Differential contributions of vision, touch and muscle proprioception to the coding of hand movements

Reinforcement Learning May Demystify the Limited Human Motor Learning Efficacy Due to Visual-Proprioceptive Mismatch

Vision and proprioception have fundamental sensory mismatches in delivering locational information, and such mismatches are critical factors limiting the efficacy of motor learning. However, it is still not clear how and to what extent this mismatch limits motor learning outcomes. To further the understanding of the effect of sensory mismatch on motor learning outcomes, a reinforcement learning algorithm and the simplified biomechanical elbow joint model were employed to mimic the motor learning process in a computational environment. By applying a reinforcement learning algorithm to the motor learning of elbow joint flexion task, simulation results successfully explained how visual-proprioceptive mismatch limits motor learning outcomes in terms of motor control accuracy and task completion speed. The larger the perceived angular offset between the two sensory modalities, the lower the motor control accuracy. Also, the more similar the peak reward amplitude of the two sensory modalities, the lower the motor control accuracy. In addition, simulation results suggest that insufficient exploration rate limits task completion speed, and excessive exploration rate limits motor control accuracy. Such a speed-accuracy trade-off shows that a moderate exploration rate could serve as another important factor in motor learning.

When proprioceptive feedback enhances visual perception of self-body movement: rehabilitation perspectives

Introduction

Rehabilitation approaches take advantage of vision's important role in kinesthesia, using the mirror paradigm as a means to reduce phantom limb pain or to promote recovery from hemiparesis. Notably, it is currently applied to provide a visual reafferentation of the missing limb to relieve amputees' pain. However, the efficiency of this method is still debated, possibly due to the absence of concomitant coherent proprioceptive feedback. We know that combining congruent visuo-proprioceptive signals at the hand level enhances movement perception in healthy people. However, much less is known about lower limbs, for which actions are far less visually controlled in everyday life than upper limbs. Therefore, the present study aimed to explore, with the mirror paradigm, the benefit of combined visuo-proprioceptive feedback from the lower limbs of healthy participants.

Methods

We compared the movement illusions driven by visual or proprioceptive afferents and tested the extent to which adding proprioceptive input to the visual reflection of the leg improved the resulting movement illusion. To this end, 23 healthy adults were exposed to mirror or proprioceptive stimulation and concomitant visuo-proprioceptive stimulation. In the visual conditions, participants were asked to voluntarily move their left leg in extension and look at its reflection in the mirror. In the proprioceptive conditions, a mechanical vibration was applied to the hamstring muscle of the leg hidden behind the mirror to simulate an extension of the leg, either exclusively or concomitantly, to the visual reflection of the leg in the mirror.

Results

(i) Visual stimulation evoked leg movement illusions but with a lower velocity than the actual movement reflection on the mirror; (ii) proprioceptive stimulation alone provided more salient illusions than the mirror illusion; and (iii) adding a congruent proprioceptive stimulation improved the saliency, amplitude, and velocity of the illusion.

Conclusion

The present findings confirm that visuo-proprioceptive integration occurs efficiently when the mirror paradigm is coupled with mechanical vibration at the lower limbs, thus providing promising new perspectives for rehabilitation.

Mechanosensory encoding dysfunction emerges from cancer-chemotherapy interaction

Persistent sensory, motor and cognitive disabilities comprise chemotherapy-induced neural disorders (CIND) that limit quality of life with little therapeutic relief for cancer survivors. Our recent preclinical study provides new insight into a condition impacting the severity of chronic CIND. We find that sensorimotor disability observed following cancer treatment exceeds that attributable to chemotherapy alone. A possible explanation for intensified disability emerged from evidence that codependent effects of cancer and chemotherapy amplify defective firing in primary sensory neurons supplying one type of low threshold mechanosensory receptor (LTMR). Here we test whether cancer's modification of chemotherapy-induced sensory defects generalizes across eight LTMR submodalities that collectively generate the signals of origin for proprioceptive and tactile perception and guidance of body movement. Preclinical study enabled controlled comparison of the independent contributions of chemotherapy and cancer to their clinically relevant combined effects. We compared data sampled from rats that were otherwise healthy or bearing colon cancer and treated, or not, with human-scaled, standard-of-care chemotherapy with oxaliplatin. Action potential firing patterns encoding naturalistic mechanical perturbations of skeletal muscle and skin were measured electrophysiologically in vivo from multiple types of LTMR neurons. All expressed aberrant encoding of dynamic and/or static features of mechanical stimuli in healthy rats treated with chemotherapy, and surprisingly also by some LTMRs in cancer-bearing rats that were not treated. By comparison, chemotherapy and cancer in combination worsened encoding aberrations, especially in slowly adapting LTMRs supplying both muscle and glabrous skin. Probabilistic modeling best predicted observed encoding defects when incorporating interaction effects of cancer and chemotherapy. We conclude that for multiple mechanosensory submodalities, the severity of encoding defects is modulated by a codependence of chemotherapy side effects and cancer's systemic processes. We propose that the severity of CIND might be reduced by therapeutically targeting the mechanisms, yet to be determined, by which cancer magnifies chemotherapy's neural side effects as an alternative to reducing chemotherapy and its life-saving benefits.

Differing Characteristics of Human-Shaped Visual Stimuli Affect Clinicians' Dosage of a Spinal Manipulative Thrust on a Low-Fidelity Model: A Cross-Sectional Study

OBJECTIVE

The purpose of this study was to determine whether chiropractic clinicians modulate spinal manipulation (SM) thrust characteristics based on visual perception of simulated human silhouette attributes.

METHODS

We performed a cross-sectional within-participant design with 8 experienced chiropractors. During each trial, participants observed a human-shaped life-sized silhouette of a mock patient and delivered an SM thrust on a low-fidelity thoracic spine model based on their visual perception. Silhouettes varied on the following 3 factors: apparent sex (male or female silhouette), height (short, average, tall), and body mass index (BMI) (underweight, healthy, obese). Each combination was presented 6 times for a total of 108 trials in random order. Outcome measures included peak thrust force, thrust duration, peak preload force, peak acceleration, time to peak acceleration, and rate of force application. A 3-way repeated measures analysis of variance model was used to for each variable, followed by Tukey's honestly significant difference on significant interactions.

RESULTS

Peak thrust force was reduced when apparent sex of the presented silhouette was female (F_1,7 = 5.70, P = .048). Thrust duration was largely invariant, except that a BMI by height interaction revealed a longer duration occurred for healthy tall participants than healthy short participants (F_4,28 = 4.34, P = .007). Compared to an image depicting obese BMI, an image appearing underweight lead to reduced peak acceleration (F_2,5 = 6.756, P = .009). Clinician time to peak acceleration was reduced in short compared to tall silhouettes (t₇ = 2.20, P = .032).

CONCLUSION

Visual perception of simulated human silhouette attributes, including apparent sex, height, and BMI, influenced SM dose characteristics through both kinetic and kinematic measures. The results suggest that visual information from mock patients affects the decision-making of chiropractic clinicians delivering SM thrusts.

Involvement of visual signals in kinaesthesia: A virtual reality study

Body movements are invariably accompanied by various proprioceptive, visual, tactile and/or motor signals. It is therefore difficult to completely dissociate these various signals from each other in order to study their specific involvement in the perception of movement (kinaesthesia). Here, we manipulated visual motion signals in a virtual reality display by using a humanoid avatar. The visual signals of movement could therefore be manipulated freely, relative to the participant's actual movement or lack of movement. After an embodiment phase in which the avatar's movements were coupled to the participant's voluntary movements, kinaesthetic illusions were evoked by moving the avatar's right forearm (flexion or extension) while the participant's right arm remained static. The avatar's left forearm was hidden from view. In parallel, somaesthetic signals could be masked by agonist-antagonist co-vibration or be amplified (by agonist vibration only or antagonist vibration only) so that the real impact of visual cues of movement in kinaesthesia could be studied. In a study of 24 participants, masking the somaesthetic signals (which otherwise provide signals indicating that the arm is static) was associated with a greater intensity and shorter latency of the visually evoked illusions. These results confirm the importance of carefully considering somaesthetic signals when assessing the contribution of vision to kinaesthesia. The use of a combination of virtual reality and somaesthetic signal manipulation might be of clinical value.

The Effect of Proprioception Training on Pain Intensity in Thumb Basal Joint Osteoarthritis: A Randomized Controlled Trial

A randomized controlled trial of forty-five females over 18 years of age with diagnosis of thumb basal osteoarthritis in their dominant hand and with a minimum pain rating of 4/10 on the Visual Analogue Scale (VAS) during activities of daily living (ADLs) were recruited from March to June 2021. The group receiving proprioception training was compared to routine conservative physiotherapy treatment. The main purpose of this clinical trial is to test the effect of proprioception training on pain intensity in subjects with thumb osteoarthritis. Primary outcome was joint position sense (JPS) for the assessment of CMC proprioception and secondary outcomes were Visual Analogue Scale (VAS) and Canadian Occupational Performance Measure (COPM) for the assessment of patient satisfaction and the Quick-DASH which assessed upper limb function. A block randomization was carried out for the control group (n = 22) and experimental group (n = 23). Participants and evaluator were blinded to the group assignment. Proprioception training produced a statistically significant reduction in pain post intervention, but this reduction was small (d = 0.1) at the 3-month follow-up. JPS accuracy demonstrated statistically significant differences between the groups (p = 0.001) post-intervention and at the 3-month follow-up (p < 0.003). Statistically significant differences between means were found in both the Quick-Dash and COPM post intervention (both, p < 0.001), as well as at the 3-month follow-up (both, p < 0.001). There was a significant time factor for the reduction of pain intensity over time but effect sizes between groups was small at the 3-month follow-up period. Proprioceptive training improves thumb JPS accuracy; however, it does not contribute to a reduction in pain intensity in the long term. The inclusion of a proprioceptive program may be beneficial for improving individuals with thumb CMC OA sensorimotor performance. The study was registered at ClinicalTrials.gov NCT04738201. No funding was provided for this study.

The Refraction Assessment and the Electronic Trial Frame Measurement during Standing or Sitting Position Can Affect Postural Stability

Vision has been shown to influence body posture. The purpose of this study is to investigate the correlations between visual acuity and body postural control both in a standing and seated position. This cohort study included 37 patients examined using Adaptica’s (Italy) Kaleidos and VisionFit. Objective refraction was measured with Kaleidos both in a standing and seated position by the same operator and in the same environmental conditions. The parameters obtained with the device were binocular refraction, monocular refraction, pupil distance, pupil size, head tilt, gaze, phorias, and tropias. The results obtained were then subjectively tested using VisionFit: an electronic trial frame with phoropter functionalities. The study’s outcome revealed that the differences in the visual acuity parameters obtained in standing and seated positions were statistically significant; the Student’s t-test showed a p-value < 0.001 in all parameter averages. Automated refraction is widely being performed and postural control can affect the visual acuity parameters; therefore, it is relevant to consider the possibility of measuring in orthostatism. It might be appropriate to take into account the possibility of measuring in orthostatism and wearing trial frames in orthostatic conditions as well as walking freely around the room, looking outside of a window, sitting, and reading.

Neural circuit mechanisms of sensorimotor disability in cancer treatment

Cancer survivors rank sensorimotor disability among the most distressing, long-term consequences of chemotherapy. Disorders in gait, balance, and skilled movements are commonly assigned to chemotoxic damage of peripheral sensory neurons without consideration of the deterministic role played by the neural circuits that translate sensory information into movement. This oversight precludes sufficient, mechanistic understanding and contributes to the absence of effective treatment for reversing chemotherapy-induced disability. We rectified this omission through the use of a combination of electrophysiology, behavior, and modeling to study the operation of a spinal sensorimotor circuit in vivo in a rat model of chronic, oxaliplatin (chemotherapy)-induced neuropathy (cOIN). Key sequential events were studied in the encoding of propriosensory information and its circuit translation into the synaptic potentials produced in motoneurons. In cOIN rats, multiple classes of propriosensory neurons expressed defective firing that reduced accurate sensory representation of muscle mechanical responses to stretch. Accuracy degraded further in the translation of propriosensory signals into synaptic potentials as a result of defective mechanisms residing inside the spinal cord. These sequential, peripheral, and central defects compounded to drive the sensorimotor circuit into a functional collapse that was consequential in predicting the significant errors in propriosensory-guided movement behaviors demonstrated here in our rat model and reported for people with cOIN. We conclude that sensorimotor disability induced by cancer treatment emerges from the joint expression of independent defects occurring in both peripheral and central elements of sensorimotor circuits.

The Effects of Sit-to-Stand Training Combined with Real-Time Visual Feedback on Strength, Balance, Gait Ability, and Quality of Life in Patients with Stroke: A Randomized Controlled Trial

This study aimed to investigate the effects of lower limbs muscles' strength, balance, walking, and quality of life through sit-to-stand training combined with real-time visual feedback (RVF-STS group) in patients with stroke and to compare the effects of classic sit-to-stand training (C-STS group). Thirty patients with stroke were randomly divided into two groups. The RVF-STS group received sit-to-stand training combined with real-time visual feedback using a Wii Balance Board (n = 15), and the C-STS group received classic sit-to-stand training (n = 15). All participants received training for 20 min once a day, 5 days a week for 6 weeks, and both groups underwent general physical therapy for 30 min before training. Before and after the training, the muscle strength of the hip flexor, abductor, and knee extensor were measured, and the Wii Balance Board was used to perform the center of pressure test and Berg Balance Scale to evaluate static and dynamic balance. Additionally, the 10 m walking test and the Timed Up and Go test were performed to evaluate gait function. The Stroke-Specific Quality of Life was used to measure the quality of life. The results showed that the lower extremity muscle strength, balance ability, walking ability, and quality of life of the RVF-STS group significantly improved in comparison of the pre- and post-differences (p < 0.05), and it also showed significant differences between groups (p < 0.05). This study showed that sit-to-stand training combined with real-time visual feedback was effective at improving the muscle strength of the lower extremities, balance, gait, and quality of life in patients with stroke. Therefore, repeating sit-to-stand training combined with real-time visual feedback could be used as an effective treatment method for patients with stroke.

Visual feedback improves movement illusions induced by tendon vibration after chronic stroke

BACKGROUND

Illusion of movement induced by tendon vibration is commonly used in rehabilitation and seems valuable for motor rehabilitation after stroke, by playing a role in cerebral plasticity. The aim was to study if congruent visual cues using Virtual Reality (VR) could enhance the illusion of movement induced by tendon vibration of the wrist among participants with stroke.

METHODS

We included 20 chronic stroke participants. They experienced tendon vibration of their wrist (100 Hz, 30 times) inducing illusion of movement. Three VR visual conditions were added to the vibration: a congruent moving virtual hand (Moving condition); a static virtual hand (Static condition); or no virtual hand at all (Hidden condition). The participants evaluated for each visual condition the intensity of the illusory movement using a Likert scale, the sensation of wrist's movement using a degree scale and they answered a questionnaire about their preferred condition.

RESULTS

The Moving condition was significantly superior to the Hidden condition and to the Static condition in terms of illusion of movement (p < 0.001) and the wrist's extension (p < 0.001). There was no significant difference between the Hidden and the Static condition for these 2 criteria. The Moving condition was considered the best one to increase the illusion of movement (in 70% of the participants). Two participants did not feel any illusion of movement.

CONCLUSIONS

This study showed the interest of using congruent cues in VR in order to enhance the consistency of the illusion of movement induced by tendon vibration among participants after stroke, regardless of their clinical severity. By stimulating the brain motor areas, this visuo-proprioceptive feedback could be an interesting tool in motor rehabilitation. Record number in Clinical Trials: NCT04130711, registered on October 17th 2019 ( https://clinicaltrials.gov/ct2/show/NCT04130711?id=NCT04130711&draw=2&rank=1 ).

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.

Integration of predictions and afferent signals in body ownership

We aimed at investigating the contribution of sensory predictions triggered by the sight of an object moving towards the body for the sense of body ownership. We used a recently developed psychophysical discrimination task to assess body ownership in the rubber hand illusion. In this task, the participants had to choose which of the two right rubber hands in view felt most like their own, and the ownership discriminations were fitted to psychometric curves. In the current study, we occluded the visual impressions of the object moving towards one of the rubber hands (during the first two-thirds of the path) and only revealed the final third of the object's movement trajectory when it touched the rubber hand (approach-occluded condition). Alternatively, we occluded only the final part so that the main part of the movement towards the model hand was visible (touch-occluded). We compared these two conditions to an illusion baseline condition where the object was visible during the entire trajectory and contact (no-occlusion). The touch-occluded condition produced equally strong hand ownership as the baseline condition with no occlusion, while ownership perception was significantly reduced when vision of the object approaching the rubber hand was occluded (approach-occluded). Our results show that tactile predictions generated from seeing an object moving towards the body are temporally exact, and they contribute to the rubber hand illusion by integrating with temporally congruent afferent sensory signals. This finding highlights the importance of multisensory predictions in peripersonal space, object permanence, and the interplay between bottom-up sensory signals and top-down predictions in body ownership.

Multisensory integration of visual cues from first- to third-person perspective avatars in the perception of self-motion

In the perception of self-motion, visual cues originating from an embodied humanoid avatar seen from a first-person perspective (1^st-PP) are processed in the same way as those originating from a person's own body. Here, we sought to determine whether the user's and avatar's bodies in virtual reality have to be colocalized for this visual integration. In Experiment 1, participants saw a whole-body avatar in a virtual mirror facing them. The mirror perspective could be supplemented with a fully visible 1^st-PP avatar or a suggested one (with the arms hidden by a virtual board). In Experiment 2, the avatar was viewed from the mirror perspective or a third-person perspective (3^rd-PP) rotated 90° left or right. During an initial embodiment phase in both experiments, the avatar's forearms faithfully reproduced the participant's real movements. Next, kinaesthetic illusions were induced on the static right arm from the vision of passive displacements of the avatar's arms enhanced by passive displacement of the participant's left arm. Results showed that this manipulation elicited kinaesthetic illusions regardless of the avatar's perspective in Experiments 1 and 2. However, illusions were more likely to occur when the mirror perspective was supplemented with the view of the 1st-PP avatar's body than with the mirror perspective only (Experiment 1), just as they are more likely to occur in the latter condition than with the 3^rd-PP (Experiment 2). Our results show that colocalization of the user's and avatar's bodies is an important, but not essential, factor in visual integration for self-motion perception.

Influence of virtual reality visual feedback on the illusion of movement induced by tendon vibration of wrist in healthy participants

Introduction

Illusion of movement induced by tendon vibration is an effective approach for motor and sensory rehabilitation in case of neurological impairments. The aim of our study was to investigate which modality of visual feedback in Virtual Reality (VR) associated with tendon vibration of the wrist could induce the best illusion of movement.

Methods

We included 30 healthy participants in the experiment. Tendon vibration inducing illusion of movement (wrist extension, 100Hz) was applied on their wrist during 3 VR visual conditions (10 times each): a moving virtual hand corresponding to the movement that the participants could feel during the tendon vibration (Moving condition), a static virtual hand (Static condition), or no virtual hand at all (Hidden condition). After each trial, the participants had to quantify the intensity of the illusory movement on a Likert scale, the subjective degree of extension of their wrist and afterwards they answered a questionnaire.

Results

There was a significant difference between the 3 visual feedback conditions concerning the Likert scale ranking and the degree of wrist’s extension (p<0.001). The Moving condition induced a higher intensity of illusion of movement and a higher sensation of wrist’s extension than the Hidden condition (p<0.001 and p<0.001 respectively) than that of the Static condition (p<0.001 and p<0.001 respectively). The Hidden condition also induced a higher intensity of illusion of movement and a higher sensation of wrist’s extension than the Static condition (p<0.01 and p<0.01 respectively). The preferred condition to facilitate movement’s illusion was the Moving condition (63.3%).

Conclusions

This study demonstrated the importance of carefully selecting a visual feedback to improve the illusion of movement induced by tendon vibration, and the increase of illusion by adding VR visual cues congruent to the illusion of movement. Further work will consist in testing the same hypothesis with stroke patients.

Functional properties of extended body representations in the context of kinesthesia

A person's internal representation of his/her body is not fixed. It can be substantially modified by neurological injuries and can also be extended (in healthy participants) to incorporate objects that have a corporeal appearance (such as fake body segments, e.g. a rubber hand), virtual whole bodies (e.g. avatars), and even objects that do not have a corporeal appearance (e.g. tools). Here, we report data from patients and healthy participants that emphasize the flexible nature of body representation and question the extent to which incorporated objects have the same functional properties as biological body parts. Our data shed new light by highlighting the involvement of visual motion information from incorporated objects (rubber hands, full body avatars and hand-held tools) in the perception of one's own movement (kinesthesia). On the basis of these findings, we argue that incorporated objects can be treated as body parts, especially when kinesthesia is involved.

Impact of Experimental Tonic Pain on Corrective Motor Responses to Mechanical Perturbations

Movement is altered by pain, but the underlying mechanisms remain unclear. Assessing corrective muscle responses following mechanical perturbations can help clarify these underlying mechanisms, as these responses involve spinal (short-latency response, 20-50 ms), transcortical (long-latency response, 50-100 ms), and cortical (early voluntary response, 100-150 ms) mechanisms. Pairing mechanical (proprioceptive) perturbations with different conditions of visual feedback can also offer insight into how pain impacts on sensorimotor integration. The general aim of this study was to examine the impact of experimental tonic pain on corrective muscle responses evoked by mechanical and/or visual perturbations in healthy adults. Two sessions (Pain (induced with capsaicin) and No pain) were performed using a robotic exoskeleton combined with a 2D virtual environment. Participants were instructed to maintain their index in a target despite the application of perturbations under four conditions of sensory feedback: (1) proprioceptive only, (2) visuoproprioceptive congruent, (3) visuoproprioceptive incongruent, and (4) visual only. Perturbations were induced in either flexion or extension, with an amplitude of 2 or 3 Nm. Surface electromyography was recorded from Biceps and Triceps muscles. Results demonstrated no significant effect of the type of sensory feedback on corrective muscle responses, no matter whether pain was present or not. When looking at the effect of pain on corrective responses across muscles, a significant interaction was found, but for the early voluntary responses only. These results suggest that the effect of cutaneous tonic pain on motor control arises mainly at the cortical (rather than spinal) level and that proprioception dominates vision for responses to perturbations, even in the presence of pain. The observation of a muscle-specific modulation using a cutaneous pain model highlights the fact that the impacts of pain on the motor system are not only driven by the need to unload structures from which the nociceptive signal is arising.

Proprioceptive Localization of the Fingers: Coarse, Biased, and Context-Sensitive

The proprioceptive sense provides somatosensory information about positions of parts of the body, information that is essential for guiding behavior and monitoring the body. Few studies have investigated the perceptual localization of individual fingers, despite their importance for tactile exploration and fine manipulation. We present two experiments assessing the performance of proprioceptive localization of multiple fingers, either alone or in combination with visual cues. In the first experiment, we used a virtual reality paradigm to assess localization of multiple fingers. Surprisingly, the errors averaged 3.7 cm per digit, which represents a significant fraction of the range of motion of any finger. Both random and systematic errors were large. The latter included participant-specific biases and participant-independent distortions that evoked similar observations from prior studies of perceptual representations of hand shape. In a second experiment, we introduced visual cues about positions of nearby fingers, and observed that this contextual information could greatly decrease localization errors. The results suggest that only coarse proprioceptive information is available through somatosensation, and that finer information may not be necessary for fine motor behavior. These findings may help elucidate human hand function, and inform new applications to the design of human-computer interfaces or interactions in virtual reality.

Diagnostic accuracy of lumbopelvic motor control tests using pressure biofeedback unit in professional swimmers: A cross-sectional study

Hypothesis

To determine the effect of receiving the visual feedback of the sphygmomanometer on lumbopelvic motor control (LPMC) tests in professional swimmers.

Method

31 professional swimmers to participate in the study. The outcome was maximum absolute mmHg variation in the pressure biofeedback unit's manometer with and without visual feedback on four LPMC tests.

Results

Test scores were significantly affected by visual feedback F = 10.07, p = 0.002, η² _p = 0.117 and the type of test F = 32.53, p < 0.001, η² _p = 0.300.

Conclusion

Visual feedback has a positive effect on the Active Straight Leg Raise Test (ASLR), the Knee Lift Abdominal Test (KLAT) scores completed by professional swimmers.

Fake hand in movement: Visual motion cues from the rubber hand are processed for kinesthesia

The feeling that a fake (e.g. rubber) hand belongs to a person's own body can be elicited by synchronously stroking the fake hand and the real hand, with the latter hidden from view. Here, we sought to determine whether visual motion signals from that incorporated rubber hand would provide relevant cues for sensing movement (i.e. kinesthesia). After 180 s of visuo-tactile synchronous or asynchronous stroking, the fake hand was moved along the lateral or the sagittal axis. After synchronous stroking, movement of the rubber hand induced illusory movement of the static (real) hand in the same direction; the illusion was slightly more frequent and more intense when the fake hand was moved along the sagittal axis. We therefore conclude that visual signals of motion originating from the rubber hand are integrated for kinesthesia by the central nervous system just as visual signals from the real hand are.

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.

A Conceptual Model of Tactile Processing across Body Features of Size, Shape, Side, and Spatial Location

The processing of touch depends of multiple factors, such as the properties of the skin and type of receptors stimulated, as well as features related to the actual configuration and shape of the body itself. A large body of research has focused on the effect that the nature of the stimuli has on tactile processing. Less research, however, has focused on features beyond the nature of the touch. In this review, we focus on some features related to the body that have been investigated for less time and in a more fragmented way. These include the symmetrical quality of the two sides of the body, the postural configuration of the body, as well as the size and shape of different body parts. We will describe what we consider three key aspects: (1) how and at which stages tactile information is integrated between different parts and sides of the body; (2) how tactile signals are integrated with online and stored postural configurations of the body, regarded as priors; (3) and how tactile signals are integrated with representations of body size and shape. Here, we describe how these different body dimensions affect integration of tactile information as well as guide motor behavior by integrating them in a single model of tactile processing. We review a wide range of neuropsychological, neuroimaging, and neurophysiological data and suggest a revised model of tactile integration on the basis of the one proposed previously by Longo et al.

Effect of kinesthetic illusion induced by visual stimulation on ankle dorsiflexion dysfunction in a stroke patient: ABAB single-case design

The purpose of this study was to investigate the effect of simultaneous intervention with the kinesthetic illusion induced by visual stimulation (KiNvis) and voluntary exercise on ankle dorsiflexion dysfunction in a patient with right-sided stroke hemiparesis. Within an ABAB single-case design, we conducted two phases each lasting five days. Phase A represented the baseline during which only voluntary ankle dorsiflexion (VAD) was performed. Phase B involved simultaneous performance of VAD and KiNvis. We measured the angle of ankle joint dorsiflexion (AJD), and the 10 m maximum walking speed (10MWS). AJD and 10MWS were significantly improved in phase B.

The respective contributions of visual and proprioceptive afferents to the mirror illusion in virtual reality

The reflection of passive arm displacement in a mirror is a powerful means of inducing a kinaesthetic illusion in the static arm hidden behind the mirror. Our recent research findings suggest that this illusion is not solely visual in origin but results from the combination of visual and proprioceptive signals from the two arms. To determine the respective contributions of visual and proprioceptive signals to this illusion, we reproduced the mirror paradigm in virtual reality. As in the physical version of the mirror paradigm, one of the participant’s arms (the left arm, in our study) could be flexed or extended passively. This movement was combined with displacements of the avatar’s left and right forearms, as viewed in a first-person perspective through a virtual reality headset. In order to distinguish between visual and proprioceptive contributions, two unimodal conditions were applied separately: displacement of the avatar’s forearms in the absence of physical displacement of the left arm (the visual condition), and displacement of the left forearm while the avatar’s forearms were masked (the proprioceptive condition). Of the 34 female participants included in the study, 28 experienced a kinaesthetic mirror illusion in their static (right) arm. The strength of the illusion (expressed in terms of speed and duration) evoked by the bimodal condition was much higher than that observed in either of the two unimodal conditions. Our present results confirm that the involvement of visual signals in the mirror illusion—often considered as a prototypic visual illusion—has been overstated. The mirror illusion also involves non-visual signals (bilateral proprioceptive-somaesthetic signals, in fact) that interact with the visual signals and strengthen the kinaesthetic effect.

Motor and sensory disturbances induced by sensorimotor conflicts during passive and active movements in healthy participants

Sensorimotor conflict induces both sensory and motor disturbances, but the specific factors playing a role in conflict-induced disturbances are still misunderstood. For example, we still do not know the role played by motor intention (vs. a purely visuo-proprioceptive conflict) or the influence of specific types of incongruent visual feedback. The objective of this study was threefold: 1- to compare the effect of passive and active movement during sensorimotor conflict on sensory disturbances measured with a questionnaire; 2- to compare the effect of three incongruent visual feedback conditions on sensory and motor (mediolateral drift and movement amplitude) disturbances; 3- to test whether conflict-induced sensory and motor disturbances were stable over time. 20 healthy participants realized active or passive cyclic upper limb movements while viewing either congruent or incongruent visual feedback about their movement using a robotized exoskeleton combined with 2D virtual reality interface. First, results showed that in condition of conflict, participants reported higher sensory disturbances during active movements compared to passive movements (p = 0.034), suggesting that the efference copy reinforces the conflict between vision and proprioception. Second, the three conditions of incongruence in the active condition induced similar sensory (all p>0.45) and motor disturbances (medio-lateral drift: all p>0.59 and amplitude: all p>0.25), suggesting that conflict induced motor disturbances could be related more to the observation of another movement rather than to a detection of conflict between motor intention and sensory feedback. Finally, both sensory and motor disturbances were stable over time (all ICCs between 0.76 and 0.87), demonstrating low variability within participants. Overall, our results suggest that the efference copy is more involved in sensory disturbances than in motor disturbances, suggesting that they might rely on independent processes.

When the vibrations allow for anticipating the force to be produced: an extend to Pfister et al. (2014)

According to the ideomotor theory, action selection is done by the mental anticipation of its perceptual consequences. If the distal information processed mainly by vision and hearing are considered essential for the representation of the action, the proximal information processed by the sense of touch and proprioception is of less importance. Recent works seem to show the opposite. Nevertheless, it is necessary to complete these results by offering a situation, more ecological, where response and effect can occur on the same effector. So, the goal of our work was to implement a more relevant spatial correspondence because to touch is not the same action that to hear or to see. To do so, participants pressed a specific key after the presentation of a stimulus. The key vibrated depending on the pressure exerted on it. In a compatible condition, high pressure on a key triggered a high vibration, while in an incompatible condition high pressure triggered a low vibration on the same effectors. As expected, the response times were faster in the compatible condition than the incompatible condition. This means that proximal information participates actively in the selection of action.

What's left of the mirror illusion when the mirror can no longer be seen? Bilateral integration of proprioceptive afferents!

Recent data suggest that manipulating the muscle afferents of one arm affects both ipsilateral and contralateral perceptual estimates. Here, we used the mirror paradigm to study the bimanual integration of kinesthetic muscle afferents. The reflection of a moving hand in a mirror positioned in the sagittal plane creates an illusion of symmetrical bimanual movement. Although vision clearly has a role in kinesthesia, its role in the mirror illusion might have been overestimated. Conversely, the role of bimanual integration of muscle afferents might have been underestimated. We hypothesized that muscle-proprioceptive afferents of the passively displaced arm (the image of which was reflected in the mirror) are involved in this illusion. We evoked in 19 healthy adult participants the mirror illusion by displacing passively their left arm, the image of which was reflected in the mirror. Once participants experienced the illusion that their hidden right arm was moving, we then either occluded their view of the mirror (using occlusive glasses) and/or prevent the passive left arm displacement. Participants' illusion characteristics (duration and kinematic) under these conditions were compared with classical mirror illusion (without visual occlusion). We found that as long as the arm was still moving, the kinesthetic illusion decayed slowly after visual occlusion. These findings suggest that the mirror illusion results from the combination of visuo-proprioceptive signals from the two arms and is not purely visual in origin. Our findings also support the more general concept whereby proprioceptive afferents are integrated bilaterally for the purpose of kinesthesia during bimanual tasks.

Muscular responses appear to be associated with existence of kinesthetic perception during combination of tendon co-vibration and motor imagery

The afferent inputs from peripheral sensory receptors and efferent signals from the central nervous system that underlie intentional movement can contribute to kinesthetic perception. Previous studies have revealed that tendon vibration to wrist muscles elicits an excitatory response-known as the antagonist vibratory response-in muscles antagonistic to the vibrated muscles. Therefore, the present study aimed to further investigate the effect of tendon vibration combined with motor imagery on kinesthetic perception and muscular activation. Two vibrators were applied to the tendons of the left flexor carpi radialis and extensor carpi radialis. When the vibration frequency was the same between flexors and extensors, no participant perceived movement and no muscle activity was induced. When participants imagined flexing their wrists during tendon vibration, the velocity of perceptual flexion movement increased. Furthermore, muscle activity of the flexor increased only during motor imagery. These results demonstrate that kinesthetic perception can be induced during the combination of motor imagery and co-vibration, even with no experience of kinesthetic perception from an afferent input with co-vibration at the same frequency. Although motor responses were observed during combined co-vibration and motor imagery, no such motor responses were recorded during either co-vibration alone or motor imagery alone, suggesting that muscular responses during the combined condition are associated with kinesthetic perception. Thus, the present findings indicate that kinesthetic perception is influenced by the interaction between afferent input from muscle spindles and the efferent signals that underlie intentional movement. We propose that the physiological behavior resulting from kinesthetic perception affects the process of modifying agonist muscle activity, which will be investigated in a future study.

Vision of the body improves inter-hemispheric integration of tactile-motor responses

Sensory input from and motor output to the two sides of the body needs to be continuously integrated between the two cerebral hemispheres. This integration can be measured through its cost in terms of processing speed. In simple detection tasks, reaction times (RTs) are faster when stimuli are presented to the side of the body ipsilateral to the body part used to respond. This advantage - the contralateral-ipsilateral difference (also known as the crossed-uncrossed difference: CUD) - is thought to reflect inter-hemispheric interactions needed for sensorimotor information to be integrated between the two hemispheres. Several studies have shown that non-informative vision of the body enhances performance in tactile tasks. However, it is unknown whether the CUD can be similarly affected by vision. Here, we investigated whether the CUD is modulated by vision of the body (i.e., the stimulated hand) by presenting tactile stimuli unpredictably on the middle fingers when one hand was visible (i.e., either the right or left hand). Participants detected the stimulus and responded as fast as possible using either their left or right foot. Consistent with previous results, a clear CUD (5.8ms) was apparent on the unseen hand. Critically, however, no such effect was found on the hand that was visible (-2.2ms). Thus, when touch is delivered to a seen hand, the usual cost in processing speed of responding with a contralateral effector is eliminated. This result suggests that vision of the body improves the interhemispheric integration of tactile-motor responses.

Muscle Vibration-Induced Illusions: Review of Contributing Factors, Taxonomy of Illusions and User’s Guide

Limb muscle vibration creates an illusory limb movement in the direction corresponding to lengthening of the vibrated muscle. Neck muscle vibration results in illusory motion of visual and auditory stimuli. Attributed to the activation of muscle spindles, these and related effects are of great interest as a tool in research on proprioception, for rehabilitation of sensorimotor function and for multisensory immersive virtual environments. However, these illusions are not easy to elicit in a consistent manner. We review factors that influence them, propose their classification in a scheme that links this area of research to perception theory, and provide practical suggestions to researchers. Local factors that determine the illusory effect of vibration include properties of the vibration stimulus such as its frequency, amplitude and duration, and properties of the vibrated muscle, such as contraction and fatigue. Contextual (gestalt) factors concern the relationship of the vibrated body part to the rest of the body and the environment. Tactile and visual cues play an important role, and so does movement, imagined or real. The best-known vibration illusions concern one’s own body and can be classified as ‘first-order’ due to a direct link between activity in muscle spindles and the percept. More complex illusions involve other sensory modalities and external objects, and provide important clues regarding the hidden role of proprioception, our ‘silent’ sense. Our taxonomy makes explicit this and other distinctions between different illusory effects. We include User’s Guide with tips for anyone wishing to conduct a vibration study.

A New Neurocognitive Interpretation of Shoulder Position Sense during Reaching: Unexpected Competence in the Measurement of Extracorporeal Space

Background. The position sense of the shoulder joint is important during reaching. Objective. To examine the existence of additional competence of the shoulder with regard to the ability to measure extracorporeal space, through a novel approach, using the shoulder proprioceptive rehabilitation tool (SPRT), during reaching. Design. Observational case-control study. Methods. We examined 50 subjects: 25 healthy and 25 with impingement syndrome with a mean age [years] of 64.52 +/− 6.98 and 68.36 +/− 6.54, respectively. Two parameters were evaluated using the SPRT: the integration of visual information and the proprioceptive afferents of the shoulder (Test 1) and the discriminative proprioceptive capacity of the shoulder, with the subject blindfolded (Test 2). These tasks assessed the spatial error (in centimeters) by the shoulder joint in reaching movements on the sagittal plane. Results. The shoulder had proprioceptive features that allowed it to memorize a reaching position and reproduce it (error of 1.22 cm to 1.55 cm in healthy subjects). This ability was lower in the impingement group, with a statistically significant difference compared to the healthy group (p < 0.05 by Mann–Whitney test). Conclusions. The shoulder has specific expertise in the measurement of the extracorporeal space during reaching movements that gradually decreases in impingement syndrome.

Two Visual Pathways in Primates Based on Sampling of Space: Exploitation and Exploration of Visual Information

Evidence is strong that the visual pathway is segregated into two distinct streams—ventral and dorsal. Two proposals theorize that the pathways are segregated in function: The ventral stream processes information about object identity, whereas the dorsal stream, according to one model, processes information about either object location, and according to another, is responsible in executing movements under visual control. The models are influential; however recent experimental evidence challenges them, e.g., the ventral stream is not solely responsible for object recognition; conversely, its function is not strictly limited to object vision; the dorsal stream is not responsible by itself for spatial vision or visuomotor control; conversely, its function extends beyond vision or visuomotor control. In their place, we suggest a robust dichotomy consisting of a ventral stream selectively sampling high-resolution/focal spaces, and a dorsal stream sampling nearly all of space with reduced foveal bias. The proposal hews closely to the theme of embodied cognition: Function arises as a consequence of an extant sensory underpinning. A continuous, not sharp, segregation based on function emerges, and carries with it an undercurrent of an exploitation-exploration dichotomy. Under this interpretation, cells of the ventral stream, which individually have more punctate receptive fields that generally include the fovea or parafovea, provide detailed information about object shapes and features and lead to the systematic exploitation of said information; cells of the dorsal stream, which individually have large receptive fields, contribute to visuospatial perception, provide information about the presence/absence of salient objects and their locations for novel exploration and subsequent exploitation by the ventral stream or, under certain conditions, the dorsal stream. We leverage the dichotomy to unify neuropsychological cases under a common umbrella, account for the increased prevalence of multisensory integration in the dorsal stream under a Bayesian framework, predict conditions under which object recognition utilizes the ventral or dorsal stream, and explain why cells of the dorsal stream drive sensorimotor control and motion processing and have poorer feature selectivity. Finally, the model speculates on a dynamic interaction between the two streams that underscores a unified, seamless perception. Existing theories are subsumed under our proposal.

Robot-Assisted Proprioceptive Training with Added Vibro-Tactile Feedback Enhances Somatosensory and Motor Performance

This study examined the trainability of the proprioceptive sense and explored the relationship between proprioception and motor learning. With vision blocked, human learners had to perform goal-directed wrist movements relying solely on proprioceptive/haptic cues to reach several haptically specified targets. One group received additional somatosensory movement error feedback in form of vibro-tactile cues applied to the skin of the forearm. We used a haptic robotic device for the wrist and implemented a 3-day training regimen that required learners to make spatially precise goal-directed wrist reaching movements without vision. We assessed whether training improved the acuity of the wrist joint position sense. In addition, we checked if sensory learning generalized to the motor domain and improved spatial precision of wrist tracking movements that were not trained. The main findings of the study are: First, proprioceptive acuity of the wrist joint position sense improved after training for the group that received the combined proprioceptive/haptic and vibro-tactile feedback (VTF). Second, training had no impact on the spatial accuracy of the untrained tracking task. However, learners who had received VTF significantly reduced their reliance on haptic guidance feedback when performing the untrained motor task. That is, concurrent VTF was highly salient movement feedback and obviated the need for haptic feedback. Third, VTF can be also provided by the limb not involved in the task. Learners who received VTF to the contralateral limb equally benefitted. In conclusion, somatosensory training can significantly enhance proprioceptive acuity within days when learning is coupled with vibro-tactile sensory cues that provide feedback about movement errors. The observable sensory improvements in proprioception facilitates motor learning and such learning may generalize to the sensorimotor control of the untrained motor tasks. The implications of these findings for neurorehabilitation are discussed.

Removal of proprioception by BCI raises a stronger body ownership illusion in control of a humanlike robot

Body ownership illusions provide evidence that our sense of self is not coherent and can be extended to non-body objects. Studying about these illusions gives us practical tools to understand the brain mechanisms that underlie body recognition and the experience of self. We previously introduced an illusion of body ownership transfer (BOT) for operators of a very humanlike robot. This sensation of owning the robot’s body was confirmed when operators controlled the robot either by performing the desired motion with their body (motion-control) or by employing a brain-computer interface (BCI) that translated motor imagery commands to robot movement (BCI-control). The interesting observation during BCI-control was that the illusion could be induced even with a noticeable delay in the BCI system. Temporal discrepancy has always shown critical weakening effects on body ownership illusions. However the delay-robustness of BOT during BCI-control raised a question about the interaction between the proprioceptive inputs and delayed visual feedback in agency-driven illusions. In this work, we compared the intensity of BOT illusion for operators in two conditions; motion-control and BCI-control. Our results revealed a significantly stronger BOT illusion for the case of BCI-control. This finding highlights BCI’s potential in inducing stronger agency-driven illusions by building a direct communication between the brain and controlled body, and therefore removing awareness from the subject’s own body.

Optimal visuotactile integration for velocity discrimination of self-hand movements

Illusory hand movements can be elicited by a textured disk or a visual pattern rotating under one's hand, while proprioceptive inputs convey immobility information (Blanchard C, Roll R, Roll JP, Kavounoudias A. PLoS One 8: e62475, 2013). Here, we investigated whether visuotactile integration can optimize velocity discrimination of illusory hand movements in line with Bayesian predictions. We induced illusory movements in 15 volunteers by visual and/or tactile stimulation delivered at six angular velocities. Participants had to compare hand illusion velocities with a 5°/s hand reference movement in an alternative forced choice paradigm. Results showed that the discrimination threshold decreased in the visuotactile condition compared with unimodal (visual or tactile) conditions, reflecting better bimodal discrimination. The perceptual strength (gain) of the illusions also increased: the stimulation required to give rise to a 5°/s illusory movement was slower in the visuotactile condition compared with each of the two unimodal conditions. The maximum likelihood estimation model satisfactorily predicted the improved discrimination threshold but not the increase in gain. When we added a zero-centered prior, reflecting immobility information, the Bayesian model did actually predict the gain increase but systematically overestimated it. Interestingly, the predicted gains better fit the visuotactile performances when a proprioceptive noise was generated by covibrating antagonist wrist muscles. These findings show that kinesthetic information of visual and tactile origins is optimally integrated to improve velocity discrimination of self-hand movements. However, a Bayesian model alone could not fully describe the illusory phenomenon pointing to the crucial importance of the omnipresent muscle proprioceptive cues with respect to other sensory cues for kinesthesia.

Anchoring the "floating arm": Use of proprioceptive and mirror visual feedback from one arm to control involuntary displacement of the other arm

Arm movement control takes advantage of multiple inputs, including those originating from the contralateral arm. In the mirror paradigm, it has been suggested that control of the unseen arm, hidden by the mirror, is facilitated by the reflection of the other, moving arm. Although proprioceptive feedback originating from the moving arm, (the image of which is reflected in the mirror), is always coupled with visual feedback in the mirror paradigm, the former has received little attention. We recently showed that the involuntary arm movement following a sustained, isometric contraction, known as the "floating arm" or "Kohnstamm phenomenon", was adjusted to the passive-motorized displacement of the other arm. However, provision of mirror feedback, that is, the reflection in the mirror of the passively moved arm, did not add to this coupling effect. Therefore, the interlimb coupling in the mirror paradigm may to a large extent have a proprioceptive origin rather than a visual origin. The objective of the present study was to decouple mirror feedback and proprioceptive feedback from the reflected, moving arm and evaluate their respective contributions to interlimb coupling in the mirror paradigm. First (in Experiment 1, under eyes-closed conditions), we found that masking the proprioceptive afferents of the passively moved arm (by co-vibrating the antagonistic biceps and triceps muscles) suppressed the interlimb coupling between involuntary displacement of one arm and passive displacement of the other. Next (in Experiment 2), we masked proprioceptive afferents of the passively moved arm and specifically evaluated mirror feedback. We found that interlimb coupling through mirror feedback (though significant) was weaker than interlimb coupling through proprioceptive feedback. Overall, the present results show that in the mirror paradigm, proprioceptive feedback is stronger and more consistent than visual-mirror feedback in terms of the impact on interlimb coupling.

Brain Regions Associated to a Kinesthetic Illusion Evoked by Watching a Video of One's Own Moving Hand

It is well known that kinesthetic illusions can be induced by stimulation of several sensory systems (proprioception, touch, vision…). In this study we investigated the cerebral network underlying a kinesthetic illusion induced by visual stimulation by using functional magnetic resonance imaging (fMRI) in humans. Participants were instructed to keep their hand still while watching the video of their own moving hand (Self Hand) or that of someone else's moving hand (Other Hand). In the Self Hand condition they experienced an illusory sensation that their hand was moving whereas the Other Hand condition did not induce any kinesthetic illusion. The contrast between the Self Hand and Other Hand conditions showed significant activation in the left dorsal and ventral premotor cortices, in the left Superior and Inferior Parietal lobules, at the right Occipito-Temporal junction as well as in bilateral Insula and Putamen. Most strikingly, there was no activation in the primary motor and somatosensory cortices, whilst previous studies have reported significant activation in these regions for vibration-induced kinesthetic illusions. To our knowledge, this is the first study that indicates that humans can experience kinesthetic perception without activation in the primary motor and somatosensory areas. We conclude that under some conditions watching a video of one's own moving hand could lead to activation of a network that is usually involved in processing copies of efference, thus leading to the illusory perception that the real hand is indeed moving.

Kinaesthetic mirror illusion and spatial congruence

Position sense and kinaesthesia are mainly derived from the integration of somaesthetic and visual afferents to form a single, coherent percept. However, visual information related to the body can play a dominant role in these perceptual processes in some circumstances, and notably in the mirror paradigm. The objective of the present study was to determine whether or not the kinaesthetic illusions experienced in the mirror paradigm obey one of the key rules of multisensory integration: spatial congruence. In the experiment, the participant's left arm (the image of which was reflected in a mirror) was either passively flexed/extended with a motorized manipulandum (to induce a kinaesthetic illusion in the right arm) or remained static. The right (unseen) arm remained static but was positioned parallel to the left arm's starting position or placed in extension (from 15° to 90°, in steps of 15°), relative to the left arm's flexed starting position. The results revealed that the frequency of the illusion decreased only slightly as the incongruence prior to movement onset between the reflected left arm and the hidden right arm grew and remained quite high even in the most incongruent settings. However, the greater the incongruence between the visually and somaesthetically specified positions of the right forearm (from 15° to 90°), the later the onset and the lower the perceived speed of the kinaesthetic illusion. Although vision dominates perception in a context of visuoproprioceptive conflict (as in the mirror paradigm), our results show that the relative weightings allocated to proprioceptive and visual signals vary according to the degree of spatial incongruence prior to movement onset.

A cutaneous positioning system

Our previous work revealed that torso cutaneous information contributes to the internal representation of the torso and plays a role in postural control. Hence, the aims of this study were to assess whether posture could be manipulated by patterns of vibrotactile stimulation and to determine whether resulting modified postures were associated with specific and consistent spatial attitudes. Ten healthy young adults stood in normal and Romberg stances with six vibrating actuators positioned on the torso in contact with the skin over the anatomical locations corresponding to left and right external oblique, internal oblique and erector spinae muscles at the L4/L5 vertebrae level. A 250-Hz tactile vibration was applied for 5 s either at a single location or consecutively at each location in clockwise or counterclockwise sequences. Kinematic analysis of the body segments indicated that postural responses observed in response to single and sequential stimulation patterns were similar, while the center of pressure remained unaltered in any situations. Moreover, torso inclinations followed rectilinear-like path segments chartered by stimuli loci during sequential stimulations. Comparison of torso attitudes with previous results obtained with co-vibration patterns of the same duration showed that torso inclination amplitudes are equivalent for single (one location) and co-vibration (pairs of locations) patterns inducing the same directional effect. Hence, torso cutaneous information exhibits kinesthetic properties, appears to provide a map of upper body spatial configuration, and could assume the role of an internal positioning system for the upper body.

Passive or simulated displacement of one arm (but not its mirror reflection) modulates the involuntary motor behavior of the other arm

Recent studies of both healthy and patient populations have cast doubt on the mirror paradigm's beneficial effect on motor behavior. Indeed, the voluntary arm displacement that accompanies reflection in the mirror may be the determining factor in terms of the motor behavior of the contralateral arm. The objective of the present study was to assess the respective effects of mirror reflection and arm displacement (whether real or simulated) on involuntary motor behavior of the contralateral arm following sustained, isometric contraction (Kohnstamm phenomenon). Our results revealed that (i) passive displacement of one arm (displacement of the left arm via a motorized manipulandum moving at 4°/s) influenced the velocity of the Kohnstamm phenomenon (forearm flexion occurring shortly after the cessation of muscle contraction) in the contralateral arm and (ii) mirror vision had no effect. Indeed, the velocity of the Kohnstamm phenomenon tended to be adjusted to match the velocity of the passive displacement of the other arm. In a second experiment, arm displacement was simulated by vibrating the triceps at 25, 50 or 75 Hz. Results showed that the velocity of the Kohnstamm phenomenon in one arm increased with the vibration frequency applied to the other arm. Our results revealed the occurrence of bimanual coupling because involuntary displacement of one arm was regulated by muscle-related information generated by the actual or simulated displacement of the other arm. In line with the literature data on voluntary motor behavior, our study failed to evidence an additional impact of mirror vision on involuntary motor behavior.

The co-constitution of the self and the world: action and proprioceptive coupling

This article proposes a theoretical reflection on the conditions for the constitution of a distinction between the self and the world by a cognitive system. The main hypothesis is the following: proprioception, as a sensory system that is habitually dedicated essentially to experience of the body, is conceived here as a coupling which is necessary for the dual and concomitant constitution of a bodily self and of a distal perceptual field. After recalling the singular characteristics of proprioceptive coupling, three lines of thought are developed. The first, which is notably inspired by research on sensory substitution, aims at emphasizing the indispensable role of action in the context of such perceptual learning. In a second part, this hypothesis is tested against opposing arguments. In particular, we shall discuss, in the context of what Braitenberg called a synthetic psychology, the emergence of oriented behaviors in simple robots that can be regulated by sensory regulations which are strictly external, since these robots do not have any form of "proprioception." In the same vein, this part also provides the opportunity to discuss the argument concerning a bijective relation between action and proprioception; it has been argued by others that because of this strict bijection it is not possible for proprioception to be the basis for the constitution of an exteriority. The third part, which is more prospective, suggests that it is important to take the measure of the phylogenetic history of this exteriority, starting from unicellular organisms. Taking into account the literature which attests the existence of proprioception even amongst the most elementary living organisms, this leads us to propose that the coupling of proprioception to action is very primitive, and that the role we propose for it in the co-constitution of an exteriority and self is probably already at work in the simplest living organisms.

Trying to move your unseen static arm modulates visually-evoked kinesthetic illusion

Although kinesthesia is known to largely depend on afferent inflow, recent data suggest that central signals originating from volitional control (efferent outflow) could also be involved and interact with the former to build up a coherent percept. Evidence derives from both clinical and experimental observations where vision, which is of primary importance in kinesthesia, was systematically precluded. The purpose of the present experiment was to assess the role of volitional effort in kinesthesia when visual information is available. Participants (n=20) produced isometric contraction (10-20% of maximal voluntary force) of their right arm while their left arm, which image was reflected in a mirror, either was passively moved into flexion/extension by a motorized manipulandum, or remained static. The contraction of the right arm was either congruent with or opposite to the passive displacements of the left arm. Results revealed that in most trials, kinesthetic illusions were visually driven, and their occurrence and intensity were modulated by whether volitional effort was congruent or not with visual signals. These results confirm the impact of volitional effort in kinesthesia and demonstrate for the first time that these signals interact with visual afferents to offer a coherent and unified percept.