I'm a believer: Illusory self-generated touch elicits sensory attenuation and somatosensory evoked potentials similar to the real self-touch

The tangled threads: Unveiling the interplay between the sense of body ownership and the sense of agency in impacting the bodily-self representation in eating disorders

The feeling of owning a body (body ownership) and controlling its actions (sense of agency) contributes to the emergence of the bodily-self representation, whose alteration is at the root of the central psychopathology of Eating Disorders (EDs). Yet, studies addressing these aspects in EDs provided inconsistent results. Here, we simultaneously test body ownership and sense of agency in EDs compared to controls by exploiting different rubber hand illusion (RHI) paradigms (i.e., classic visuo-tactile, passive and active visuo-motor versions). In any RHI versions, no differences in the susceptibility to the illusion between EDs patients and controls emerged at the body ownership questionnaire, thus suggesting a normal multisensory integration mechanism. Crucially, correlation analysis revealed that a higher level of body dissatisfaction is associated with increased susceptibility to RHI, as measured by the body ownership questionnaire. Interestingly, patients with a bulimic variant of EDs reported agency toward the fake hand in the visuo-tactile RHI, revealing an abnormal sense of agency in absence of voluntary movement. Moreover, in the visuo-motor RHI, EDs patients exhibited a proprioceptive drift both in synchronous and asynchronous conditions. Hence, our results revealed a dissociation between explicit and implicit RHI measures, showing a more plastic bodily-self representation when the RHI enlists hand movements, leading to a stronger visual-capture of proprioception. This study contributes to understanding the intricate link between body ownership and agency, shedding light on the role of voluntary actions in driving the sense of self in EDs.

Sensory attenuation of self-initiated tactile feedback is modulated by stimulus strength and temporal delay in a virtual reality environment

Despite extensive research across various modalities, the precise mechanisms of sensory attenuation (SA) remain debated. Specifically, it remains unclear to what extent SA is influenced by stimulus predictability alone, as opposed to the distinct impact of self-generated actions. Forward models suggest that efference copies of motor commands enable the brain to predict and distinguish anticipated changes in self-initiated sensory input. Predictive processing proposes that predictions about upcoming changes in sensory input are not solely based on efference copies, but rather generated in the form of a generative model integrating external, contextual factors, as well. This study investigated the underlying mechanisms of SA in the tactile domain, specifically examining self-initiation and temporal predictions within a virtual reality (VR) framework. This setup allowed for precise control over sensory feedback in response to movement. Participants (N = 33) engaged in an active condition, moving their hands to elicit a virtual touch. Importantly, visual perception was modified in VR, so that participants touched their rendered-but not physical-hands. The virtual touch triggered the test vibrations on a touch controller (intensities: 0.2, 0.35, 0.5, 0.65, 0.8; in arbitrary units.), the intensity of which was then compared to that of a standard stimulus (intensity: 0.5). In the passive condition, vibrations were presented without movement and were preceded by a visual cue. Further, test vibrations appeared either immediately or after a variable onset delay (700-800ms). Our results revealed a significant effect of the factor "onset delay" on perceived vibration intensity. In addition, we observed interactions between the factors "agency" and "test vibration intensity" and between the factors "agency" and "onset delay," with attenuation effects for immediate vibrations at high intensities and enhancement effects for delayed vibrations at low intensities. These findings emphasize the impact of external, contextual factors and support the notion of a broader, attention-oriented predictive mechanism for the perception of self-initiated stimuli.

Body Ownership and the Motor System: Rapid Facilitation of Embodied Fake Hand Movement on Actual Movement Execution

Body ownership-the perception that one's body belongs to oneself-has been explored using a rubber hand illusion, in which individuals misperceive a fake hand as their own (i.e., embodiment of the fake hand) when an unseen real hand and a visible fake hand are stroked synchronously. Thus, the movement of an embodied fake body may be represented in one's own sensorimotor system. Using a combination of the rubber hand illusion and a motor task, we investigated whether simple movement of the embodied fake hand influenced the subsequent movement of the participants' hand. The participants lifted their own index finger immediately upon observing the index finger lifting on the embodied (rubber hand illusion) or non-embodied (non-rubber hand illusion) fake hand (Experiment 1), and a light-emitting diode turning on near the fake hand (Experiment 2). The reaction times, peak velocities, and peak acceleration were extracted from the participants' finger-lifting movements. In Experiment 1, the reaction time was significantly shorter in the rubber hand illusion condition than in the non-rubber hand illusion condition, suggesting the rapid facilitation effect of embodied fake hand movement on actual movement. However, no such motor facilitation was observed in Experiment 2, confirming that the improved reaction time in Experiment 1 resulted from the visual movement of the fake hand rather than attention to the fake hand itself. In contrast to the reaction time, the peak velocity and acceleration did not differ significantly in either experiment. These findings reflect the similar sensorimotor representations of illusory and actual self-movement.

Investigation of sensory attenuation in the somatosensory domain using EEG in a novel virtual reality paradigm

We are not only passively immersed in a sensorial world, but we are active agents that directly produce stimulations. Understanding what is unique about sensory consequences can give valuable insight into the action-perception-cycle. Sensory attenuation is the phenomenon that self-produced stimulations are perceived as less intense compared to externally-generated ones. Studying this phenomenon, however, requires considering a plethora of factors that could otherwise interfere with its interpretation, such as differences in stimulus properties, attentional resources, or temporal predictability. We therefore developed a novel Virtual Reality (VR) setup which allows control over several of these confounding factors. Furthermore, we modulated the expectation of receiving a somatosensory stimulation across self-production and passive perception through a simple probabilistic learning task, allowing us to test to what extent the electrophysiological correlates of sensory attenuation are impacted by stimulus expectation. Therefore, the aim of the present study was twofold: first we aimed validating a novel VR paradigm during electroencephalography (EEG) recoding to investigate sensory attenuation in a highly controlled setup; second, we tested whether electrophysiological differences between self- and externally-generated sensations could be better explained by stimulus predictability factors, corroborating the validity of sensory attenuation. Results of 26 participants indicate that early (P100), mid-latency (P200) and later negative contralateral potentials were significantly attenuated by self-generated sensations, independent of the stimulus expectation. Moreover, a component around 200 ms post-stimulus at frontal sites was found to be enhanced for self-produced stimuli. The P300 was influenced by stimulus expectation, regardless of whether the stimulation was actively produced or passively attended. Together, our results demonstrate that VR opens up new possibilities to study sensory attenuation in more ecological valid yet well-controlled paradigms, and that sensory attenuation is not significantly modulated by stimulus predictability, suggesting that sensory attenuation relies on motor-specific predictions about their sensory outcomes. This not only supports the phenomenon of sensory attenuation, but is also consistent with previous research and the concept that action actually plays a crucial role in perception.

Rejecting unfairness enhances the implicit sense of agency in the human brain

Sense of agency (SoA) describes the feeling of control over one's actions and their consequences. One proposed index of implicit SoA is temporal compression, which refers to the phenomenon that voluntary actions and their outcomes are perceived as closer in time than they actually are. The present study measured temporal compression in the social norm violation situation. In two experiments participants joined in an Ultimatum game (UG), in which they were presented with offers that varied in fairness and they could choose to accept or reject the offers by pressing buttons. A neutral sound would occur after their choices in the UG and the participants had to estimate the time interval between their button pressing and the occurrence of the sound, and EEG signals were recorded during the task. Experiment 1 demonstrated that rejecting unfair offers decreased the perceived interval between action and outcome compared to accepting fair offers, suggesting a higher level of SoA after rejecting unfair offers. Experiment 2 replicated these results and further revealed an attenuated N1 in response to the sound following rejections of unfairness. Taken together, these results highlight the importance of social norms in affecting people's behaviors and agency experiences.

Brain Encoding of Naturalistic, Continuous, and Unpredictable Tactile Events

Studies employing EEG to measure somatosensory responses have been typically optimized to compute event-related potentials in response to discrete events. However, tactile interactions involve continuous processing of nonstationary inputs that change in location, duration, and intensity. To fill this gap, this study aims to demonstrate the possibility of measuring the neural tracking of continuous and unpredictable tactile information. Twenty-seven young adults (females, 15) were continuously and passively stimulated with a random series of gentle brushes on single fingers of each hand, which were covered from view. Thus, tactile stimulations were unique for each participant and stimulated fingers. An encoding model measured the degree of synchronization between brain activity and continuous tactile input, generating a temporal response function (TRF). Brain topographies associated with the encoding of each finger stimulation showed a contralateral response at central sensors starting at 50 ms and peaking at ∼140 ms of lag, followed by a bilateral response at ∼240 ms. A series of analyses highlighted that reliable tactile TRF emerged after just 3 min of stimulation. Strikingly, topographical patterns of the TRF allowed discriminating digit lateralization across hands and digit representation within each hand. Our results demonstrated for the first time the possibility of using EEG to measure the neural tracking of a naturalistic, continuous, and unpredictable stimulation in the somatosensory domain. Crucially, this approach allows the study of brain activity following individualized, idiosyncratic tactile events to the fingers.

Disembodiment and Affective Resonances in Esketamine Treatment of Depersonalized Depression Subtype: Two Case Studies

INTRODUCTION

Dissociative experiences are considered undesirable ketamine's adverse events. However, they might be crucial for ketamine's antidepressant effects, at least in some depression subtypes. Current understandings of ketamine's therapeutic potentials converge on the so-called "relaxed prior hypothesis," suggesting that glutamatergic blockage up-weights bottom-up surprising somatosensory/affective states. As a result, ketamine improves short-term plasticity in depression by enhancing sensitivity to interoceptive signals.

METHODS

We selected 2 case studies for their paradigmatic description of "depersonalized depression" (Entfremdungsdepression) symptoms. Patients were included in a 6-month-long esketamine program for treatment resistant depression, during which we collected their spontaneous experience with esketamine. According to a neurophenomenological approach, we combined subjective reports from unstructured clinical interviews and the review of previous objective neuroimaging results and neurocomputational models to unveil the relation between esketamine antidepressant effects and interoceptive sensitivity.

RESULTS

According to our clinical observations, esketamine-induced dissociation might be particularly effective in the depersonalized depression subtype, in which interoceptive awareness and interaffectivity are particularly compromised. Ketamine and esketamine's dissociative effects and particularly disembodiment might suspend previously acquired patterns of feeling, sensing, and behaving.

CONCLUSIONS

Coherently with previous research, we suggest that esketamine-induced disembodiment allows for a transient window of psychological plasticity and enhanced sensitivity, where the body recovers its permeability to affective affordances.

Balancing the Senses: Electrophysiological Responses Reveal the Interplay between Somatosensory and Visual Processing During Body-Related Multisensory Conflict

In the study of bodily awareness, the predictive coding theory has revealed that our brain continuously modulates sensory experiences to integrate them into a unitary body representation. Indeed, during multisensory illusions (e.g., the rubber hand illusion, RHI), the synchronous stroking of the participant's concealed hand and a fake visible one creates a visuotactile conflict, generating a prediction error. Within the predictive coding framework, through sensory processing modulation, prediction errors are solved, inducing participants to feel as if touches originated from the fake hand, thus ascribing the fake hand to their own body. Here, we aimed to address sensory processing modulation under multisensory conflict, by disentangling somatosensory and visual stimuli processing that are intrinsically associated during the illusion induction. To this aim, we designed two EEG experiments, in which somatosensory- (SEPs; Experiment 1; N = 18; F = 10) and visual-evoked potentials (VEPs; Experiment 2; N = 18; F = 9) were recorded in human males and females following the RHI. Our results show that, in both experiments, ERP amplitude is significantly modulated in the illusion as compared with both control and baseline conditions, with a modality-dependent diametrical pattern showing decreased SEP amplitude and increased VEP amplitude. Importantly, both somatosensory and visual modulations occur in long-latency time windows previously associated with tactile and visual awareness, thus explaining the illusion of perceiving touch at the sight location. In conclusion, we describe a diametrical modulation of somatosensory and visual processing as the neural mechanism that allows maintaining a stable body representation, by restoring visuotactile congruency under the occurrence of multisensory conflicts.

Prime-induced illusion of control: The influence of unconscious priming on self-initiated actions and the role of regression to the mean

To what degree human cognition is influenced by subliminal stimuli is a controversial empirical question. One striking example was reported by Linser and Goschke (2007): participants overestimated how much control they had over objectively uncontrollable stimuli when masked congruent primes were presented immediately before the action. Critically, however, unawareness of the masked primes was established by post hoc data selection. In our preregistered study we sought to explore these findings while adjusting prime visibility based on individual thresholds, so that each participant underwent both visible and non-visible conditions. In experiment 1, N = 39 participants engaged in a control judgement task: following the presentation of a semantic prime, they freely selected between two keys, which triggered the appearance of a colored circle. The color of the circles, however, was independent of the key-press. Subsequently, participants assessed their perceived control over the circle's color, based on their key-presses, via a rating scale that ranged from 0 % (no control) to 100 % (complete control). Contrary to Linser and Goschke (2007)'s findings, this experiment demonstrated that predictive information influenced the experience of agency only when primes were consciously processed. In experiment 2, utilizing symbolic (arrow) primes, N = 35 participants had to rate their feeling of control over the effect-stimulus' identity during a two-choice identification paradigm (i.e., they were instructed to press a key corresponding to a target stimulus; with a contingency between target and effect stimulus of 75 %/25 %). The results revealed no significant influence of subliminal priming on agency perceptions. In summary, this study implies that unconscious stimuli may not exert a substantial influence on the conscious experience of agency, underscoring the need for careful consideration of methodological aspects and experimental design's impact on observed phenomena.

Shared attention in virtual immersive reality enhances electrophysiological correlates of implicit sensory learning

Shared attention effects on learning and memory demonstrate that experiences are amplified when we are not alone. Virtual reality poses new challenges to the study of co-presence. Above all, is coattending together with someone else's avatar in an immersive VR setting comparable with shared experiences at a neural processing level? In the present study we investigate shared attention effects in VR for the first time. We recorded mismatch negativities (MMN) during an auditory roving paradigm, a well-known index of implicit perceptual learning. EEG responses to deviant and standard sounds were registered while subjects were alone (Solo condition) or together (Other condition) with a virtual avatar (Virtual scenario) or physically present confederate (Physical scenario). We found an overall main effect of co-presence on MMN revealed by a point-by-point 2 × 2 ANOVA, thereby replicating previous studies on physical co-presence. Additionally, we found no significant interaction between the scenario (Physical vs. Virtual) and co-presence (Solo vs. Other). Our results indicate that virtual immersive co-presence mimics physical co-presence.

Modulation of vestibular input by short-term head-down bed rest affects somatosensory perception: implications for space missions

Introduction

On Earth, self-produced somatosensory stimuli are typically perceived as less intense than externally generated stimuli of the same intensity, a phenomenon referred to as somatosensory attenuation (SA). Although this phenomenon arises from the integration of multisensory signals, the specific contribution of the vestibular system and the sense of gravity to somatosensory cognition underlying distinction between self-generated and externally generated sensations remains largely unknown. Here, we investigated whether temporary modulation of the gravitational input by head-down tilt bed rest (HDBR)-a well-known Earth-based analog of microgravity-might significantly affect somatosensory perception of self- and externally generated stimuli.

Methods

In this study, 40 healthy participants were tested using short-term HDBR. Participants received a total of 40 non-painful self- and others generated electrical stimuli (20 self- and 20 other-generated stimuli) in an upright and HDBR position while blindfolded. After each stimulus, they were asked to rate the perceived intensity of the stimulation on a Likert scale.

Results

Somatosensory stimulations were perceived as significantly less intense during HDBR compared to upright position, regardless of the agent administering the stimulus. In addition, the magnitude of SA in upright position was negatively correlated with the participants' somatosensory threshold. Based on the direction of SA in the upright position, participants were divided in two subgroups. In the subgroup experiencing SA, the intensity rating of stimulations generated by others decreased significantly during HDBR, leading to the disappearance of the phenomenon of SA. In the second subgroup, on the other hand, reversed SA was not affected by HDBR.

Conclusion

Modulation of the gravitational input by HDBR produced underestimation of somatosensory stimuli. Furthermore, in participants experiencing SA, the reduction of vestibular inputs by HDBR led to the disappearance of the SA phenomenon. These findings provide new insights into the role of the gravitational input in somatosensory perception and have important implications for astronauts who are exposed to weightlessness during space missions.

Extensive Sensorimotor Training Predetermines Central Pain Changes During the Development of Prolonged Muscle Pain

Repetitive movements (RM) are a main risk factor for musculoskeletal pain, which is partly explained by the overloading of musculoskeletal structures. However, RM may also drive brain plasticity, leading to maladaptive changes in sensorimotor areas and altered pain processing. This study aimed to understand whether individuals performing extensive RM (musicians) exhibit altered brain processing to prolonged experimental muscle pain. Nineteen healthy musicians and 20 healthy nontrained controls attended 3 sessions (Day 1-Day 3-Day 8). In each session, event-related potentials (ERPs) to non-nociceptive superficial and nociceptive intraepidermal electrical stimulation, reaction times, electrical detection thresholds, and pressure pain thresholds (PPTs) were recorded. In all participants, prolonged muscle pain was induced by intramuscular injection of nerve growth factor (NGF) into the right first dorsal interosseous muscle at the end of Day1. Pain intensity was assessed on a numerical rating scale (NRS) and was lower in musicians compared to non-musicians (P < .007). Moreover, in musicians, the higher amount of weekly training was associated with lower NRS pain scores on Day 3 to Day 8 (P < .037). Compared with Day1, NGF reduced PPTs on Day 3 to Day 8 (P < .001) and non-nociceptive P200 and P300 ERP amplitudes on Day 8 (P < .044) in both groups. Musicians compared to controls showed secondary hyperalgesia to electrical stimulation on Day 3 to Day 8 (P < .004) and reduced nociceptive P200 ERP amplitudes on Day 8 (P < .005). Across participants, ERP components correlated with pain detection reaction times, sensitivity (PPTs and electrical detection thresholds), and severity (NRS), (all P < .043). These results show that repetitive sensorimotor training leads to brain changes in the processing of prolonged pain, biasing the cortical response to nociceptive inputs. PERSPECTIVE: Repetitive sensorimotor training may increase the responsiveness of nociceptive inputs during the development of prolonged muscle pain. These novel data highlight the role of repetitive sensorimotor practice as a source for interindividual variability in central pain processing.

Virtual occlusion effects on the perception of self-initiated visual stimuli

Virtual reality (VR) has established itself as a useful tool in the study of human perception in the laboratory. A recent study introduced a new approach to examine visual sensory attenuation (SA) effects in VR. Hand movements triggered the appearance of Gabor stimuli, which were either presented behind the participant's hand - not rendered in VR ("virtual occlusion") - or elsewhere on the display. Virtual occlusion led to a rightward shift of the psychometric curve, suggesting that self-generated hand movements reduced the perceived contrast of the stimulus. Since such attenuation effects might provide a window into the predictive processing of the sensory and cognitive apparatus, we sought to better understand the nature of the virtual occlusion effects. In our study, the presentation of test stimuli was either self-initiated, self-initiated with a variable delay, or triggered externally; the test stimuli were occluded or not. In conflict with our hypothesis, we found moderate to strong evidence for an absence of any horizontal shifts between the psychometric curves. However, virtual occlusion was associated with a decrease in the slope of the psychometric function. Our results suggest that virtual occlusion attenuated the relative perceptual sensitivity, so that participants had more difficulty discriminating contrast differences when the test stimulus was presented behind the hand. We tentatively conclude that, in the visual domain, the discriminability of stimulus intensity is modified by internal predictive cues (i.e., proprioception), possibly linked to shifts in covert spatial attention.

Physical but not virtual presence of others potentiates implicit and explicit learning

E-learning activities are becoming more and more common. Whilst it is well known that the physical presence of others motivates individuals to engage in perceptual and learning tasks, systematic investigations comparing the effects of physical and virtual co-presence of others on knowledge acquisition are still scarce. Here we investigate the effects of physical and virtual co-presence of others on explicit and implicit learning. In Experiment 1 (discovery sample), retrieval accuracy in a spatial memory task and EEG indexes (mismatch negativity-MMN) of implicit perceptual learning were recorded when participants were alone or in presence of another individual. In Experiment 2 (replicating sample), we added a "virtual" condition, where the same tasks were performed during a video-conference call. In both experiments, MMN was demonstrated to encode for perceptual learning as revealed by the significant correlation with Bayesian Surprise (a consolidated information-theoretic index of Bayesian learning). Furthermore, In Experiments 1 and 2 physical co-presence systematically ameliorated memorization performances and increased MMN indexes related to implicit learning. These positive effects were absent in the virtual condition, thus suggesting that only physical, but not virtual co-presence is effective in potentiating learning dynamics.

Sensory attenuation from action observation

A central claim of many embodied approaches to cognition is that understanding others' actions is achieved by covertly simulating the observed actions and their consequences in one's own motor system. If such a simulation occurs, it may be accomplished through forward models, a component of the motor system already known to perform simulations of actions and their consequences in order to support sensory-monitoring of one's own actions. Forward-model simulations cause an attenuation of sensory intensity, so if the simulations hypothesized by embodied cognition are indeed provided by forward models, then action observation should trigger this sensory attenuation. To test this hypothesis, the experiments reported here measured the perceived intensity of a touch sensation on the finger when participants observed an active touch (a finger reaching to touch a ball) vs. a passive touch (a ball rolling to touch an unmoving finger). The touch sensation was perceived as less intense during observation of active touch in comparison with observation of passive touch, providing evidence that forward models are indeed engaged during action observation. The strength of this sensory attenuation is compared and contrasted with a well-established sensory-amplification effect caused by visual attention. This sensory-amplification effect has not generally been considered in studies related to sensory attenuation in action observation, which may explain conflicting results reported in the field.

Electrophysiological correlates of action monitoring in brain-damaged patients: A systematic review

Action monitoring is crucial to the successful execution of an action and understanding the actions of others. It is often impaired due to brain lesions, in particular after stroke. This systematic review aims to map the literature on the neurophysiological correlates of action monitoring in patients with brain lesions. Eighteen studies were identified and divided into two groups: studies on monitoring of one's own actions and studies on monitoring of the actions of others. The first group included EEG studies on monitoring of self-performed erroneous and correct actions. Impaired error detection (decreased error-related negativity) was observed in patients with lesions in the performance-monitoring network, as compared to healthy controls. Less consistent results were shown for error positivity and behavioral error monitoring performance. The second group of studies on monitoring of others' actions reported decreased mu frequency suppression, impaired readiness potential in the affected hemisphere and decreased EEG indices of error observation (observed error positivity and theta power) in stroke patients. As a whole, these results indicate distinct patterns of impaired neurophysiological activity related to monitoring one's own versus others' actions in patients with brain lesions. EEG recordings of this dissociation in the same patients might be a useful index of motor recovery, and therefore, potentially also beneficial in rehabilitation protocols.

Non-motor cues do not generate the perception of self-agency: A critique of cue-integration

How does one know that (s)he is the causal agent of their motor actions? Earlier theories of sense of agency have attributed the capacity for perception of self-agency to the comparator process of the motor-control/action system. However, with the advent of the findings implying a role of non-motor cues (like affective states, beliefs, primed concepts, and social instructions or previews of actions) in the sense of agency literature, the perception of self-agency is hypothesized to be generated even by non-motor cues (based on their relative reliability or weighting estimate); and, this theory is come to be known as the cue-integration of sense of agency. However, the cue-integration theory motivates skepticism about whether it is falsifiable and whether it is plausible that non-motor cues that are sensorily unrelated to typical sensory processes of self-agency have the capacity to produce a perception of self-agency. To substantiate this skepticism, I critically analyze the experimental operationalizations of cue-integration-with the (classic) vicarious agency experiment as a case study-to show that (1) the participants in these experiments are ambiguous about their causal agency over motor actions, (2) thus, these participants resort to reports of self-agency as heuristic judgments (under ambiguity) rather than due to cue-integration per se, and (3) there might not have occurred cue-integration based self-agency reports if these experimental operationalizations had eliminated ambiguity about the causal agency. Thus, I conclude that the reports of self-agency (observed in typical non-motor cues based cue-integration experiments) are not instances of perceptual effect-that are hypothesized to be produced by non-motor cues-but are of heuristic judgment effect.

"Mine Works Better": Examining the Influence of Embodiment in Virtual Reality on the Sense of Agency During a Binary Motor Imagery Task With a Brain-Computer Interface

Motor imagery-based brain-computer interfaces (MI-BCI) have been proposed as a means for stroke rehabilitation, which combined with virtual reality allows for introducing game-based interactions into rehabilitation. However, the control of the MI-BCI may be difficult to obtain and users may face poor performance which frustrates them and potentially affects their motivation to use the technology. Decreases in motivation could be reduced by increasing the users' sense of agency over the system. The aim of this study was to understand whether embodiment (ownership) of a hand depicted in virtual reality can enhance the sense of agency to reduce frustration in an MI-BCI task. Twenty-two healthy participants participated in a within-subject study where their sense of agency was compared in two different embodiment experiences: 1) avatar hand (with body), or 2) abstract blocks. Both representations closed with a similar motion for spatial congruency and popped a balloon as a result. The hand/blocks were controlled through an online MI-BCI. Each condition consisted of 30 trials of MI-activation of the avatar hand/blocks. After each condition a questionnaire probed the participants' sense of agency, ownership, and frustration. Afterwards, a semi-structured interview was performed where the participants elaborated on their ratings. Both conditions supported similar levels of MI-BCI performance. A significant correlation between ownership and agency was observed (r = 0.47, p = 0.001). As intended, the avatar hand yielded much higher ownership than the blocks. When controlling for performance, ownership increased sense of agency. In conclusion, designers of BCI-based rehabilitation applications can draw on anthropomorphic avatars for the visual mapping of the trained limb to improve ownership. While not While not reducing frustration ownership can improve perceived agency given sufficient BCI performance. In future studies the findings should be validated in stroke patients since they may perceive agency and ownership differently than able-bodied users.

Multivariate Analysis of Evoked Responses during the Rubber Hand Illusion Suggests a Temporal Parcellation into Manipulation and Illusion-Specific Correlates

The neurophysiological processes reflecting body illusions such as the rubber hand remain debated. Previous studies investigating the neural responses evoked by the illusion-inducing stimulation have provided diverging reports as to when these responses reflect the illusory state of the artificial limb becoming embodied. One reason for these diverging reports may be that different studies contrasted different experimental conditions to isolate potential correlates of the illusion, but individual contrasts may reflect multiple facets of the adopted experimental paradigm and not just the illusory state. To resolve these controversies, we recorded EEG responses in human participants and combined multivariate (cross-)classification with multiple Illusion and non-Illusion conditions. These conditions were designed to probe for markers of the illusory state that generalize across the spatial arrangements of limbs or the specific nature of the control object (a rubber hand or participant’s real hand), hence which are independent of the precise experimental conditions used as contrast for the illusion. Our results reveal a parcellation of evoked responses into a temporal sequence of events. Around 125 and 275 ms following stimulus onset, the neurophysiological signals reliably differentiate the illusory state from non-Illusion epochs. These results consolidate previous work by demonstrating multiple neurophysiological correlates of the rubber hand illusion and illustrate how multivariate approaches can help pinpointing those that are independent of the precise experimental configuration used to induce the illusion.

Behavioural evidence of altered sensory attenuation in obesity

Body ownership (i.e., the conscious belief of owning a body) and sense of agency (i.e., being the agent of one's own movements) are part of a pre-reflective experience of bodily self, which grounds on low-level complex sensory-motor processes. Although previous literature had already investigated body ownership in obesity, sense of agency was never explored. Here, we exploited the sensory attenuation effect (i.e., an implicit marker of the sense of agency; SA effect) to investigate whether the sense of agency was altered in a sample of 18 individuals affected by obesity as compared with 18 healthy-weight individuals. In our experiment, participants were asked to rate the perceived intensity of self-generated and other-generated tactile stimuli. Healthy-weight individuals showed a significantly greater SA effect than participants affected by obesity. Indeed, while healthy-weight participants perceived self-generated stimuli as significantly less intense as compared to externally generated ones, this difference between stimuli was not reported by affected participants. Our results relative to the SA effect pinpointed an altered sense of agency in obesity. We discussed this finding within the motor control framework with reference to obesity. We encouraged future research to further explore such effect and its role in shaping the clinical features of obesity.

Sensory Attenuation in the Auditory Modality as a Window Into Predictive Processing

Self-generated auditory input is perceived less loudly than the same sounds generated externally. The existence of this phenomenon, called Sensory Attenuation (SA), has been studied for decades and is often explained by motor-based forward models. Recent developments in the research of SA, however, challenge these models. We review the current state of knowledge regarding theoretical implications about the significance of Sensory Attenuation and its role in human behavior and functioning. Focusing on behavioral and electrophysiological results in the auditory domain, we provide an overview of the characteristics and limitations of existing SA paradigms and highlight the problem of isolating SA from other predictive mechanisms. Finally, we explore different hypotheses attempting to explain heterogeneous empirical findings, and the impact of the Predictive Coding Framework in this research area.

Self-other distinction modulates the social softness illusion

The social softness illusion (i.e., the tendency to perceive another person's skin as softer than our own) is thought to promote the sharing of social-emotional experiences because of the rewarding properties of receiving and giving social affective touch. Here we investigated whether the ability to distinguish someone else's body from our own modulates the social softness illusion. In particular, we tested whether the spatial perspective taken by the participants and seeing or not the touched arms could alter this illusion. Pairs of female participants were assigned the roles of either the giver (i.e., delivering the touches) or the receiver (i.e., being touched). We manipulated the location of the touch (palm or forearm), the spatial perspective of the receiver's body with respect to the giver's body (egocentric or allocentric perspective), and the vision of the touched body part (the giver could either see both her own and the receiver's body part, or she was blindfolded). Consistently with previous findings, the skin of another person was perceived as softer than the own one. Additionally, the illusion was present for both the forearm and the palm, and it was stronger in allocentric compared to the egocentric perspective (i.e., when the self-other distinction was clearer). These findings show that the mechanisms underpinning the ability to represent another person's body as distinct from our own modulates the social softness illusion, and thus support the role of the social softness illusion in fostering social relationships.

Staying in touch with our bodies: Stronger sense of ownership during self- compared to other touch despite temporal mismatches

Self-touch is considered important for bodily self-consciousness and self-other distinction and has been reported to improve clinical symptoms of disembodiment. To investigate the link between self-touch and disembodiment in healthy participants, we studied the effect of self-touch versus touch produced by another person (other-touch) on experimentally induced disembodiment. In a mixed reality paradigm, across two experiments, participants could see their own body and surroundings with a controllable visual delay and either stroked their own hand with a paintbrush or were stroked with it by the experimenter. Experiment 1 first assessed the sensitivity to temporal multimodal mismatches and delay-induced changes in the sense of body ownership in three conditions, namely self-touch, other-touch and hidden-self-touch (visually occluding the touching hand). In a second block, we compared phenomenological and physiological (threat response) measures of disembodiment between the self-touch and other-touch conditions. Experiment 2 roughly replicated the first block of Experiment 1 but included a condition in which participants performed the self-touch gesture without touching their hand. Such experiment attempted to control for the potential role of efferent signals. Our results show that increasing visual delay generally enhances the feeling of disembodiment, yet the decrease of body ownership is less pronounced during self-touch. For sensitivity to delay between conditions, however, diverging findings are discussed. This study provides evidence for the importance of self-touch in sustaining a healthy sense of body in the context of disembodiment.

Owning a virtual body entails owning the value of its actions in a detection-of-deception procedure

The feeling of owning one's body underlies human self-awareness. Body-ownership illusions allow temporarily modulating body ownership, which has observable effects on the behavior and cognitive processes. However, the extent of those effects is unclear. Here, we investigated whether illusory ownership of a virtual body extended to ownership of the value/meaning of its actions. A variation of detection-of-deception procedure (Concealed Information Test) was performed by an embodied virtual avatar (first-person perspective, 1PP), or a non-embodied one (third-person perspective, 3PP), while the skin conductance responses (SCRs) were recorded from passively observing participants. Target stimuli (i.e., concealed information) evoked significantly larger SCRs than the neutral ones only when the avatar was embodied (in 1PP). Such pattern of SCR differences corresponds to that observed when participants perform the task themselves, thus suggesting that the sole experience of owning a virtual body can trigger physiological responses related to the subjective significance of the body's actions.