digiti minimi ( n = 3), quadratus plantae ( n = 3), plantar interossei ( n = 4), flexor digitorum brevis ( n = 3), dorsal interossei ( n = 2), and lumbricals ( n = 2), with one each supplying abductor hallucis, adductor hallucis, and flexor hallucis brevis. The identity of another two muscle afferents was unknown. The majority of the units were silent at rest, only seven (27%) being spontaneously active. Because of the anatomic constraints of the foot, some spindles supplying muscles acting on the toes responded to movements of one or more digits. In the second study, 12 muscle spindle afferents were examined during standing. The ongoing discharge of eight spindle afferents covaried with changes in the center of pressure during postural sway. We conclude that the majority of spindle endings in the small muscles of the foot are silent at rest, which may allow them to encode changes in conformation of the foot when it is loaded during standing. Moreover, these muscle spindle afferents can provide useful proprioceptive information during standing and postural sway. NEW & NOTEWORTHY We have characterized the firing properties of muscle spindles in the intrinsic muscles of the human foot for the first time. The majority of the spindle endings are silent in seated subjects, and most fire tonically during standing, their discharge covarying with center of pressure during postural sway. We conclude that spindle endings in the intrinsic muscles of the foot provide useful proprioceptive information during free standing.

Neural Coding of Leg Proprioception in Drosophila

Animals rely on an internal sense of body position and movement to effectively control motor behavior. This sense of proprioception is mediated by diverse populations of mechanosensory neurons distributed throughout the body. Here, we investigate neural coding of leg proprioception in Drosophila, using in vivo two-photon calcium imaging of proprioceptive sensory neurons during controlled movements of the fly tibia. We found that the axons of leg proprioceptors are organized into distinct functional projections that contain topographic representations of specific kinematic features. Using subclass-specific genetic driver lines, we show that one group of axons encodes tibia position (flexion/extension), another encodes movement direction, and a third encodes bidirectional movement and vibration frequency. Overall, our findings reveal how proprioceptive stimuli from a single leg joint are encoded by a diverse population of sensory neurons, and provide a framework for understanding how proprioceptive feedback signals are used by motor circuits to coordinate the body.

Heritability of proprioceptive senses

Heritability studies using the twin model have provided the basis to disentangle genetic and environmental factors that contribute to several complex human traits. However, the relative importance of these factors to individual differences in proprioception is largely unknown despite the fact that proprioceptive senses are of great importance, allowing us to respond to stimuli stemming from the space around us and react to altering circumstances. Hence, a total of 44 healthy male twins (11 MZ and 11 DZ pairs), 19-28 yr old, were examined for movement, position, and force sense at the elbow joint, and their heritability estimates were computed. Results showed that genetic factors explained 1) 72 and 76% of the total variance of movement sense at the start and the end of the movement, respectively, 2) 60 to 77% of the total variance of position sense, depending on the angle of elbow flexion and whether forearm positioning was active or passive, and 3) 73 and 70% of the total variance of the force sense at 90 and 60° of elbow flexion, respectively. It is concluded that proprioception assessed by these conscious sensations is to a substantial degree genetically dependent, with heritability indexes ranging from 0.60 to 0.77, depending on the task. NEW & NOTEWORTHY Proprioceptive acuity varies among people, but it is not known how much of this variability is due to differences in their genes. This study is the first to report that proprioception, expressed as movement sense, position sense, and force sense, is substantially heritable, and it is conceivable that this may have implications for motor learning and control, neural development, and neurorehabilitation.

Functional properties of human muscle spindles

Muscle spindles are ubiquitous encapsulated mechanoreceptors found in most mammalian muscles. There are two types of endings, primary and secondary, and both are sensitive to changes in muscle length and velocity, with the primary endings having a greater dynamic sensitivity. Unlike other mechanoreceptors in the somatosensory system, muscle spindles are unique in possessing motor innervation, via γ-motoneurons (fusimotor neurons), that control their sensitivity to stretch. Much of what we know about human muscles spindles comes from studying the behavior of their afferents via intraneural microelectrodes (microneurography) inserted into accessible peripheral nerves. We review the functional properties of human muscle spindles, comparing and contrasting with what we know about the functions of muscle spindles studied in experimental animals. As in the cat, many human muscle spindles possess a background discharge that is related to the degree of muscle stretch, but mean firing rates are much lower (~10 Hz). They can faithfully encode changes in muscle fascicle length in passive conditions, but higher level extraction of information is required by the central nervous system to measure changes in muscle length during muscle contraction. Moreover, although there is some evidence supporting independent control of human muscle spindles via fusimotor neurons, any effects are modest compared with the clearly independent control of fusimotor neurons observed in the cat.

Robotic assessment of the contribution of motor commands to wrist position sense

Assessing joint position sense for rehabilitation after neurological injury provides a prognostic factor in recovery and long-term functional outcomes. A common method for testing joint position sense involves the active replication of a joint configuration presented via a passive movement. However, recent evidence showed how this sense is mediated by the centrally generated signals of motor command, such that movements produced volitionally may be coded differently from passive movements and accuracy may be different when matching targets presented actively. To verify this hypothesis we asked ten participants to actively replicate a target wrist angle with the help of a visual feedback in two conditions, which differed in the mode of target presentation: active (aaJPM) or passive (paJPM). The accuracy of target matching, directional bias and variability were analyzed, as well as speed and smoothness of the matching movement and criterion movement in the aaJPM. Overall results indicate higher accuracy and lower variability in the paJPM, while directional bias showed the tendency to overshoot the target regardless of condition. The speed did not differ in the two conditions and movements were smoother in the aaJPM, suggesting a higher confidence by participants in their matching ability. In conclusion, this study suggests that motor commands negatively affect the accuracy of joint position sense when matching involves the integration of visual and proprioceptive information.

From the motor cortex to the movement and back again

The motor cortex controls motor behaviors by generating movement-specific signals and transmitting them through spinal cord circuits and motoneurons to the muscles. Precise and well-coordinated muscle activation patterns are necessary for accurate movement execution. Therefore, the activity of cortical neurons should correlate with movement parameters. To investigate the specifics of such correlations among activities of the motor cortex, spinal cord network and muscles, we developed a model for neural control of goal-directed reaching movements that simulates the entire pathway from the motor cortex through spinal cord circuits to the muscles controlling arm movements. In this model, the arm consists of two joints (shoulder and elbow), whose movements are actuated by six muscles (4 single-joint and 2 double-joint flexors and extensors). The muscles provide afferent feedback to the spinal cord circuits. Cortical neurons are defined as cortical "controllers" that solve an inverse problem based on a proposed straight-line trajectory to a target position and a predefined bell-shaped velocity profile. Thus, the controller generates a motor program that produces a task-specific activation of low-level spinal circuits that in turn induce the muscle activation realizing the intended reaching movement. Using the model, we describe the mechanisms of correlation between cortical and motoneuronal activities and movement direction and other movement parameters. We show that the directional modulation of neuronal activity in the motor cortex and the spinal cord may result from direction-specific dynamics of muscle lengths. Our model suggests that directional modulation first emerges at the level of muscle forces, augments at the motoneuron level, and further increases at the level of the motor cortex due to the dependence of frictional forces in the joints, contractility of the muscles and afferent feedback on muscle lengths and/or velocities.

Emergence of gamma motor activity in an artificial neural network model of the corticospinal system

Muscle spindle discharge during active movement is a function of mechanical and neural parameters. Muscle length changes (and their derivatives) represent its primary mechanical, fusimotor drive its neural component. However, neither the action nor the function of fusimotor and in particular of γ-drive, have been clearly established, since γ-motor activity during voluntary, non-locomotor movements remains largely unknown. Here, using a computational approach, we explored whether γ-drive emerges in an artificial neural network model of the corticospinal system linked to a biomechanical antagonist wrist simulator. The wrist simulator included length-sensitive and γ-drive-dependent type Ia and type II muscle spindle activity. Network activity and connectivity were derived by a gradient descent algorithm to generate reciprocal, known target α-motor unit activity during wrist flexion-extension (F/E) movements. Two tasks were simulated: an alternating F/E task and a slow F/E tracking task. Emergence of γ-motor activity in the alternating F/E network was a function of α-motor unit drive: if muscle afferent (together with supraspinal) input was required for driving α-motor units, then γ-drive emerged in the form of α-γ coactivation, as predicted by empirical studies. In the slow F/E tracking network, γ-drive emerged in the form of α-γ dissociation and provided critical, bidirectional muscle afferent activity to the cortical network, containing known bidirectional target units. The model thus demonstrates the complementary aspects of spindle output and hence γ-drive: i) muscle spindle activity as a driving force of α-motor unit activity, and ii) afferent activity providing continuous sensory information, both of which crucially depend on γ-drive.

An Assessment of Six Muscle Spindle Models for Predicting Sensory Information during Human Wrist Movements

Background: The muscle spindle is an important sensory organ for proprioceptive information, yet there have been few attempts to use Shannon information theory to quantify the capacity of human muscle spindles to encode sensory input.

Methods: Computer simulations linked kinematics, to biomechanics, to six muscle spindle models that generated predictions of firing rate. The predicted firing rates were compared to firing rates of human muscle spindles recorded during a step-tracking (center-out) task to validate their use. The models were then used to predict firing rates during random movements with statistical properties matched to the ergonomics of human wrist movements. The data were analyzed for entropy and mutual information.

Results: Three of the six models produced predictions that approximated the firing rate of human spindles during the step-tracking task. For simulated random movements these models predicted mean rates of 16.0 ± 4.1 imp/s (mean ± SD), peak firing rates <50 imp/s and zero firing rate during an average of 25% of the movement. The average entropy of the neural response was 4.1 ± 0.3 bits and is an estimate of the maximum information that could be carried by muscles spindles during ecologically valid movements. The information about tendon displacement preserved in the neural response was 0.10 ± 0.05 bits per symbol; whereas 1.25 ± 0.30 bits per symbol of velocity input were preserved in the neural response of the spindle models.

Conclusions: Muscle spindle models, originally based on cat experiments, have predictive value for modeling responses of human muscle spindles with minimal parameter optimization. These models predict more than 10-fold more velocity over length information encoding during ecologically valid movements. These results establish theoretical parameters for developing neuroprostheses for proprioceptive function.

Changes in the Spinal Neural Circuits are Dependent on the Movement Speed of the Visuomotor Task

Previous studies have shown that spinal neural circuits are modulated by motor skill training. However, the effects of task movement speed on changes in spinal neural circuits have not been clarified. The aim of this research was to investigate whether spinal neural circuits were affected by task movement speed. Thirty-eight healthy subjects participated in this study. In experiment 1, the effects of task movement speed on the spinal neural circuits were examined. Eighteen subjects performed a visuomotor task involving ankle muscle slow (nine subjects) or fast (nine subjects) movement speed. Another nine subjects performed a non-visuomotor task (controls) in fast movement speed. The motor task training lasted for 20 min. The amounts of D1 inhibition and reciprocal Ia inhibition were measured using H-relfex condition-test paradigm and recorded before, and at 5, 15, and 30 min after the training session. In experiment 2, using transcranial magnetic stimulation (TMS), the effects of corticospinal descending inputs on the presynaptic inhibitory pathway were examined before and after performing either a visuomotor (eight subjects) or a control task (eight subjects). All measurements were taken under resting conditions. The amount of D1 inhibition increased after the visuomotor task irrespective of movement speed (P < 0.01). The amount of reciprocal Ia inhibition increased with fast movement speed conditioning (P < 0.01), but was unchanged by slow movement speed conditioning. These changes lasted up to 15 min in D1 inhibition and 5 min in reciprocal Ia inhibition after the training session. The control task did not induce changes in D1 inhibition and reciprocal Ia inhibition. The TMS conditioned inhibitory effects of presynaptic inhibitory pathways decreased following visuomotor tasks (P < 0.01). The size of test H-reflex was almost the same size throughout experiments. The results suggest that supraspinal descending inputs for controlling joint movement are responsible for changes in the spinal neural circuits, and that task movement speed is one of the critical factors for inducing plastic changes in reciprocal Ia inhibition.

Information processing in the hemisphere of the cerebellar cortex for control of wrist movement

A region of cerebellar lobules V and VI makes strong loop connections with the primary motor (M1) and premotor (PM) cortical areas and is assumed to play essential roles in limb motor control. To examine its functional role, we compared the activities of its input, intermediate, and output elements, i.e., mossy fibers (MFs), Golgi cells (GoCs), and Purkinje cells (PCs), in three monkeys performing wrist movements in two different forearm postures. The results revealed distinct steps of information processing. First, MF activities displayed temporal and directional properties that were remarkably similar to those of M1/PM neurons, suggesting that MFs relay near copies of outputs from these motor areas. Second, all GoCs had a stereotyped pattern of activity independent of movement direction or forearm posture. Instead, GoC activity resembled an average of all MF activities. Therefore, inhibitory GoCs appear to provide a filtering function that passes only prominently modulated MF inputs to granule cells. Third, PCs displayed highly complex spatiotemporal patterns of activity, with coordinate frames distinct from those of MF inputs and directional tuning that changed abruptly before movement onset. The complexity of PC activities may reflect rapidly changing properties of the peripheral motor apparatus during movement. Overall, the cerebellar cortex appears to transform a representation of outputs from M1/PM into different movement representations in a posture-dependent manner and could work as part of a forward model that predicts the state of the peripheral motor apparatus.

Sensorimotor control during peripheral muscle vibration: an experimental study

OBJECTIVE

The aims of this study were to determine whether the application of vibration on a postural lower limb muscle altered the sensorimotor control of its joint as measured by isometric force production parameters and to compare present findings with previous work conducted on trunk muscle.

METHODS

Twenty healthy adults were asked to reproduce submaximal isometric plantar flexion under 3 different conditions: no vibration and vibration frequencies of 30 and 80 Hz on the soleus muscle. Time to peak torque, variable error, as well as constant error and absolute error in peak torque were calculated and compared across conditions.

RESULTS

Under vibration, participants were significantly less accurate in the force reproduction task, as they mainly undershot the target torque. Applying an 80-Hz vibration resulted in a significantly higher negative constant error than lower-frequency vibration (30 Hz) or no-vibration condition. Decreases in isometric force production accuracy under vibration influence were also observed in a previous study conducted on trunk muscle. However, no difference in constant error was found between 30- and 80-Hz vibration conditions.

CONCLUSION

The results suggest that acute soleus muscle vibration interferes with plantar flexion torque generation by distorting proprioceptive information, leading to decreases in accuracy of a force reproduction task. Similar results in an isometric trunk extension force reproduction task were found with vibration applied on erector spinae muscle. However, high-frequency vibration applied on soleus muscle elicited higher force reproduction errors than low-frequency stimulation.

Context-dependent inhibition of unloaded muscles during the long-latency epoch

A number of studies have highlighted the sophistication of corrective responses in lengthened muscles during the long-latency epoch. However, in various contexts, unloading can occur, which requires corrective actions from a shortened muscle. Here, we investigate the sophistication of inhibitory responses in shortened muscles due to unloading. Our first experiment quantified the inhibitory responses following an unloading torque that displaced the hand either into or away from a peripheral target. We observed larger long-latency inhibitory responses when perturbed into the peripheral target compared with away from the target. In our second experiment, we characterized the degree of inhibition following unloading with respect to different levels of preperturbation muscle activity. We initially observed that the inhibitory activity during the short-latency epoch scaled with increased levels of preperturbation muscle activity. However, this scaling peaked early in the R2 epoch (∼ 50 ms) but then quickly diminished through the rest of the long-latency epoch. Finally, in experiment 3, we investigated whether inhibitory perturbation responses consider intersegmental dynamics of the limb. We quantified unloading responses for either pure shoulder or pure elbow torques that evoked similar motion at the shoulder but different elbow motion. The long-latency inhibitory response in the shoulder, unlike the short-latency, was greater for the shoulder torque compared with the response following an elbow torque, as previously observed for a loading response. Taken together, these results illustrate that the long-latency unloading response is capable of a similar level of complexity as observed when loads are applied to the limb.

Decoding of the spike timing of primary afferents during voluntary arm movements in monkeys

Understanding the mechanisms of encoding forelimb kinematics in the activity of peripheral afferents is essential for developing a somatosensory neuroprosthesis. To investigate whether the spike timing of dorsal root ganglion (DRG) neurons could be estimated from the forelimb kinematics of behaving monkeys, we implanted two multi-electrode arrays chronically in the DRGs at the level of the cervical segments in two monkeys. Neuronal activity during voluntary reach-to-grasp movements were recorded simultaneously with the trajectories of hand/arm movements, which were tracked in three-dimensional space using a motion capture system. Sixteen and 13 neurons, including muscle spindles, skin receptors, and tendon organ afferents, were recorded in the two monkeys, respectively. We were able to reconstruct forelimb joint kinematics from the temporal firing pattern of a subset of DRG neurons using sparse linear regression (SLiR) analysis, suggesting that DRG neuronal ensembles encoded information about joint kinematics. Furthermore, we estimated the spike timing of the DRG neuronal ensembles from joint kinematics using an integrate-and-fire model (IF) incorporating the SLiR algorithm. The temporal change of firing frequency of a subpopulation of neurons was reconstructed precisely from forelimb kinematics using the SLiR. The estimated firing pattern of the DRG neuronal ensembles encoded forelimb joint angles and velocities as precisely as the originally recorded neuronal activity. These results suggest that a simple model can be used to generate an accurate estimate of the spike timing of DRG neuronal ensembles from forelimb joint kinematics, and is useful for designing a proprioceptive decoder in a brain machine interface.

Three tools for the real-time simulation of embodied spiking neural networks using GPUs

This paper presents a toolbox of solutions that enable the user to construct biologically-inspired spiking neural networks with tens of thousands of neurons and millions of connections that can be simulated in real time, visualized in 3D and connected to robots and other devices. NeMo is a high performance simulator that works with a variety of neural and oscillator models and performs parallel simulations on either GPUs or multi-core processors. SpikeStream is a visualization and analysis environment that works with NeMo and can construct networks, store them in a database and visualize their activity in 3D. The iSpike library provides biologically-inspired conversion between real data and spike representations to support work with robots, such as the iCub. Each of the tools described in this paper can be used independently with other software, and they also work well together.

Sensory systems in the control of movement

Animal movement is immensely varied, from the simplest reflexive responses to the most complex, dexterous voluntary tasks. Here, we focus on the control of movement in mammals, including humans. First, the sensory inputs most closely implicated in controlling movement are reviewed, with a focus on somatosensory receptors. The response properties of the large muscle receptors are examined in detail. The role of sensory input in the control of movement is then discussed, with an emphasis on the control of locomotion. The interaction between central pattern generators and sensory input, in particular in relation to stretch reflexes, timing, and pattern forming neuronal networks is examined. It is proposed that neural signals related to bodily velocity form the basic descending command that controls locomotion through specific and well-characterized relationships between muscle activation, step cycle phase durations, and biomechanical outcomes. Sensory input is crucial in modulating both the timing and pattern forming parts of this mechanism.

Predictive modeling by the cerebellum improves proprioception

Because sensation is delayed, real-time movement control requires not just sensing, but also predicting limb position, a function hypothesized for the cerebellum. Such cerebellar predictions could contribute to perception of limb position (i.e., proprioception), particularly when a person actively moves the limb. Here we show that human cerebellar patients have proprioceptive deficits compared with controls during active movement, but not when the arm is moved passively. Furthermore, when healthy subjects move in a force field with unpredictable dynamics, they have active proprioceptive deficits similar to cerebellar patients. Therefore, muscle activity alone is likely insufficient to enhance proprioception and predictability (i.e., an internal model of the body and environment) is important for active movement to benefit proprioception. We conclude that cerebellar patients have an active proprioceptive deficit consistent with disrupted movement prediction rather than an inability to generally enhance peripheral proprioceptive signals during action and suggest that active proprioceptive deficits should be considered a fundamental cerebellar impairment of clinical importance.

The time windows of the sense of agency

The sense of agency depends on some internal cues that derive from action control, as well as external cues like contextual information and prior information (degree of contingency between an action and is effect). We assessed whether external agency cues are combined with internal agency cues to affect the sense of agency. In two experiments participants performed a movement (button press) that elicited, after a varying delay, an effect (ball appearing on a screen), and reported their sense of agency over the effect (full, partial or no-agency) while internal cues (premotor information) and external cues (contextual and prior information) were manipulated. We assessed the effect of agency cues on the delays at which the sense of agency varied. The delays were increased with premotor signals but were decreased with contextual information. These findings favour a model of integration of internal and external agency cues over time.

Perceptual learning in sensorimotor adaptation

Motor learning often involves situations in which the somatosensory targets of movement are, at least initially, poorly defined, as for example, in learning to speak or learning the feel of a proper tennis serve. Under these conditions, motor skill acquisition presumably requires perceptual as well as motor learning. That is, it engages both the progressive shaping of sensory targets and associated changes in motor performance. In the present study, we test the idea that perceptual learning alters somatosensory function and in so doing produces changes to human motor performance and sensorimotor adaptation. Subjects in these experiments undergo perceptual training in which a robotic device passively moves the subject's arm on one of a set of fan-shaped trajectories. Subjects are required to indicate whether the robot moved the limb to the right or the left and feedback is provided. Over the course of training both the perceptual boundary and acuity are altered. The perceptual learning is observed to improve both the rate and extent of learning in a subsequent sensorimotor adaptation task and the benefits persist for at least 24 h. The improvement in the present studies varies systematically with changes in perceptual acuity and is obtained regardless of whether the perceptual boundary shift serves to systematically increase or decrease error on subsequent movements. The beneficial effects of perceptual training are found to be substantially dependent on reinforced decision-making in the sensory domain. Passive-movement training on its own is less able to alter subsequent learning in the motor system. Overall, this study suggests perceptual learning plays an integral role in motor learning.

The primate cerebellum selectively encodes unexpected self-motion

BACKGROUND

The ability to distinguish sensory signals that register unexpected events (exafference) from those generated by voluntary actions (reafference) during self-motion is essential for accurate perception and behavior. The cerebellum is most commonly considered in relation to its contributions to the fine tuning of motor commands and sensorimotor calibration required for motor learning. During unexpected motion, however, the sensory prediction errors that drive motor learning potentially provide a neural basis for the computation underlying the distinction between reafference and exafference.

RESULTS

Recording from monkeys during voluntary and applied self-motion, we demonstrate that individual cerebellar output neurons encode an explicit and selective representation of unexpected self-motion by means of an elegant computation that cancels the reafferent sensory effects of self-generated movements. During voluntary self-motion, the sensory responses of neurons that robustly encode unexpected movement are canceled. Neurons with vestibular and proprioceptive responses to applied head and body movements are unresponsive when the same motion is self-generated. When sensory reafference and exafference are experienced simultaneously, individual neurons provide a precise estimate of the detailed time course of exafference.

CONCLUSIONS

These results provide an explicit solution to the longstanding problem of understanding mechanisms by which the brain anticipates the sensory consequences of our voluntary actions. Specifically, by revealing a striking computation of a sensory prediction error signal that effectively distinguishes between the sensory consequences of self-generated and externally produced actions, our findings overturn the conventional thinking that the sensory errors coded by the cerebellum principally contribute to the fine tuning of motor activity required for motor learning.

Responses of somatosensory area 2 neurons to actively and passively generated limb movements

Control of reaching movements requires an accurate estimate of the state of the limb, yet sensory signals are inherently noisy, because of both noise at the receptors themselves and the stochastic nature of the information representation by neural discharge. One way to derive an accurate representation from noisy sensor data is to combine it with the output of a forward model that considers both the previous state estimate and the noisy input. We recorded from primary somatosensory cortex (S1) in macaques (Macaca mulatta) during both active and passive movements to investigate how the proprioceptive representation of movement in S1 may be modified by the motor command (through efference copy). We found neurons in S1 that respond to one or both movement types covering a broad distribution from active movement only, to both, to passive movement only. Those neurons that responded to both active and passive movements responded with similar directional tuning. Confirming earlier results, some, but not all, neurons responded before the onset of volitional movements, possibly as a result of efference copy. Consequently, many of the features necessary to combine the forward model with proprioceptive feedback appear to be present in S1. These features would not be expected from combinations of afferent receptor responses alone.

Population coding of forelimb joint kinematics by peripheral afferents in monkeys

Various peripheral receptors provide information concerning position and movement to the central nervous system to achieve complex and dexterous movements of forelimbs in primates. The response properties of single afferent receptors to movements at a single joint have been examined in detail, but the population coding of peripheral afferents remains poorly defined. In this study, we obtained multichannel recordings from dorsal root ganglion (DRG) neurons in cervical segments of monkeys. We applied the sparse linear regression (SLiR) algorithm to the recordings, which selects useful input signals to reconstruct movement kinematics. Multichannel recordings of peripheral afferents were performed by inserting multi-electrode arrays into the DRGs of lower cervical segments in two anesthetized monkeys. A total of 112 and 92 units were responsive to the passive joint movements or the skin stimulation with a painting brush in Monkey 1 and Monkey 2, respectively. Using the SLiR algorithm, we reconstructed the temporal changes of joint angle, angular velocity, and acceleration at the elbow, wrist, and finger joints from temporal firing patterns of the DRG neurons. By automatically selecting a subset of recorded units, the SLiR achieved superior generalization performance compared with a regularized linear regression algorithm. The SLiR selected not only putative muscle units that were responsive to only the passive movements, but also a number of putative cutaneous units responsive to the skin stimulation. These results suggested that an ensemble of peripheral primary afferents that contains both putative muscle and cutaneous units encode forelimb joint kinematics of non-human primates.

Temporal features of human tendon vibration illusions

Muscle spindles provide information about the position and movement of our bodies. One method for investigating spindle signals is tendon vibration. Vibration of flexor tendons can produce illusions of extension, and vibration of extensor tendons can produce illusions of flexion. Here we estimate the temporal resolution and persistence of these illusions. In Experiments 1 and 2, sequences of alternating vibration of wrist flexor and extensor tendons produced position illusions that varied with alternation period. When vibrations alternated at 1 Hz or slower, perceived position at the end of the sequence depended on the last vibration. When vibrations alternated every 0.3 s, perceived position was independent of the last vibration. Experiment 2 verified and extended these results using more trials and concurrent electromyographic recording. Although tendon vibrations sometimes induce reflexive muscle activity, we found no evidence that such activity contributed to these effects. Experiment 3 investigated how long position sense is retained when not updated by current information from spindles. Our first experiments suggested that vibrating antagonistic tendons simultaneously could produce conflicting inputs, leaving position sense reliant on memory of position prior to vibration onset. We compared variability in position sense after different durations of such double vibration. After 12 s of double vibration, variability across trials exceeded levels predicted from vibrations of flexor or extensor tendons alone. This suggests that position sense memory had decayed too much to substitute for the current conflicting sensory information. Together, our results provide novel, quantitative insight into the temporal properties of tendon vibration illusions.

Ankle joint movements are encoded by both cutaneous and muscle afferents in humans

We analyzed the cutaneous encoding of two-dimensional movements by investigating the coding of movement velocity for differently oriented straight-line movements and the coding of complex trajectories describing cursive letters. The cutaneous feedback was then compared with that of the underlying muscle afferents previously recorded during the same "writing-like" movements. The unitary activity of 43 type II cutaneous afferents was recorded in the common peroneal nerve in healthy subjects during imposed ankle movements. These movements consisted first of ramp-and-hold movements imposed at two different and close velocities in seven directions and secondly of "writing-like" movements. In both cases, the responses were analyzed using the neuronal population vector model. The results show that movement velocity encoding depended on the direction of the ongoing movement. Discriminating between two velocities therefore involved processing the activity of afferent populations located in the various skin areas surrounding the moving joint, as shown by the statistically significant difference observed in the amplitude of the sum vectors. Secondly, "writing-like" movements induced cutaneous neuronal patterns of activity, which were reproducible and specific to each trajectory. Lastly, the "cutaneous neuronal trajectories," built by adding the sum vectors tip-to-tail, nearly matched both the movement trajectories and the "muscle neuronal trajectories," built from previously recorded muscle afferents. It was concluded that type II cutaneous and the underlying muscle afferents show similar encoding properties of two-dimensional movement parameters. This similarity is discussed in relation to a central gating process that would for instance increase the gain of cutaneous inputs when muscle information is altered by the fusimotor drive.

Enhanced propriospinal excitation from hand muscles to wrist flexors during reach-to-grasp in humans

In humans, propriospinal neurons located at midcervical levels receive peripheral and corticospinal inputs and probably participate in the control of grip tasks, but their role in reaching movements, as observed in cats and primates, is still an open question. The effect of ulnar nerve stimulation on flexor carpi radialis (FCR) motor evoked potential (MEP) was tested during reaching tasks and tonic wrist flexion. Significant MEP facilitation was observed at the end of reach during reach-to-grasp but not during grasp, reach-to-point, or tonic contractions. MEP facilitation occurred at a longer interstimulus interval than expected for convergence of corticospinal and afferent volleys at motoneuron level and was not paralleled by a change in the H-reflex. These findings suggest convergence of the two volleys at propriospinal level. Ulnar-induced MEP facilitation was observed when conditioning stimuli were at 0.75 motor response threshold (MT), but not 1 MT. This favors an increased excitability of propriospinal neurons rather than depression of their feedback inhibition, as has been observed during tonic power grip tasks. It is suggested that the ulnar-induced facilitation of FCR MEP during reach may be due to descending activation of propriospinal neurons, assisting the early recruitment of large motoneurons for rapid movement. Because the feedback inhibitory control is still open, this excitation can be truncated by cutaneous inputs from the palmar side of the hand during grasp, thus assisting movement termination. It is concluded that the feedforward activation of propriospinal neurons and their feedback control may be involved in the internal model, motor planning, and online adjustments for reach-to-grasp movements in humans.

CSF Bcl-2 and cytochrome C temporal profiles in outcome prediction for adults with severe TBI

The biochemical cascades associated with cell death after traumatic brain injury (TBI) involve both pro-survival and pro-apoptotic proteins. We hypothesized that elevated cerebrospinal fluid (CSF) Bcl-2 and cytochrome C (CytoC) levels over time would reflect cellular injury response and predict long-term outcomes after TBI. Cerebrospinal fluid Bcl-2 and CytoC levels were measured for 6 days after injury for adults with severe TBI (N=76 subjects; N=277 samples). Group-based trajectory analysis was used to generate distinct temporal biomarker profiles that were compared with Glasgow Outcome Scale (GOS) and Disability Rating Scale (DRS) scores at 6 and 12 months after TBI. Subjects with persistently elevated temporal Bcl-2 and CytoC profiles compared with healthy controls had the worst outcomes at 6 and 12 months (P≤0.027). Those with CytoC profiles near controls had better long-term outcomes, and those with declining CytoC levels over time had intermediate outcomes. Subjects with Bcl-2 profiles that remained near controls had better outcomes than those with consistently elevated Bcl-2 profiles. However, subjects with Bcl-2 values that started near controls and steadily rose over time had 100% good outcomes by 12 months after TBI. These results show the prognostic value of Bcl-2 and CytoC profiles and suggest a dynamic apoptotic and pro-survival response to TBI.

Contributions of skin and muscle afferent input to movement sense in the human hand

In the stationary hand, static joint-position sense originates from multimodal somatosensory input (e.g., joint, skin, and muscle). In the moving hand, however, it is uncertain how movement sense arises from these different submodalities of proprioceptors. In contrast to static-position sense, movement sense includes multiple parameters such as motion detection, direction, joint angle, and velocity. Because movement sense is both multimodal and multiparametric, it is not known how different movement parameters are represented by different afferent submodalities. In theory, each submodality could redundantly represent all movement parameters, or, alternatively, different afferent submodalities could be tuned to distinctly different movement parameters. The study described in this paper investigated how skin input and muscle input each contributes to movement sense of the hand, in particular, to the movement parameters dynamic position and velocity. Healthy adult subjects were instructed to indicate with the left hand when they sensed the unseen fingers of the right hand being passively flexed at the metacarpophalangeal (MCP) joint through a previously learned target angle. The experimental approach was to suppress input from skin and/or muscle: skin input by anesthetizing the hand, and muscle input by unexpectedly extending the wrist to prevent MCP flexion from stretching the finger extensor muscle. Input from joint afferents was assumed not to play a significant role because the task was carried out with the MCP joints near their neutral positions. We found that, during passive finger movement near the neutral position in healthy adult humans, both skin and muscle receptors contribute to movement sense but qualitatively differently. Whereas skin input contributes to both dynamic position and velocity sense, muscle input may contribute only to velocity sense.

The adaptability of self-action perception and movement control when the limb is passively versus actively moved

Research suggests that perceptual experience of our movements adapts together with movement control when we are the agents of our actions. Is this agency critical for perceptual and motor adaptation? We had participants view cursor feedback during elbow extension-flexion movements when they (1) actively moved their arm, or (2) had their arm passively moved. We probed adaptation of movement perception by having participants report the reversal point of their unseen movement. We probed adaptation of movement control by having them aim to a target. Perception and control of active movement were influenced by both types of exposure, although adaptation was stronger following active exposure. Furthermore, both types of exposure led to a change in the perception of passive movements. Our findings support the notion that perception and control adapt together, and they suggest that some adaptation is due to recalibrated proprioception that arises independently of active engagement with the environment.

Predicting Any Arm Movement Feedback to Induce Three-Dimensional Illusory Movements in Humans

Our sense of body posture and movement is mainly mediated by densely packed populations of tiny mechanoreceptors present in the muscles. Signals triggered in muscle spindles by our own actions contribute crucially to our consciousness of positions and movements by continuously feeding and updating dynamic sensorimotor maps. Deciphering the coding rules whereby the nervous system integrates this proprioceptive information perceptually could help to elucidate the mechanisms underlying kinesthesia. The aim of the present study was to test the validity of a “propriomimetic method” of predicting the proprioceptive streams emitted by each of the muscles involved in two- (2D) and three-dimensional (3D) arm movements. This method was based on the functional properties of muscle spindle populations previously recorded microneurographically in behaving humans. Ia afferent patterns mimicking those evoked when the “arm–forearm” ensemble is drawing straight lines, graphic symbols, and complex 3D figures were calculated. These simulated patterns were then delivered to the main elbow and shoulder muscle tendons of motionless volunteers via a set of vibrators. Results show that the simulated proprioceptive patterns applied induced, in passive subjects, illusory 2D and 3D arm movements, the trajectories of which were very similar to the expected ones. These simulated patterns can therefore be said to be a substitute for the Ia proprioceptive feedback evoked by any human arm movement and this method can certainly be extended to other musculoskeletal ensembles. The illusory movements induced when these proprioceptive patterns are applied to muscle groups via sets of vibrators may provide useful tools for sensorimotor rehabilitation purposes.

Mapping proprioception across a 2D horizontal workspace

Relatively few studies have been reported that document how proprioception varies across the workspace of the human arm. Here we examined proprioceptive function across a horizontal planar workspace, using a new method that avoids active movement and interactions with other sensory modalities. We systematically mapped both proprioceptive acuity (sensitivity to hand position change) and bias (perceived location of the hand), across a horizontal-plane 2D workspace. Proprioception of both the left and right arms was tested at nine workspace locations and in 2 orthogonal directions (left-right and forwards-backwards). Subjects made repeated judgments about the position of their hand with respect to a remembered proprioceptive reference position, while grasping the handle of a robotic linkage that passively moved their hand to each judgement location. To rule out the possibility that the memory component of the proprioceptive testing procedure may have influenced our results, we repeated the procedure in a second experiment using a persistent visual reference position. Both methods resulted in qualitatively similar findings. Proprioception is not uniform across the workspace. Acuity was greater for limb configurations in which the hand was closer to the body, and was greater in a forward-backward direction than in a left-right direction. A robust difference in proprioceptive bias was observed across both experiments. At all workspace locations, the left hand was perceived to be to the left of its actual position, and the right hand was perceived to be to the right of its actual position. Finally, bias was smaller for hand positions closer to the body. The results of this study provide a systematic map of proprioceptive acuity and bias across the workspace of the limb that may be used to augment computational models of sensory-motor control, and to inform clinical assessment of sensory function in patients with sensory-motor deficits.

Where is your arm? Variations in proprioception across space and tasks

The sense of limb position is crucial for movement control and environmental interactions. Our understanding of this fundamental proprioceptive process, however, is limited. For example, little is known about the accuracy of arm proprioception: Does it vary with changes in arm configuration, since some peripheral receptors are engaged only when joints move toward extreme angles? Are these variations consistent across different tasks? Does proprioceptive ability change depending on what we try to localize (e.g., fingertip position vs. elbow angle)? We used a robot exoskeleton to study proprioception in 14 arm configurations across three tasks, asking healthy subjects to 1) match a pointer to elbow angles after passive movements, 2) match a pointer to fingertip positions after passive movements, and 3) actively match their elbow angle to a pointer. Across all three tasks, subjects overestimated more extreme joint positions; this may be due to peripheral sensory signals biasing estimates as a safety mechanism to prevent injury. We also found that elbow angle estimates were more precise when used to judge fingertip position versus directly reported, suggesting that the brain has better access to limb endpoint position than joint angles. Finally, precision of elbow angle estimates improved in active versus passive movements, corroborating work showing that efference copies of motor commands and alpha-gamma motor neuron coactivation contribute to proprioceptive estimates. In sum, we have uncovered fundamental aspects of normal proprioceptive processing, demonstrating not only predictable biases that are dependent on joint configuration and independent of task but also improved precision when integrating information across joints.

Inducing Any Virtual Two-Dimensional Movement in Humans by Applying Muscle Tendon Vibration

In humans, tendon vibration evokes illusory sensation of movement. We developed a model mimicking the muscle afferent patterns corresponding to any two-dimensional movement and checked its validity by inducing writing illusory movements through specific sets of muscle vibrators. Three kinds of illusory movements were compared. The first was induced by vibration patterns copying the responses of muscle spindle afferents previously recorded by microneurography during imposed ankle movements. The two others were generated by the model. Sixteen different vibratory patterns were applied to 20 motionless volunteers in the absence of vision. After each vibration sequence, the participants were asked to name the corresponding graphic symbol and then to reproduce the illusory movement perceived. Results showed that the afferent patterns generated by the model were very similar to those recorded microneurographically during actual ankle movements ( r = 0.82). The model was also very efficient for generating afferent response patterns at the wrist level, if the preferred sensory directions of the wrist muscle groups were first specified. Using recorded and modeled proprioceptive patterns to pilot sets of vibrators placed at the ankle or wrist levels evoked similar illusory movements, which were correctly identified by the participants in three quarters of the trials. Our proprioceptive model, based on neurosensory data recorded in behaving humans, should then be a useful tool in fields of research such as sensorimotor learning, rehabilitation, and virtual reality.

Discharges in human muscle receptor afferents during block grasping

Human grasping relies on feedforward control that is monitored and corrected on-line by means of sensory feedback. While much of the sensory mechanisms underpinning hand-object interaction are known, information has been lacking about muscle receptor responses during the phases before and after actual object contact. We therefore let subjects use their thumb and fingers to grasp blocks presented to them while we recorded muscle afferents from the thumb and finger extensor muscles along with wrist and digit kinematics, and electromyographic activity. The kinematics of the task was indistinguishable from "normal" grasping. None of the afferents encoded either object contact or finger apposition. Both primary and secondary afferents were more phase advanced on the parent muscle lengths than expected from previous studies as well as from their responses to imposed length changes of their parent muscles. Thus, the discharges of both primary and secondary afferents were well correlated to the tendon velocity of their parent muscles and that of primary afferents also to acceleration whereas neither appeared to encode muscle length as such. Decoding the velocity of muscle length changes were significantly improved if the discharge of Golgi tendon organ afferents were taken into account along with that of the muscle spindle afferents. We propose that these findings may be explained by the biomechanical properties of contracting muscles. Moreover, we conclude that it seems unlikely that the muscle spindle afferents recorded in this task have any role in providing "proprioceptive" information pertaining to the size of an object grasped.

Discharges in human muscle spindle afferents during a key-pressing task

Most manual tasks demand a delicate control of the wrist. Sensory information for this control, e.g. about the position and movement velocity of the hand, is assumed to be primarily provided by muscle spindle afferents. It is known that human muscle spindles in relaxed muscles behave as stretch receptors but it is unclear how they discharge during 'natural' hand movements, since their discharges can also be affected by extrafusal contractions and fusimotor activity. We therefore let subjects perform a centre-out-centre key-pressing task on buttons laid out in a 3 x 3 pattern, a task that allowed unconstrained hand and finger movements and required precise control of the wrist. Microneurography recordings from muscle spindle afferents of the wrist extensor muscles were obtained along with wrist kinematics and electromyographic signals. The discharge rates of afferents were more phase advanced than expected on the length of the radial wrist extensor, which acted as an anti-gravity muscle in the key-pressing task. As such, both acceleration and velocity had significant impacts on the discharge rate of primary afferents, velocity on that of secondary afferents, and length had no impact on either afferent type. The response patterns were different for the two types of muscle spindle afferents from the predominantly eccentrically contracting ulnar wrist extensor: muscle length and velocity had significant impacts on the ensemble response of secondary afferents whereas the primary afferents showed highly variable responses. Accordingly, good predictions of the radial ulnar angular velocity were possible from spindle ensemble responses (R(2) = 0.85) whereas length could be predicted only for phases with lengthening of the ulnar wrist extensor. There are several possible explanations for the unexpectedly large phase advance of spindle afferents in the radial wrist extensor. Given the compliance of tendons, for instance, the phase relationship between the muscle fascicle length and the whole muscle length is conjectured to depend on the load. While additional phase advances are advantageous in motor control, it is concluded that if the central nervous system estimates length or velocity of a muscle from its muscle spindle discharges, this would require additional information about not only the concomitant extrafusal and fusimotor drive but also about the mechanical properties of the load on which the muscle acts.

Proprioception contributes to the sense of agency during visual observation of hand movements: evidence from temporal judgments of action

The ability to recognize visually one's own movement is important for motor control and, through attribution of agency, for social interactions. Agency of actions may be decided by comparisons of visual feedback, efferent signals, and proprioceptive inputs. Because the ability to identify one's own visual feedback from passive movements is decreased relative to active movements, or in some cases is even absent, the role of proprioception in self-recognition has been questioned. Proprioception during passive and active movements may, however, differ, and so to address any role for proprioception in the sense of agency, the active movement condition must be examined. Here we tested a chronically deafferented man (I.W.) and an age-matched group of six healthy controls in a task requiring judgement of the timing of action. Subjects performed finger movements and watched a visual cursor that moved either synchronously or asynchronously with a random delay, and reported whether or not they felt they controlled the cursor. Movement accuracy was matched between groups. In the absence of proprioception, I.W. was less able than the control group to discriminate self- from computer-produced cursor movement based on the timing of movement. In a control visual discrimination task with concurrent similar finger movements but no agency detection, I.W. was unimpaired, suggesting that this effect was task specific. We conclude that proprioception does contribute to the visual identification of ownership during active movements and, thus, to the sense of agency.

Afferent input, efference copy, signal noise, and biases in perception of joint angle during active versus passive elbow movements

Psychophysical studies have reported an overestimation of limb position in the direction of movement during the early part of active movements. The main hypothesis tested in this study is that the overestimation results from a process of forward prediction of limb state driven by an efference copy of the outgoing motor command. This hypothesis predicts that position overestimation should decrease or disappear during passive movements, for which there should be no efference copy. Seven subjects were asked to remember and to report the perceived angle of their elbow joint at different times during active and passive movements. They showed a highly velocity-dependent overestimation of the elbow joint angle near the beginning of the movement in both active and passive trials. Toward the end of the movement, subjects showed a relatively velocity-independent underestimation of their elbow angle in all trials. Contrary to the prediction of the efference copy hypothesis, the amplitude and the velocity-dependent slope of the elbow angle overestimation were both greater during the early part of passive movements than active movements. This indicates that psychophysical evidence of early overestimation of arm position on its own is not a sufficient proof of forward prediction based on an efference copy, at least under the conditions of this study. Decreased errors during active movements suggest that an efference copy can improve the accuracy of state estimation during active movements. Error patterns seem to parallel the likely level of sensorimotor noise, suggesting a probabilistic mechanism for position estimation.

Cutaneous afferents provide a neuronal population vector that encodes the orientation of human ankle movements

The aim of this study was to analyse the directional coding of two-dimensional limb movements by cutaneous afferents from skin areas covering a multidirectional joint, the ankle. The activity of 89 cutaneous afferents was recorded in the common peroneal nerve, and the mean discharge frequency of each unit was measured during the outward phase of ramp and hold movements imposed in 16 different directions. Forty-two afferents responded to the movements in the following decreasing order (SA2, n = 24/27; FA2, n = 13/17; FA1, n = 3/24; SA1, n = 2/21). All the units activated responded to a specific range of directions, defining their 'preferred sector', within which their response peaked in a given direction, their 'preferred direction'. Based on the distribution of the preferred directions, two populations of afferents, and hence two skin areas were defined: the anterior and the external lateral parts of the leg. As the directional tuning of each population was cosine shaped, the neuronal population vector model was applied and found to efficiently describe the movement direction encoded by cutaneous afferents, as it has been previously reported for muscle afferents. The responses of cutaneous afferents were then considered with respect to those of the afferents from the underlying muscles, which were previously investigated, and an almost perfect matching of directional sensitivity was observed. It is suggested that the common movement-encoding characteristics exhibited by cutaneous and muscle afferents, as early as the peripheral level, may facilitate the central co-processing of their feedbacks subserving kinaesthesia.

The Ia afferent feedback of a given movement evokes the illusion of the same movement when returned to the subject via muscle tendon vibration

The aim of the present study was to further investigate the contribution of primary muscle spindle feedback to proprioception and higher brain functions, such as movement trajectory recognition. For this purpose, complex illusory movements were evoked in subjects by applying patterns of muscle tendon vibration mimicking the natural Ia afferent pattern. Ia afferent messages were previously recorded using microneurographic method from the six main muscle groups acting on the ankle joint during imposed "writing like" movements. The mean Ia afferent pattern was calculated for each muscle group and used as a template to pilot each vibrator. Eleven different vibratory patterns were applied to ten volunteers. Subjects were asked both to copy the perceived illusory movements by hand on a digitizing tablet and to recognize and name the corresponding graphic symbol. The results show that the Ia afferent feedback of a given movement evokes the illusion of the same movement when it is applied to the subject via the appropriate pattern of muscle tendon vibration. The geometry and the kinematic parameters of the imposed and illusory movements are very similar and the so-called "two-thirds power law" is present in the reproduction of the vibration-induced illusory movements. Vibrations within the "natural" frequency range of Ia fibres firing (around 30 Hz) produce clear illusions of movements in all the tested subjects. In addition, increasing the mean frequency of the vibration patterns resulted in a linear increase in the size of the illusory movements. Lastly, the subjects were able to recognize and name the symbols evoked by the vibration-induced primary muscle spindle afferent patterns in 83% of the trials. These findings suggest that the "proprioceptive signature" of a given movement is associated with the corresponding "perceptual signature". The neural mechanisms possibly underlying the sensory to perceptual transformation are discussed in the general framework of "the neuronal population vector model".