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.

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.

Soft Embodiment for Engineering Artificial Limbs.

We highlight two alternative, yet complementary, solutions for harnessing available neural resources for improving integration of artificial limbs (ALs) through embodiment. 'Hard' embodiment exploits neural and cognitive body mechanisms by closely mimicking their original biological functions. 'Soft' embodiment exploits these same mechanisms by recycling them to support a different function altogether.

How do the body schema and the body image interact?

Despite their differences, body schema and the body image representations are not only consistent in everyday life, but also sometimes consistent in pathological disorders, such as in Alice in Wonderland syndrome and anorexia nervosa. The challenge is to understand how they achieve such consistency. Recently, we suggested that these two representations were co-constructed (Pitron & Vignemont, 2017). In his reply, Gadsby (2018) invited us to clarify how this co-construction works and to what extent the body schema and the body image can reshape each other. Here we motivate conceptual grounds for a model on which these two forms of representation modify one another and explore theoretical options for the way(s) in which they might do so. In particular, we highlight the virtues of a serial model in which the body schema has some primacy over the body image, while also acknowledging the special role played by the body image.

Reachability and the sense of embodiment in amputees using prostheses.

Amputated patients are hardly satisfied with upper limb prostheses, and tend to favour the use of their contralateral arm to partially compensate their disability. This may seem surprising in light of recent evidences that external objects (rubber hand or tool) can easily be embodied, namely incorporated in the body representation. We investigated both implicit body representations (by evaluating the peripersonal space using a reachability judgement task) and the quality of bodily integration of the patient's prosthesis (assessed via questionnaires). As expected, the patients estimated that they could reach further while wearing their prosthesis, showing an embodiment of their prosthesis in their judgement. Yet, the real reaching space was found to be smaller with their prosthesis than with their healthy limb, showing a large error between reachability judgement and actual capacity. An overestimation was also found on the healthy side (comparatively to healthy subjects) suggesting a bilateral modification of body representation in amputated patients. Finally, a correlation was found between the quality of integration of the prosthesis and the way the body representation changed. This study therefore illustrates the multifaceted nature of the phenomenon of prosthesis integration, which involves its incorporation as a tool, but also various specific subjective aspects.

Beyond differences between the body schema and the body image: insights from body hallucinations.

The distinction between the body schema and the body image has become the stock in trade of much recent work in cognitive neuroscience and philosophy. Yet little is known about the interactions between these two types of body representations. We need to account not only for their dissociations in rare cases, but also for their convergence most of the time. Indeed in our everyday life the body we perceive does not conflict with the body we act with. Are the body image and the body schema then somehow reshaping each other or are they relatively independent and do they only happen to be congruent? On the basis of the study of bodily hallucinations, we consider which model can best account for the body schema/body image interactions.

Shared body representations and the 'Whose' system.

Mirroring has been almost exclusively analysed in motor terms with no reference to the body that carries the action. According to the standard view, one activates motor representations upon seeing other people moving. However, one does not only see movements, one also sees another individual's body. The following questions then arise. To what extent does one recruit body representations in social context? And does it imply that body representations are shared between self and others? This latter question is all the more legitimate since recent evidence indicates the existence of shared cortical networks for bodily sensations, including pain (e.g., Singer et al., 2004) and touch (e.g., Keysers et al., 2004; Blakemore, Bristow, Bird, Frith, & Ward, 2005). But if body representations are shared, then it seems that their activation cannot suffice to discriminate between one's body and other people's bodies. Does one then need a 'Whose' system to recognise one's body as one's own, in the same way that Jeannerod argues that one needs a 'Who' system to recognise one's actions as one's own?

Embodiment, ownership and disownership.

There are two main pathways to investigate the sense of body ownership, (i) through the study of the conditions of embodiment for an object to be experienced as one's own and (ii) through the analysis of the deficits in patients who experience a body part as alien. Here, I propose that E is embodied if some properties of E are processed in the same way as the properties of one's body. However, one must distinguish among different types of embodiment, and only self-specific embodiment can lead to feelings of ownership. I address issues such as the functional role and the dynamics of embodiment, degrees and measures of ownership, and shared body representations between self and others. I then analyse the interaction between ownership and disownership. On the one hand, I show that there is no evidence that in the Rubber Hand Illusion, the rubber hand replaces the biological hand. On the other hand, I argue that the sense of disownership experienced by patients towards their body part cannot be reduced to the mere lack of ownership.

Cooling the thermal grill illusion through self-touch.

Acute peripheral pain is reduced by multisensory interactions at the spinal level [1]. Central pain is reduced by reorganization of cortical body representations [2, 3]. We show here that acute pain can also be reduced by multisensory integration through self-touch, which provides proprioceptive, thermal, and tactile input forming a coherent body representation [4, 5]. We combined self-touch with the thermal grill illusion (TGI) [6]. In the traditional TGI, participants press their fingers on two warm objects surrounding one cool object. The warm surround unmasks pain pathways, which paradoxically causes the cool object to feel painfully hot. Here, we warmed the index and ring fingers of each hand while cooling the middle fingers. Immediately after, these three fingers of the right hand were touched against the same three fingers on the left hand. This self-touch caused a dramatic 64% reduction in perceived heat. We show that this paradoxical release from paradoxical heat cannot be explained by low-level touch-temperature interactions alone. To reduce pain, we often clutch a painful hand with the other hand. We show here that self-touch not only gates pain signals reaching the brain [7-9] but also, via multisensory integration, increases coherence of cognitive body representations to which pain afferents project [10].

The weight of representing the body: addressing the potentially indefinite number of body representations in healthy individuals.

There is little consensus about the characteristics and number of body representations in the brain. In the present paper, we examine the main problems that are encountered when trying to dissociate multiple body representations in healthy individuals with the use of bodily illusions. Traditionally, task-dependent bodily illusion effects have been taken as evidence for dissociable underlying body representations. Although this reasoning holds well when the dissociation is made between different types of tasks that are closely linked to different body representations, it becomes problematic when found within the same response task (i.e., within the same type of representation). Hence, this experimental approach to investigating body representations runs the risk of identifying as many different body representations as there are significantly different experimental outputs. Here, we discuss and illustrate a different approach to this pluralism by shifting the focus towards investigating task-dependency of illusion outputs in combination with the type of multisensory input. Finally, we present two examples of behavioural bodily illusion experiments and apply Bayesian model selection to illustrate how this different approach of dissociating and classifying multiple body representations can be applied.

Body schema and body image--pros and cons.

There seems to be no dimension of bodily awareness that cannot be disrupted. To account for such variety, there is a growing consensus that there are at least two distinct types of body representation that can be impaired, the body schema and the body image. However, the definition of these notions is often unclear. The notion of body image has attracted most controversy because of its lack of unifying positive definition. The notion of body schema, onto which there seems to be a more widespread agreement, also covers a variety of sensorimotor representations. Here, I provide a conceptual analysis of the body schema contrasting it with the body image(s) as well as assess whether (i) the body schema can be specifically impaired, while other types of body representation are preserved; and (ii) the body schema obeys principles that are different from those that apply to other types of body representation.

Losing one's hand: visual-proprioceptive conflict affects touch perception.

BACKGROUND: While the sense of bodily ownership has now been widely investigated through the rubber hand illusion (RHI), very little is known about the sense of disownership. It has been hypothesized that the RHI also affects the ownership feelings towards the participant's own hand, as if the rubber hand replaced the participant's actual hand. Somatosensory changes observed in the participants' hand while experiencing the RHI have been taken as evidence for disownership of their real hand. Here we propose a theoretical framework to disambiguate whether such somatosensory changes are to be ascribed to the disownership of the real hand or rather to the anomalous visuo-proprioceptive conflict experienced by the participant during the RHI. METHODOLOGY/PRINCIPAL FINDINGS: In experiment 1, reaction times (RTs) to tactile stimuli delivered to the participants' hand slowed down following the establishment of the RHI. In experiment 2, the misalignment of visual and proprioceptive inputs was obtained via prismatic displacement, a situation in which ownership of the seen hand was doubtless. This condition slowed down the participants' tactile RTs. Thus, similar effects on touch perception emerged following RHI and prismatic displacement. Both manipulations also induced a proprioceptive drift, toward the fake hand in the first experiment and toward the visual position of the participants' hand in the second experiment. CONCLUSIONS/SIGNIFICANCE: These findings reveal that somatosensory alterations in the experimental hand resulting from the RHI result from cross-modal mismatch between the seen and felt position of the hand. As such, they are not necessarily a signature of disownership.

Is social cognition embodied?

Theories of embodied cognition abound in the literature, but it is often unclear how to understand them. We offer several interpretations of embodiment, the most interesting being the thesis that mental representations in bodily formats (B-formats) have an important role in cognition. Potential B-formats include motoric, somatosensory, affective and interoceptive formats. The literature on mirroring and related phenomena provides support for a limited-scope version of embodied social cognition under the B-format interpretation. It is questionable, however, whether such a thesis can be extended. We show the limits of embodiment in social cognition.

Is this hand for real? Attenuation of the rubber hand illusion by transcranial magnetic stimulation over the inferior parietal lobule.

In the rubber hand illusion (RHI), participants incorporate a rubber hand into a mental representation of one's body. This deceptive feeling of ownership is accompanied by recalibration of the perceived position of the participant's real hand toward the rubber hand. Neuroimaging data suggest involvement of the posterior parietal lobule during induction of the RHI, when recalibration of the real hand toward the rubber hand takes place. Here, we used off-line low-frequency repetitive transcranial magnetic stimulation (rTMS) in a double-blind, sham-controlled within-subjects design to investigate the role of the inferior posterior parietal lobule (IPL) in establishing the RHI directly. Results showed that rTMS over the IPL attenuated the strength of the RHI for immediate perceptual body judgments only. In contrast, delayed perceptual responses were unaffected. Furthermore, ballistic action responses as well as subjective self-reports of feeling of ownership over the rubber hand remained unaffected by rTMS over the IPL. These findings are in line with previous research showing that the RHI can be broken down into dissociable bodily sensations. The illusion does not merely affect the embodiment of the rubber hand but also influences the experience and localization of one's own hand in an independent manner. Finally, the present findings concur with a multicomponent model of somatosensory body representations, wherein the IPL plays a pivotal role in subserving perceptual body judgments, but not actions or higher-order affective bodily judgments.

Segmenting the body into parts: evidence from biases in tactile perception.

How do we individuate body parts? Here, we investigated the effect of body segmentation between hand and arm in tactile and visual perception. In a first experiment, we showed that two tactile stimuli felt farther away when they were applied across the wrist than when they were applied within a single body part (palm or forearm), indicating a "category boundary effect". In the following experiments, we excluded two hypotheses, which attributed tactile segmentation to other, nontactile factors. In Experiment 2, we showed that the boundary effect does not arise from motor cues. The effect was reduced during a motor task involving flexion and extension movements of the wrist joint. Action brings body parts together into functional units, instead of pulling them apart. In Experiments 3 and 4, we showed that the effect does not arise from perceptual cues of visual discontinuities. We did not find any segmentation effect for the visual percept of the body in Experiment 3, nor for a neutral shape in Experiment 4. We suggest that the mental representation of the body is structured in categorical body parts delineated by joints, and that this categorical representation modulates tactile spatial perception.

Spatial organisation in passive tactile perception: is there a tactile field?

The perceptual field is a cardinal concept of sensory psychology. 'Field' refers to a representation in which perceptual contents have spatial properties and relations which derive from the spatial properties and relations of corresponding stimuli. It is a matter of debate whether a perceptual field exists in touch analogous to the visual field. To study this issue, we investigated whether tactile stimuli on the palm can be perceived as complex stimulus patterns, according to basic spatial principles. Subjects judged the intensity of a target stimulus to the palm, ignoring two brief preceding touches at nearby flanker locations. We found that the judgements of the target intensity were boosted by flankers when the target lay on the line joining the flankers in comparison to when the target lay away from this line. Therefore, we suggest that a tactile spatial organisation, i.e. a tactile field, exists; the field supports the relation of collinearity; it is automatically and implicitly activated by touch, and it groups spatially coherent perceptual contents.

Frames of reference in social cognition.

How is mindreading affected by social context? It is often implicitly assumed that there is one single way to understand others, whatever the situation or the identity of the person. In contrast, I emphasize the duality of functions of mindreading depending on the context (social interaction and social observation), as well as the duality of social frames of reference (egocentric and allocentric). I argue in favour of a functional distinction between knowledge-oriented mindreading and interaction-oriented mindreading. They both aim at understanding other people's behaviour. But they do so using different strategies. However, to say that mindreading has two functions does not suffice to show that there are two kinds of mindreading. One and the same ability could accomplish different functions. Unfortunately, there has been almost no experimental data on a possible dissociation between two kinds of mindreading abilities. Nonetheless, I discuss a few results that point towards a dual ability.

When do we empathize?

According to a motor theory of empathy, empathy results from the automatic activation of emotion triggered by the observation of someone else's emotion. It has been found that the subjective experience of emotions and the observation of someone else experiencing the same emotion activate overlapping brain areas. These shared representations of emotions (SRE) could be the key for the understanding of empathy. However, if the automatic activation of SRE suffices to induce empathy, we would be in a permanent emotional turmoil. In contrast, it seems intuitively that we do not empathize all the time and that far from being automatic, empathy should be explained by a complex set of cognitive and motivational factors. I will provide here a new account of the automaticity of empathy, starting from a very simple question: when do we empathize? We need to distinguish clearly the activation of SRE and empathy. I will provide a model that accounts both for the automaticity of the activation of SRE and for the selectiveness of empathy. As Prinz (2002) says about imitation, the problem is not so much to account for the ubiquitous occurrence of empathy, but rather for its notorious non-occurrence in many situations.

The empathic brain: how, when and why?

Recent imaging results suggest that individuals automatically share the emotions of others when exposed to their emotions. We question the assumption of the automaticity and propose a contextual approach, suggesting several modulatory factors that might influence empathic brain responses. Contextual appraisal could occur early in emotional cue evaluation, which then might or might not lead to an empathic brain response, or not until after an empathic brain response is automatically elicited. We propose two major roles for empathy; its epistemological role is to provide information about the future actions of other people, and important environmental properties. Its social role is to serve as the origin of the motivation for cooperative and prosocial behavior, as well as help for effective social communication.

Mental rotation in schizophrenia.

Motor imagery provides a direct insight into action representations. The aim of the present study was to investigate the level of impairment of action monitoring in schizophrenia by evaluating the performance of schizophrenic patients on mental rotation tasks. We raised the following questions: (1) Are schizophrenic patients impaired in motor imagery both at the explicit and at the implicit level? (2) Are body parts more difficult for them to mentally rotate than objects? (3) Is there any link between the performance and the hallucinating symptom profile? The schizophrenic patients (n = 13) displayed the same pattern of performance as the control subjects (n = 13). More particularly, schizophrenic patients' reaction time varied as a function of the angular disparity of the stimuli. On the other hand, they were significantly slower and less accurate. Interestingly, patients suffering from hallucinations made significantly more errors than non-hallucinatory patients. We discussed these latter results in terms of deficit of the forward model. We emphasized the necessity to distinguish different levels of action, more or less impaired in schizophrenia.