Optimal visuotactile integration for velocity discrimination of self-hand movements

Visuotactile integration in individuals with fibromyalgia

Our brain constantly integrates afferent information, such as visual and tactile information, to perceive the world around us. According to the maximum-likelihood estimation (MLE) model, imprecise information will be weighted less than precise, making the multisensory percept as precise as possible. Individuals with fibromyalgia (FM), a chronic pain syndrome, show alterations in the integration of tactile information. This could lead to a decrease in their weight in a multisensory percept or a general disruption of multisensory integration, making it less beneficial. To assess multisensory integration, 15 participants with FM and 18 pain-free controls performed a temporal-order judgment task in which they received pairs of sequential visual, tactile (unisensory conditions), or visuotactile (multisensory condition) stimulations on the index and the thumb of the non-dominant hand and had to determine which finger was stimulated first. The task enabled us to measure the precision and accuracy of the percept in each condition. Results indicate an increase in precision in the visuotactile condition compared to the unimodal conditions in controls only, although we found no intergroup differences. The observed visuotactile precision was correlated to the precision predicted by the MLE model in both groups, suggesting an optimal integration. Finally, the weights of the sensory information were not different between the groups; however, in the group with FM, higher pain intensity was associated with smaller tactile weight. This study shows no alterations of the visuotactile integration in individuals with FM, though pain may influence tactile weight in these participants.

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.

Correctly establishing evidence for cue combination via gains in sensory precision: Why the choice of comparator matters

Studying how sensory signals from different sources (sensory cues) are integrated within or across multiple senses allows us to better understand the perceptual computations that lie at the foundation of adaptive behaviour. As such, determining the presence of precision gains - the classic hallmark of cue combination - is important for characterising perceptual systems, their development and functioning in clinical conditions. However, empirically measuring precision gains to distinguish cue combination from alternative perceptual strategies requires careful methodological considerations. Here, we note that the majority of existing studies that tested for cue combination either omitted this important contrast, or used an analysis approach that, unknowingly, strongly inflated false positives. Using simulations, we demonstrate that this approach enhances the chances of finding significant cue combination effects in up to 100% of cases, even when cues are not combined. We establish how this error arises when the wrong cue comparator is chosen and recommend an alternative analysis that is easy to implement but has only been adopted by relatively few studies. By comparing combined-cue perceptual precision with the best single-cue precision, determined for each observer individually rather than at the group level, researchers can enhance the credibility of their reported effects. We also note that testing for deviations from optimal predictions alone is not sufficient to ascertain whether cues are combined. Taken together, to correctly test for perceptual precision gains, we advocate for a careful comparator selection and task design to ensure that cue combination is tested with maximum power, while reducing the inflation of false positives.

Proprioceptive uncertainty promotes the rubber hand illusion

Body ownership is the multisensory perception of a body as one's own. Recently, the emergence of body ownership illusions like the visuotactile rubber hand illusion has been described by Bayesian causal inference models in which the observer computes the probability that visual and tactile signals come from a common source. Given the importance of proprioception for the perception of one's body, proprioceptive information and its relative reliability should impact this inferential process. We used a detection task based on the rubber hand illusion where participants had to report whether the rubber hand felt like their own or not. We manipulated the degree of asynchrony of visual and tactile stimuli delivered to the rubber hand and the real hand under two levels of proprioceptive noise using tendon vibration applied to the lower arm's antagonist extensor and flexor muscles. As hypothesized, the probability of the emergence of the rubber hand illusion increased with proprioceptive noise. Moreover, this result, well fitted by a Bayesian causal inference model, was best described by a change in the a priori probability of a common cause for vision and touch. These results offer new insights into how proprioceptive uncertainty shapes the multisensory perception of one's own body.

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.

Kinesthesia and Temporal Experience: On the 'Knitting and Unknitting' Process of Bodily Subjectivity in Schizophrenia

This paper proposes a phenomenological hypothesis that psychosis entails a disturbance of the two-fold process of the indication function of kinesthesia and the presentification function of touch that affects the constitution of bodily subjectivity. Recent functional connectivity studies showed that the increased synchrony between the right anterior insula and the default mode network are associated with psychosis. This association is proposed to be correlated with the disrupted dynamics between the pre-reflective and reflective temporal experience in psychotic patients. The paper first examines the dynamic nature of kinesthesia and the influence touch and vision exert on it, and then the reciprocal influence with temporal experience focusing on the body's cyclic sense of temporality and its impact on physiology and phenomenology. Affectivity and self-affection are considered in their basic bodily expressions mainly through the concepts of responsivity and receptivity. The overall constitutive processes referred to throughout the article are proposed as a roadmap to develop body-based therapeutic work.

Influence of hand-arm self-avatar motion delay on the directional perception induced by an illusory sensation of being twisted

Sensory information from movements of body parts can alter their position when exposed to external physical stimuli. Visual information monitors the position and movement of body parts from an exterior perspective, whereas somatosensory information monitors them from an internal viewpoint. However, how such sensory data are integrated is unclear. In this study, a virtual reality (VR) system was used to evaluate the influence of the temporal difference between visual and somatosensory information from hand movements on the directional perception of a torque while modifying the visual appearance (human hand vs. non-human object) and visuohaptic congruency (congruent vs. incongruent) of self-avatars. Visual information was provided by the movement of the self-avatars in a VR environment, while somatosensory information was provided by vibrations with asymmetrical amplitudes that gave the participants the sensation of being continuously pushed or pulled without actually moving any body part. Delaying the movement of the avatar by 50 ms resulted in the sensitivity of the force direction perception to be lower with human hands than with non-human avatars, whereas a delay of 200 ms resulted in a higher sensitivity. This study can contribute to applications requiring multisensory integration in a VR environment.

Sustained rubber hand illusion after the end of visuotactile stimulation with a similar time course for the reduction of subjective ownership and proprioceptive drift

The rubber hand illusion is a perceptual illusion in which participants experience an inanimate rubber hand as their own when they observe this model hand being stroked in synchrony with strokes applied to the person’s real hand, which is hidden. Earlier studies have focused on the factors that determine the elicitation of this illusion, the relative contribution of vision, touch and other sensory modalities involved and the best ways to quantify this perceptual phenomenon. Questionnaires serve to assess the subjective feeling of ownership, whereas proprioceptive drift is a measure of the recalibration of hand position sense towards the rubber hand when the illusion is induced. Proprioceptive drift has been widely used and thought of as an objective measure of the illusion, although the relationship between this measure and the subjective illusion is not fully understood. Here, we examined how long the illusion is maintained after the synchronous visuotactile stimulation stops with the specific aim of clarifying the temporal relationship in the reduction of both subjective ownership and proprioceptive drift. Our results show that both the feeling of ownership and proprioceptive drift are sustained for tens of seconds after visuotactile stroking has ceased. Furthermore, our results indicate that the reduction of proprioceptive drift and the feeling of ownership follow similar time courses in their reduction, suggesting that the two phenomena are temporally correlated. Collectively, these findings help us better understand the relationships of multisensory stimulation, subjective ownership, and proprioceptive drift in the rubber hand illusion.

Visual pursuit biases tactile velocity perception

During a smooth pursuit eye movement of a target stimulus, a briefly flashed stationary background appears to move in the opposite direction as the eye's motion-an effect known as the Filehne illusion. Similar illusions occur in audition, in the vestibular system, and in touch. Recently, we found that the movement of a surface perceived from tactile slip was biased if this surface was sensed with the moving hand. The analogy between these two illusions suggests similar mechanisms of motion processing between the vision and touch. In the present study, we further assessed the interplay between these two sensory channels by investigating a novel paradigm that associated an eye pursuit of a visual target with a tactile motion over the skin of the fingertip. We showed that smooth pursuit eye movements can bias the perceived direction of motion in touch. Similarly to the classical report from the Filehne illusion in vision, a static tactile surface was perceived as moving rightward with a leftward eye pursuit movement, and vice versa. However, this time the direction of surface motion was perceived from touch. The biasing effects of eye pursuit on tactile motion were modulated by the reliability of the tactile and visual stimuli, consistently with a Bayesian model of motion perception. Overall, these results support a modality- and effector-independent process with common representations for motion perception.NEW & NOTEWORTHY The study showed that smooth pursuit eye movement produces a bias in tactile motion perception. This phenomenon is modulated by the reliability of the tactile estimate and by the presence of a visual background, in line with the predictions of the Bayesian framework of motion perception. Overall, these results support the hypothesis of shared representations for motion perception.

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.

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.

Which hand is mine? Discriminating body ownership perception in a two-alternative forced-choice task

The experience of one's body as one's own is referred to as the sense of body ownership. This central part of human conscious experience determines the boundary between the self and the external environment, a crucial distinction in perception, action, and cognition. Although body ownership is known to involve the integration of signals from multiple sensory modalities, including vision, touch, and proprioception, little is known about the principles that determine this integration process, and the relationship between body ownership and perception is unclear. These uncertainties stem from the lack of a sensitive and rigorous method to quantify body ownership. Here, we describe a two-alternative forced-choice discrimination task that allows precise and direct measurement of body ownership as participants decide which of two rubber hands feels more like their own in a version of the rubber hand illusion. In two experiments, we show that the temporal and spatial congruence principles of multisensory stimulation, which determine ownership discrimination, impose tighter constraints than previously thought and that texture congruence constitutes an additional principle; these findings are compatible with theoretical models of multisensory integration. Taken together, our results suggest that body ownership constitutes a genuine perceptual multisensory phenomenon that can be quantified with psychophysics in discrimination experiments.

Functional brain changes in the elderly for the perception of hand movements: A greater impairment occurs in proprioception than touch

Unlike age-related brain changes linked to motor activity, neural alterations related to self-motion perception remain unknown. Using fMRI data, we investigated age-related changes in the central processing of somatosensory information by inducing illusions of right-hand rotations with specific proprioceptive and tactile stimulation. Functional connectivity during resting-state (rs-FC) was also compared between younger and older participants. Results showed common sensorimotor activations in younger and older adults during proprioceptive and tactile illusions, but less deactivation in various right frontal regions and the precuneus were found in the elderly. Older participants exhibited a less-lateralized pattern of activity across the primary sensorimotor cortices (SM1) in the proprioceptive condition only. This alteration of the interhemispheric balance correlated with declining individual performance in illusion velocity perception from a proprioceptive, but not a tactile, origin. By combining task-related data, rs-FC and behavioral performance, this study provided consistent results showing that hand movement perception was altered in the elderly, with a more pronounced deterioration of the proprioceptive system, likely due to the breakdown of inhibitory processes with aging. Nevertheless, older people could benefit from an increase in internetwork connectivity to overcome this kinesthetic decline.

Neuronal Encoding of Multisensory Motion Features in the Rat Associative Parietal Cortex

Motion perception is facilitated by the interplay of various sensory channels. In rodents, the cortical areas involved in multisensory motion coding remain to be identified. Using voltage-sensitive-dye imaging, we revealed a visuo-tactile convergent region that anatomically corresponds to the associative parietal cortex (APC). Single unit responses to moving visual gratings or whiskers deflections revealed a specific coding of motion characteristics strikingly found in both sensory modalities. The heteromodality of this region was further supported by a large proportion of bimodal neurons and by a classification procedure revealing that APC carries information about motion features, sensory origin and multisensory direction-congruency. Altogether, the results point to a central role of APC in multisensory integration for motion perception.

Effect of Cutaneous Feedback on the Perception of Virtual Object Weight during Manipulation

Haptic interface technologies for virtual reality applica have been developed to increase the reality and manipulability of a virtual object by creating a diverse tactile sensation. Most evaluation of the haptic technologies, however, have been limited to the haptic perception of the tactile stimuli via static virtual objects. Noting this, we investigated the effect of lateral cutaneous feedback, along with kinesthetic feedback on the perception of virtual object weight during manipulation. We modeled the physical interaction between a participant’s finger avatars and virtual objects. The haptic stimuli were rendered with custom-built haptic feedback systems that can provide kinesthetic and lateral cutaneous feedback to the participant. We conducted two virtual object manipulation experiments, 1. a virtual object manipulation with one finger, and 2. the pull-out and lift-up of a virtual object grasped with a precision grip. The results of Experiment 1 indicate that the participants felt the virtual object rendered with lateral cutaneous feedback significantly heavier than with only kinesthetic feedback (p < 0.05 for m_ref = 100 and 200 g). Similarly, the participants of Experiment 2 felt the virtual objects significantly heavier when lateral cutaneous feedback was available (p < 0.05 for m_ref = 100, 200, and 300 g). Therefore, the additional lateral cutaneous feedback to the force feedback led the participants to feel the virtual object heavier than without the cutaneous feedback. The results also indicate that the contact force applied to a virtual object during manipulation can be a function of the perceived object weight p = 0.005 for Experiment 1 and p = 0.2 for Experiment 2.

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.

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.

Kinesthetic Senses

The kinesthetic senses are the senses of position and movement of the body, senses we are aware of only on introspection. A method used to study kinesthesia is muscle vibration, which engages afferents of muscle spindles to trigger illusions of movement and changed position. When vibrating elbow flexors, it generates sensations of forearm extension, when vibrating extensors, sensations of forearm flexion. Vibrating the elbow joint produces no illusion. Vibrating flexors and extensors together at the same frequency also produces no illusion, because what is perceived is the signal difference between antagonist muscles of each arm and between arms. The size of the illusion depends on how the muscle has been conditioned beforehand, due to a property of muscle called thixotropy. When measuring the illusion, blindfolded subjects may carry out a matching or pointing task. In pointing, signals from muscle spindles are less important than in matching. Afferent signals from kinesthetic receptors project to areas of somatosensory cortex to generate sensations of detection and location. This is referred to the body model, which provides information about size and shape of body parts. Kinesthesia, together with vision and touch, is associated with the sense of body ownership. All three can combine or each, on its own, can generate ownership. Related is the sense of agency, the sense of being responsible for one's own actions. In recent times, much progress has been made using neuroimaging techniques to identify the various areas of the brain likely to be responsible for generating these sensations. © 2017 American Physiological Society. Compr Physiol 8:1157-1183, 2018.

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.

Integration of visual and tactile information in reproduction of traveled distance

In the natural world, self-motion always stimulates several different sensory modalities. Here we investigated the interplay between a visual optic flow stimulus simulating self-motion and a tactile stimulus (air flow resulting from self-motion) while human observers were engaged in a distance reproduction task. We found that adding congruent tactile information (i.e., speed of the air flow and speed of visual motion are directly proportional) to the visual information significantly improves the precision of the actively reproduced distances. This improvement, however, was smaller than predicted for an optimal integration of visual and tactile information. In contrast, incongruent tactile information (i.e., speed of the air flow and speed of visual motion are inversely proportional) did not improve subjects' precision indicating that incongruent tactile information and visual information were not integrated. One possible interpretation of the results is a link to properties of neurons in the ventral intraparietal area that have been shown to have spatially and action-congruent receptive fields for visual and tactile stimuli.NEW & NOTEWORTHY This study shows that tactile and visual information can be integrated to improve the estimates of the parameters of self-motion. This, however, happens only if the two sources of information are congruent-as they are in a natural environment. In contrast, an incongruent tactile stimulus is still used as a source of information about self-motion but it is not integrated with visual information.