Somatosensory-guided tool use modifies arm representation for action

Proprioceptive acuity for locating and controlling movements of a hand-held tool

We investigated proprioceptive acuity for location and motion of a never seen hand-held tool (30 cm long rod) and the accuracy of movements to place tool parts in the location of remembered visual targets. Ten blindfolded right-handed subjects (5 females) reached with the tool held in the right hand to touch the tip and midpoint to the stationary and moving left index-tip, to the right and left ear lobes and to remembered visual target locations. We also tested accuracy of left hand rod reaches to the ear lobes to determine if rod dimensions and control of tool movements experienced during right hand tool use could be used to accurately localize the rod parts when held in the left hand. Errors for right hand-held rod-tip movements to touch the stationary and moving left index-tip averaged only about 1 cm larger than observed previously for right hand movements to touch its index-tip to the left index-tip. The tool-tip was localized with lower mean distance errors (about 1 cm) than the tool-midpoint (5.5-6.5 cm) when reaching to touch the ear lobes with the rod in right and left hands. Right hand reaches to place the tool- tip and midpoint in remembered visual target locations were inaccurate with large overshoots of close targets and undershoots of far targets, similar to previous reports for reaching with the right hand to remembered visual targets. These results support the distalization hypothesis, that the tool endpoint becomes the effective upper limb endpoint when using the tool.

Eyes and hand are both reliable at localizing somatosensory targets

Body representations (BR) for action are critical to perform accurate movements. Yet, behavioral measures suggest that BR are distorted even in healthy people. However, the upper limb has mostly been used as a probe so far, making difficult to decide whether BR are truly distorted or whether this depends on the effector used as a readout. Here, we aimed to assess in healthy humans the accuracy of the eye and hand effectors in localizing somatosensory targets, to determine whether they may probe BR similarly. Twenty-six participants completed two localization tasks in which they had to localize an unseen target (proprioceptive or tactile) with either their eyes or hand. Linear mixed model revealed in both tasks a larger horizontal (but not vertical) localization error for the ocular than for the manual localization performance. However, despite better hand mean accuracy, manual and ocular localization performance positively correlated to each other in both tasks. Moreover, target position also affected localization performance for both eye and hand responses: accuracy was higher for the more flexed position of the elbow in the proprioceptive task and for the thumb than for the index finger in the tactile task, thus confirming previous results of better performance for the thumb. These findings indicate that the hand seems to beat the eyes along the horizontal axis when localizing somatosensory targets, but the localization patterns revealed by the two effectors seemed to be related and characterized by the same target effect, opening the way to assess BR with the eyes when upper limb motor control is disturbed.

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.

Three Sliding Probes Placed on Forelimb Skin for Proprioceptive Feedback Differentially yet Complementarily Contribute to Hand Gesture Detection and Object-Size Discrimination

The purpose was to assess the effectiveness of three sliding tactile probes placed on the forelimb skin to provide proprioceptive feedback for the detection of hand gestures and discrimination of object size. Tactile contactors representing the first three fingers were driven along the proximodistal axis by linear servo motors. Twenty healthy subjects were involved in the gesture detection test, with 10 of them also participating in the object-size discrimination task. Motors were controlled by computer in the first four sessions of the gesture detection experiment, while the fifth session utilized a sensorized glove. Both the volar and dorsal sides of the forearm were examined. In the object-size discrimination experiment, the method was exclusively assessed on the volar surface under four distinct feedback conditions, including all fingers and each finger separately. The psychophysical data were further analyzed using a structural equation model (SEM) to evaluate the specific contributions of each individual contactor. Subjects consistently outperformed the chance level in detecting gestures. Performance improved up to the third session, with better results obtained on the volar side. The performances were similar in the fourth and fifth sessions. The just noticeable difference for achieving a 75% discrimination accuracy was found to be 2.90 mm of movement on the skin. SEM analysis indicated that the contactor for the index finger had the lowest importance in gesture detection, while it played a more significant role in object-size discrimination. However, all fingers were found to be significant predictors of subjects' responses in both experiments, except for the thumb, which was deemed insignificant in object-size discrimination. The study highlights the importance of considering the partial contribution of each degree of freedom in a sensory feedback system, especially concerning the task, when designing such systems.

A conceptual framework on body representations and their relevance for mental disorders

Many mental disorders are accompanied by distortions in the way the own body is perceived and represented (e.g., eating disorders, body dysmorphic disorder including muscle dysmorphia, or body integrity dysphoria). We are interested in the way these distortions develop and aim at better understanding their role in mental health across the lifespan. For this purpose, we first propose a conceptual framework of body representation that defines this construct and integrates different perspectives (e.g., cognitive neuroscience, clinical psychology) on body representations. The framework consists of a structural and a process model of body representation emphasizing different goals: the structural model aims to support researchers from different disciplines to structure results from studies and help collectively accumulate knowledge about body representations and their role in mental disorders. The process model is reflecting the dynamics during the information processing of body-related stimuli. It aims to serve as a motor for (experimental) study development on how distorted body representations emerge and might be changed. Second, we use this framework to review the normative development of body representations as well as the development of mental disorders that relate to body representations with the aim to further clarify the potential transdiagnostic role of body representations.

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.

Impact of supplementary sensory feedback on the control and embodiment in human movement augmentation

In human movement augmentation, the number of controlled degrees of freedom could be enhanced by the simultaneous and independent use of supernumerary robotic limbs (SRL) and natural ones. However, this poses several challenges, that could be mitigated by encoding and relaying the SRL status. Here, we review the impact of supplementary sensory feedback on the control and embodiment of SRLs. We classify the main feedback features and analyse how they improve control performance. We report the feasibility of pushing body representation beyond natural human morphology and suggest that gradual SRL embodiment could make multisensory incongruencies less disruptive. We also highlight shared computational bases between SRL motor control and embodiment and suggest contextualizing them within the same theoretical framework. Finally, we argue that a shift towards long term experimental paradigms is necessary for successfully integrating motor control and embodiment.

Supernumerary robotic limbs are robotic devices providing additional limbs to the user. Mattia Pinardi and colleagues review the impact of supplementary sensory feedback on the control performance and embodiment of supernumerary robotic limbs.

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.

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.

Tool-use training in augmented reality: plasticity of forearm body schema does not predict sense of ownership or agency in older adults

In young adults (YA) who practised controlling a virtual tool in augmented reality (AR), the emergence of a sense of body ownership over the tool was associated with the integration of the virtual tool into the body schema (BS). Agency emerged independent of BS plasticity. Here we aimed to replicate these findings in older adults (OA). Although they are still able to learn new motor tasks, brain plasticity and learning capacity are reduced in OA. We predicted that OA would be able to gain control over the virtual tool indicated by the emergence of agency but would show less BS plasticity as compared to YA. Still, an association between BS plasticity and body ownership was expected. OA were trained in AR to control a virtual gripper to enclose and touch a virtual object. In the visuo-tactile (VT) but not the vision-only (V) condition, vibro-tactile feedback was applied through a CyberTouch II glove when the tool touched the object. BS plasticity was assessed with a tactile distance judgement task where participants judged distances between two tactile stimuli applied to their right forearm. Participants further rated their perceived ownership and agency after training. As expected, agency emerged during the use of the tool. However, results did not indicate any changes in the BS of the forearm after virtual tool-use training. Also, an association between BS plasticity and the emergence of body ownership could not be confirmed for OA. Similar to YA, the practice effect was stronger in the visuo-tactile feedback condition compared with the vision-only condition. We conclude that a sense of agency may strongly relate to improvement in tool-use in OA independent of alterations in the BS, while ownership did not emerge due to a lack of BS plasticity.

Comparing end-effector position and joint angle feedback for online robotic limb tracking

Somatosensation greatly increases the ability to control our natural body. This suggests that supplementing vision with haptic sensory feedback would also be helpful when a user aims at controlling a robotic arm proficiently. However, whether the position of the robot and its continuous update should be coded in a extrinsic or intrinsic reference frame is not known. Here we compared two different supplementary feedback contents concerning the status of a robotic limb in 2-DoFs configuration: one encoding the Cartesian coordinates of the end-effector of the robotic arm (i.e., Task-space feedback) and another and encoding the robot joints angles (i.e., Joint-space feedback). Feedback was delivered to blindfolded participants through vibrotactile stimulation applied on participants' leg. After a 1.5-hour training with both feedbacks, participants were significantly more accurate with Task compared to Joint-space feedback, as shown by lower position and aiming errors, albeit not faster (i.e., similar onset delay). However, learning index during training was significantly higher in Joint space feedback compared to Task-space feedback. These results suggest that Task-space feedback is probably more intuitive and more suited for activities which require short training sessions, while Joint space feedback showed potential for long-term improvement. We speculate that the latter, despite performing worse in the present work, might be ultimately more suited for applications requiring long training, such as the control of supernumerary robotic limbs for surgical robotics, heavy industrial manufacturing, or more generally, in the context of human movement augmentation.

Relating different Dimensions of Bodily Experiences: Review and proposition of an integrative model relying on phenomenology, predictive brain and neuroscience of the self

How we mentally experience our body has been studied in a variety research domains. Each of these domains focuses in its own ways on different aspects of the body, namely the neurophysiological, perceptual, affective or social components, and proposes different conceptual taxonomies. It is therefore difficult to find one's way through this vast literature and to grasp the relationships between the different dimensions of bodily experiences. In this narrative review, we summarize the existing research directions and present their limits. We propose an integrative framework, grounded in studies on phenomenal consciousness, self-consciousness and bodily self-consciousness, that can provide a common basis for evaluating findings on different dimensions of bodily experiences. We review the putative mechanisms, relying on predictive processes, and neural substrates that support this model. We discuss how this model enables a conceptual assessment of the interrelationships between multiple dimensions of bodily experiences and potentiate interdisciplinary approaches.

Embodiment for Robotic Lower-Limb Exoskeletons: A Narrative Review

Research on embodiment of objects external to the human body has revealed important information about how the human nervous system interacts with robotic lower limb exoskeletons. Typical robotic exoskeleton control approaches view the controllers as an external agent intending to move in coordination with the human. However, principles of embodiment suggest that the exoskeleton controller should ideally coordinate with the human such that the nervous system can adequately model the input-output dynamics of the exoskeleton controller. Measuring embodiment of exoskeletons should be a necessary step in the exoskeleton development and prototyping process. Researchers need to establish high fidelity quantitative measures of embodiment, rather than relying on current qualitative survey measures. Mobile brain imaging techniques, such as high-density electroencephalography, is likely to provide a deeper understanding of embodiment during human-machine interactions and advance exoskeleton research and development. In this review we show why future exoskeleton research should include quantitative measures of embodiment as a metric of success.

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

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

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.

Psychophysical Evaluation of Proprioceptive Feedback Through a Probe Sliding on the Forearm Skin of Healthy Humans

We have tested the forearm skin of humans as a target organ to deliver proprioceptive feedback via a tactile sensory substitution method. In the proposed method, a contactor probe was actuated by a linear servo motor and moved on the skin in proximo-distal axis depending on the angle of a virtual joint moving on a 180° arc. Twenty healthy subjects were tested to stop the joint at a given target under no-feedback, visual feedback, and tactile (dorsal and volar) feedback conditions. The absolute difference between the target and the response angle was recorded. Tests were repeated 4 times with ~ 1-week intervals. Two joint movement speeds were tested. The subjects performed best with visual feedback, and worst if no feedback was provided. Their performances with tactile feedback were not as good as in the visual feedback condition, but better than in the no-feedback condition. Subjects equally performed with volar and dorsal tactile feedback. The movement speed had no significant effects on tactile feedback. The performance improved with training only in tactile feedback conditions. The proprioceptive information from a motorized prosthesis can be provided through probes moving on the forearm skin, while the efficacy of the feedback may improve with extensive training.

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.

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.

Body schema plasticity is altered in Developmental Coordination Disorder

Developmental Coordination Disorder (DCD) is a pathological condition characterized by impaired motor skills. Current theories advance that a deficit of the internal models is mainly responsible for DCD children's altered behavior. Yet, accurate movement execution requires not only correct movement planning, but also integration of sensory feedback into body representation for action (Body Schema) to update the state of the body. Here we advance and test the hypothesis that the plasticity of this body representation is altered in DCD. To probe Body Schema (BS) plasticity, we submitted a well-established tool-use paradigm to seventeen DCD children, required to reach for an object with their hand before and after tool use, and compared their movement kinematics to that of a control group of Typically Developing (TD) peers. We also asked both groups to provide explicit estimates of their arm length to probe plasticity of their Body Image (BI). Results revealed that DCD children explicitly judged their arm shorter after tool use, showing changes in their BI comparable to their TD peers. Unlike them, though, DCD did not update their implicit BS estimate: kinematics showed that tool use affected their peak amplitudes, but not their latencies. Remarkably, the kinematics of tool use showed that the motor control of the tool was comparable between groups, both improving with practice, confirming that motor learning abilities are preserved in DCD. This study thus brings evidence in favor of an alternative theoretical account of the DCD etiology. Our findings point to a deficit in the plasticity of the body representation used to plan and execute movements. Though not mutually exclusive, this widens the theoretical perspective under which DCD should be considered: DCD may not be limited to a problem affecting the internal models and their motor functions, but may concern the state of the effector they have to use.

Expertise in Tool Use Promotes Tool Embodiment

Body representations are known to be dynamically modulated or extended through tool use. Here, we review findings that demonstrate the importance of a user's tool experience or expertise for successful tool embodiment. Examining expert tool users, such as individuals who use tools in professional sports, people who use chopsticks at every meal, or spinal injury patients who use a wheelchair daily, offers new insights into the role of expertise in tool embodiment: Not only does tool embodiment differ between novices and experts, but experts may experience enhanced changes to their body representation when interacting with their own, personal tool. The findings reviewed herein reveal the importance of assessing tool skill in future studies of tool embodiment.

Somatosensory deafferentation reveals lateralized roles of proprioception in feedback and adaptive feedforward control of movement and posture

Proprioception provides crucial information necessary for determining limb position and movement, and plausibly also for updating internal models that might underlie the control of movement and posture. Seminal studies of upper-limb movements in individuals living with chronic, large fiber deafferentation have provided evidence for the role of proprioceptive information in the hypothetical formation and maintenance of internal models to produce accurate motor commands. Vision also contributes to sensorimotor functions but cannot fully compensate for proprioceptive deficits. More recent work has shown that posture and movement control processes are lateralized in the brain, and that proprioception plays a fundamental role in coordinating the contributions of these processes to the control of goal-directed actions. In fact, the behavior of each limb in a deafferented individual resembles the action of a controller in isolation. Proprioception, thus, provides state estimates necessary for the nervous system to efficiently coordinate multiple motor control processes.

Embodied cognition: So flexible as to be "disembodied"?

This review aims to explore what I call the "Embodiment Cost Hypothesis" (ECH), according to which, when humans "embody" a part of the world other than their bodies, a measurable cost is detectable on their real bodies. The review analyzes experimental evidence in favor of the ECH by examining studies from different research fields, including studies of action observation (2), tool-use (3), rubber hand illusion (4), and full-body illusions (5). In light of this literature, this review argues that embodiment effects can profitably be seen as phenomena associated with both benefits (resulting from the embodiment of external objects/bodies) and costs (resulting from the disembodiment at various levels of the subject's own body). Implications are discussed in relation to the ongoing debate on the embodied cognition (EC) approach.

The long developmental trajectory of body representation plasticity following tool use

Humans evolution is distinctly characterized by their exquisite mastery of tools, allowing them to shape their environment in more elaborate ways compared to other species. This ability is present ever since infancy and most theories indicate that children become proficient with tool use very early. In adults, tool use has been shown to plastically modify metric aspects of the arm representation, as indexed by changes in movement kinematics. To date, whether and when the plastic capability of updating the body representation develops during childhood remains unknown. This question is particularly important since body representation plasticity could be impacted by the fact that the human body takes years to achieve a stable metric configuration. Here we assessed the kinematics of 90 young participants (8-21 years old) required to reach for an object before and after tool use, as a function of their pubertal development. Results revealed that tool incorporation, as indexed by the adult typical kinematic pattern, develops very slowly and displays a u-shaped developmental trajectory. From early to mid puberty, the changes in kinematics following tool use seem to reflect a shortened arm representation, opposite to what was previously reported in adults. This pattern starts reversing after mid puberty, which is characterized by the lack of any kinematics change following tool use. The typical adult-like pattern emerges only at late puberty, when body size is stable. These findings reveal the complex dynamics of tool incorporation across development, possibly indexing the transition from a vision-based to a proprioception-based body representation plasticity.

Online proprioception feeds plasticity of arm representation following tool-use in healthy aging

Following tool-use, the kinematics of free-hand movements are altered. This modified kinematic pattern has been taken as a behavioral hallmark of the modification induced by tool-use on the effector representation. Proprioceptive inputs appear central in updating the estimated effector state. Here we questioned whether online proprioceptive modality that is accessed in real time, or offline, memory-based, proprioception is responsible for this update. Since normal aging affects offline proprioception only, we examined a group of 60 year-old adults for proprioceptive acuity and movement's kinematics when grasping an object before and after tool-use. As a control, participants performed the same movements with a weight-equivalent to the tool-weight-attached to their wrist. Despite hampered offline proprioceptive acuity, 60 year-old participants exhibited the typical kinematic signature of tool incorporation: Namely, the latency of transport components peaks was longer and their amplitude reduced after tool-use. Instead, we observed no kinematic modifications in the control condition. In addition, online proprioception acuity correlated with tool incorporation, as indexed by the amount of kinematics changes observed after tool-use. Altogether, these findings point to the prominent role played by online proprioception in updating the body estimate for the motor control of tools.

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.

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.