Cutaneous Receptors Contribute to Kinesthesia at the Index Finger, Elbow, and Knee

Reduction in motor error by presenting subthreshold somatosensory information during visuomotor tracking tasks

Weak sensory noise acts on the nervous system and promotes sensory and motor functions. This phenomenon is called stochastic resonance and is expected to be applied for improving biological functions. This study investigated the effect of electrical stimulation on grip force adjustment ability. The coefficient of variation and absolute motor error in grip force was measured during a visuomotor tracking task under different intensities of somatosensory noise. Depending on the style of force exertion, the grip movement used in the visuomotor tracking task consisted of force generation (FG), force relaxation (FR), and constant contraction (Constant) phases. The subthreshold condition resulted in significantly lower coefficient of variation in the Constant phase and motor errors in the FG and Constant phases than the no-noise condition. However, the differences among the other conditions were insignificant. Additionally, we examined the correlation between the motor error in the condition without electrical stimulation and the change in motor error induced by subthreshold electrical stimulation. Significant negative correlations were observed in all FG, FR, and Constant phases. These results indicated that somatosensory noise had a strong effect on subjects with large motor errors and enhanced the grip force adjustment ability. By contrast, subjects with small motor errors had weak improvement in motor control. Although the effect of subthreshold noise varies depending on the individual differences, stochastic resonance is effective in improving motor control ability.

Creation of a biological sensorimotor interface for bionic reconstruction

Neuromuscular control of bionic arms has constantly improved over the past years, however, restoration of sensation remains elusive. Previous approaches to reestablish sensory feedback include tactile, electrical, and peripheral nerve stimulation, however, they cannot recreate natural, intuitive sensations. Here, we establish an experimental biological sensorimotor interface and demonstrate its potential use in neuroprosthetics. We transfer a mixed nerve to a skeletal muscle combined with glabrous dermal skin transplantation, thus forming a bi-directional communication unit in a rat model. Morphological analyses indicate reinnervation of the skin, mechanoreceptors, NMJs, and muscle spindles. Furthermore, sequential retrograde labeling reveals specific sensory reinnervation at the level of the dorsal root ganglia. Electrophysiological recordings show reproducible afferent signals upon tactile stimulation and tendon manipulation. The results demonstrate the possibility of surgically creating an interface for both decoding efferent motor control, as well as encoding afferent tactile and proprioceptive feedback, and may indicate the way forward regarding clinical translation of biological communication pathways for neuroprosthetic applications.

Hierarchical and dynamic relationships between body part ownership and full-body ownership

What is the relationship between experiencing individual body parts and the whole body as one's own? We theorised that body part ownership is driven primarily by the perceptual binding of visual and somatosensory signals from specific body parts, whereas full-body ownership depends on a more global binding process based on multisensory information from several body segments. To examine this hypothesis, we used a bodily illusion and asked participants to rate illusory changes in ownership over five different parts of a mannequin's body and the mannequin as a whole, while we manipulated the synchrony or asynchrony of visual and tactile stimuli delivered to three different body parts. We found that body part ownership was driven primarily by local visuotactile synchrony and could be experienced relatively independently of full-body ownership. Full-body ownership depended on the number of synchronously stimulated parts in a nonlinear manner, with the strongest full-body ownership illusion occurring when all parts received synchronous stimulation. Additionally, full-body ownership influenced body part ownership for nonstimulated body parts, and skin conductance responses provided physiological evidence supporting an interaction between body part and full-body ownership. We conclude that body part and full-body ownership correspond to different processes and propose a hierarchical probabilistic model to explain the relationship between part and whole in the context of multisensory awareness of one's own body.

Are blind individuals immune to bodily illusions? Somatic rubber hand illusion in the blind revisited

Multisensory awareness of one's own body relies on the integration of signals from various sensory modalities such as vision, touch, and proprioception. But how do blind individuals perceive their bodies without visual cues, and does the brain of a blind person integrate bodily senses differently from a sighted person? To address this question, we aimed to replicate the only two previous studies on this topic, which claimed that blind individuals do not experience the somatic rubber hand illusion, a bodily illusion triggered by the integration of correlated tactile and proprioceptive signals from the two hands. We used a larger sample size than the previous studies and added Bayesian analyses to examine statistical evidence in favor of the lack of an illusion effect. Moreover, we employed tests to investigate whether enhanced tactile acuity and cardiac interoceptive accuracy in blind individuals could also explain the weaker illusion. We tested 36 blind individuals and 36 age- and sex-matched sighted volunteers. The results show that blind individuals do not experience the somatic rubber hand illusion based on questionnaire ratings and behavioral measures that assessed changes in hand position sense toward the location of the rubber hand. This conclusion is supported by Bayesian evidence in favor of the null hypothesis. The findings confirm that blind individuals do not experience the somatic rubber hand illusion, indicating that lack of visual experience leads to permanent changes in multisensory bodily perception. In summary, our study suggests that changes in multisensory integration of tactile and proprioceptive signals may explain why blind individuals are "immune" to the nonvisual version of the rubber hand illusion.

Sensorimotor control in the congenital absence of functional muscle spindles

Hereditary sensory and autonomic neuropathy type III (HSAN III), also known as familial dysautonomia or Riley-Day syndrome, results from an autosomal recessive genetic mutation that causes a selective loss of specific sensory neurones, leading to greatly elevated pain and temperature thresholds, poor proprioception, marked ataxia and disturbances in blood pressure control. Stretch reflexes are absent throughout the body, which can be explained by the absence of functional muscle spindle afferents - assessed by intraneural microelectrodes inserted into peripheral nerves in the upper and lower limbs. This also explains the greatly compromised proprioception at the knee joint, as assessed by passive joint-angle matching. Moreover, there is a tight correlation between loss of proprioceptive acuity at the knee and the severity of gait impairment. Surprisingly, proprioception is normal at the elbow, suggesting that participants are relying more on sensory cues from the overlying skin; microelectrode recordings have shown that myelinated tactile afferents in the upper and lower limbs appear to be normal. Nevertheless, the lack of muscle spindles does affect sensorimotor control in the upper limb: in addition to poor performance in the finger-to-nose test, manual performance in the Purdue pegboard task is much worse than in age-matched healthy controls. Unlike those rare individuals with large-fibre sensory neuropathy, in which both muscle spindle and cutaneous afferents are absent, those with HSAN III present as a means of assessing sensorimotor control following the selective loss of muscle spindle afferents.

Contralateral versus ipsilateral protective effect against muscle damage of the elbow flexors and knee extensors induced by maximal eccentric exercise

The present study compared the ipsilateral repeated bout effect (IL-RBE) and contralateral repeated bout effect (CL-RBE) of the elbow flexors (EF) and knee flexors (KF) for the same interval between bouts to shed light on their mechanisms. Fifty-two healthy sedentary young (20-28 years) men were randomly assigned to the IL-EF, IL-KF, CL-EF, and CL-KF groups (n = 13/group). Thirty maximal eccentric contractions of the EF were performed in IL-EF and CL-EF, and 60 maximal eccentric contractions of the KF were performed in IL-KF and CL-KF, with a 2-week interval between bouts. Changes in muscle damage markers such as maximal voluntary contraction (MVC) torque, muscle soreness, and plasma creatine kinase activity, and proprioception measures before to 5 days post-exercise were compared between groups. Changes in all variables were greater (p < 0.05) after the first than second bout for all groups, and the changes were greater (p < 0.05) for the EF than KF. The changes in all variables after the second bout were greater (p < 0.05) for the CL than IL condition for both EF and KF. The magnitude of the average protective effect was similar between CL-EF (33%) and CL-KF (32%), but slightly greater (p < 0.05) for IL-EF (67%) than IL-KF (61%). These demonstrate that the magnitude of CL-RBE relative to IL-RBE was similar between the EF and KF (approximately 50%), regardless of the greater muscle damage for the EF than KF. It appears that the CL-RBE is more associated with neural adaptations at cerebrum, cerebellum, interhemispheric inhibition, and coricospinal tract, but the IL-RBE is induced by additional adaptations at muscles.

Cyclic relationship of mechanical likelihood: Coupling perception-action states in extended haptic accuracy

The present study investigates the dynamic nature of haptic accuracy in racket sports, specifically focusing on self-produced movements in participants with different skill levels (novice vs. expert). The study examines performance accuracy using indicators such as absolute error size and the coefficient of restitution as measures of haptic accuracy. To collect and analyze the data, custom-made devices, including shock and vibration sensors and Qualisys Track Manager, were used. The results indicate that skilled participants demonstrated higher accuracy, reflected by smaller absolute error sizes, and exhibited reduced variability in impulse vibration during self-produced movements. Moreover, employing maximum likelihood estimation and differential equations, we reveal cyclic relationships among these mechanical features. These findings provide valuable insights into perception-action coupling within different haptic skill levels, contributing to a comprehensive understanding of expertise in racket sports. By shedding light on the intricate relationship between haptic accuracy and performance, this research offers a valuable framework for studying perception-action coupling in racket sports and can potentially guide future investigations.

Integration of proprioception in upper limb prostheses through non-invasive strategies: a review

Proprioception plays a key role in moving our body dexterously and effortlessly. Nevertheless, the majority of investigations evaluating the benefits of providing supplemental feedback to prosthetics users focus on delivering touch restitution. These studies evaluate the influence of touch sensation in an attempt to improve the controllability of current robotic devices. Contrarily, investigations evaluating the capabilities of proprioceptive supplemental feedback have yet to be comprehensively analyzed to the same extent, marking a major gap in knowledge within the current research climate. The non-invasive strategies employed so far to restitute proprioception are reviewed in this work. In the absence of a clearly superior strategy, approaches employing vibrotactile, electrotactile and skin-stretch stimulation achieved better and more consistent results, considering both kinesthetic and grip force information, compared with other strategies or any incidental feedback. Although emulating the richness of the physiological sensory return through artificial feedback is the primary hurdle, measuring its effects to eventually support the integration of cumbersome and energy intensive hardware into commercial prosthetic devices could represent an even greater challenge. Thus, we analyze the strengths and limitations of previous studies and discuss the possible benefits of coupling objective measures, like neurophysiological parameters, as well as measures of prosthesis embodiment and cognitive load with behavioral measures of performance. Such insights aim to provide additional and collateral outcomes to be considered in the experimental design of future investigations of proprioception restitution that could, in the end, allow researchers to gain a more detailed understanding of possibly similar behavioral results and, thus, support one strategy over another.

A reassessment of the role of joint receptors in human position sense

In the past, the peripheral sense organs responsible for generating human position sense were thought to be the slowly adapting receptors in joints. More recently, our views have changed and the principal position sensor is now believed to be the muscle spindle. Joint receptors have been relegated to the lesser role of acting as limit detectors when movements approach the anatomical limit of a joint. In a recent experiment concerned with position sense at the elbow joint, measured in a pointing task over a range of forearm angles, we have observed falls in position errors as the forearm was moved closer to the limit of extension. We considered the possibility that as the arm approached full extension, a population of joint receptors became engaged and that they were responsible for the changes in position errors. Muscle vibration selectively engages signals of muscle spindles. Vibration of elbow muscles undergoing stretch has been reported to lead to perception of elbow angles beyond the anatomical limit of the joint. The result suggests that spindles, by themselves, cannot signal the limit of joint movement. We hypothesise that over the portion of the elbow angle range where joint receptors become active, their signals are combined with those of spindles to produce a composite that contains joint limit information. As the arm is extended, the growing influence of the joint receptor signal is evidenced by the fall in position errors.

Effect of external lumbar supports on joint position sense, postural control, and postural adjustment: a systematic review

PURPOSE

To review the effects of external lumbar supports on various aspects of sensorimotor function including joint position sense (JPS), postural control, anticipatory postural adjustments (APAs), and compensatory postural adjustments (CPAs).

METHODS

A systematic literature search was performed in PubMed, EMBASE, Scopus, Ovid, Cochrane library, and Web of Science. Two reviewers selected studies which assessed the effect of lumbosacral orthosis or kinesio-tape on JPS, postural control or APAs/CPAs in subjects with and without low back pain (LBP). The methodological quality of included studies was assessed using a modified version of Downs and Black's checklist.

RESULTS

Findings demonstrated moderate effects of lumbosacral orthosis on specific aspects of sensorimotor control including JPS and to a lesser extent standing stability. These domains were not or minimally affected by application of kinesio-tape. Both orthosis and kinesio-tape had negligible effects on APAs and CPAs.

CONCLUSIONS

The positive effects of lumbar orthosis on JPS or postural control were mostly observed in conditions where sources of proprioceptive feedback are impaired (such as LBP) or absent (standing with eyes closed on an unstable surface). However, evidence does not prove significant positive effects for the application of kinesio-tape to improve sensorimotor control.IMPLICATIONS FOR REHABILITATIONWearing lumbar orthosis leads to an improvement in joint position sense.Postural stability seems to be affected to some extent by utilizing lumbar orthosis.Clinicians can administer orthosis to improve sensorimotor adaptation, especially in conditions with poor proprioception.Kinesio-tape had negligible effects on all domains of sensorimotor control.Improvement of sensorimotor function as a result of application of kinesio-tape is questionable.

Kinesthetic Coordination Abilities in 6-Year-Old Children: School Quintile, Gender, and Hand Dominance Differences

Proprioceptive kinaesthetic control underpins motor movements of developing children and can be influenced by several factors. The main aim of this study was to establish proprioceptive kinaesthetic coordination differences in six-year-olds from different school quintiles, of different genders, and with different handedness. A total of 193 six-year-olds from 10 schools of different quintiles in the Motheo District, Mangaung, were included, of which 97 (50.3%) were boys and 96 (49.7%) were girls. A quantitative cross-sectional study design was followed to determine proprioceptive kinaesthetic coordination differences. Right-handed participants performed significantly better than left-handed participants in the Finger-to-Nose task (p = 0.0125) when moving and positioning their dominant arm and hand. When using their dominant arm, significant differences in the shoulder-level-arm-raise (p = 0.0288) favoured boys. Girls showed superior execution of the force perception task (p = 0.0322). In conclusion, significant proprioceptive kinaesthetic coordination differences in six-year-olds were mainly not evident. Future work should explore proprioceptive kinaesthetic coordination differences in children of other ages and determine the practical implications of identified differences.

Biomechanically-Consistent Skin Stretch as an Intuitive Mechanism for Sensory Feedback: A Preliminary Investigation in the Lower Limb

The proprioceptive loss accompanied by lower limb amputation can impair function and mobility. We explore a simple, mechanical skin-stretch array configured to generate superficial tissue behaviour that might occur with movement about an intact joint. Four adhesive pads attached around the circumference of the lower leg were connected via cords to a remote "foot" mounted on a ball joint attached to the underside of a fracture boot, such that "foot" reorientation would result in skin stretch. In two discrimination experiments performed with and without the connection, with no view of the mechanism, and with minimal training, unimpaired adults (i) estimated foot orientation following passive foot rotations (eight directions), either with or without contact between the lower leg and boot, and (ii) actively lowered the "foot" to estimate slope orientation (four directions). In (i), 56-60% of responses (depending on contact condition) were correct and 88-94% were either correct or one of the two adjacent choices. In (ii), 56% of responses were correct. In contrast, without the connection, participants performed near or no different to chance. A biomechanically-consistent skin stretch array may be an intuitive means to convey proprioceptive information from an artificial or poorly innervated joint.

Taping-induced cutaneous stimulation to the ankle tendons reduces minimum toe clearance variability

Large variability of minimum toe clearance (MTC) leads to a higher risk of tripping. Visual feedback-based gait training systems have been used to regulate MTC distribution, but these systems are expensive and bulky. Furthermore, the effect of such training lasts only for a short period of time. Considering the efficacy of elastic adhesive tape-induced cutaneous stimulation to the ankle tendons in improving proprioception and movement detection, we hypothesize that application of tapes to the ankle tendons as a practical method for modifying MTC distribution. To test this hypothesis, we recruited 13 young and healthy adults and instructed them to walk on a treadmill under four conditions: no taping, taping the tibialis anterior tendon, taping the Achilles tendon, and taping both tendons. We measured MTC distribution, lower limb joint angles and muscle activations of the tibialis anterior and gastrocnemius medialis, and compared these outcomes under the four conditions. The application of elastic adhesive tape to the ankle tendons had no significant effect on the average MTC height, but tapes applied to the Achilles tendon and both tendons significantly reduced MTC variability. Taping decreased the variability of some lower limb joint angles, but taping did not induce significant changes in the activation levels of the shank muscles. These results demonstrate that elastic adhesive tape applied to the shank can reduce MTC variability with minimal resistance, inertia and cumbersomeness.

The Role of the Skin in Interoception: A Neglected Organ?

In the past 2 decades, interoception has received increasing attention in the fields of psychology and cognitive science, as well as neuroscience and physiology. A plethora of studies adopted the perception of cardiac signals as a proxy for interoception. However, recent findings have cast doubt on the methodological and intrinsic validity of the tasks used thus far. Therefore, there is an ongoing effort to improve the existing cardiac interoceptive tasks and to identify novel channels to target the perception of the physiological state of the body. Amid such scientific abundancy, one could question whether the field has been partially neglecting one of our widest organs in terms of dimensions and functions: the skin. According to some views grounded on anatomical and physiological evidence, skin-mediated signals such as affective touch, pain, and temperature have been redefined as interoceptive. However, there is no agreement in this regard. Here, we discuss some of the anatomical, physiological, and experimental arguments supporting the scientific study of interoception by means of skin-mediated signals. We argue that more attention should be paid to the skin as a sensory organ that monitors the bodily physiological state and further propose thermosensation as a particularly attractive model of skin-mediated interoception.

Positional transversal release is effective as stretching on range of movement, performance and balance: a cross-over study

Background

The aim of this study was to compare the positional transversal release (PTR) technique to stretching and evaluate the acute effects on range of movement (ROM), performance and balance.

Methods

Thirty-two healthy individuals (25.3 ± 5.6 years; 68.8 ± 12.5 kg; 172.0 ± 8.8 cm) were tested on four occasions 1 week apart. ROM through a passive straight leg raise, jumping performance through a standing long jump (SLJ) and balance through the Y-balance test were measured. Each measure was assessed before (T0), immediately after (T1) and after 15 min (T2) of the provided intervention. On the first occasion, no intervention was administered (CG). The intervention order was randomized across participants and comprised static stretching (SS), proprioceptive neuromuscular facilitation (PNF) and the PTR technique. A repeated measure analysis of variance was used for comparisons.

Results

No differences across the T0 of the four testing sessions were observed. No differences between T0, T1 and T2 were present for the CG session. A significant time × group interaction for ROM in both legs from T0 to T1 (mean increase of 5.4° and 4.9° for right and left leg, respectively) was observed for SS, PNF and the PTR. No differences for all groups were present between T1 and T2. No differences in the SLJ and in measures of balance were observed across interventions.

Conclusions

The PTR is equally effective as SS and PNF in acutely increasing ROM of the lower limbs. However, the PTR results less time-consuming than SS and PNF. Performance and balance were unaffected by all the proposed interventions.

Human muscle spindles are wired to function as controllable signal-processing devices

Muscle spindles are encapsulated sensory organs found in most of our muscles. Prevalent models of sensorimotor control assume the role of spindles is to reliably encode limb posture and movement. Here, I argue that the traditional view of spindles is outdated. Spindle organs can be tuned by spinal γ motor neurons that receive top-down and peripheral input, including from cutaneous afferents. A new model is presented, viewing γ motor activity as an intermediate coordinate transformation that allows multimodal information to converge on spindles, creating flexible coordinate representations at the level of the peripheral nervous system. That is, I propose that spindles play a unique overarching role in the nervous system: that of a peripheral signal-processing device that flexibly facilitates sensorimotor performance, according to task characteristics. This role is compatible with previous findings and supported by recent studies with naturalistically active humans. Such studies have so far shown that spindle tuning enables the independent preparatory control of reflex muscle stiffness, the selective extraction of information during implicit motor adaptation, and for segmental stretch reflexes to operate in joint space. Incorporation of advanced signal-processing at the periphery may well prove a critical step in the evolution of sensorimotor control theories.

A Study on Artificial Kinesthesia Generation by Simultaneous Stimulation of Mechanical Vibration and Mechanical Skin Stretch

Artificially controlled kinesthesia can be applied to many situations because kinesthesia is essential to recognizing body movements. It could be used to generate artificial kinesthesia in rehabilitation or daily motion assist to improve self-efficacy of the robot user. Moreover, the controlled artificial kinesthesia could make people feel as if they are performing the actions of the robotic limbs with their own limbs. Mechanical vibration stimulation is one of the candidates to artificially control kinesthesia. It is known that mechanical vibration stimulation on human muscles or tendons from skin surface evokes an illusion of movement as if the stimulated muscles are extended. That effect of artificial kinesthesia is called Kinesthetic Illusion (KI). In this paper, a method to increase the amount of KI without changing the frequency of the vibration stimulation is investigated by applying mechanical skin stretch stimulation at the same time with the mechanical vibration stimulation. The experiment was conducted by generating KI for flexion motion of the elbow joint on a horizontal plane to evaluate the proposed approach. In the experiments, three out of five subjects showed obvious increase in the amount of KI when skin stretch stimulation was applied at the same time with the mechanical vibration stimulation. The results of this study provide a first step toward artificial kinesthesia control using a wearable robotic device using the mechanical vibration stimulation.

Effects of Intensive Vibratory Treatment with a Robotic System on the Recovery of Sensation and Function in Patients with Subacute and Chronic Stroke: A Non-Randomized Clinical Trial

Background: Sensory–motor deficits are frequent and affect the functionality after stroke. The use of robotic systems to improve functionality and motor performance is advisable; therefore, the aim of the present study was to evaluate the effects of intensive, high-frequency vibration treatment administered with a robotic system in subacute and chronic stroke patients in terms of upper limb sensitivity, motor function, quantity and quality of movement, and quality of life. Methods: A simple-blind, non-randomized controlled trial was conducted. The control group received conventional rehabilitation treatment and the experimental group received robotic treatment with an Amadeo^® robot in addition to their conventional rehabilitation sessions. Results: Intragroup analysis identified significant improvements in the experimental group in hand (p = 0.012), arm (p = 0.018), and shoulder (p = 0.027) sensitivity, as well as in motor function (FMA-UEmotor function, p = 0.028), integration of the affected limb (MAL-14amount scale, p = 0.011; MAL-14How well scale, p = 0.008), and perceived quality of life (SIS-16, p = 0.008). The measures between the control and experimental groups showed statistically significant differences in motor performance and spontaneous use of the affected limb (MAL-14amount scale, p = 0.021; MAL-14How well scale, p = 0.037). Conclusions: Intensive, high-frequency vibration with a robotic system, in combination with conventional intervention, improves the recovery of upper limb function in terms of quantity and quality of movement in patients with subacute and chronic stroke.

PIEZO2 ion channels in proprioception

PIEZO2 is a stretch-gated ion channel that is expressed at high levels in somatosensory neurons. Humans with rare mutations in the PIEZO2 gene have profound mechanosensory deficits that include a loss of the sense of proprioception. These striking phenotypes match those seen in conditional knockout mouse models demonstrating the highly conserved function for this gene. Here, we review the ramifications of loss of PIEZO2 function on normal daily activities and what studies like these have revealed about proprioception at the molecular and cellular level. Additionally, we highlight recent work that has uncovered the surprising functional and molecular diversity of proprioceptors. Together, these findings pioneer a path toward determining how the detection of mechanosensory input from muscles and tendons is used to control posture and refine motor performance.

The myokinetic stimulation interface: activation of proprioceptive neural responses with remotely actuated magnets implanted in rodent forelimb muscles

OBJECTIVE

Proprioception is the sense of one's position, orientation, and movement in space, and it is of fundamental importance for motor control. When proprioception is impaired or absent, motor execution becomes error-prone, leading to poorly coordinated movements. The kinaesthetic illusion, which creates perceptions of limb movement in humans through non-invasively applying vibrations to muscles or tendons, provides an avenue for studying and restoring the sense of joint movement (kinaesthesia). This technique, however, leaves ambiguity between proprioceptive percepts that arise from muscles versus those that arise from skin receptors. Here we propose the concept of a stimulation system to activate kinaesthesia through the untethered application of localized vibration through implanted magnets.

APPROACH

In this proof-of-concept study, we use two simplified 1-DoF systems to show the feasibility of eliciting muscle-sensory responses in an animal model across multiple frequencies, including those that activate the kinaesthetic illusion (70 - 115 Hz). Furthermore, we generalized the concept by developing a 5-DoF prototype system capable of generating directional, frequency-selective vibrations with desired displacement profiles.

MAIN RESULTS

In-vivo tests with the 1-DoF systems demonstrated the feasibility to elicit muscle sensory neural responses in the median nerve of an animal model. Instead, in-vitro tests with the 5-DoF prototype demonstrated high accuracy in producing directional and frequency selective vibrations along different magnet axes.

SIGNIFICANCE

These results provide evidence for a new technique that interacts with the native neuro-muscular anatomy to study proprioception and eventually pave the way towards the development of advanced limb prostheses or assistive devices for the sensory impaired.

Alteration of Ligamento-Muscular Reflex Patterns After Cutaneous and Periarticular Desensitization of the Basal Thumb Joint: An Electromyographic Study

PURPOSE

Stimulation of the dorsoradial ligament (DRL) of the first carpometacarpal joint (CMC-1) has shown a ligamento-muscular reflex pathway between the DRL and CMC-1 stabilizing muscles in healthy volunteers. However, it remains unclear how this ligamento-muscular reflex pattern is altered after anesthetizing sensory skin receptors and administering a further periarticular block around the CMC-1 joint, which may influence the dynamic aspects of joint stability.

METHODS

Ligamento-muscular reflexes were obtained from the extensor pollicis longus, abductor pollicis longus, abductor pollicis brevis, and the first dorsal interosseous muscles in 10 healthy participants after establishing superficial anesthesia of the skin around the CMC-1. The DRL was stimulated with a fine wire electrode while EMG activities were recorded during isometric tip, key, and palmar pinch. The measurements were repeated after an additional periarticular CMC-1 block using 5 ml of 1% lidocaine. Average EMG values were analyzed to compare the prestimulus and poststimulus activity.

RESULTS

Statistically significant changes in poststimulus EMG activity were observed in all 4 muscles and all 3 tested thumb positions. A markedly reduced activity in all 4 muscles was observed in the palmar position, followed by the tip and key pinch positions. Almost no reactions were observed in the first 20 ms poststimulus for all muscles in all positions.

CONCLUSIONS

Superficial skin anesthesia and an additional periarticular CMC-1 block anesthesia resulted in a reduced ligamento-muscular reflex pattern in all 4 muscles.

CLINICAL RELEVANCE

Ligamento-muscular reflexes play an important role in dynamic CMC-1 joint stability. The elimination of early reactions, those considered joint-protective reflexes, is a potential risk factor for developing osteoarthritis or injury because it results in an inability to adequately protect and stabilize the joint in sudden movements.

Immediate voluntary activation deficits following submaximal eccentric contractions of knee extensors are associated with alterations of the sense of movement

The mechanisms underlying movement sense alterations following repeated eccentric contractions remain unclear. This study concomitantly investigated the effects of unilateral eccentric contractions on movement sense and on neuromuscular function at the knee before, immediately after (POST), 24 (POST24) and 48 (POST48) h after the exercise. Twelve participants performed sets of submaximal knee extensors (KE) eccentric contractions until a 20% decrease in maximal voluntary isometric contraction (MVIC) torque was reached. Threshold to detect passive movement (TTDPM) tasks were used to assess movement sense during both knee flexion (TTDPM_FLEX) and extension (TTDPM_EXT). KE fatigability was assessed using the interpolated twitch technique. TTDPM values expressed in seconds and the percentage of unsuccessful trials only increased at POST during TTDPM_FLEX and TTDPM_EXT. The 20%-MVIC decrease was associated with significant decreases in voluntary activation level (- 12.7%, p < 0.01) and potentiated doublet torque at 100 Hz (- 18.1%, p < 0.001). At POST24, despite persistent reductions of maximal voluntary and electrically evoked torques associated with increased perceived muscle soreness, TTDPM values and the percentage of unsuccessful trials returned to baseline values. Consequently, movement sense alterations were only observed in the presence of voluntary activation deficits, suggesting that some exercise-induced central alterations may affect the somatosensory function.

Proprioceptive sensitivity to imposed finger deflections

Hand proprioception, the sense of the posture and movements of the wrist and digits, is critical to dexterous manual behavior and to stereognosis, the ability to sense the three-dimensional structure of objects held in the hand. To better understand this sensory modality and its role in hand function, we sought to characterize the acuity with which the postures and movements of finger joints are sensed. To this end, we measured the ability of human subjects to discriminate changes in posture and speed around the three joints of the index finger. In these experiments, we isolated the sensory component by imposing the postures on an otherwise still hand, to complement other studies in which subjects made judgments on actively achieved postures. We found that subjects could reliably sense 12-16% changes in joint angle and 18-32% changes in joint speed. Furthermore, the acuity for posture and speed was comparable across the three joints of the finger. Finally, task performance was unaffected by the presence of a vibratory stimulus, calling into question the role of cutaneous cues in hand proprioception.NEW & NOTEWORTHY Manual dexterity and stereognosis are supported by two exquisite sensory systems, namely touch and proprioception. Here, we measure the sensitivity of hand proprioception and show that humans can sense the posture and movements of the fingers with great accuracy. We also show that application of a skin vibration does not impair sensitivity, suggesting that proprioceptive acuity relies primarily on receptors in the muscles (and possibly tendons) rather than the skin.

Heating the Skin Over the Knee Improves Kinesthesia During Knee Extension

To determine how heating affects dynamic joint position sense at the knee, participants (n = 11; F = 6) were seated in a HUMAC NORM dynamometer. The leg was passively moved through extension and flexion, and participants indicated when the 90° reference position was perceived, both at baseline (28.74 ± 2.43 °C) and heated (38.05 ± 0.16 °C) skin temperatures. Day 2 of testing reduced knee skin feedback with lidocaine. Directional error (actual leg angle–target angle) and absolute error (AE) were calculated. Heating reduced extension AE (baseline AE = 5.46 ± 2.39°, heat AE = 4.10 ± 1.97°), but not flexion. Lidocaine did not significantly affect flexion AE or extension AE. Overall, increased anterior knee-skin temperature improves dynamic joint position sense during passive knee extension, where baseline matching is poorer. Limited application of lidocaine to the anterior thigh, reducing some skin input, did not influence dynamic joint position sense, suggesting cutaneous receptors may play only a secondary role to spindle information during kinesthetic tasks. Importantly, cutaneous input from adjacent thigh regions cannot be ruled out as a contributor.

SEBT in individuals with patellofemoral pain: an integrative review

Abstract Introduction: Decreased postural stability can be observed in individuals with patellofemoral pain (PP). The Star Excursion Balance Test (SEBT) is widely used to assess deficits that need to be improved, with different application methods and result presentation formats. Objective: To map SEBT use in individuals with PP, characterizing the studies that applied it to identify different application methods and result presentation formats. Methods: The review included randomized and non-randomized clinical trials, cross-sectional, case-control and cohort studies. Searches were performed in Pubmed and SciElo databases. Data extracted from eligible studies were divided into categories: (I) study characterization (II) SEBT application methods and result presentation formats. Results: A total of 177 studies were identified in the databases, 13 of which were selected. There are a growing number of new studies that assess the dynamic postural control of individuals with PP using the SEBT, and a variety of test application and result presentation formats. Conclusion: The SEBT is a useful, easy-to-apply test that identifies changes in dynamic postural control in individuals with PP. Different application and result presentation formats are in accordance with the literature, but it is recommended that future studies apply the protocols most widely used in previous studies that exhibit a low risk of bias, in order to improve repeatability and comparisons between studies.

Comparison of Wide-Band Vibrotactile and Friction Modulation Surface Gratings

This article seeks to understand conditions under which virtual gratings produced via vibrotaction and friction modulation are perceived as similar and to find physical origins in the results. To accomplish this, we developed two single-axis devices, one based on electroadhesion and one based on out-of-plane vibration. The two devices had identical touch surfaces, and the vibrotactile device used a novel closed-loop controller to achieve precise control of out-of-plane plate displacement under varying load conditions across a wide ranget of frequencies. A first study measured the perceptual intensity equivalence curve of gratings generated under electroadhesion and vibrotaction across the 20-400 Hz frequency range. A second study assessed the perceptual similarity between two forms of skin excitation given the same driving frequency and same perceived intensity. Our results indicate that it is largely the out-of-plane velocity that predicts vibrotactile intensity relative to shear forces generated by friction modulation. A high degree of perceptual similarity between gratings generated through friction modulation and through vibrotaction is apparent and tends to scale with actuation frequency suggesting perceptual indifference to the manner of fingerpad actuation in the upper frequency range.

The interaction between motion and texture in the sense of touch

Besides providing information on elementary properties of objects, like texture, roughness, and softness, the sense of touch is also important in building a representation of object movement and the movement of our hands. Neural and behavioral studies shed light on the mechanisms and limits of our sense of touch in the perception of texture and motion, and of its role in the control of movement of our hands. The interplay between the geometrical and mechanical properties of the touched objects, such as shape and texture, the movement of the hand exploring the object, and the motion felt by touch, will be discussed in this article. Interestingly, the interaction between motion and textures can generate perceptual illusions in touch. For example, the orientation and the spacing of the texture elements on a static surface induces the illusion of surface motion when we move our hand on it or can elicit the perception of a curved trajectory during sliding, straight hand movements. In this work we present a multiperspective view that encompasses both the perceptual and the motor aspects, as well as the response of peripheral and central nerve structures, to analyze and better understand the complex mechanisms underpinning the tactile representation of texture and motion. Such a better understanding of the spatiotemporal features of the tactile stimulus can reveal novel transdisciplinary applications in neuroscience and haptics.

The effect of muscle fatigue on wrist joint position sense in healthy adults

STUDY DESIGN

Pretest and posttest experimental study.

INTRODUCTION

The effect of muscle fatigue on wrist joint position sense (JPS) has yet to be determined.

PURPOSE OF THE STUDY

The primary aim was to determine whether muscle fatigue affects wrist JPS in healthy adults. The secondary aims were to compare the effect of muscle fatigue on younger and older adults JPS and determine the association between JPS rate of change and total muscle fatigue (TMF) rates postexercise.

METHODS

Forty male and female healthy adults were assigned into younger (18-40 years) and older (41-65 years) groups. Preexercise and postexercise testing consisted of active wrist JPS, handgrip, and wrist extensor strength assessments. Muscle fatigue was induced via a calibrated gripper and wrist extension dumbbell exercises. Dependent variables were the JPS rate of change (ie, preexercise and postexercise), TMF rate (ie, grip and wrist extension average strength decline), and Borg Rating of Perceived Exertion scale scores.

RESULTS

Postexercise wrist JPS test scores were significantly higher than preexercise. Exercises induced statistically significant TMF rates and Borg Rating of Perceived Exertion scores among all participants. No statistically significant age-group differences on JPS rate of change, and TMF rate was found. A statistically significant mild correlation (r = 0.425) existed between JPS rate of change and TMF rates.

DISCUSSION

Postexercise fatigue significantly impairs wrist JPS in both younger and older adults. On average, an 18% muscle strength decline led to 215% wrist JPS deficit.

CONCLUSIONS

Significant wrist proprioception deficits persist for ≤5 min following exertional exercises, regardless of age level.

Encoding of limb state by single neurons in the cuneate nucleus of awake monkeys

The cuneate nucleus (CN) is among the first sites along the neuraxis where proprioceptive signals can be integrated, transformed, and modulated. The objective of the study was to characterize the proprioceptive representations in CN. To this end, we recorded from single CN neurons in three monkeys during active reaching and passive limb perturbation. We found that many neurons exhibited responses that were tuned approximately sinusoidally to limb movement direction, as has been found for other sensorimotor neurons. The distribution of their preferred directions (PDs) was highly nonuniform and resembled that of muscle spindles within individual muscles, suggesting that CN neurons typically receive inputs from only a single muscle. We also found that the responses of proprioceptive CN neurons tended to be modestly amplified during active reaching movements compared to passive limb perturbations, in contrast to cutaneous CN neurons whose responses were not systematically different in the active and passive conditions. Somatosensory signals thus seem to be subject to a "spotlighting" of relevant sensory information rather than uniform suppression as has been suggested previously.NEW & NOTEWORTHY The cuneate nucleus (CN) is the somatosensory gateway into the brain, and only recently has it been possible to record these signals from an awake animal. We recorded single CN neurons in monkeys. Proprioceptive CN neurons appear to receive input from very few muscles, and their sensitivity to movement changes reliably during reaching relative to passive arm perturbations. Sensitivity is generally increased, but not exclusively so, as though CN "spotlights" critical proprioceptive information during reaching.

A new approach of inducing proprioceptive illusion by transcutaneous electrical stimulation

BACKGROUND

Neurotraumas or neurodegenerative diseases often result in proprioceptive deficits, which makes it challenging for the nervous system to adapt to the compromised sensorimotor conditions. Also, in human machine interactions, such as prosthesis control and teleoperation, proprioceptive mismatch limits accuracy and intuitiveness of controlling active joints in robotic agents. To address these proprioceptive deficits, several invasive and non-invasive approaches like vibration, electrical nerve stimulation, and skin stretch have been introduced. However, proprioceptive modulation is still challenging as the current solutions have limitations in terms of effectiveness, usability, and consistency. In this paper, we propose a new way of modulating proprioception using transcutaneous electrical stimulation. We hypothesized that transcutaneous electrical stimulation on elbow flexor muscles will induce illusion of elbow joint extension.

METHOD

Eight healthy human subjects participated in the study to test the hypothesis. Transcutaneous electrodes were placed on different locations targeting elbow flexor muscles on human subjects and experiments were conducted to identify the best locations for electrode placement, and best electrical stimulation parameters, to maximize induced proprioceptive effect. Arm matching experiments and Pinocchio illusion test were performed for quantitative and qualitative analysis of the observed effects. One-way repeated ANOVA test was performed on the data collected in arm matching experiment for statistical analysis.

RESULTS

We identified the best location for transcutaneous electrodes to induce the proprioceptive illusion, as one electrode on the muscle belly of biceps brachii short head and the other on the distal myotendinous junction of brachioradialis. The results for arm-matching and Pinocchio illusion tests showed that transcutaneous electrical stimulation using identified electrode location and electrical stimulation parameters evoked the illusion of elbow joint extension for all eight subjects, which supports our hypothesis. On average, subjects reported 6.81° angular illusion of elbow joint extension in arm-matching tests and nose elongated to 1.78 × height in Pinocchio illusion test.

CONCLUSIONS

Transcutaneous electrical stimulation, applied between the the synergistic elbow flexor muscles, consistently modulated elbow joint proprioception with the illusion of elbow joint extension, which has immense potential to be translated into various real-world applications, including neuroprosthesis, rehabilitation, teleoperation, mixed reality, and etc.

Proprioceptive Acuity is Enhanced During Arm Movements Compared to When the Arm is Stationary: A Study of Young and Older Adults

Proprioception in old age is thought to be poorer due to degeneration of the central (CNS) and peripheral nervous systems (PNS). We tested whether community-dwelling older adults (65-83 years) make larger proprioceptive errors than young adults (18-22 years) using a natural reaching task. Subjects moved the right arm to touch the index fingertip to the stationary or moving left index fingertip. The range of locations of the target index fingertip was large, sampling the natural workspace of the human arm. The target arm was moved actively by the subject or passively by the experimenter and reaching arm movements towards the target were made under visual guidance, or with vision blocked (proprioceptive guidance). Subjects did not know the direction or speed of upcoming target hand motion in the passive conditions. Mean 3D distance errors between the right and left index finger tips were small in both groups and only slightly larger when vision was blocked than when allowed, but averaged 2-5 mm larger in older than in younger adults in moving (p = 0.002) and stationary (p = 0.07) conditions, respectively. Variable errors were small and similar in the two groups (p > 0.35). Importantly, clearly larger errors were observed for reaching to the stationary than to the moving index fingertip in both groups, demonstrating that dynamic proprioceptive information during movement permits more accurate localization of the endpoint of the moving arm. This novel finding demonstrates the importance of dynamic proprioceptive information in movement guidance and bimanual coordination.

Normative values of the vibration perception thresholds at finger pulps and metatarsal heads in healthy adults

AIMS

To establish normative values of vibration perception thresholds (VPTs), using multi-frequency vibrometry at finger pulps and at metatarsal heads of the foot in healthy adults. We also aimed to investigate factors that could potentially affect VPTs such as age, sex, height, weight, foot- or handedness and skin temperature.

METHODS

VPTs were examined in 924 healthy and randomly selected subjects in the southern Sweden (mean 46 years; 628 women and 296 men). Inclusion criterias were adult subjects (>18 years) in considerable health without diabetes mellitus or other nerve affecting disorders. VPTs were measured at the finger pulps of index and little finger, as well as the first and fifth metatarsal heads of the foot, through multi-frequency vibrometry using the VibroSense Meter® I device. Patient characteristics were recorded and skin temperature was measured before assessment of VPTs.

RESULTS

We present normative values of VPTs for a large population of both male and female subjects in various ages. VPTs detoriated as age increased (0.09-0.59 dB per year; p<0.001), i.e. progressing with normal aging. Increasing skin temperature affected VPTs in finger pulps, but not at metatarsal heads, with -0.2 to -1.6 dB, i.e. vibration perception improved with higher temperatures. Height was only found to affect the VPTs of metatarsal heads (250 Hz: 0.42 dB per cm). Sex, weight and handedness did not affect the VPTs.

CONCLUSION

We investigated the normative values of VPTs and presented affecting factors as age, skin temperature and height. With these results, VPT testing through multi-frequency vibrometry is enabled to be used in a clinical practice as a diagnostic tool when investigating neuropathy and other neurological disorders.

Coordinated Reset Vibrotactile Stimulation Induces Sustained Cumulative Benefits in Parkinson's Disease

BACKGROUND

Abnormal synchronization of neuronal activity in dopaminergic circuits is related to motor impairment in Parkinson's disease (PD). Vibrotactile coordinated reset (vCR) fingertip stimulation aims to counteract excessive synchronization and induce sustained unlearning of pathologic synaptic connectivity and neuronal synchrony. Here, we report two clinical feasibility studies that examine the effect of regular and noisy vCR stimulation on PD motor symptoms. Additionally, in one clinical study (study 1), we examine cortical beta band power changes in the sensorimotor cortex. Lastly, we compare these clinical results in relation to our computational findings.

METHODS

Study 1 examines six PD patients receiving noisy vCR stimulation and their cortical beta power changes after 3 months of daily therapy. Motor evaluations and at-rest electroencephalographic (EEG) recordings were assessed off medication pre- and post-noisy vCR. Study 2 follows three patients for 6+ months, two of whom received daily regular vCR and one patient from study 1 who received daily noisy vCR. Motor evaluations were taken at baseline, and follow-up visits were done approximately every 3 months. Computationally, in a network of leaky integrate-and-fire (LIF) neurons with spike timing-dependent plasticity, we study the differences between regular and noisy vCR by using a stimulus model that reproduces experimentally observed central neuronal phase locking.

RESULTS

Clinically, in both studies, we observed significantly improved motor ability. EEG recordings observed from study 1 indicated a significant decrease in off-medication cortical sensorimotor high beta power (21-30 Hz) at rest after 3 months of daily noisy vCR therapy. Computationally, vCR and noisy vCR cause comparable parameter-robust long-lasting synaptic decoupling and neuronal desynchronization.

CONCLUSION

In these feasibility studies of eight PD patients, regular vCR and noisy vCR were well tolerated, produced no side effects, and delivered sustained cumulative improvement of motor performance, which is congruent with our computational findings. In study 1, reduction of high beta band power over the sensorimotor cortex may suggest noisy vCR is effectively modulating the beta band at the cortical level, which may play a role in improved motor ability. These encouraging therapeutic results enable us to properly plan a proof-of-concept study.

Stability of Action and Kinesthetic Perception in Parkinson's Disease

We present a review of action and perception stability within the theoretical framework based on the idea of control with spatial referent coordinates for the effectors at a number of hierarchical levels. Stability of salient variables is ensured by synergies, neurophysiological structures that act in multi-dimensional spaces of elemental variables and limit variance to the uncontrolled manifold during action and iso-perceptual manifold during perception. Patients with Parkinson’s disease show impaired synergic control reflected in poor stability (low synergy indices) and poor agility (low indices of anticipatory synergy adjustments prior to planned quick actions). They also show impaired perception across modalities, including kinesthetic perception. We suggest that poor stability at the level of referent coordinates can be the dominant factor leading to poor stability of percepts.

Relative Contribution of Proprioceptive and Vestibular Sensory Systems to Locomotion: Opportunities for Discovery in the Age of Molecular Science

Locomotion is a fundamental animal behavior required for survival and has been the subject of neuroscience research for centuries. In terrestrial mammals, the rhythmic and coordinated leg movements during locomotion are controlled by a combination of interconnected neurons in the spinal cord, referred as to the central pattern generator, and sensory feedback from the segmental somatosensory system and supraspinal centers such as the vestibular system. How segmental somatosensory and the vestibular systems work in parallel to enable terrestrial mammals to locomote in a natural environment is still relatively obscure. In this review, we first briefly describe what is known about how the two sensory systems control locomotion and use this information to formulate a hypothesis that the weight of the role of segmental feedback is less important at slower speeds but increases at higher speeds, whereas the weight of the role of vestibular system has the opposite relation. The new avenues presented by the latest developments in molecular sciences using the mouse as the model system allow the direct testing of the hypothesis.

Relationship between lamellar sensory corpuscles distributed along the upper arm's deep arteries and pulsating sensation of blood vessels

Vibration is detected by mechanoreceptors, including Pacinian corpuscles (PCs), which are widely distributed in the human body including the adventitia of large blood vessels. Although the distribution of PCs around large limb vessels has been previously reported, there remains no consensus on their distribution in the adventitia of the human deep blood vessels in the upper arm. In addition, the physiological functions of PCs located around the deep limb blood vessels remain largely unknown. This study aimed to elucidate detailed anatomical features and physiological function of lamellar sensory corpuscles structurally identified as PCs using the immunohistochemical methods around the deep vessels in the upper arm. We identified PCs in the connective tissue adjacent to the deep vessels in the upper arm using histological analysis and confirmed that PCs are located in the vascular sheath of the artery and its accompanying vein as well as in the connective tissue surrounding the vascular sheath and nerves. PCs were densely distributed on the distal side of deep vessels near the elbow. We also examined the relationship between vascular sound and pulsating sensation to evaluate the PCs functions around deep arteries and veins and found that the vascular sound made by pressing the brachial arteries in the upper arm was associated with the pulsating sensation of the examinee. Our results suggest that PCs, around deep vessels, function as bathyesthesia sensors by detecting vibration from blood vessels.

Interjoint coupling of position sense reflects sensory contributions of biarticular muscles

Perception of limb position and motion combines sensory information from spindles in muscles that span one joint (monoarticulars) and two joints (biarticulars). This anatomical organization should create interactions in estimating limb position. We developed two models, one with only monoarticulars and one with both monoarticulars and biarticulars, to explore how biarticulars influence estimates of arm position in hand (x, y) and joint (shoulder, elbow) coordinates. In hand coordinates, both models predicted larger medial-lateral than proximal-distal errors, although the model with both muscle groups predicted that biarticulars would reduce this bias. In contrast, the two models made significantly different predictions in joint coordinates. The model with only monoarticulars predicted that errors would be uniformly distributed because estimates of angles at each joint would be independent. In contrast, the model that included biarticulars predicted that errors would be coupled between the two joints, resulting in smaller errors for combinations of flexion or extension at both joints and larger errors for combinations of flexion at one joint and extension at the other joint. We also carried out two experiments to examine errors made by human subjects during an arm position matching task in which a robot passively moved one arm to different positions and the subjects moved their other arm to mirror-match each position. Errors in hand coordinates were similar to those predicted by both models. Critically, however, errors in joint coordinates were only similar to those predicted by the model with monoarticulars and biarticulars. These results highlight how biarticulars influence perceptual estimates of limb position by helping to minimize medial-lateral errors.NEW & NOTEWORTHY It is unclear how sensory information from muscle spindles located within muscles spanning multiple joints influences perception of body position and motion. We address this issue by comparing errors in estimating limb position made by human subjects with predicted errors made by two musculoskeletal models, one with only monoarticulars and one with both monoarticulars and biarticulars. We provide evidence that biarticulars produce coupling of errors between joints, which help to reduce errors.

Effect of primary motor cortex excitability changes after quadripulse transcranial magnetic stimulation on kinesthetic sensitivity: A preliminary study

Muscle spindles provide the greatest contribution to kinesthetic perception. Primary motor cortex (M1) excitability changes in parallel with the intensity of kinesthetic perception inputs from muscle spindles; M1 is therefore involved in kinesthetic perception. However, the causal relationship between changes in kinesthetic sensitivity and M1 excitability is unclear. The purpose of this study was to test whether artificially and sustainably modulated M1 excitability causes changes in kinesthetic sensitivity in healthy individuals. We evaluated motor evoked potentials (MEP) in Experiment 1 and joint motion detection thresholds (JMDT) in Experiment 2 before and after quadripulse transcranial magnetic stimulation (QPS). Nine healthy right-handed male volunteers were recruited. In each experiment, participants received QPS or sham stimulation (Sham) on separate days. MEP amplitude and JMDT were recorded before and at 0, 15, 30, 45, and 60 min after QPS and Sham. Our results showed that M1 excitability and kinesthetic sensitivity increased after QPS, whereas neither changed after Sham. In the five subjects who participated in both experiments, there was a significant moderate correlation between M1 excitability and kinesthetic sensitivity. Thus, the long-lasting change in kinesthetic sensitivity may be due to changes in M1 excitability. In addition, M1 may play a gain adjustment role in the neural pathways of muscle spindle input.

Somatosensory target information is used for reaching but not for saccadic eye movements

A systematic investigation of contributions of different somatosensory modalities (proprioception, kinesthesia, tactile) for goal-directed movements is missing. Here we demonstrate that while eye movements are not affected by different types of somatosensory information, reach precision improves when two different types of information are available. Moreover, reach accuracy and gaze precision to unseen somatosensory targets improve when performing coordinated eye-hand movements, suggesting bidirectional contributions of efferent information in reach and eye movement control.

Contributions of exercise-induced fatigue versus intertrial tendon vibration on visual-proprioceptive weighting for goal-directed movement

It has been argued that exercise-induced muscle fatigue and tendon vibration can alter proprioceptive estimates of limb position. While exercise-induced muscle fatigue may also affect central efferent processes related to limb position sense, tendon vibration specifically targets peripheral afferent signals. It is unclear, however, whether either of these perturbations (i.e., muscle fatigue or tendon vibration) can alter the multisensory weighting processes preceding goal-directed movements. The current study sought to specifically explore visual-proprioceptive weighting before or after eccentric exercise-induced antagonist muscle fatigue (experiment 1) versus with or without intertrial simultaneous agonist-antagonist tendon vibration (experiment 2). To assess sensory weighting, a visual-proprioceptive mismatch between the participant's actual initial starting position and the associated visual cursor position was employed. This method provides an estimate of the participant's reliance on the proprioceptive or visual starting limb position for their aiming movements. Although there was clear evidence of muscle fatigue, there was no systematic alteration of proprioceptive weighting after eccentric exercise and no relationship between sensory weighting and the level of fatigue. On the other hand, participants' reliance on their actual (proprioceptive) limb position was systematically reduced when exposed to agonist-antagonist tendon vibration before each aiming movement. These findings provide seminal evidence that intertrial tendon vibration, but not exercise-induced fatigue, can alter the reliability of proprioceptive estimates and the relative contributions of visual and proprioceptive information for goal-directed movement.NEW & NOTEWORTHY Previous work has used muscle fatigue or tendon vibration to perturb proprioceptive limb position estimates. This study sought to determine whether exercise-induced muscle fatigue versus intertrial tendon vibration can alter multisensory weighting for upper limb-aiming movements. By introducing a discrepancy between participants' actual proprioceptive and visual finger position, this study provides seminal evidence for the reduction of proprioceptive-to-visual weighting using intertrial tendon vibration but no evidence for a systematic reduction following exercise-induced fatigue.

Robotic tests for position sense and movement discrimination in the upper limb reveal that they each are highly reproducible but not correlated in healthy individuals

BACKGROUND

Robotic technologies for neurological assessment provide sensitive, objective measures of behavioural impairments associated with injuries or disease such as stroke. Previous robotic tasks to assess proprioception typically involve single limbs or in some cases both limbs. The challenge with these approaches is that they often rely on intact motor function and/or working memory to remember/reproduce limb position, both of which can be impaired following stroke. Here, we examine the feasibility of a single-arm Movement Discrimination Threshold (MDT) task to assess proprioception by quantifying thresholds for sensing passive limb movement without vision. We use a staircase method to adjust movement magnitude based on subject performance throughout the task in order to reduce assessment time. We compare MDT task performance to our previously-designed Arm Position Matching (APM) task. Critically, we determine test-retest reliability of each task in the same population of healthy controls.

METHOD

Healthy participants (N = 21, age = 18-22 years) completed both tasks in the End-Point Kinarm robot. In the MDT task the robot moved the dominant arm left or right and participants indicated the direction moved. Movement displacement was systematically adjusted (decreased after correct answers, increased after incorrect) until the Discrimination Threshold was found. In the APM task, the robot moved the dominant arm and participants "mirror-matched" with the non-dominant arm.

RESULTS

Discrimination Threshold for direction of arm displacement in the MDT task ranged from 0.1-1.3 cm. Displacement Variability ranged from 0.11-0.71 cm. Test-retest reliability of Discrimination Threshold based on ICC confidence intervals was moderate to excellent (range, ICC = 0.78 [0.52-0.90]). Interestingly, ICC values for Discrimination Threshold increased to 0.90 [0.77-0.96] (good to excellent) when the number of trials was reduced to the first 50. Most APM parameters had ICC's above 0.80, (range, ICC = [0.86-0.88]) with the exception of variability (ICC = 0.30). Importantly, no parameters were significantly correlated across tasks as Spearman rank correlations across parameter-pairings ranged from - 0.27 to 0.30.

CONCLUSIONS

The MDT task is a feasible and reliable task, assessing movement discrimination threshold in ~ 17 min. Lack of correlation between the MDT and a position-matching task (APM) indicates that these tasks assess unique aspects of proprioception that are not strongly related in young, healthy individuals.

Elbow proprioception is normal in patients with a congenital absence of functional muscle spindles

KEY POINTS

Individuals with hereditary sensory and autonomic neuropathy type III (HSAN III), also known as Riley-Day syndrome or familial dysautonomia, do not have functional muscle spindle afferents but do have essentially normal cutaneous mechanoreceptors. Lack of muscle spindle feedback from the legs may account for the poor proprioception at the knee and the ataxic gait typical of HSAN III. Given that functional muscle spindle afferents are also absent in the upper limb, we assessed whether proprioception at the elbow was likewise compromised. Passive joint angle matching showed that proprioception was normal at the elbow, suggesting that individuals with HSAN III rely more on cutaneous afferents around the elbow.

ABSTRACT

Hereditary sensory and autonomic neuropathy type III (HSAN III) is a rare neurological condition that features a marked ataxic gait that progressively worsens over time. We have shown that functional muscle spindle afferents are absent in the upper and lower limbs in HSAN III, and we have argued that this may account for the ataxia. We recently used passive joint angle matching to demonstrate that proprioception of the knee joint is very poor in HSAN III but can be improved towards normal by application of elastic kinesiology tape across the knee joints, which we attribute to the presence of intact cutaneous mechanoreceptors. Here we assessed whether proprioception was equally compromised at the elbow joint, and whether it could be improved through taping. Proprioception at the elbow joint was assessed using passive joint angle matching in 12 HSAN III patients and 12 age-matched controls. There was no difference in absolute error, gradient or correlation coefficient of the relationship between joint angles of the reference and indicator arms. Unlike at the knee, taping did not improve elbow proprioception in either group. Clearly, the lack of muscle spindles compromised proprioception at the knee but not at the elbow, and we suggest that the HSAN III patients rely more on proprioceptive signals from the skin around the elbow.