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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.

In the rubber hand illusion (RHI), the feeling that a fake hand belongs to oneself can be induced by the simultaneous, congruent touch of the fake visible hand and one's own hidden hand. This condition is also associated with a recalibration of the perceived location of the real hand. A cortical network, including premotor and temporo-parietal areas, has been proposed as the basis of the RHI. However, the causal contribution of these areas to the discrete illusory components remains unclear. We used transcranial direct current stimulation (tDCS) to assess the contribution of the right premotor cortex (rPMc) and the right temporo-parietal junction (rTPJ) to the RHI and explored the role of these areas in modulating the subjective experience of embodiment and the misperception of the hand position. We found that anodal tDCS of both rPMc and rTPJ increased the misjudgement of the real hand location towards the fake hand. Crucially, the difference in proprioceptive displacement evoked by the congruent and incongruent visuo-tactile stroking was minimised when tDCS was applied over the rPMc, while it was amplified when the rTPJ was targeted. The parietal effects of tDCS also extended to the self-report components of the RHI. These findings suggest that the tDCS of rTPJ modulates the RHI depending on the temporal congruency of the visuo-tactile stimulation, while the tDCS of rPMc induces a general recalibration of hand coordinates, regardless of the visuo-tactile congruency. The present results are discussed in the view of a multicomponent model of the RHI.

Perceived limb position is known to rely on sensory signals and motor commands. Another potential source of input is a standard representation of body posture, which may bias perceived limb position toward more stereotyped positions. Recent results show that tactile stimuli are processed more efficiently when delivered to a thumb in a relatively low position or an index finger in a relatively high position. This observation suggests that we may have a standard posture of the body that promotes a more efficient interaction with the environment. In this study, we mapped the standard posture of the entire hand by characterizing the spatial associations of all 5 digits. Moreover, we show that the effect is not an artifact of intermanual integration. Results showed that the thumb is associated with low positions, while the other fingers are associated with upper locations. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

The knowledge of the size of our own body parts is essential for accurately moving in space and efficiently interact with objects. A distorted perceptual representation of the body size often represents a core diagnostic criterion for some psychopathological conditions. The metric representation of the body was shown to depend on somatosensory afferences: local deafferentation indeed causes a perceptual distortion of the size of the anesthetized body part. A specular effect can be induced by altering the cortical map of body parts in the primary somatosensory cortex. Indeed, the present study demonstrates, in healthy adult participants, that repetitive Transcranial Magnetic Stimulation to the somatosensory cortical map of the hand in both hemispheres causes a perceptual distortion (i.e., an overestimation) of the size of the participants' own hand (Experiments 1-3), which does not involve other body parts (i.e., the foot, Experiment 2). Instead, the stimulation of the inferior parietal lobule of both hemispheres does not affect the perception of the own body size (Experiment 4). These results highlight the role of the primary somatosensory cortex in the building up and updating of the metric of body parts: somatosensory cortical activity not only shapes our somatosensation, it also affects how we perceive the dimension of our body.

Morphology and functional aspects of the tool have been proposed to be critical factors modulating tool use-induced plasticity. However, how these aspects contribute to changing body representation has been underinvestigated. In the arm bisection task, participants have to estimate the length of their own arm by indicating its midpoint, a paradigm used to investigate the representation of the metric properties of the body. We employed this paradigm to investigate the impact of different actions onto tool embodiment. Our findings suggest that a training requiring actions mostly with proximal (shoulder) or distal (wrist) parts induces a different shift in the perceived arm midpoint. This effect is independent of, but enhanced by, the use of the tool during the training and in part influenced by specific demands of the task. These results suggest that specific motor patterns required by the training can induce different changes of body representation, calling for rethinking the concept of tool embodiment, which would be characterized not simply by the morphology of the tools, but also by the actions required for their specific use.

Previous evidence has shown that active tool-use can reshape one's own body schema, extend peripersonal space and modulate the representation of related body parts. Here we investigate the effect of tool-use training on length representation of the contralesional forearm in brain-damaged hemiplegic patients who manifested a pathological embodiment of other people body parts. Four patients and 20 aged-matched healthy-controls were asked to estimate the mid-point of their contralesional forearm before and after 15. min of tool-use training (i.e. retrieving targets with a garbage plier). In the case of patients, training was always performed by the examiner's (alien) arm acting in two different positions, aligned (where the pathological embodiment occurs; E+ condition) or misaligned (where the pathological embodiment does not occur; E- condition) relative to the patients' shoulder. Healthy controls performed tool-use training either with their own arm (action condition) or observing the examiner's arm performing the task (observation condition), handling (observation with-tool condition) or not (observation without-tool condition) a similar tool. Crucially, in the E+ condition, when patients were convinced to perform the tool-use training with their own paralyzed arm, a significant overestimation effect was found (as in the Action condition with normal subjects): patients mislocated their forearm midpoint more proximally to the hand in the post- than in the pre-training phase. Conversely, in the E- condition, they did not show any overestimation effect, similarly to healthy subjects in the observation condition (neither in the with-tool nor in the without-tool condition significant overestimation effects were found). These findings show the existence of a tight link between spatial, motor and bodily representations and provide strong evidence that a pathological sense of body ownership can extend to intentional motor processes and modulate the sensory map of action-related body parts.

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.

How to reduce pain is a fundamental clinical and experimental question. Acute pain is a complex experience which seems to emerge from the co-activation of two main processes, namely the nociceptive/discriminative analysis and the affective/cognitive evaluation of the painful stimulus. Recently it has been found that pain threshold increases following the visual magnification of the body part targeted by the painful stimulation. This finding is compatible with the well-known notion that body representation and perceptual experience relay on complex, multisensory factors. However, the level of cognitive processing and the physiological mechanisms underlying this analgesic effect are still to be investigated. In the present work we found that following the visual magnification of a body part, the Skin Conductance Responses (SCR), to an approaching painful stimulus increases before contact and decreases following the real stimulation, compared to the non-distorted view of the hand. By contrast, an unspecific SCR increase is found when the hand is visually shrunk. Moreover a reduction of subjective pain experience was found specifically for the magnified hand in explicit pain ratings. These findings suggest that the visual increase of body size enhances the cognitive, anticipatory component of pain processing; such an anticipatory reaction reduces the response to the following contact with the noxious stimulus. The present results support the idea that cognitive aspects of pain experience relay on the multisensory representation of the body, and that could be usefully exploited for inducing a significant reduction of subjective pain experience.

Disruption of motor control in the alien hand syndrome might result from a dissociation between intentions and sensory information. We hypothesized that voluntary motor control in this condition could improve by restoring the congruency between motor intentions and visual feedback. The present study shows that, in one patient with right alien hand syndrome, the use of a mirror box paradigm improved motor speed. We speculate that the visual feedback provided by the mirror increases the sense of congruence between intention and sensory feedback, leading to motor improvement.

Looking at one's own body has been shown to induce analgesia. In the present work we investigated whether illusory self-identification with an avatar, as induced experimentally through visuo-tactile stimulation, modulates the response to painful stimuli. In 30 healthy volunteers, a robotic device was used to stroke the participants' back, while they viewed either the body of an avatar, a non-body object (control object), or a body avatar with scrambled body parts (control body). All were visually stimulated in either congruent or incongruent fashion with the participant's body. We collected physiological responses (skin conductance response: SCR) to painful stimuli delivered to the participant's hand and responses to a questionnaire inquiring about self-identification with the avatar. We expected reduced physiological responses to pain during the observation of a body avatar only during synchronous visuo-tactile stroking and no reduction for the control object and the control body. Results showed a reduced SCR to painful stimuli when participants observed the normal body avatar being stroked synchronously that was also associated with largest self-identification ratings recordable already during the pain anticipation. Moreover, a negative correlation between self-identification and SCR was observed, suggesting that a greater degree of self-identification with the avatar was associated with larger decreases in SCR. These results suggest that during states of illusory self-identification with the avatar, the vision of an alien body (anatomically compatible for the vision and congruently stroked for the touch) is effective in modulating physiological responses to painful stimuli. (C) 2014 Elsevier B.V. All rights reserved.

The way in which humans represent their own bodies is critical in guiding their interactions with the environment. To achieve successful body–space interactions, the body representation is strictly connected with that of the space immediately surrounding it through efficient visuo-tactile crossmodal integration. Such a body–space integrated representation is not fixed, but can be dynamically modulated by the use of external tools. Our study aims to explore the effect of using a complex tool, namely a functional prosthesis, on crossmodal visuo-tactile spatial interactions in healthy participants. By using the crossmodal visuo-tactile congruency paradigm, we found that prolonged training with a mechanical hand capable of distal hand movements and providing sensory feedback induces a pattern of interference, which is not observed after a brief training, between visual stimuli close to the prosthesis and touches on the body. These results suggest that after extensive, but not short, training the functional prosthesis acquires a visuo-tactile crossmodal representation akin to real limbs. This finding adds to previous evidence for the embodiment of functional prostheses in amputees, and shows that their use may also improve the crossmodal combination of somatosensory feedback delivered by the prosthesis with visual stimuli in the space around it, thus effectively augmenting the patients' visuomotor abilities. & 2013 Elsevier Ltd. All rights reserved.

The representation of body parts holds a special status in the brain, due to their prototypical shape and the contribution of multisensory (visual and somatosensory-proprioceptive) information. In a previous study (Sposito et al., 2010), we showed that patients with left unilateral spatial neglect exhibit a rightward bias in setting the mid-point of their left forearm, which becomes larger when bisecting a cylindrical object comparable in size. This body part advantage, found also in control participants, suggests partly different processes for computing the extent of body parts and objects. In this study we tested 16 right-brain-damaged patients, and 10 unimpaired participants, on a manual bisection task of their own (real) left forearm, or a size-matched fake forearm. We then explored the effects of adaptation to rightward displacing prism exposure, which brings about leftward aftereffects. We found that all participants showed prism adaptation and aftereffects, with right-brain-damaged patients exhibiting a reduction of the rightward bias for both real and fake forearm, with no overall differences between them. Second, correlation analyses highlighted the role of visual and proprioceptive information for the metrics of body parts. Third, single-patient analyses showed dissociations between real and fake forearm bisections, and the effects of prism adaptation, as well as a more frequent impairment with fake body parts. In sum, the rightward bias shown by right-brain-damaged patients in bisecting body parts is reduced by prism exposure, as other components of the neglect syndrome; discrete spatial representations for real and fake body parts, for which visual and proprioceptive codes play different roles, are likely to exist. Multisensory information seems to render self bodily segments more resistant to the disruption brought about by right-hemisphere injury.

Body parts are represented in the brain in a very specific fashion, as compared to other three-dimensional objects, with reference to their prototypic shape and multisensory coding. However, evidence is lacking about the spatial representation of body parts. To address this issue, in Experiment I we first compared the metric representation of body parts and of non-bodily objects in 14 right-brain-damaged patients with left unilateral spatial neglect (USN), and in 14 neurologically unimpaired control participants. Participants bisected, by manual pointing, a three-dimensional object, or their own left forearm. Patients showed the well-known ipsilesional rightward displacement of the subjective midpoint, in both forearm and solid bisection. Both USN patients and control participants were overall more accurate in the bisection of their own forearm, relative to the extracorporeal object. in four patients this advantage of the forearm was significant in a single-case analysis, while two patients showed the opposite dissociation, being more accurate with the solid object. In Experiment 2 neurologically unimpaired participants were more accurate in the bisection of a fake forearm, as well as of their own forearm, as compared to the extrapersonal object. Overall, the results indicate that the representation of the metric of the body is more reliable than that of extrapersonal objects, and also more resistant to the disruption of spatial representations brought about by USN, possibly due to the prototypical shape of body parts. Furthermore, the double dissociation found in USN patients suggests that the metrics of body parts and of extrapersonal objects are supported by independent spatial processes. (C) 2009 Elsevier Ltd. All rights reserved.

Il nostro corpo è percepito solitamente come un'entità unica attraverso la quale possiamo interagire con successo con l'ambiente esterno. Inoltre, il corpo è particolarmente speciale, poiché è sia l'oggetto che il soggetto di ogni esperienza. Infatti, attraverso i nostri sensi, possiamo agire e interagire nell'ambiente ma anche percepire noi stessi dall' "interno". Poiché la percezione e la conoscenza del nostro corpo provengono da diversi recettori e modalità sensoriali, numerosi aspetti sono coinvolti nella rappresentazione del corpo. Le rappresentazioni del corpo sono intrinsecamente multimodali. Ad esempio, il senso di ownership verso il proprio corpo, cioè la sensazione che il proprio corpo appartenga a se stessi, deriva dall’integrazione dei diversi segnali sensoriali che raggiungono le aree multimodali. Inoltre, le informazioni sensoriali vengono combinate per generare rappresentazioni corporee multiple: pesi diversi assegnati a diverse modalità sensoriali danno origine a rappresentazioni corporee distinte. Lo scambio tra sensi e rappresentazione corporea sembra essere bidirezionale. In effetti, la rappresentazione corporea avrebbe anche un ruolo nell'influenzare l'elaborazione delle informazioni provenienti da diverse modalità sensoriali. Pertanto, le rappresentazioni di diverse caratteristiche corporee, come la forma, le dimensioni, le proporzioni e la postura, possono derivare dall'integrazione di varie informazioni sensoriali. Allo stesso tempo, la rappresentazione corporea può essere il riferimento nell'elaborazione delle informazioni sensoriali, influenzando così la percezione del mondo esterno. La presente tesi di dottorato si propone di sottolineare questa relazione bidirezionale tra rappresentazione corporea ed elaborazione sensoriale: l'integrazione sensoriale può modificare la rappresentazione corporea e, allo stesso tempo, la rappresentazione corporea può influenzare l’elaborazione percettiva. In accordo, la prima parte della tesi si concentra su come la rappresentazione sensomotoria del proprio corpo nello spazio e la rappresentazione dello spazio circostante il corpo siano costantemente aggiornate e modulate in base alle informazioni sensoriali in arrivo (Studio 1 e 2). In particolare, lo Studio 1 mostra che un conflitto tra informazioni tattili e visive può influenzare la percezione del proprio corpo e dello spazio potenziale di azione, enfatizzando la reciprocità dell’interazione tra corpo e sensi. Mentre, lo Studio 2 suggerisce come il semplice mantenimento in mano di uno strumento, senza eseguire alcun movimento, possa influenzare la rappresentazione morfologica del corpo (cioè, incorporazione online dello strumento). La seconda parte della tesi (Studio 3 e 4) mira a verificare l'influenza della rappresentazione corporea sull'elaborazione delle informazioni sensoriali in entrata (cioè aptiche e visive). In particolare, lo Studio 3, utilizzando il noto aftereffect delle dimensioni (Illusione aptica di Uznadze) mostra che le informazioni propriocettive relative alla posizione delle braccia possono modulare la percezione aptica della dimensione degli stimoli. Infine, lo Studio 4 considera come l'elaborazione visiva delle dimensioni differisca in presenza di stimoli corporei durante l'Illusione visiva di Uznadze, sottolineando l'importanza del corpo come riferimento nell’elaborazione sensoriale. In conclusione, i risultati degli esperimenti di questa tesi evidenziano l’influenza reciproca tra la percezione del corpo e del mondo esterno, avvalorando l'idea di una relazione bidirezionale tra come il corpo è rappresentato e come le informazioni sensoriali in entrata sono elaborate. Our body is generally perceived as a unique entity through which we can successfully interact with the external environment. Moreover, the body is particularly special as it can be considered both the object and subject of all experiences. Indeed, through our senses, we can act and interact in the environment but also perceive ourselves from the "inside." Since the perception and knowledge of our body come from different receptors and sensory modalities, numerous aspects are involved in representing our body. Body representation is intrinsically multimodal. Indeed, different sensory signals reach multimodal areas where the multisensory integration occurs to generate the sense of body ownership, i.e., the feeling that one's own body belongs to oneself. Furthermore, sensory information is combined to generate multiple body representations: different weights assigned to different sensory modalities originate distinct body representations. The exchange between senses and body representation seems to be bidirectional. Indeed, body representation would also play a role in influencing the information processing of different sensory modalities. Therefore, representations of different body features, such as shape, size, proportions, and posture, can result from integrating various sensory information. At the same time, the body representation can be the reference in processing sensory signals, thus influencing the perception of the external world. The present doctoral thesis aims to highlight this critical bidirectional relation between body representation and sensory processing: multisensory integration can modify body representation, and, simultaneously, body representation may influence perceptual phenomena. Accordingly, the first part of the thesis focuses on how the sensorimotor representation of one's own body in space and the representation of the space surrounding the body are constantly updated and modulated based on incoming sensory information (Study 1 and 2). Specifically, Study 1 reveals that a conflict between tactile and visual information can influence the perception of one's own body and of the potential space of action, emphasizing the reciprocity of the interaction between body and senses. Whereas Study 2 suggests how simply holding a tool in the hand, without performing any movement, can affect the morphological body representation (i.e., online incorporation of the tool). The second part of the thesis aims to test how body representation influences the processing of incoming sensory information (i.e., haptic and visual). More specifically, Study 3, using the established size-contrast aftereffect (i.e., Uznadze haptic Illusion), finds that the proprioceptive information about the arms position could modulate haptic size perception. Finally, Study 4 showed how visual size processing differs in the presence of bodily stimuli during the Uznadze visual Illusion, emphasizing the importance of the body in influencing sensory processing. In conclusion, the results of the experiments in this thesis highlight the critical reciprocal influence between the perception of the body and the external world, supporting the idea of a bidirectional relation between how the body is represented and how incoming sensory information is processed.

Bodily self consciousness is a blooming field of research where a lot of questions are still unsolved. From a psychological point of view de Vignemont (2011) proposed an agenda of main issues wondering what is the fundamental role of body ownership, what grounds the sense of ownership, how is the sense of ownership related to bodily sensations, action and emotion, and whether it is possible to feel a sense of ownership for any extracorporeal object. In the present thesis I am going to present a progression of seven studies where I tried to characterize how body representation interacts with pain processing, investigating either patients presenting with body representation disorders and healthy people experiencing bodily misperceptions, by means of recording responses from the autonomous nervous system and ratings of pain experience under different conditions of body representation distortion. Taken together the studies presented show three main findings: an anticipatory response to incoming noxious stimuli is recordable when stimuli enter in the peripersonal space; an intact body representation is necessary to properly monitor noxious stimuli approaching to our own body; when the sense of ownership is transferred to an external object or a virtual body, a visual treat to that external object elicits anticipatory responses akin to those elicited by a menace directed to our own body. The present set of studies provide novel experimental evidence showing the critical influence of body representation for the mapping of sensory experience, in particular pain processing, and how the sense of ownership critically governs this interaction. Overall, the key contribution of the present work is to provide converging data, gathered from different models ranging from the study of pathological populations to the assessment of neurologically intact individuals by means of experimental manipulations, about the role of body representation for the efficient and safe interaction with the world around us, by correctly anticipating potentially dangerous incoming stimuli.