Extension of perceived arm length following tool-use: Clues to plasticity of body metrics

Multisensory conflict affects body schema and reaching space

Multisensory integration plays a crucial role in building the sense of body ownership, i.e., the perceptual status of one's body for which the body is perceived as belonging to oneself. Temporal and spatial mismatching of visual and tactile signals coming from one's body can reduce ownership feelings towards the body and its parts, i.e., produce disownership feelings. Here, we investigated whether visuo-tactile conflict also affects the sensorimotor representation of the body in space (i.e., body schema) and the perception of the space around the body in terms of action potentiality (i.e., reaching space). In two experiments, body schema (Experiment 1) and reaching space (Experiment 2) were assessed before and after either synchronous or asynchronous visuo-tactile stimulation. Results showed that the asynchronous condition, provoking multisensory conflict, caused disownership over one's hand and concurrently affected the body schema and the reaching space. These findings indicate that body schema and reaching space could be dynamically shaped by the multisensory regularities that build up the sense of body ownership.

Have I Been Touched? Subjective and Objective Aspects of Tactile Awareness

Somatosensory tactile experience is a key aspect of our interaction with the environment. It is involved in object manipulation, in the planning and control of actions and, in its affective components, in the relationships with other individuals. It is also a foundational component of body awareness. An intriguing aspect of sensory perception in general and tactile perception in particular is the way in which stimulation comes to consciousness. Indeed, although being aware of something seems a rather self-evident and monolithic aspect of our mental states, sensory awareness may be in fact modulated by many different processes that impact on the mere stimulation of the skin, including the way in which we perceive our bodies as belonging to us. In this review, we first took into consideration the pathological conditions of absence of phenomenal experience of touch, in the presence of implicit processing, as initial models for understanding the neural bases of conscious tactile experience. Subsequently, we discussed cases of tactile illusions both in normal subjects and in brain-damaged patients which help to understand which high-order processes impact tactile awareness. Finally, we discussed the observations reported in the review in light of some influential models of touch and body representation.

Rewiring the evolution of the human hand: How the embodiment of a virtual bionic tool improves behavior

Humans are the most versatile tool users among animals. Accordingly, our manual skills evolved alongside the shape of the hand. In the future, further evolution may take place: humans may merge with their tools, and technology may integrate into our biology in a way that blurs the line between the two. So, the question is whether humans can embody a bionic tool (i.e., experience it as part of their body) and thus if this would affect behavior. We investigated in virtual reality how the substitution of the hand with a virtual grafting of an end-effector, either non-naturalistic (a bionic tool) or naturalistic (a hand), impacts embodiment and behavior. Across four experiments, we show that the virtual grafting of a bionic tool elicits a sense of embodiment similar to or even stronger than its natural counterpart. In conclusion, the natural usage of bionic tools can rewire the evolution of human behavior.

The different impact of attention, movement, and sensory information on body metric representation

A growing body of research investigating the relationship between body representation and tool-use has shown that body representation is highly malleable. The nature of the body representation does not consist only of sensory attributes but also of motor action-oriented qualities, which may modulate the subjective experience of our own body. However, how these multisensory factors and integrations may specifically guide and constrain body reorientation's plasticity has been under-investigated. In this study, we used a forearm bisection task to selectively investigate the contribution of motor, sensory, and attentional aspects in guiding body representation malleability. Results show that the perceived forearm midpoint deviates from the real one. This shift is further modulated by a motor task but not by a sensory task, whereas the attentional task generates more uncertain results. Our findings provide novel insight into the individual role of movement, somatosensation, and attention in modulating body metric representation.

Perception, action, and the body model

In 1992, Goodale and Milner proposed to study the visual system based on function, thus dissociating vision for perception (ventral stream) and vision for action (dorsal stream). This became known as the Perception and Action model (PAM). Following the PAM in the visual system, a somatosensory PAM was proposed including a body representation for perception and a separate for action. This review explores the body model of the hand and how it relates to the PAM. The body model refers to the internal representation of the body that is responsible for position sense. Previous research has shown that the representation of the hand features systematic distortions: an overestimation of hand width and an underestimation of finger length. These distortions have been reported using different paradigms, different body parts, and in various settings. Thus, body model distortions appear to be a characteristic of human body representation. If the body model of the hand is distorted, how can actions like reaching and grasping be accurate? We review evidence that body model distortions may in fact provide a functional benefit to our actions, that cortical maps in the somatosensory and motor cortices reflect these distortions, and that actions rely on a distorted body model. We argue that the body model is a product of both the ventral and dorsal somatosensory streams. Further, we suggest that the body model is an example of the inextricable link between the two streams.

Alpha oscillations reflect similar mapping mechanisms for localizing touch on hands and tools

It has been suggested that our brain re-uses body-based computations to localize touch on tools, but the neural implementation of this process remains unclear. Neural oscillations in the alpha and beta frequency bands are known to map touch on the body in external and skin-centered coordinates, respectively. Here, we pinpointed the role of these oscillations during tool-extended sensing by delivering tactile stimuli to either participants' hands or the tips of hand-held rods. To disentangle brain responses related to each coordinate system, we had participants' hands/tool tips crossed or uncrossed at their body midline. We found that midline crossing modulated alpha (but not beta) band activity similarly for hands and tools, also involving a similar network of cortical regions. Our findings strongly suggest that the brain uses similar oscillatory mechanisms for mapping touch on the body and tools, supporting the idea that body-based neural processes are repurposed for tool use.

Forearm bisection task suggests an alteration in body schema in patients with functional movement disorders (motor conversion disorders)

OBJECTIVES

To explore potential alterations of the Body Schema, the implicit sensorimotor representation of one's own body, in patients with Functional Movement Disorders (FMD, Motor Conversion Disorders), characterized by neurological symptoms of altered voluntary motor function that cannot be explained by typical medical conditions. This investigation is prompted by the potential dissociation from their reportedly intact sense of ownership.

METHODS

10 FMD patients and 11 healthy controls (HC) underwent the Forearm Bisection Task, aimed at assessing perceived body metrics, which consists in asking the subject, blindfolded, to repeatedly point at the perceived middle point of their dominant forearm with the index finger of their contralateral hand, and a psychometric assessment for anxiety, depression, alexithymia, and tendency to dissociation.

RESULTS

FMD patients bisected their forearm more proximally (with an increased shift towards their elbow equal to 7.5%) with respect to HC; average bisection point was positively associated with anxiety levels in the whole sample, and with the tendency to dissociation in the FMD group.

CONCLUSIONS

FMD patients perceive their forearm as shorter than HC, suggesting an alteration of their Body Schema. The Body Schema can go through short- and long-term updates in the life course, mainly related to the use of each body segment; we speculate that, despite FMD being a disorder of functional nature, characterized by variability and fluctuations in symptomatology, the lack of sense of agency over a body part might be interpreted by the nervous system as disuse and hence influence the Body Schema, as deficits of organic etiology do.

A Somatosensory Computation That Unifies Limbs and Tools

It is often claimed that tools are embodied by their user, but whether the brain actually repurposes its body-based computations to perform similar tasks with tools is not known. A fundamental computation for localizing touch on the body is trilateration. Here, the location of touch on a limb is computed by integrating estimates of the distance between sensory input and its boundaries (e.g., elbow and wrist of the forearm). As evidence of this computational mechanism, tactile localization on a limb is most precise near its boundaries and lowest in the middle. Here, we show that the brain repurposes trilateration to localize touch on a tool, despite large differences in initial sensory input compared with touch on the body. In a large sample of participants, we found that localizing touch on a tool produced the signature of trilateration, with highest precision close to the base and tip of the tool. A computational model of trilateration provided a good fit to the observed localization behavior. To further demonstrate the computational plausibility of repurposing trilateration, we implemented it in a three-layer neural network that was based on principles of probabilistic population coding. This network determined hit location in tool-centered coordinates by using a tool's unique pattern of vibrations when contacting an object. Simulations demonstrated the expected signature of trilateration, in line with the behavioral patterns. Our results have important implications for how trilateration may be implemented by somatosensory neural populations. We conclude that trilateration is likely a fundamental spatial computation that unifies limbs and tools.

Hypnotic suggestion versus sensory modulation of bodily awareness

Bodily awareness arises from somatosensory, vestibular, and visual inputs but cannot be reduced to these incoming sensory signals. Cognitive factors are known to also impact bodily awareness, though their specific influence is poorly understood. Here we systematically compared the effects of sensory (bottom-up) and cognitive (top-down) manipulations on the estimated size of body parts. Toward this end, in a repeated-measures design, we sought to induce the illusion that the right index finger was elongating by vibrating the biceps tendon of the left arm whilst participants grasped the tip of their right index finger (Lackner illusion; bottom-up) and separately by hypnotic suggestion (top-down), with a sham version of the Lackner illusion as an active control condition. The effects of these manipulations were assessed with perceptual and motor tasks to capture different components of the representation of body size. We found that hypnotic suggestion significantly induced the illusion in both tasks relative to the sham condition. The magnitudes of these effects were stronger than those in the Lackner illusion condition, which only produced a significantly stronger illusion than the sham condition in the perceptual task. We further observed that illusion magnitude significantly correlated across tasks and conditions, suggesting partly shared mechanisms. These results are in line with theories of separate but interacting representational processes for perception and action and highlight the influence of cognitive factors on low-level body representations.

Knowing your boundaries: no effect of tool-use on body representation following a gather-and-sort task

Internal representations of the body have received considerable attention in recent years, particularly in the context of tool-use. Results have supported the notion that these representations are plastic and tool-use engenders an extension of the internal representation of the arm. However, the limitations of the literature underlying this tool embodiment process have not been adequately considered or tested. For example, there is some evidence that tool-use effects do not extend beyond simplistic tool-use tasks. To further clarify this issue, 66 participants engaged in a period of tool-augmented reaches in a speeded gather-and-sort task. If task characteristics inherent to simplistic tasks are relevant to putative embodiment effects, it was predicted that there would be no effect of tool-use on tactile distance judgments or forearm bisections. A Bayesian analysis found considerable support for the null hypothesis in both outcome measures, suggesting that some of the evidence for tool embodiment may be based in task characteristics inherent in the narrow range of tool-use tasks used to study them, rather than a tool incorporation process. Potential sources of influence stemming from these characteristics are discussed.

Paying attention to the outcome of others' actions has dissociated effects on observer's peripersonal space representation and exploitation

The representation of peripersonal space (PPS representation) and the selection of motor actions within it (PPS exploitation) are influenced by action outcomes and reward prospects. The present study tested whether observing the outcome of others' actions altered the observer's PPS representation and exploitation. Participants (observers) performed a reachability-judgement task (assessing PPS representation) before and after having observed a confederate (actors) performing a stimuli-selection task on a touch-screen table. In the stimuli-selection task, the stimuli selected could either yield a reward or not, but the probability to select a reward-yielding stimulus was biased in space, being either 50%, 25% or 75% in the actor's proximal or distal space. After the observation phase, participants performed the stimuli-selection task (assessing PPS exploitation), but with no spatial bias in the distribution of reward-yielding stimuli. Results revealed an effect of actors' actions outcome on observers' PPS representation, which changed according to the distribution of reward-yielding stimuli in the actors' proximal and distal spaces. No significant effect of actors' actions outcome was found on observers' PPS exploitation. As a whole, the results suggest dissociated effects of observing the outcome of others' actions on PPS representation and exploitation.

Emergence of sense of body ownership but not agency during virtual tool-use training is associated with an altered body schema

In this study we examined if training with a virtual tool in augmented reality (AR) affects the emergence of ownership and agency over the tool and whether this relates to changes in body schema (BS). 34 young adults learned controlling a virtual gripper to grasp a virtual object. In the visuo-tactile (VT) but not the vision-only (V) condition, vibro-tactile feedback was applied to the palm, thumb and index fingers through a CyberTouch II glove when the tool touched the object. Changes in the forearm BS were assessed with a tactile distance judgement task (TDJ) where participants judged distances between two tactile stimuli applied to their right forearm either in proximodistal or mediolateral orientation. Participants further rated their perceived ownership and agency after training. TDJ estimation errors were reduced after training for proximodistal orientations, suggesting that stimuli oriented along the arm axis were perceived as closer together. Higher ratings for ownership were associated with increasing performance level and more BS plasticity, i.e., stronger reduction in TDJ estimation error, and after training in the VT as compared to the V feedback condition, respectively. Agency over the tool was achieved independent of BS plasticity. We conclude that the emergence of a sense of ownership but not agency depends on performance level and the integration of the virtual tool into the arm representation.

Seeing our hand or a tool during visually-guided actions: Different effects on the somatosensory and visual cortices

The processing of proprioceptive information in the context of a conflict between visual and somatosensory feedbacks deteriorates motor performance. Previous studies have shown that seeing one's hand increases the weighting assigned to arm somatosensory inputs. In this light, we hypothesized that the sensory conflict, when tracing the contour of a shape with mirror-reversed vision, will be greater for participants who trace with a stylus seen in their hand (Hand group, n = 17) than for participants who trace with the tip of rod without seen their hand (Tool group, n = 15). Based on this hypothesis, we predicted that the tracing performance with mirror vision will be more deteriorated for the Hand group than for the Tool group, and we predicted a greater gating of somatosensory information for the Hand group to reduce the sensory conflict. The participants of both groups followed the outline of a shape in two visual conditions. Direct vision: the participants saw the hand or portion of a light 40 cm rod directly. Mirror Vision: the hand or the rod was seen through a mirror. We measured tracing performance using a digitizing tablet and the cortical activity with electroencephalography. Behavioral analyses revealed that the tracing performance of both groups was similarly impaired by mirror vision. However, contrasting the spectral content of the cortical oscillatory activity between the Mirror and Direct conditions, we observed that tracing with mirror vision resulted in significantly larger alpha (8-12 Hz) and beta (15-25 Hz) powers in the somatosensory cortex for participants of the Hand group. The somatosensory alpha and beta powers did not significantly differ between Mirror and Direct vision conditions for the Tool group. For both groups, tracing with mirror vision altered the activity of the visual cortex: decreased alpha power for the Hand group, decreased alpha and beta power for the Tool group. Overall, these results suggest that seeing the hand enhanced the sensory conflict when tracing with mirror vision and that the increase of alpha and beta powers in the somatosensory cortex served to reduce the weight assigned to somatosensory information. The increased activity of the visual cortex observed for both groups in the mirror vision condition suggests greater visual processing with increased task difficulty. Finally, the fact that the participants of the Tool group did not show better tracing performance than those of the Hand group suggests that tracing deterioration resulted from a sensorimotor conflict (as opposed to a visuo-proprioceptive conflict).

On the Role of Interoception in Body and Object Perception: A Multisensory-Integration Account

Various "embodied perception" phenomena suggest that what people sense of their body shapes what they perceive of the environment and that what they perceive of the environment shapes what they perceive of their bodies. For example, an observer's own hand can be felt where a fake hand is seen, events produced by own body movements seem to occur earlier than they did, and feeling a heavy weight at an observer's back may prompt hills to look steeper. Here we argue that such and various other phenomena are instances of multisensory integration of interoceptive signals from the body and exteroceptive signals from the environment. This overarching view provides a mechanistic description of what embodiment in perception means and how it works. It suggests new research questions while questioning a special role of the body itself and various phenomenon-specific explanations in terms of ownership, agency, or action-related scaling of visual information.

The distorted hand: systematic but 'independent' distortions in both explicit and implicit hand representations in young female adults

It has long been assumed that an accurate representation of the size and shape of one's body is necessary to successfully interact with the environment. Previous research has shown accurate representations when healthy participants make overt judgments (i.e. explicit) about the size of their bodies. However, when body size is judged implicitly, studies have shown systematic distortions. One suggestion for these differences, is that explicit and implicit representations are informed by different sensory modalities. Explicit representations rely on vision whereas implicit representations are informed by haptics. We designed an experiment to investigate if explicit representations that are informed by haptics are more like implicit representation featuring systematic distortions. We asked female participants to estimate the size of their fingers and hands in three different tasks: an explicit-haptic, an implicit, and an explicit-vision task. The results showed that all three representations were distorted and furthermore, the distortions for each representation were different from one another. These results suggest that inaccurate finger and hand length are a stereotypical feature of body representation that is present in both visual and haptic domains. We discuss the results in relation to theories of body representation.

Top-down effect of body representation on pain perception

Many studies on body representation intend to change the perceived size, material, and structure of the body. However, whether the perception of a stimulus can be modified by manipulating body representation remains largely unexplored. Thus, the current study investigated the relationship between transparency of body representation and pain perception. Using augmented reality technology, we made the participants' limbs transparent and analyzed changes in body representation. Using a questionnaire, we confirmed that the participants perceived their limb as transparent. Simultaneously, their sense of ownership of the limb decreased, because they felt that it no longer belonged to their body. The participants were given an electrical stimulus to assess their subjective perception of pain intensity. An increase in limb opacity decreased the perception of pain, which, in turn, increased the feeling of transparency. These results suggested that the feeling of transparency in their limb favored the decrease in perceived pain. This effect was modified by body ownership, where high levels reinforced the analgesic effect. However, body ownership displayed a positive relationship with perceived pain. The study suggests that body transparency may constitute a strategy for decreasing refractory pain given that body ownership is retained at a high level.

The Significance of Touch in Pediatric Physiotherapy

Change in theoretical framework over the last decades and recent research in pediatric physiotherapy, has created a debate surrounding therapeutic touch. What is the role of or is there a need for handling and hands-on facilitated guidance (facilitation)? Does it limit and/or interfere with children's learning and development? It is frequently argued that therapeutic touch represents a passive and/or static approach that restricts disabled children's participation during interaction and activity in clinical encounters leading to decreased home, school and community participation. Touch may even appear as coercive and controlling. In this context, therapeutic touch is largely associated with physical hands-on activities. However, therapeutic touch can also be understood as an intersubjective phenomenon that arises from a deep connection between movement, perception, and action. We believe the significance of therapeutic touch and its impact on physiotherapy for children has not been considered from this broader, holistic perspective. In this theoretical paper, we will apply enactive concepts of embodiment, sensory-motor agency, coordination, and emergence to explore the concept and importance of touch in physiotherapists‘ clinical face-to face encounters with children. We will frame the discussion within the context of the typical sensorimotor development of children from the fetal stage to birth on and into adulthood. Moreover, we will rely on biological, physiological, and phenomenological insights to provide an extended understanding of the importance of touch and the significance of touch in clinical practice.

Are tools truly incorporated as an extension of the body representation?: Assessing the evidence for tool embodiment

The predominant view on human tool-use suggests that an action-oriented body representation, the body schema, is altered to fit the tool being wielded, a phenomenon termed tool embodiment. While observations of perceptual change after tool-use purport to support this hypothesis, several issues undermine their validity in this context, discussed at length in this critical review. The primary measures used as indicators of tool embodiment each face unique challenges to their construct validity. Further, the perceptual changes taken as indicating extension of the body representation only appear to account for a fraction of the tool's size in any given experiment, and do not demonstrate the covariance with tool length that the embodiment hypothesis would predict. The expression of tool embodiment also appears limited to a narrow range of tool-use tasks, as deviations from a simple reaching paradigm can mollify or eliminate embodiment effects altogether. The shortcomings identified here generate important avenues for future research. Until the source of the kinematic and perceptual effects that have substantiated tool embodiment is disambiguated, the hypothesis that the body representation changes to fit tools during tool-use should not be favored over other possibilities such as the formation of separable internal tool models, which seem to offer a more complete account of human tool-use behaviors. Indeed, studies of motor learning have observed analogous perceptual changes as aftereffects to adaptation despite the absence of handheld tool-use, offering a compelling alternative explanation, though more work is needed to confirm this possibility.

Experience-Dependent Modulation of Rubber Hand Illusion in Badminton Players

Badminton players have a plastic modification of their arm representation in the brain due to the prolonged use of their racket. However, it is not known whether their arm representation can be altered through short-term visuotactile integration. The neural representation of the body is easily altered when multiple sensory signals are integrated in the brain. One of the most popular experimental paradigms for investigating this phenomenon is the "rubber hand illusion." This study was designed to investigate the effect of prolonged use of a racket on the modulation of arm representation during the rubber hand illusion in badminton players. When badminton players hold the racket, their badminton experience in years is negatively correlated with the magnitude of the rubber hand illusion. This finding suggests that tool embodiment obtained by the prolonged use of the badminton racket is less likely to be disturbed when holding the racket.

Alpha Oscillations Are Involved in Localizing Touch on Handheld Tools

The sense of touch is not restricted to the body but can also extend to external objects. When we use a handheld tool to contact an object, we feel the touch on the tool and not in the hand holding the tool. The ability to perceive touch on a tool actually extends along its entire surface, allowing the user to accurately localize where it is touched similarly as they would on their body. Although the neural mechanisms underlying the ability to localize touch on the body have been largely investigated, those allowing to localize touch on a tool are still unknown. We aimed to fill this gap by recording the electroencephalography signal of participants while they localized tactile stimuli on a handheld rod. We focused on oscillatory activity in the alpha (7–14 Hz) and beta (15–30 Hz) ranges, as they have been previously linked to distinct spatial codes used to localize touch on the body. Beta activity reflects the mapping of touch in skin-based coordinates, whereas alpha activity reflects the mapping of touch in external space. We found that alpha activity was solely modulated by the location of tactile stimuli applied on a handheld rod. Source reconstruction suggested that this alpha power modulation was localized in a network of fronto-parietal regions previously implicated in higher-order tactile and spatial processing. These findings are the first to implicate alpha oscillations in tool-extended sensing and suggest an important role for processing touch in external space when localizing touch on a tool.

I am the metre: The representation of one's body size affects the perception of tactile distances on the body

Humans must ground the perception of one's body in a mental representation to move in space and interact with objects. This representation can be temporarily altered artificially. In the full-body illusion (FBI), participants see a virtual (or filmed) body receiving a tactile stimulation. When participants receive touches on their body similarly to the seen one (i.e., homologous location and synchronous timing), they embody the seen alien body. While the subjective embodiment of alien bodies of different sizes has been already manipulated with the FBI, it remains unexplored whether the body-metric perception is impacted too. We first developed a new setup for the FBI using 360° videos to favour the embodiment. The FBI was induced for bodies of three sizes adopting anatomical and non-anatomical viewpoints, and we measured the subjective embodiment. The results suggest that humans can embody normal size or bigger bodies seen from anatomical viewpoints, but not smaller ones. We then investigated if the FBI modulates the body-metric representation. We found that the resized bodies' vision affects the perception of one's body-metric representation, but this was independent of the embodiment, suggesting that the FBI alters the body representation at different levels with a specific impact.

From embodying tool to embodying alien limb: sensory-motor modulation of personal and extrapersonal space

Years ago, it was demonstrated (e.g., Rizzolatti et al. in Handbook of neuropsychology, Elsevier Science, Amsterdam, 2000) that the brain does not encode the space around us in a homogeneous way, but through neural circuits that map the space relative to the distance that objects of interest have from the body. In monkeys, relatively discrete neural systems, characterized by neurons with specific neurophysiological responses, seem to be dedicated either to represent the space that can be reached by the hand (near/peripersonal space) or to the distant space (far/extrapersonal space). It was also shown that the encoding of spaces has dynamic aspects because they can be remapped by the use of tools that trigger different actions (e.g., Iriki et al. 1998). In this latter case, the effect of the tool depends on the modulation of personal space, that is the space of our body. In this paper, I will review and discuss selected research, which demonstrated that also in humans: 1 spaces are encoded in a dynamic way; 2 encoding can be modulated by the use of tool that the system comes to consider as parts of the own body; 3 body representations are not fixed, but they are fragile and subject to change to the point that we can incorporate not only the tools necessary for action, but even limbs belonging to other people. What embodiment of tools and of alien limb tell us about body representations is then briefly discussed.

Familiarity with a Tool Influences Peripersonal Space and Primary Motor Cortex Excitability of Muscles Involved in Haptic Contact

Long-term experience with a tool stably enlarges peripersonal space (PPS). Also, gained experience with a tool modulates internal models of action. The aim of this work was to understand whether the familiarity with a tool influences both PPS and motor representation. Toward this goal, we tested in 13 expert fencers through a multisensory integration paradigm the embodiment in their PPS of a personal (pE) or a common (cE) épée. Then, we evaluated the primary motor cortex excitability of proximal (ECR) and distal (APB) muscles during a motor imagery (MI) task of an athletic gesture when athletes handled these tools. Results showed that pE enlarges subjects' PPS, while cE does not. Moreover, during MI, handling tools increased cortical excitability of ECR muscle. Notably, APB's cortical excitability during MI only increased with pE as a function of its embodiment in PPS. These findings indicate that the familiarity with a tool specifically enlarges PPS and modulates the cortical motor representation of those muscles involved in the haptic contact with it.

Back in control of intentional action: Improvement of ideomotor apraxia by mirror box treatment

OBJECTIVES

A novel method of rehabilitation for ideomotor apraxia (IMA), using a modified version of the mirror box (MB), is proposed. The rationale is based on the theory that disrupted body representation occurs in IMA and that MB training may improve body representation. In the present MB training, patients observed and reproduced movements made by the experimenter in a mirror. The visual perspective gave the illusory sensation of seeing one's own affected hand in the mirror.

METHODS

Thirteen patients were included in the study; apraxia was measured four times: i) at baseline; ii) after a week of unspecific poststroke rehabilitation (rest); iii) after a week of imitation training for apraxia, used as a control; and iv) after a week of MB training. Imitation and mirror box training were presented in counterbalanced order between participants. The effect of the mirror box on a measure of body representation was also assessed.

RESULTS

The results show that MB training improved apraxia when compared to the outcomes in both the imitation and rest conditions. The improvement correlates with the impact of the mirror box on the body representation (i.e., the degree of embodiment).

CONCLUSIONS

MB training shows promising effects in promoting recovery from apraxia. The hypothesis is that the mirror box triggers a quickly generated sense of embodiment of the reflected moving arm into the observer's body representation. This embodiment of the visuomotor features of the observed movements would positively affect motor programming, promoting motor improvement. Crucially, this effect seems to extend to actions performed outside the mirror box setup, enhancing patients' performance on an apraxia test.

Same action in different spatial locations induces selective modulation of body metric representation

Recent studies have hypothesized that the stereotypical representation of the body may reflect some functional aspects of routine actions that are performed in specific peripersonal domains. For example, the lower and upper limbs tend to 'act' in different peripersonal spaces and perform different functions. The present study aims to directly investigate the relationship between body representation and the spatial context where actions are performed. By means of a modified version of the body image task, we investigated body representation before and after a sorting task training in two groups of participants who were asked to carry out the same task/actions in two different spaces: on a table or on the floor, while sitting on a chair. Findings showed that a significant recalibration of the perceived upper arms' length occurred when participants were asked to perform a motor task on the floor. These results seem to suggest that the modulation of the body representation reflects an increase action capabilities driven by the contribution of motor training, and importantly, the location in which the action occurs. Furthermore, the modulation was not limited to the body part actively involved in the action (the arms), it extended to other upper body parts (the torso) to maintain, we propose, a functionally coherent representation of the upper body.

Body size perception in stroke patients with paresis

Recent studies have suggested that people’s intent and ability to act also can influence their perception of their bodies’ peripersonal space. Vice versa one could assume that the inability to reach toward and grasp an object might have an impact on the subject’s perception of reaching distance. Here we tested this prediction by investigating body size and action capability perception of neurological patients suffering from arm paresis after stroke, comparing 32 right-brain-damaged patients (13 with left-sided arm paresis without additional spatial neglect, 10 with left-sided arm paresis and additional spatial neglect, 9 patients had neither arm paresis nor neglect) and 27 healthy controls. Nineteen of the group of right hemisphere stroke patients could be re-examined about five months after initial injury. Arm length was estimated in three different methodological approaches: explicit visual, explicit tactile/proprioceptive, and implicit reaching. Results fulfilled the working hypothesis. Patients with an arm paresis indeed perceived their bodies differently. We found a transient overestimation of the length of the contralesional, paretic arm after stroke. Body size and action capability perception for the extremities thus indeed seem to be tightly linked in humans.

Embodied Cooperation to Promote Forgiving Interactions With Autonomous Machines

During every waking moment, we must engage with our environments, the people around us, the tools we use, and even our own bodies to perform actions and achieve our intentions. There is a spectrum of control that we have over our surroundings that spans the extremes from full to negligible. When the outcomes of our actions do not align with our goals, we have a tremendous capacity to displace blame and frustration on external factors while forgiving ourselves. This is especially true when we cooperate with machines; they are rarely afforded the level of forgiveness we provide our bodies and often bear much of our blame. Yet, our brain readily engages with autonomous processes in controlling our bodies to coordinate complex patterns of muscle contractions, make postural adjustments, adapt to external perturbations, among many others. This acceptance of biological autonomy may provide avenues to promote more forgiving human-machine partnerships. In this perspectives paper, we argue that striving for machine embodiment is a pathway to achieving effective and forgiving human-machine relationships. We discuss the mechanisms that help us identify ourselves and our bodies as separate from our environments and we describe their roles in achieving embodied cooperation. Using a representative selection of examples in neurally interfaced prosthetic limbs and intelligent mechatronics, we describe techniques to engage these same mechanisms when designing autonomous systems and their potential bidirectional interfaces.

The Interrelation Between Peripersonal Action Space and Interpersonal Social Space: Psychophysiological Evidence and Clinical Implications

The peripersonal space is an adaptive and flexible interface between the body and the environment that fulfills a dual-motor function: preparing the body for voluntary object-oriented actions to interact with incentive stimuli and preparing the body for defensive responses when facing potentially harmful stimuli. In this position article, we provide arguments for the sensorimotor rooting of the peripersonal space representation and highlight the variables that contribute to its flexible and adaptive characteristics. We also demonstrate that peripersonal space represents a mediation zone between the body and the environment contributing to not only the control of goal-directed actions but also the organization of social life. The whole of the data presented and discussed led us to the proposal of a new theoretical framework linking the peripersonal action space and the interpersonal social space and we highlight how this theoretical framework can account for social behaviors in populations with socio-emotional deficits.

The toolish hand illusion: embodiment of a tool based on similarity with the hand

A tool can function as a body part yet not feel like one: Putting down a fork after dinner does not feel like losing a hand. However, studies show fake body-parts are embodied and experienced as parts of oneself. Typically, embodiment illusions have only been reported when the fake body-part visually resembles the real one. Here we reveal that participants can experience an illusion that a mechanical grabber, which looks scarcely like a hand, is part of their body. We found changes in three signatures of embodiment: the real hand’s perceived location, the feeling that the grabber belonged to the body, and autonomic responses to visible threats to the grabber. These findings show that artificial objects can become embodied even though they bear little visual resemblance to the hand.

How ageing shapes body and space representations: A comparison study between healthy young and older adults

To efficiently interact with the external world, the brain needs to represent the size of the involved body parts - body representations (BR) - and the space around the body in which the interactions with the environment take place - peripersonal space representation (PPS). BR and PPS are both highly flexible, being updated by the continuous flow of sensorimotor signals between the brain and the body, as observed for example after tool-use or immobilization. The progressive decline of sensorimotor abilities typically described in ageing could thus influence BR and PPS representations in the older adults. To explore this hypothesis, we compared BR and PPS in healthy young and older participants. By focusing on the upper limb, we adapted tasks previously used to evaluate BR and PPS plasticity, i.e., the body-landmarks localization task and audio-tactile interaction task, together with a new task targeting explicit BR (avatar adjustment task, AAT). Results show significantly higher distortions in the older rather than young participants in the perceived metric characteristic of the upper limbs. We found significant modifications in the implicit BR of the global shape (length and width) of both upper limbs, together with an underestimation in the arm length. Similar effects were also observed in the AAT task. Finally, both young and older adults showed equivalent multisensory facilitation in the space close to the hand, suggesting an intact PPS representation. Together, these findings demonstrated significant alterations of implicit and explicit BR in the older participants, probably associated with a less efficient contribution of bodily information typically subjected to age-related decline, whereas the comparable PPS representation in both groups could be supported by preserved multisensory abilities in older participants. These results provide novel empirical insight on how multiple representations of the body in space, subserving actions and perception, are shaped by the normal course of life.

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.

Body illusion and affordances: the influence of body representation on a walking imagery task in virtual reality

It is well known that our body works as a fundamental reference when we perform visuo-perceptual judgements in spatial surroundings, and that body illusions can modify our perception of size and distance of objects in space. To date, however, few studies have evaluated whether or not a body illusion could have a significant impact on the way individuals perceive to move within the environment. Here, we used a full-body illusion paradigm to verify the hypothesis that an altered representation of the legs of the individuals influences their time-to-walk estimation while imaging to reach objects in a virtual environment. To do so, we asked a group of young healthy volunteers to perform a task in which they were required to imagine walking towards a previously seen target location in a virtual environment, soon after receiving the body illusion; we required participants to use a response button to time their imagined walk from start to end. We found that participants imagined walking faster following the illusion elicited by the vision of longer legs presented from an anatomical perspective, as compared to when experiencing standard legs in the same position.This difference in imagined walking distance decreased when the object to reach was displayed farther, suggesting a fading effect. Furthermore, taking into consideration the baseline error in walking time estimation in VR, we noticed a specific influence of the long anatomical legs in reducing the perceived time needed to reach an object and a general increase in the percentage of error when the same legs are presented in a non-anatomical orientation. These findings provide evidence that body illusions could influence the way individuals perceive their locomotion in the spatial surrounding.

Effect of tool-use observation on metric body representation and peripersonal space

In everyday life, we constantly act and interact with objects and with others' people through our body. To properly perform actions, the representations of the dimension of body-parts (metric body representation, BR) and of the space surrounding the body (peripersonal space, PPS) need to be constantly updated. Previous evidence has shown that BR and PPS representation are highly flexible, being modulated by sensorimotor experiences, such as the active use of tools to reach objects in the far space. In this study, we investigate whether the observation of another person using a tool to interact with objects located in the far space is sufficient to influence the plasticity of BR and PPS representation in a similar way to active tool-use. With this aim, two groups of young healthy participants were asked to perform 20 min trainings based on the active use of a tool to retrieve far cubes (active tool-use) and on the first-person observation of an experimenter doing the same tool-use training (observational tool-use). Behavioural tasks adapted from literature were used to evaluate the effects of the active and observational tool-use on BR (body-landmarks localization task-group 1), and PPS (audio-tactile interaction task - group 2). Results show that after active tool-use, participants perceived the length of their arm as longer than at baseline, while no significant differences appear after observation. Similarly, significant modifications in PPS representation, with comparable multisensory facilitation on tactile responses due to near and far sounds, were seen only after active tool-use, while this did not occur after observation. Together these results suggest that a mere observational training could not be sufficient to significantly modulate BR or PPS. The dissociation found in the active and observational tool-use points out differences between action execution and action observation, by suggesting a fundamental role of the motor planning, the motor intention, and the related sensorimotor feedback in driving BR and PPS plasticity.

Defective Embodiment of Alien Hand Uncovers Altered Sensorimotor Integration in Schizophrenia

The observation that people with schizophrenia misattribute the source of their own actions has led to the hypothesis that they suffer from altered sensorimotor processes underlying sense of agency. Furthermore, rubber hand studies suggest an abnormal experience of embodiment in schizophrenia. However, this latter finding is based on a procedure that elicits ownership sensations for a fake hand by visuo-tactile stimulation, leaving the agency subcomponent of embodiment relatively untouched. By using a visuo-motor version of the embodiment illusion able to actively elicit also sense of agency for an alien hand, we tested whether the putative sensorimotor deficits are also involved in altering embodiment sensations in schizophrenia. Subjective (questionnaire) and perceptual (forearm bisection performance) indexes of the embodiment illusion were collected. Differently from controls, both the explicit agency component and the implicit body metrics update were not modulated by the extent of visuo-motor congruency in participants with schizophrenia. We conclude that motor prediction and/or temporal binding window impairments may alter the feeling of embodiment and body representation in schizophrenia.

Tool-Use Training Induces Changes of the Body Schema in the Limb Without Using Tool

Previous studies have shown that tool use affects the plasticity of the body schema. In other words, people will perceive the tool as a part of their body, and thus feel like they have “longer limbs” after using tools. However, it is unclear whether tool embodiment could spread to a limb that is not using the tool, and whether other limbs could utilize the proprioception of a limb. In Experiment 1, blindfolded participants were asked to search with a cane (Condition 1) or to walk with a cane (Condition 2). The results in Condition 1 illustrated that the tactile distance perception on the forearm was lengthened after tool use, while other body parts did not significantly change. In Condition 2, the tactile distance perception on the hand and forearm extended significantly after using tools. Additionally, tool-use training even induced an increased perception of the calf that was not using the tool. Possible interference from the difference between walking and standing was excluded in Experiment 2. These results demonstrate that the proprioception information of one limb could be exploited by another limb to extend the body schema even though that limb was not using a tool. It was also observed that the effect of direction was task-dependent in the tactile perception task.

The Embodiment of Objects: Review, Analysis, and Future Directions

Here we offer a thorough review of the empirical literature on the conditions under which an object, such as a tool or a prosthetic (whether real or virtual), can be experienced as being in some sense a part or extension of one's body. We discuss this literature both from the standpoint of the apparent malleability of our body representations, and also from within the framework of radical embodied cognition, which understands the phenomenon to result not from an alteration to a representation, but rather from the achievement of a certain kind of sensory/motor coupling. We highlight both the tensions between these frameworks, and also areas where they can productively complement one another for future research.

How Tool-Use Shapes Body Metric Representation: Evidence From Motor Training With and Without Robotic Assistance

Previous evidence has shown that tool-use can reshape one's own body schema, extending peripersonal space and modulating the representation of related body parts. Here, we investigated the role of tool action in shaping the body metric representation, by contrasting two different views. According to a first view, the shaping would rely on the mere execution of tool action, while the second view suggests that the shaping induced by tool action on body representation would primarily depend on the representation of the action goals to be accomplished. To this aim, we contrasted a condition in which participants voluntarily accomplish the movement by representing the program and goal of a tool action (i.e., active tool-use training) with a condition in which the tool-use training was produced without any prior goal representation (i.e., passive tool-use training by means of robotic assistance). If the body metric representation primarily depends on the coexistence between goal representation and bodily movements, we would expect an increase of the perceived forearm length in the post- with respect to the pre-training phase after the active training phase only. Healthy participants were asked to estimate the midpoint of their right forearm before and after 20 min of tool-use training. In the active condition, subjects performed "enfold-and-push" movements using a rake to prolong their arm. In the passive condition, subjects were asked to be completely relaxed while the movements were performed with robotic assistance. Results showed a significant increase in the perceived arm length in the post- with respect to the pre-training phase only in the active task. Interestingly, only in the post-training phase, a significant difference was found between active and passive conditions, with a higher perceived arm length in the former than in the latter. From a theoretical perspective, these findings suggest that tool-use may shape body metric representation only when action programs are motorically represented and not merely produced. From a clinical perspective, these results support the use of robots for the rehabilitation of brain-damaged hemiplegic patients, provided that robot assistance during the exercises is present only "as-needed" and that patients' motor representation is actively involved.

Tool Use Modulates Somatosensory Cortical Processing in Humans

Tool use leads to plastic changes in sensorimotor body representations underlying tactile perception. The neural correlates of this tool-induced plasticity in humans have not been adequately characterized. This study used ERPs to investigate the stage of sensory processing modulated by tool use. Somatosensory evoked potentials, elicited by median nerve stimulation, were recorded before and after two forms of object interaction: tool use and hand use. Compared with baseline, tool use-but not use of the hand alone-modulated the amplitude of the P100. The P100 is a mid-latency component that indexes the construction of multisensory models of the body and has generators in secondary somatosensory and posterior parietal cortices. These results mark one of the first demonstrations of the neural correlates of tool-induced plasticity in humans and suggest that tool use modulates relatively late stages of somatosensory processing outside primary somatosensory cortex. This finding is consistent with what has been observed in tool-trained monkeys and suggests that the mechanisms underlying tool-induced plasticity have been preserved across primate evolution.

Impact of action planning on visual and body perception in a virtual grasping task

The present study examined mutual influences of visual and body-related signals during planning of an object-oriented action. Participants were to enclose a visual target object using two cursors controlled by the movements of their fingers. During movement preparation, they were asked to judge either the size of the object or a certain finger distance. Both types of judgments were systematically affected by the transformation of finger movements into the movements of visual cursors. We suggest that these biases are perceptual consequences of sensory integration of visual and body-related signals relating to the same external object.

Body representations and cognitive ontology: Drawing the boundaries of the body image

The distinction between body image and body schema has been incredibly influential in cognitive neuroscience. Recently, researchers have begun to speculate about the relationship between these representations (Gadsby, 2017, 2018; Pitron & de Vignemont, 2017; Pitron et al., 2018). Within this emerging literature, Pitron et al. (2018) proposed that the long-term body image and long-term body schema co-construct one another, through a process of reciprocal interaction. In proposing this model, they make two assumptions: that the long-term body image incorporates the spatial characteristics of tools, and that it is distorted in the case of Alice in wonderland syndrome. Here, I challenge these assumptions, with a closer examination of what the term "long-term body image" refers to. In doing so, I draw out some important taxonomic principles for research into body representation.

Enlarged representation of peripersonal space in pregnancy

Our ability to maintain a coherent bodily self despite continuous changes within and outside our body relies on the highly flexible multisensory representation of the body, and of the space surrounding it: the peripersonal space (PPS). The aim of our study was to investigate whether during pregnancy - when extremely rapid changes in body size and shape occur - a likewise rapid plastic reorganization of the neural representation of the PPS occurs. We used an audio-tactile integration task to measure the PPS boundary at different stages of pregnancy. We found that in the second trimester of pregnancy and postpartum women did not show differences in their PPS size as compared to the control group (non-pregnant women). However, in the third trimester the PPS was larger than the controls’ PPS and the shift between representation of near and far space was more gradual. We therefore conclude that during pregnancy the brain adapts to the sudden bodily changes, by expanding the representation of the space around the body. This may represent a mechanism to protect the vulnerable abdomen from injury from surrounding objects.

Somatosensory-guided tool use modifies arm representation for action

Tool-use changes both peripersonal space and body representations, with several effects being nowadays termed tool embodiment. Since somatosensation was typically accompanied by vision in most previous tool use studies, whether somatosensation alone is sufficient for tool embodiment remains unknown. Here we address this question via a task assessing arm length representation at an implicit level. Namely, we compared movement’s kinematics in blindfolded healthy participants when grasping an object before and after tool-use. Results showed longer latencies and smaller peaks in the arm transport component after tool-use, consistent with an increased length of arm representation. No changes were found in the hand grip component and correlations revealed similar kinematic signatures in naturally long-armed participants. Kinematics changes did not interact with target object position, further corroborating the finding that somatosensory-guided tool use may increase the represented size of the participants’ arm. Control experiments ruled out alternative interpretations based upon altered hand position sense. In addition, our findings indicate that tool-use effects are specific for the implicit level of arm representation, as no effect was observed on the explicit estimate of the forearm length. These findings demonstrate for the first time that somatosensation is sufficient for incorporating a tool that has never been seen, nor used before.

Tool Embodiment: The Tool's Output Must Match the User's Input

The embodiment of tools and rubber hands is believed to involve the modification of two separate body representations: the body schema and the body image, respectively. It is thought that tools extend the capabilities of the body’s action schema, whereas prosthetics like rubber hands are incorporated into the body image itself. Contrary to this dichotomy, recent research demonstrated that chopsticks can be embodied perceptually during a modified version of the rubber hand illusion (RHI) in which tools are held by the rubber hand and by the participant. In the present research, two experiments examined tool morpho-functional (tool output affordance, e.g., precision grasping) and sensorimotor (tool input, e.g., precision grip) match as a mechanism for this tool-use dependent change to the body image. Proprioceptive drift in the RHI occurred when the tool’s output and the user’s input matched, but not when this match was absent. This suggests that this factor may be necessary for tools to interact with the body image in the RHI.

When social and action spaces diverge: A study in children with typical development and autism

The space around the body has been defined as action space ( peripersonal space) and a social space ( interpersonal space). Within the current debate about the characteristics of these spaces, here we investigated the functional properties and plasticity of action and social space in developmental age. To these aims, children with typical development and autism spectrum disorders were submitted to Reaching- and Comfort-distance tasks, to assess peripersonal and interpersonal space, respectively. Participants approached a person (confederate) or an object and stopped when they thought they could reach the stimulus (Reaching-distance task), or they felt comfortable with stimulus’ proximity (Comfort-distance task). Both tasks were performed before and after a cooperative tool-use training, in which participant and confederate actively cooperated to reach tokens by using either a long (Experiment 1) or a short (Experiment 2) tool. Results showed that in both groups, peripersonal space extended following long-tool-use but not short-tool-use training. Conversely, in typical development, but not in autism spectrum disorders children, interpersonal space toward confederate reduced following the cooperative tool-use training. These findings reveal that action and social spaces are functionally dissociable both in typical and atypical development, and that action but not social space regulation is intact in children with autism.

'See Me, Feel Me': Prismatic Adaptation of an Alien Limb Ameliorates Spatial Neglect in a Patient Affected by Pathological Embodiment

Pathological embodiment (E+) is a specific contralesional delusion of body ownership, observed following brain damage, in which patients embody someone else's arm and its movements within their own body schema whenever the contralesional 'alien' arm is presented in a body-congruent position (i.e., 1st person perspective and aligned with the patient's shoulder). This disorder is often associated with spatial neglect, a neurological syndrome in which patients are unaware of stimuli presented in the contralesional (often the left) space. Capitalizing on previous evidence demonstrating that prismatic adaptation of the ipsilesional arm to right-deviating prisms is effective in ameliorating neglect symptoms, here we investigated whether such amelioration also occurs in E+ patients with neglect when prismatic training is performed by the 'alien' embodied arm. Four left neglect patients (one with and three without pathological embodiment) underwent visuomotor prismatic training performed by an 'alien' arm. Specifically, while patients were wearing prismatic goggles shifting the visual field rightward, a co-experimenter's left arm presented in a body-congruent perspective was repeatedly moved toward visual targets by another examiner. In a control condition, the co-experimenter's arm was moved toward the targets from a body-incongruent position (i.e., 3rd person perspective). Neglect symptoms were assessed before and after training through paper-and-pencil tasks. In the E+ patient, neglect improved significantly more in 1st than in 3rd person perspective training, suggesting that prismatic adaptation of the 'alien' embodied arm is effective in modulating spatial representation. Conversely, for control E- patients (not embodying the 'alien' arm), we observed more limited improvements following training. These findings indicate that the 'alien' embodied arm is so deeply embedded in the patient body and motor schema that adaptation to prismatic lenses can affect multiple processing stages, from low level sensory-motor correspondences, to higher level body, motor and spatial maps, similarly as it occurs in normal subjects and neglect patients without pathological embodiment.