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The peripersonal space (PPS) is a special portion of space immediately surrounding the body, where the integration between tactile stimuli delivered on the body and auditory or visual events emanating from the environment occurs. Interestingly, PPS can widen if a tool is employed to interact with objects in the far space. However, electrophysiological evidence of such tool-use dependent plasticity in the human brain is scarce. Here, in a series of three experiments, participants were asked to respond to tactile stimuli, delivered to their right hand, either in isolation (unimodal condition) or combined with auditory stimulation, which could occur near (bimodal-near) or far from the stimulated hand (bimodal-far). According to multisensory integration spatial rule, when bimodal stimuli are presented at the same location, we expected a response enhancement (response time – RT – facilitation and event-related potential – ERP – super-additivity). In Experiment 1, we verified that RT facilitation was driven by bimodal input spatial congruency, independently from auditory stimulus intensity. In Experiment 2, we showed that our bimodal task was effective in eliciting the magnification of ERPs in bimodal conditions, with significantly larger responses in the near as compared to far condition. In Experiment 3 (main experiment), we explored tool-use driven PPS plasticity. Our audio-tactile task was performed either following tool-use (a 20-minute reaching task, performed using a 145 cm-long rake) or after a control cognitive training (a 20-minute visual discrimination task) performed in the far space. Following the control training, faster RTs and greater super-additive ERPs were found in bimodal-near as compared to bimodal-far condition (replicating Experiment 2 results). Crucially, this far-near differential response was significantly reduced after tool-use. Altogether our results indicate a selective effect of tool-use remapping in extending the boundaries of PPS. The present finding might be considered as an electrophysiological evidence of tool-use dependent plasticity in the human brain.

Individuals with autism spectrum conditions (ASC) are less susceptible to multisensory delusions, such as rubber hand illusion (RHI). Here, we investigate whether a monochannel variant of RHI is more effective in inducing an illusory feeling of ownership in ASC. To this aim, we exploit a non-visual variant of the RHI that, excluding vision, leverages only on the somatosensory channel. While the visual-tactile RHI does not alter the perceived hand position in ASC individuals, the tacto-tactile RHI effectively modulates proprioception to a similar extent as that found in typical development individuals. These findings suggest a more effective integration of multiple inputs originating from the same sensory channel in ASC, revealing a monochannel preference in this population.

The ability to identify our own body and its boundaries is crucial for survival. Ideally, the sooner we learn to discriminate external stimuli occurring close to our body from those occurring far from it, the better (and safer) we may interact with the sensory environment. However, when this mechanism emerges within ontogeny is unknown. Is it something acquired throughout infancy, or is it already present soon after birth? The presence of a spatial modulation of multisensory integration (MSI) is considered a hallmark of a functioning representation of the body position in space. Here, we investigated whether MSI is present and spatially organized in 18- to 92-h-old newborns. We compared electrophysiological responses to tactile stimulation when concurrent auditory events were delivered close to, as opposed to far from, the body in healthy newborns and in a control group of adult participants. In accordance with previous studies, adult controls showed a clear spatial modulation of MSI, with greater superadditive responses for multisensory stimuli close to the body. In newborns, we demonstrated the presence of a genuine electrophysiological pattern of MSI, with older newborns showing a larger MSI effect. Importantly, as for adults, multisensory superadditive responses were modulated by the proximity to the body. This finding may represent the electrophysiological mechanism responsible for a primitive coding of bodily self boundaries, thus suggesting that even just a few hours after birth, human newborns identify their own body as a distinct entity from the environment.

During the rubber hand illusion (RHI), the synchronous stroking of the participants’ concealed hand and a visible rubber hand induces a conflict among visuo-tactile inputs, leading healthy subjects to perceive the illusion of being touched on the rubber hand, as if it were part of their body. The predictive coding theory suggests that the RHI emerges to settle the conflict, attenuating somatosensory inputs in favour of visual ones, which “capture” tactile sensations. Here, we employed the psychophysical measure of perceptual threshold to measure a behavioural correlate of the somatosensory and visual modulations, to better understand the mechanisms underpinning the illusion. Before and after the RHI, participants underwent a tactile (Experiment 1) and a visual (Experiment 2) task, wherein they had to detect stimuli slightly above the perceptual threshold. According to the predictive coding framework, we found a significant decrease of tactile detection (i.e. increased tactile perceptual threshold) and a significant increase of visual detection (i.e. decreased visual perceptual threshold), suggesting a diametrical modulation of somatosensory and visual perceptual processes. These findings provide evidence of how our system plastically adapts to uncertainty, attributing different weights to sensory inputs to restore a coherent representation of the own body.

The ability to identify our own body is considered a pivotal marker of self-awareness. Previous research demonstrated that subjects are more efficient in the recognition of images representing self rather than others' body effectors (self-advantage). Here, we verified whether, at an electrophysiological level, bodily-self recognition modulates change detection responses. In a first EEG experiment (discovery sample), event-related potentials (ERPs) were elicited by a pair of sequentially presented visual stimuli (vS1; vS2), representing either the self-hand or other people's hands. In a second EEG experiment (replicating sample), together with the previously described visual stimuli, also a familiar hand was presented. Participants were asked to decide whether vS2 was identical or different from vS1. Accuracy and response times were collected. In both experiments, results confirmed the presence of the self-advantage: participants responded faster and more accurately when the self-hand was presented. ERP results paralleled behavioral findings. Anytime the self-hand was presented, we observed significant change detection responses, with a larger N270 component for vS2 different rather than identical to vS1. Conversely, when the self-hand was not included, and even in response to the familiar hand in Experiment 2, we did not find any significant modulation of the change detection responses. Overall our findings, showing behavioral self-advantage and the selective modulation of N270 for the self-hand, support the existence of a specific mechanism devoted to bodily-self recognition, likely relying on the multimodal (visual and sensorimotor) dimension of the bodily-self representation. We propose that such a multimodal self-representation may activate the salience network, boosting change detection effects specifically for the self-hand.

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.