Seeing your own touched hands in a mirror modulates cross-modal interactions

Beyond peripersonal boundaries: insights from crossmodal interactions

We experience our self as a body located in space. However, how information about self-location is integrated into multisensory processes underlying the representation of the peripersonal space (PPS), is still unclear. Prior studies showed that the presence of visual information related to oneself modulates the multisensory processes underlying PPS. Here, we used the crossmodal congruency effect (CCE) to test whether this top-down modulation depends on the spatial location of the body-related visual information. Participants responded to tactile events on their bodies while trying to ignore a visual distractor presented on the mirror reflection of their body (Self) either in the peripersonal space (Near) or in the extrapersonal space (Far). We found larger CCE when visual events were presented on the mirror reflection in the peripersonal space, as compared to the extrapersonal space. These results suggest that top-down modulation of the multisensory bodily self is only possible within the PPS.

Do Changes in the Body-Part Compatibility Effect Index Tool-Embodiment?

Tool-embodiment is said to occur when the representation of the body extends to incorporate the representation of a tool following goal-directed tool-use. The present study was designed to determine if tool-embodiment-like phenomenon emerges following different interventions. Participants completed body-part compatibility task in which they responded with foot or hand presses to colored targets presented on the foot or hand of a model, or on a rake held by the model. This response time (RT) task was performed before and after one of four interventions. In the Virtual-Tangible and the Virtual-Keyboard interventions, participants used customized controllers or keyboards, respectively, to move a virtual rake and ball around a course. Participants in the Tool-Perception intervention manually pointed to targets presented on static images of the virtual tool-use task. Participants in the Tool-Absent group completed math problems and were not exposed to a tool task. Results revealed that all four interventions lead to a pattern of pre-/post-intervention changes in RT thought to indicate the emergence of tool-embodiment. Overall, the study indicated that tool-embodiment can occur through repeated exposure to the body-part compatibility paradigm in the absence of any active tool-use, and that the paradigm may tap into more than just body schema.

Psychological and physiological evidence for an initial 'Rough Sketch' calculation of personal space

Personal space has been defined as “the area individuals maintain around themselves into which others cannot intrude without arousing discomfort”. However, the precise relationship between discomfort (or arousal) responses as a function of distance from an observer remains incompletely understood. Also the mechanisms involved in recognizing conspecifics and distinguishing them from other objects within personal space have not been identified. Accordingly, here we measured personal space preferences in response to real humans and human-like avatars (in virtual reality), using well-validated “stop distance” procedures. Based on threshold measurements of personal space, we examined within-subject variations in discomfort-related responses across multiple distances (spanning inside and outside each individual’s personal space boundary), as reflected by psychological (ratings) and physiological (skin conductance) responses to both humans and avatars. We found that the discomfort-by-distance functions for both humans and avatars were closely fit by a power law. These results suggest that the brain computation of visually-defined personal space begins with a ‘rough sketch’ stage, which generates responses to a broad range of human-like stimuli, in addition to humans. Analogous processing mechanisms may underlie other brain functions which respond similarly to both real and simulated human body parts.

Virtual Mirror and Beyond: The Psychological Basis for Avatar Embodiment via a Mirror

In virtual reality (VR), a virtual mirror is often used to display the VR avatar to the user for enhancing the embodiment. The reflected image of the synchronization of the virtual body with the user’s movement is expected to be recognized as the user’s own reflection. In addition to the visuo-motor synchrony, there are some mirror reflection factors that are probably involved in avatar embodiment. This paper reviews literature on the psychological studies that involve mirror-specific self-identification and embodied perception to clarify how the reflected image of the virtual body is embodied. Furthermore, subjective misconceptions about mirror reflections reported in naïve optics have also been reviewed to discuss the potential of virtual mirror displays to modulate avatar embodiment.

Visual body-size adaptation and estimation of tactile distance

Estimating the size of bodies is crucial for interactions with physical and social environments. Body-size perception is malleable and can be altered using visual adaptation paradigms. However, it is unclear whether such visual adaptation effects also transfer to other modalities and influence, for example, the perception of tactile distances. In this study, we employed a visual adaptation paradigm. Participants were exposed to images of expanded or contracted versions of self- or other-identity bodies. Before and after this adaptation, they were asked to manipulate the width of body stimuli to appear as 'normal' as possible. We replicated an effect of visual adaptation such that the body-size selected as most 'normal' was larger after exposure to expanded and thinner after exposure to contracted adaptation stimuli. In contrast, we did not find evidence that this adaptation effect transfers to distance estimates for paired tactile stimuli delivered to the abdomen. A Bayesian analysis showed that our data provide moderate evidence that there is no effect of visual body-size adaptation on the estimation of spatial parameters in a tactile task. This suggests that visual body-size adaptation effects do not transfer to somatosensory body-size representations.

Whatever Next and Close to My Self-The Transparent Senses and the "Second Skin": Implications for the Case of Depersonalization

In his paper “Whatever next? Predictive brains, situated agents, and the future of cognitive science,” Andy Clark seminally proposed that the brain's job is to predict whatever information is coming “next” on the basis of prior inputs and experiences. Perception fundamentally subserves survival and self-preservation in biological agents, such as humans. Survival however crucially depends on rapid and accurate information processing of what is happening in the here and now. Hence, the term “next” in Clark's seminal formulation must include not only the temporal dimension (i.e., what is perceived now) but also the spatial dimension (i.e., what is perceived here or next-to-my-body). In this paper, we propose to focus on perceptual experiences that happen “next,” i.e., close-to-my-body. This is because perceptual processing of proximal sensory inputs has a key impact on the organism's survival. Specifically, we focus on tactile experiences mediated by the skin and what we will call the “extended skin” or “second skin,” that is, immediate objects/materials that envelop closely to our skin, namely, clothes. We propose that the skin and tactile experiences are not a mere border separating the self and world. Rather, they simultaneously and inherently distinguish and connect the bodily self to its environment. Hence, these proximal and pervasive tactile experiences can be viewed as a “transparent bridge” intrinsically relating and facilitating exchanges between the self and the physical and social world. We conclude with potential implications of this observation for the case of Depersonalization Disorder, a condition that makes people feel estranged and detached from their self, body, and the world.

Visuomotor impairments in complex regional pain syndrome during pointing tasks

ABSTRACT

Complex regional pain syndrome (CRPS) is thought to be characterized by cognitive deficits affecting patients' ability to represent, perceive, and use their affected limb as well as its surrounding space. This has been tested, among others, by straight-ahead tasks testing oneself's egocentric representation, but such experiments lead to inconsistent results. Because spatial cognitive abilities encompass various processes, we completed such evaluations by varying the sensory inputs used to perform the task. Complex regional pain syndrome and matched control participants were asked to assess their own body midline either visually (ie, by means of a moving visual cue) or manually (ie, by straight-ahead pointing with one of their upper limbs) and to reach and point to visual targets at different spatial locations. Although the 2 former tasks only required one single sensory input to be performed (ie, either visual or proprioceptive), the latter task was based on the ability to coordinate perception of the position of one's own limb with visuospatial perception. However, in this latter task, limb position could only be estimated by proprioception, as vision of the limb was prevented. Whereas in the 2 former tasks CRPS participants' performance was not different from that of controls, they made significantly more deviations errors during the visuospatial task, regardless of the limb used to point or the direction of pointing. Results suggest that CRPS patients are not specifically characterized by difficulties in representing their body but, more particularly, in integrating somatic information (ie, proprioception) during visually guided movements of the limb.

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.

Higher-Order Cognition Does Not Affect Multisensory Distractor Processing

Multisensory processing is required for the perception of the majority of everyday objects and events. In the case of irrelevant stimuli, the multisensory processing of features is widely assumed to be modulated by attention. In the present study, we investigated whether the processing of audiovisual distractors is also modulated by higher-order cognition. Participants fixated a visual distractor viewed via a centrally-placed mirror and responded to a laterally-presented audiovisual target. Critically, a distractor tone was presented from the same location as the mirror, while the visual distractor feature was presented at an occluded location, visible only indirectly via mirror reflection. Consequently, it appeared as though the visual and auditory features were presented from the same location though, in fact, they actually originated from different locations. Nevertheless, the results still revealed that the visual and auditory distractor features were processed together just as in the control condition, in which the audiovisual distractor features were both actually presented from fixation. Taken together, these results suggest that the processing of irrelevant multisensory information is not influenced by higher-order cognition.

Seeing or not Seeing Where Your Hands Are. The Influence of Visual Feedback About Hand Position on the Interaction Between Nociceptive and Visual Stimuli

Examining the mechanisms underlying crossmodal interaction between nociceptive and visual stimuli is crucial to understand how humans handle potential bodily threats in their environment. It has recently been shown that nociceptive stimuli can affect the perception of visual stimuli, provided that they occur close together in external space. The present study addresses the question whether these crossmodal interactions between nociceptive and visual stimuli are mediated by the visually perceived proximity between the visual stimuli and the limb on which nociceptive stimuli are applied, by manipulating the presence vs. absence of visual feedback about the position of the stimulated limb. Participants performed temporal order judgments on pairs of visual stimuli, shortly preceded by nociceptive stimuli, either applied on one hand or both hands simultaneously. The hands were placed near the visual stimuli and could either be seen directly, seen through a glass barrier, or hidden from sight with a wooden board. Unilateral nociceptive stimuli induced spatial biases to the advantage of visual stimuli presented near the stimulated hand, which were greater in the conditions in which the hands were seen than in the condition in which vision was prevented. Spatial biases were not modulated by the presence of the glass barrier, minimizing the possibility that the differential effect between the vision and no-vision conditions is solely due to the presence of the barrier between the hands and the visual stimuli. These findings highlight the importance of visual feedback for determining spatial mapping between nociceptive and visual stimuli for crossmodal interaction.

Seeing a Work of Art Indirectly: When a Reproduction Is Better Than an Indirect View, and a Mirror Better Than a Live Monitor

Visiting a museum and seeing an original artwork can be a special experience. We use a survey and a set of hypothetical questions to explore how such experience would be affected by changes in how the artwork is seen. In a first study, participants imagined that they had traveled to see a painting that they particularly like. They discover that it is impossible to directly see the original painting. Three alternatives are offered: seeing an optical reflection (using a mirror), seeing a video screening (a closed-circuit camera), or seeing a reproduction. In all cases, it is made clear that the size, brightness, and resolution will match that of the original. In addition, these options could be within the same room as the original, in the room next door, or in a different building. Results show that physical distance did not affect significantly the responses. However, there was an overall preference for seeing a reproduction as opposed to an optical or digital image. Contrary to the idea that the original is always superior to a copy, many people felt that a direct view of a copy is a preferable experience than an indirect view. The second study was focused directly on the comparison between a mirror and a monitor. Here we highlighted the fact that for the mirror light coming from the mirror originated from the painting. Data were collected in Britain and in China. In both cases, there was a clear preference for the mirror over the monitor.

Peri-hand space expands beyond reach in the context of walk-and-reach movements

The brain incorporates sensory information across modalities to be able to interact with our environment. The peripersonal space (PPS), defined by a high level of crossmodal interaction, is centered on the relevant body part, e.g. the hand, but can spatially expand to encompass tools or reach targets during goal-directed behavior. Previous studies considered expansion of the PPS towards goals within immediate or tool-mediated reach, but not the translocation of the body as during walking. Here, we used the crossmodal congruency effect (CCE) to quantify the extension of the PPS and test if PPS can also expand further to include far located walk-and-reach targets accessible only by translocation of the body. We tested for orientation specificity of the hand-centered reference frame, asking if the CCE inverts with inversion of the hand orientation during reach. We show a high CCE with onset of the movement not only towards reach targets but also walk-and-reach targets. When participants must change hand orientation, the CCE decreases, if not vanishes, and does not simply invert. We conclude that the PPS can expand to the action space beyond immediate or tool-mediated reaching distance but is not purely hand-centered with respect to orientation.

How Visual Body Perception Influences Somatosensory Plasticity

The study of somatosensory plasticity offers unique insights into the neuronal mechanisms that underlie human adaptive and maladaptive plasticity. So far, little attention has been paid on the specific influence of visual body perception on somatosensory plasticity and learning in humans. Here, we review evidence on how visual body perception induces changes in the functional architecture of the somatosensory system and discuss the specific influence the social environment has on tactile plasticity and learning. We focus on studies that have been published in the areas of human cognitive and clinical neuroscience and refer to animal studies when appropriate. We discuss the therapeutic potential of socially mediated modulations of somatosensory plasticity and introduce specific paradigms to induce plastic changes under controlled conditions. This review offers a contribution to understanding the complex interactions between social perception and somatosensory learning by focusing on a novel research field: socially mediated sensory plasticity.

Responding to sounds from unseen locations: crossmodal attentional orienting in response to sounds presented from the rear

To date, most of the research on spatial attention has focused on probing people's responses to stimuli presented in frontal space. That is, few researchers have attempted to assess what happens in the space that is currently unseen (essentially rear space). In a sense, then, 'out of sight' is, very much, 'out of mind'. In this review, we highlight what is presently known about the perception and processing of sensory stimuli (focusing on sounds) whose source is not currently visible. We briefly summarize known differences in the localizability of sounds presented from different locations in 3D space, and discuss the consequences for the crossmodal attentional and multisensory perceptual interactions taking place in various regions of space. The latest research now clearly shows that the kinds of crossmodal interactions that take place in rear space are very often different in kind from those that have been documented in frontal space. Developing a better understanding of how people respond to unseen sound sources in naturalistic environments by integrating findings emerging from multiple fields of research will likely lead to the design of better warning signals in the future. This review highlights the need for neuroscientists interested in spatial attention to spend more time researching what happens (in terms of the covert and overt crossmodal orienting of attention) in rear space.

Assimilation of virtual legs and perception of floor texture by complete paraplegic patients receiving artificial tactile feedback

Spinal cord injuries disrupt bidirectional communication between the patient’s brain and body. Here, we demonstrate a new approach for reproducing lower limb somatosensory feedback in paraplegics by remapping missing leg/foot tactile sensations onto the skin of patients’ forearms. A portable haptic display was tested in eight patients in a setup where the lower limbs were simulated using immersive virtual reality (VR). For six out of eight patients, the haptic display induced the realistic illusion of walking on three different types of floor surfaces: beach sand, a paved street or grass. Additionally, patients experienced the movements of the virtual legs during the swing phase or the sensation of the foot rolling on the floor while walking. Relying solely on this tactile feedback, patients reported the position of the avatar leg during virtual walking. Crossmodal interference between vision of the virtual legs and tactile feedback revealed that patients assimilated the virtual lower limbs as if they were their own legs. We propose that the addition of tactile feedback to neuroprosthetic devices is essential to restore a full lower limb perceptual experience in spinal cord injury (SCI) patients, and will ultimately, lead to a higher rate of prosthetic acceptance/use and a better level of motor proficiency.

Approaching threat modulates visuotactile interactions in peripersonal space

The region surrounding our body (i.e. peripersonal space) is coded in a multimodal representation by fronto-parietal bimodal neurons integrating tactile stimuli on the body with nearby visual stimuli. This has often been suggested to serve a defensive purpose, which we propose could be mediated through visuotactile predictions. An approaching threat would then be of particular interest to peripersonal space processing. To investigate this, we asked participants to respond as fast as possible to a tactile stimulus on the hand, while looking at an animation of an approaching or receding spider or butterfly. Tactile stimulation was applied at one of 25 possible time points during the animation. Tactile reaction times were faster when an approaching stimulus was closer to the hand at the time of tactile presentation. Critically, this effect of distance on reaction times was larger when participants saw an approaching spider compared to an approaching butterfly, but only for participants who were afraid of spiders. This finding demonstrates that the perceived threat of an approaching stimulus modulates visuotactile interactions in peripersonal space and is consistent with the idea that visuotactile predictions are important for defensive purposes and maintaining bodily integrity.

Owning the body in the mirror: The effect of visual perspective and mirror view on the full-body illusion

Mirrors allow us to view our own body from a third-person (observer) perspective. However, how viewing ourselves through a mirror affects central body representations compared with true third-person perspective is not fully understood. Across a series of experiments, multisensory full-body illusions were used to modulate feelings of ownership over a mannequin body that was viewed from a third-person perspective through a mirror, from a third-person perspective without a mirror, and from a first-person perspective. In contrast to non-mirror third-person perspective, synchronously touching the participant’s actual body and the mannequin body viewed in the mirror elicited strong feelings of ownership over the mannequin and increased physiological responses to the mannequin being threatened compared to the equivalent asynchronous (non-ownership) control condition. Subjective reports of ownership viewing the mannequin through a mirror were also statistically equivalent to those following the first-person perspective illusion. These findings suggest that mirrors have a special role for viewing the self. The results also support the importance of egocentric reference frames for body ownership and suggest that mirror reflections of one’s own body are related to peripersonal space, which enables updating of central body representations.

Those are Your Legs: The Effect of Visuo-Spatial Viewpoint on Visuo-Tactile Integration and Body Ownership

Experiencing a body part as one's own, i.e., body ownership, depends on the integration of multisensory bodily signals (including visual, tactile, and proprioceptive information) with the visual top-down signals from peripersonal space. Although it has been shown that the visuo-spatial viewpoint from where the body is seen is an important visual top-down factor for body ownership, different studies have reported diverging results. Furthermore, the role of visuo-spatial viewpoint (sometime also called first-person perspective) has only been studied for hands or the whole body, but not for the lower limbs. We thus investigated whether and how leg visuo-tactile integration and leg ownership depended on the visuo-spatial viewpoint from which the legs were seen and the anatomical similarity of the visual leg stimuli. Using a virtual leg illusion, we tested the strength of visuo-tactile integration of leg stimuli using the crossmodal congruency effect (CCE) as well as the subjective sense of leg ownership (assessed by a questionnaire). Fifteen participants viewed virtual legs or non-corporeal control objects, presented either from their habitual first-person viewpoint or from a viewpoint that was rotated by 90°(third-person viewpoint), while applying visuo-tactile stroking between the participants legs and the virtual legs shown on a head-mounted display. The data show that the first-person visuo-spatial viewpoint significantly boosts the visuo-tactile integration as well as the sense of leg ownership. Moreover, the viewpoint-dependent increment of the visuo-tactile integration was only found in the conditions when participants viewed the virtual legs (absent for control objects). These results confirm the importance of first person visuo-spatial viewpoint for the integration of visuo-tactile stimuli and extend findings from the upper extremity and the trunk to visuo-tactile integration and ownership for the legs.

How we remember what we can do

According to the motor simulation theory, the knowledge we possess of what we can do is based on simulation mechanisms triggered by an off-line activation of the brain areas involved in motor control. Action capabilities memory does not work by storing some content, but consists in the capacity, rooted in sensory-motor systems, to reenact off-line action sequences exhibiting the range of our powers. In this paper, I present several arguments from cognitive neuropsychology, but also first-person analysis of experience, against this hypothesis. The claim that perceptual access to affordances is mediated by motor simulation processes rests on a misunderstanding of what affordances are, and comes up against a computational reality principle. Motor simulation cannot provide access to affordances because (i) the affordances we are aware of at each moment are too many for their realization to be simulated by the brain and (ii) affordances are not equivalent to currently or personally feasible actions. The explanatory significance of the simulation theory must then be revised downwards compared to what is claimed by most of its advocates. One additional challenge is to determine the prerequisite, in terms of cognitive processing, for the motor simulation mechanisms to work. To overcome the limitations of the simulation theory, I propose a new approach: the direct content specification hypothesis. This hypothesis states that, at least for the most basic actions of our behavioral repertoire, the action possibilities we are aware of through perception are directly specified by perceptual variables characterizing the content of our experience. The cognitive system responsible for the perception of action possibilities is consequently far more direct, in terms of cognitive processing, than what is stated by the simulation theory. To support this hypothesis I review evidence from current neuropsychological research, in particular data suggesting a phenomenon of ‘fossilization’ of affordances. Fossilization can be defined as a gap between the capacities that are treated as available by the cognitive system and the capacities this system really has at its disposal. These considerations do not mean that motor simulation cannot contribute to explain how we gain perceptual knowledge of what we can do based on the memory of our past performances. However, when precisely motor simulation plays a role and what it is for exactly currently remain largely unknown.

The multisensory body revealed through its cast shadows

One key issue when conceiving the body as a multisensory object is how the cognitive system integrates visible instances of the self and other bodies with one's own somatosensory processing, to achieve self-recognition and body ownership. Recent research has strongly suggested that shadows cast by our own body have a special status for cognitive processing, directing attention to the body in a fast and highly specific manner. The aim of the present article is to review the most recent scientific contributions addressing how body shadows affect both sensory/perceptual and attentional processes. The review examines three main points: (1) body shadows as a special window to investigate the construction of multisensory body perception; (2) experimental paradigms and related findings; (3) open questions and future trajectories. The reviewed literature suggests that shadows cast by one's own body promote binding between personal and extrapersonal space and elicit automatic orienting of attention toward the body-part casting the shadow. Future research should address whether the effects exerted by body shadows are similar to those observed when observers are exposed to other visual instances of their body. The results will further clarify the processes underlying the merging of vision and somatosensation when creating body representations.

Common coding and dynamic interactions between observed, imagined, and experienced motor and somatosensory activity

Motor imagery and perception - considered generally as forms of motor simulation - share overlapping neural representations with motor production. While much research has focused on the extent of this "common coding," less attention has been paid to how these overlapping representations interact. How do imagined, observed, or produced actions influence one another, and how do we maintain control over our perception and behavior? In the first part of this review we describe interactions between motor production and motor simulation, and explore apparent regulatory mechanisms that balance these processes. Next, we consider the somatosensory system. Numerous studies now support a "sensory mirror system" comprised of neural representations activated by either afferent sensation or vicarious sensation. In the second part of this review we summarize evidence for shared representations of sensation and sensory simulation (including imagery and observed sensation), and suggest that similar interactions and regulation of simulation occur in the somatosensory domain as in the motor domain. We suggest that both motor and somatosensory simulations are flexibly regulated to support simulations congruent with our sensorimotor experience and goals and suppress or separate the influence of those that are not. These regulatory mechanisms are frequently revealed by cases of brain injury but can also be employed to facilitate sensorimotor rehabilitation.

The robot hand illusion: inducing proprioceptive drift through visuo-motor congruency

The representation of one's own body sets the border of the self, but also shapes the space where we interact with external objects. Under particular conditions, such as in the rubber hand illusion external objects can be incorporated in one's own body representation, following congruent visuo-tactile stroking of one's own and a fake hand. This procedure induces an illusory sense of ownership for the fake hand and a shift of proprioceptive localization of the own hand towards the fake hand. Here we investigated whether pure visuo-motor, instead of visuo-tactile, congruency between one's own hand and a detached myoelectric-controlled robotic hand can induce similar embodiment effects. We found a shift of proprioceptive hand localization toward the robot hand, only following synchronized real hand/robot hand movements. Notably, no modulation was found of the sense of ownership following either synchronous or asynchronous-movement training. Our findings suggest that visuo-motor synchrony can drive the localization of one's own body parts in space, even when somatosensory input is kept constant and the experience of body ownership is maintained.

Itch relief by mirror scratching. A psychophysical study

Objective

The goal of this study was to test whether central mechanisms of scratching-induced itch attenuation can be activated by scratching the limb contralateral to the itching limb when the participant is made to visually perceive the non-itching limb as the itching limb by means of mirror images.

Methods

Healthy participants were asked to assess the intensity of an experimentally induced itch at their right forearm while they observed externally guided scratch movements either at their right (itching) or left (non-itching) forearm which were either mirrored or not mirrored. In the first experiment, a mirror placed between the participant’s forearms was used to create the visual illusion that the participant’s itching (right) forearm was being scratched while in fact the non-itching (left) forearm was scratched. To control visibility of the left (non-mirrored) forearm, a second experiment was performed in which unflipped and flipped real-time video displays of the participant’s forearms were used to create experimental conditions in which the participant visually perceived scratching either on one forearm only, on both forearms, or no scratching at all.

Results

In both experiments, scratching the non-itching limb attenuated perceived itch intensity significantly and selectively in the mirror condition, i.e., when the non-itching forearm was visually perceived as the itching limb.

Discussion

These data provide evidence that the visual illusion that an itching limb is being scratched while in fact the non-itching limb contralateral to the itching limb is scratched, can lead to significant itch relief. This effect might be due to a transient illusionary intersensory perceptual congruency of visual, tactile and pruriceptive signals. “Mirror scratching” might provide an alternative treatment to reduce itch perception in focal skin diseases with persistent pruritus without causing additional harm to the affected skin and might therefore have significant clinical impact.

Multisensory integration across the menstrual cycle

Evidence suggests that spatial processing changes across time in naturally cycling women, which is likely due to neuromodulatory effects of steroid hormones. Yet, it is unknown whether crossmodal spatial processes depend on steroid hormones as well. In the present experiment, the crossmodal congruency task was used to assess visuo-tactile interactions in naturally cycling women, women using hormonal contraceptives and men. Participants adopted either a crossed or uncrossed hands posture. It was tested whether a postural effect of hand crossing on multisensory interactions in the crossmodal congruency task is modulated by women's cycle phase. We found that visuotactile interactions changed according to cycle phase. Naturally cycling women showed a significant difference between the menstrual and the luteal phase for crossed, but not for uncrossed hands postures. The two control groups showed no test sessions effects. Regression analysis revealed a positive relation between estradiol levels and the size of crossmodal congruency effects (CCE), indicating that estradiol seems to have a neuromodulatory effect on posture processing.

Plasticity in unimodal and multimodal brain areas reflects multisensory changes in self-face identification

Nothing provides as strong a sense of self as seeing one's face. Nevertheless, it remains unknown how the brain processes the sense of self during the multisensory experience of looking at one's face in a mirror. Synchronized visuo-tactile stimulation on one's own and another's face, an experience that is akin to looking in the mirror but seeing another's face, causes the illusory experience of ownership over the other person's face and changes in self-recognition. Here, we investigate the neural correlates of this enfacement illusion using fMRI. We examine activity in the human brain as participants experience tactile stimulation delivered to their face, while observing either temporally synchronous or asynchronous tactile stimulation delivered to another's face on either a specularly congruent or incongruent location. Activity in the multisensory right temporo-parietal junction, intraparietal sulcus, and the unimodal inferior occipital gyrus showed an interaction between the synchronicity and the congruency of the stimulation and varied with the self-reported strength of the illusory experience, which was recorded after each stimulation block. Our results highlight the important interplay between unimodal and multimodal information processing for self-face recognition, and elucidate the neurobiological basis for the plasticity required for identifying with our continuously changing visual appearance.

To blink or not to blink: fine cognitive tuning of the defensive peripersonal space

The blink reflex elicited by the electrical stimulation of the median nerve at the wrist [hand blink reflex (HBR)] is a subcortical, defensive response that is enhanced when the stimulated hand is inside the peripersonal space of the face. Such enhancement results from a tonic, top-down modulation of the excitability of the brainstem interneurons mediating the HBR. Here we aim to (1) characterize the somatotopical specificity of this top-down modulation and investigate its dependence on (2) cognitive expectations and (3) the presence of objects protecting the face, in healthy humans. Experiment 1 showed that the somatotopical specificity of the HBR enhancement is partially homosegmental, i.e., it is greater for the HBR elicited by the stimulation of the hand near the face compared with the other hand, always kept far from the face. Experiment 2 showed that the HBR is enhanced only when participants expect to receive stimuli on the hand close to the face and is thus strongly dependent on cognitive expectations. Experiment 3 showed that the HBR enhancement by hand-face proximity is suppressed when a thin wooden screen is placed between the participants' face and their hand. Thus, the screen reduces the extension of the defensive peripersonal space, so that the hand is never inside the peripersonal space of the face, even in the "near" condition. Together, these findings indicate a fine somatotopical and cognitive tuning of the excitability of brainstem circuits subserving the HBR, whose strength is adjusted depending on the context in a purposeful manner.

Reflections on the hand: the use of a mirror highlights the contributions of interpreted and retinotopic representations in the rubber-hand illusion

In the rubber-hand illusion, observing a rubber hand stroked in synchrony with one's own hand results in mislocalisation of the own hand, which is perceived as being located closer to the rubber hand. This illusion depends on having the rubber hand placed at a plausible egocentric orientation with respect to the observer. In the present study, we took advantage of this finding in order to compare the relative influence on the illusion of the rubber hand's perceived retinotopic image against its real-world position. The rubber hand was positioned egocentrically (fingers away from the participant) or allocentrically (fingers towards the participant), while participants viewed it either directly or via a mirror that was placed facing the participant. In the mirror conditions, the orientation of the retinotopic image of the hand (either egocentric or allocentric) was opposed to its real-world orientation. We found that the illusion was elicited in both mirror conditions, to roughly the same extent. Thus either of two representations can elicit the rubber-hand illusion: a world-centred understanding of the scene, resulting from the inferred position of the hand based on its mirror reflection, or a purely visual retinotopic representation of the viewed hand. In the mirror conditions, the illusion was somewhat weaker than in the typical directly viewed egocentric condition. We attribute this to competition between two incompatible representations introduced by the presence of the mirror. Finally, in two control experiments we ruled out that this reduction was due to two properties of mirror reflections: the increased perceived distance of items and the reversal of the apparent handedness of the rubber hand.

When far is near: ERP correlates of crossmodal spatial interactions between tactile and mirror-reflected visual stimuli

Visuo-tactile integration occurs in a privileged way in peripersonal space, namely when visual and tactile stimuli are in spatial proximity. Here, we investigated whether crossmodal spatial effects (i.e. stronger crossmodal interactions for spatially congruent compared to incongruent visual and tactile stimuli) are also present when visual stimuli presented near the body are indirectly viewed in a mirror, thus appearing in far space. Participants had to attend to one of their hands throughout a block of stimuli in order to detect infrequent tactile target stimuli at that hand while ignoring tactile targets at the unattended hand, all tactile non-target stimuli, and any visual stimuli. Visual stimuli were presented simultaneously with tactile stimuli, in the same (congruent) or opposite (incongruent) hemispace with respect to the tactile stimuli. In one group of participants the visual stimuli were delivered near the participants' hands and were observed as indirect mirror reflections ('mirror' condition), while in the other group these were presented at a distance from the hands ('far' condition). The main finding was that crossmodal spatial modulations of ERPs recorded over and close to somatosensory cortex were present in the 'mirror' condition but not the 'far' condition. That is, ERPs were enhanced in response to tactile stimuli coupled with spatially congruent versus incongruent visual stimuli when the latter were viewed through a mirror. These effects emerged around 190 ms after stimuli onset, and were modulated by the focus of spatial attention. These results provide evidence that visual stimuli observed in far space via a mirror are coded as near-the-body stimuli according to their known rather than to their perceived location. This suggests that crossmodal interactions between vision and touch may be modulated by previous knowledge of reflecting surfaces (i.e. top-down processing).

Visual capture influences body-based indications of visual extent

Visual information regarding limb location can override proprioceptive information when there is conflict between the two-a phenomenon referred to as visual capture. In three experiments, we employed the "mirror illusion," in which the perceived location of one's hand is influenced by the visual information specified by the mirror reflection of the other hand, to test whether visual capture influences body-based indications of the extent of objects. Participants viewed their visible hand and its reflection in a mirror after the unseen hand was positioned at one of four locations on a tabletop. The unseen hand's location appeared to be the same distance from the mirror as the visible hand's location. After viewing the visible hand and its reflection while simultaneously performing simple finger movements with both hands, participants viewed a block and had to move their unseen hand to a position that would allow them to grasp the block between their two hands. Movements of the unseen hand relative to the visible hand were biased by the visual information, reflecting errors in moved hand position given visual-proprioceptive conflict. In Experiment 1, visual capture influenced the indications of object extent for objects within reach and aligned with the viewer's midline. Experiments 2 and 3 extended these findings to indications of extent for objects outside the viewer's reach (Experiment 2) and misaligned with the viewer's midline (Experiment 3). These results suggest that visual body information has a generalizable effect on actions used to indicate space perception that extends beyond egocentric spatial localization tasks.

Mirror, mirror, on the wall, is that even my hand at all? Changes in the afterimage of one's reflection in a mirror in response to bodily movement

Successful mirror self-recognition has often been considered a correlate of self-awareness in human development and phylogeny (Gallup, 1982). Studies have also shown that vision and touch interact such that objects viewed in a mirror's reflection are recoded as originating from a location within reachable, or peripersonal, space (Maravita et al. 2002). However, the association of mirror self-recognition and self-awareness is controversial, and the mechanism that underlies the recoding of visual information into peripersonal space remains an open question. In the present study, we address these issues through the novel use of an old paradigm: positive afterimages. It has been shown that when a positive afterimage is induced, and a limb is displaced from its apparent location in the afterimage, the afterimage of the limb fades or "crumbles" (Davies, 1973). We reproduced this effect in conditions where subjects viewed the afterimage of their arms' reflection using a frontally placed mirror and mirror box (Ramachandran & Rogers-Ramachandran, 1996). Our results suggest that the explicit knowledge that one is looking at a mirror as well as online visual feedback from bodily movement are unlikely to be responsible for previously observed interactions between vision and touch. Instead, we propose that a sense of ownership, and (bodily) self-awareness, might in part explain these interactions between vision and proprioception, which provides a partial vindication of the inference from successful mirror self-recognition to self-awareness.

Crossmodal visual-tactile extinction: Modulation by posture implicates biased competition in proprioceptively reconstructed space

Extinction is a common consequence of unilateral brain injury: contralesional events can be perceived in isolation, yet are missed when presented concurrently with competing events on the ipsilesional side. This can arise crossmodally, where a contralateral touch is extinguished by an ipsilateral visual event. Recent studies showed that repositioning the hands in visible space, or making visual events more distant, can modulate such crossmodal extinction. Here, in a detailed single-case study, we implemented a novel spatial manipulation when assessing crossmodal extinction. This was designed not only to hold somatosensory inputs and hand/arm-posture constant, but also to hold (retinotopic) visual inputs constant, yet while still changing the spatial relationship of tactile and visual events in the external world. Our right hemisphere patient extinguished left-hand touches due to visual stimulation of the right visual field (RVF) when tested in the usual default posture with eyes/head directed straight ahead. But when her eyes/head were turned to the far left (and any visual events shifted along with this), such that the identical RVF retinal stimulation now fell at the same external location as the left-hand touch, crossmodal extinction was eliminated. Since only proprioceptive postural cues could signal this changed spatial relationship for the critical condition, our results show for the first time that such postural cues alone are sufficient to modulate crossmodal extinction. Identical somatosensory and retinal inputs can lead to severe crossmodal extinction, or none, depending on current posture.

Multisensory processing and neural oscillatory responses: separation of visuotactile congruency effect and corresponding electroencephalogram activities

By using electroencephalogram, we aimed at identifying the neural oscillations corresponding to two subcomponents of the visuotactile congruency process, facilitation and interference effects. The participants identified the location of the tactile stimulus presented at their index finger or thumb while ignoring simultaneous visual stimuli presented at the same (congruent) or different (incongruent) finger. Significant differences in reaction time were found between the congruent or incongruent condition and the unimodal baseline condition, indicating two different effects of visuotactile interaction. Electroencephalogram results showed significant increases in the gamma-band response in the congruent condition and in the theta-band response in the incongruent condition. These results suggest two systems of multisensory interaction.

Keeping in touch with one's self: multisensory mechanisms of self-consciousness

BACKGROUND

The spatial unity between self and body can be disrupted by employing conflicting visual-somatosensory bodily input, thereby bringing neurological observations on bodily self-consciousness under scientific scrutiny. Here we designed a novel paradigm linking the study of bodily self-consciousness to the spatial representation of visuo-tactile stimuli by measuring crossmodal congruency effects (CCEs) for the full body.

METHODOLOGY/PRINCIPAL FINDINGS

We measured full body CCEs by attaching four vibrator-light pairs to the trunks (backs) of subjects who viewed their bodies from behind via a camera and a head mounted display (HMD). Subjects made speeded elevation (up/down) judgments of the tactile stimuli while ignoring light stimuli. To modulate self-identification for the seen body subjects were stroked on their backs with a stick and the felt stroking was either synchronous or asynchronous with the stroking that could be seen via the HMD. We found that (1) tactile stimuli were mislocalized towards the seen body (2) CCEs were modulated systematically during visual-somatosensory conflict when subjects viewed their body but not when they viewed a body-sized object, i.e. CCEs were larger during synchronous than during asynchronous stroking of the body and (3) these changes in the mapping of tactile stimuli were induced in the same experimental condition in which predictable changes in bodily self-consciousness occurred.

CONCLUSIONS/SIGNIFICANCE

These data reveal that systematic alterations in the mapping of tactile stimuli occur in a full body illusion and thus establish CCE magnitude as an online performance proxy for subjective changes in global bodily self-consciousness.