The vestibular system is increasingly recognized for its role in higher-order cognitive functions, including bodily self-representation. However, its involvement in self-face representation remains unexplored. In the present study, we investigated whether artificial modulation of the vestibular system via Galvanic Vestibular Stimulation (GVS) can influence the representation of one's own and others' faces. Fifty healthy individuals performed two face-recognition tasks (explicit and implicit) while receiving sinusoidal bipolar GVS or Sham stimulation. During tasks, images of faces belonging to Self (S), Familiar-Others (FO), and Stranger-Others (SO) were presented. Participants were randomly assigned to one of two groups according to GVS polarity: left-anodal/right-cathodal (L-GVS) or right-anodal/left-cathodal (R-GVS). In both tasks, a robust Self Advantage (SA) emerged, with faster reaction times (RTs) for self-faces than familiar-other and stranger-other faces. Interestingly, an interaction between Identity, Stimulation, and GVS polarity was observed only in the explicit task. Specifically, R-GVS slowed recognition of self-faces relative to Sham. In contrast, L-GVS slowed recognition of stranger-other faces while facilitating recognition of familiar-other faces. Notably, under L-GVS RTs for self-faces no longer differ from those of familiar-other faces, effectively shifting the SA toward a more general familiarity advantage. Galvanic Vestibular Stimulation modulates face recognition and self-other distinction, in a polarity-dependent manner, providing causal evidence for vestibular involvement in self-face representation and identity processing.

Tactile experience can be non-veridical, i.e., not related to the actual stimulation of one's body. Recently, using a mirror box procedure in healthy subjects, we found that during bilateral asymmetrical touches, the vision of the right-hand being stimulated, reflected in the mirror, elicited on the real left-hand the feeling of being touched in the same position as the right-hand. Because these errors resemble synchiria, we called these false feelings ‘synchiric errors' (SEs). Here, we investigated both the role of top-down feeling of body ownership (BO) over the mirrored hand—using explicit (BO questionnaires) and implicit (Electrodermal Activity) measures of BO- and bottom-up visual processing—by manipulating the presence/absence of visual feedback—in generating SEs during the Tactile Quadrant Stimulation Test (TQS). In TQS, subjects had to indicate the position of a tactile stimulus, applied in asymmetrical quadrants on the dorsum of the two hands, under three conditions: Baseline (no vision), Mirror Condition Vision (MCV; full visual feedback), and Mirror Condition Blind (MCB; visual feedback occluded). We tested 35 healthy individuals. First, measures of BO indicated that most subjects felt the right reflected hand as their own left hand. Moreover, we found a significant presence of SEs in the MCV. Crucially, SEs were significantly higher for the left hand in MCV compared to Baseline and MCB, confirming the critical role of vision in inducing the non-veridical tactile experience. Moreover, the absence of a correlation between SEs and body ownership measures, and a double dissociation between them, indicate that SEs are driven by vision and not by an alteration of BO.

The vestibular system, one of the earliest sensory systems in vertebrates, is crucial for encoding head and trunk movements. Research in stroke patients with spatial attention deficits and studies manipulating vestibular signals in healthy individuals suggest that the vestibular system is also involved in orienting visuospatial attention. However, the specific interactions between vestibular and attentional systems and the consequences of vestibular pathologies on attentional functions are still poorly understood. In this study, we investigated the impact of vestibular disorders on the orienting of automatic (exogenous) and voluntary (endogenous) attention in patients (N = 16) with acute (AVS), episodic (EVS) or chronic (CVS) vestibular syndromes, who were compared to a control group (N = 16) of age-matched healthy participants. The two groups were assessed using endogenous and exogenous versions of the Posner cueing task. Cognitive functioning and anxiety were evaluated with the Montreal Cognitive Assessment (MoCA) and the Beck Anxiety Inventory (BAI), respectively. Vestibular patients exhibited selective impairments in maintaining voluntary attention in the endogenous task, particularly those with EVS and CVS, compared to the controls. In addition, vestibular alterations influenced the automatic right-lateralized attention system, as evidenced by a reduced rightward attentional bias in the exogenous task in vestibular patients. Anxiety, aging, or the overall cognitive function did not influence the observed attention deficits. The evidence that vestibular disorders differentially affect voluntary and automatic orienting of visuospatial attention has relevant implications for the assessment and treatment of patients with vestibular disorders, the rehabilitation of stroke patients with spatial attention deficits, and space research.

Background Action recognition is a fundamental aspect of human interaction. This process is mediated by the activation of shared sensorimotor representations during action execution and observation. Although complex movements involving balance or head and trunk rotations require vestibular signals for effective execution, their role in the recognition of others' actions is still unknown. Objective To investigate the causal involvement of the vestibular system in the discrimination of actions performed by others and whether this is influenced by motor familiarity. Methods In a single-blind design involving 25 healthy participants, Galvanic Vestibular Stimulation (GVS) was administered during an Action Discrimination Task (ADT), in which videos of actions categorized as vestibular/non-vestibular and familiar/unfamiliar were presented. Following each video, participants were required to identify the climax of the previously viewed action between two image options, using a two-alternative forced choice paradigm. The ADT was performed in active and sham GVS conditions, with left or right anodal montages. Response Times (RTs), Accuracy, and subjective motor familiarity were recorded for each action category. Results In sham GVS condition, an overall familiarity effect was observed, where RTs for familiar actions were faster than RTs for unfamiliar ones, regardless of vestibular engagement (p<.001; ηp2=.80). Conversely, under active GVS, a selective interference of the identification of vestibular familiar actions was observed compared to sham. Specifically, GVS prolonged RTs for recognizing familiar vestibular actions (p=.004, d=.59) while concurrently enhancing visual sensitivity (d’) for the same actions (p=.03, r=.21). Conclusion These findings demonstrate the contribution of the vestibular system to action recognition. GVS disrupted the sensorimotor representation of vestibular actions and led to increased reliance on an alternative processing system focused on visual analysis of limb positions. This dissociation provides valuable insights for future investigations into the complex relationship between vestibular signals and cognitive processes involved in action identification, essential for developing innovative GVS interventions, particularly for individuals with sensorimotor or vestibular disorders.

Somatosensory tactile experience is a key aspect of our interaction with the environment. It is involved in object manipulation, in the planning and control of actions and, in its affective components, in the relationships with other individuals. It is also a foundational component of body awareness. An intriguing aspect of sensory perception in general and tactile perception in particular is the way in which stimulation comes to consciousness. Indeed, although being aware of something seems a rather self-evident and monolithic aspect of our mental states, sensory awareness may be in fact modulated by many different processes that impact on the mere stimulation of the skin, including the way in which we perceive our bodies as belonging to us. In this review, we first took into consideration the pathological conditions of absence of phenomenal experience of touch, in the presence of implicit processing, as initial models for understanding the neural bases of conscious tactile experience. Subsequently, we discussed cases of tactile illusions both in normal subjects and in brain-damaged patients which help to understand which high order processes impact tactile awareness. Finally, we discussed the observations reported in the review in light of some influential models of touch and body representation.

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.

Previous studies on the mechanisms underlying willed actions reported that the premotor cortex may be involved in the construction of motor awareness. However, its exact role is still under investigation. Here, we investigated the role of the dorsal premotor cortex (PMd) in motor awareness by modulating its activity applying inhibitory rTMS to PMd, before a specific motor awareness task (under three conditions: without stimulation, after rTMS and after Sham stimulation). During the task, subjects had to trace straight lines to a given target, receiving visual feedback of the line trajectories on a computer screen. Crucially, in most trials, the trajectories on the screen were deviated, and to produce straight lines, subjects had to correct their movements towards the opposite direction. After each trial, participants were asked to judge whether the line seen on the computer screen corresponded to the line actually drawn. Results show that participants in the No Stimulation condition did not recognize the perturbation until 14 degrees of deviation. Importantly, active, but not Sham, rTMS significantly modulated motor awareness, decreasing the amplitude of the angle at which participants became aware of the trajectory correction. These results suggest that PMd plays a crucial role in action self-monitoring.

This chapter examines some philosophical and psychological quandaries about mental images. Then it points to some arguments supporting the hypothesis of a functional similarity between perceptual and representational processes and shortly reviews suggestions for a neural implementation of structures in which confluence of these processes might take place. The chapter focuses on clinical observations which might constitute a critical neurological contribution to the understanding of mental images. Much of the controversy about the pictoriality of mental representations has apparently developed without a clear understanding of premises such as those formulated above or with the deliberate purpose of preventively undermining any imaginal notion of cognitive processes. Hedging idioms on both fronts leads to the absurd consequence of presenting the two antagonistic forms of representation as “non-propositional” and “nonimaginal” respectively. Anyway, non-imaginal, viz. “sentence-analog,” representations are usually said or implied to inhabit the sanctum of cognition.

The ability to discriminate between one's own and others' body parts can be lost after brain damage, as in patients who misidentify someone else's hand as their own (pathological embodiment). Surprisingly, these patients do not use visual information to discriminate between the own and the alien hand. We asked whether this impaired visual discrimination emerges only in the ecological evaluation when the pathological embodiment is triggered by the physical alien hand (the examiner's one) or whether it emerges also when hand images are displayed on a screen. Forty right brain‐damaged patients, with (E+ = 20) and without (E− = 20) pathological embodiment, and 24 healthy controls underwent two tasks in which stimuli depicting self and other hands was adopted. In the Implicit task, where participants judged which of two images matched a central target, the self‐advantage (better performance with Self than Other stimuli) selectively emerges in controls, but not in patients. Moreover, E+ patients show a significantly lower performance with respect to both controls and E− patients, whereas E− patients were comparable to controls. In the Explicit task, where participants judged which stimuli belonged to themselves, both E− and E+ patients performed worst when compared to controls, but only E+ patients hyper‐attributed others' hand to themselves (i.e., false alarms) as observed during the ecological evaluation. The VLSM revealed that SLF damage was significantly associated with the tendency of committing false alarm errors. We demonstrate that, in E+ patients, the ability to visually recognize the own body is lost, at both implicit and explicit level.

Years ago, it was demonstrated (e.g., Rizzolatti et al. in Handbook of neuropsychology, Elsevier Science, Amsterdam, 2000) that the brain does not encode the space around us in a homogeneous way, but through neural circuits that map the space relative to the distance that objects of interest have from the body. In monkeys, relatively discrete neural systems, characterized by neurons with specific neurophysiological responses, seem to be dedicated either to represent the space that can be reached by the hand (near/peripersonal space) or to the distant space (far/extrapersonal space). It was also shown that the encoding of spaces has dynamic aspects because they can be remapped by the use of tools that trigger different actions (e.g., Iriki et al. 1998). In this latter case, the effect of the tool depends on the modulation of personal space, that is the space of our body. In this paper, I will review and discuss selected research, which demonstrated that also in humans: 1 spaces are encoded in a dynamic way; 2 encoding can be modulated by the use of tool that the system comes to consider as parts of the own body; 3 body representations are not fixed, but they are fragile and subject to change to the point that we can incorporate not only the tools necessary for action, but even limbs belonging to other people. What embodiment of tools and of alien limb tell us about body representations is then briefly discussed.

In the present article, we investigated the possibility of inducing phantom tactile sensations in healthy individuals similar to those that we observed in patients after stroke. On the basis of previous research, we assumed that manipulating visual feedbacks may guide and influence, under certain conditions, the phenomenal experience of touch. To this aim, we used the Tactile Quadrant Stimulation (TQS) test in which subjects, in the crucial condition, must indicate whether and where they perceive a double tactile stimulation applied simultaneously in different quadrants of the two hands (asymmetrical Double Simultaneous Stimulation trial, Asym-DSS). The task was performed with the left-hand out of sight and the right-hand reflected in a mirror so that the right-hand reflected in the mirror looks like the own left-hand. We found that in the Asym-DSS trial, the vision of the right-hand reflected in the mirror and stimulated by a tactile stimulus elicited on the left-hand the sensation of having been touched in the same quadrant as the right-hand. In other words, we found in healthy subjects the same phantom touch effect that we previously found in patients. We interpreted these results as modulation of tactile representation by bottom-up (multisensory integration of stimuli coming from the right real and the right reflected hand) and possibly top-down (body ownership distortion) processing triggered by our experimental setup, unveiling bilateral representation of touch.

Orienting attention in the space around us is a fundamental prerequisite for willed actions. On Earth, at 1 g, orienting attention requires the integration of vestibular signals and vision, although the specific vestibular contribution to voluntary and automatic components of visuospatial attention remains largely unknown. Here, we show that unweighting of the otolith organ in zero gravity during parabolic flight, selectively enhances stimulus-driven capture of automatic visuospatial attention, while weakening voluntary maintenance of covert attention. These findings, besides advancing our comprehension of the basic influence of the vestibular function on voluntary and automatic components of visuospatial attention, may have operational implications for the identification of effective countermeasures to be applied in forthcoming human deep space exploration and habitation, and on Earth, for patients’ rehabilitation.

Researchers have widely studied peripersonal space (the space within reach) in the last 20 years with a focus on its plasticity following the use of tools and, more recently, social interactions. Ensemble music is a sophisticated joint action that is typically explored in its temporal rather than spatial dimensions, even within embodied approaches. We, therefore, devised a new paradigm in which two musicians could perform a jazz standard either in a cooperative (correct harmony) or uncooperative (incorrect harmony) condition, under the hypothesis that their peripersonal spaces are modulated by the interaction. We exploited a well-established audio-tactile integration task as a proxy for such a space. After the performances, we measured reaction times to tactile stimuli on the subjects' right hand and auditory stimuli delivered at two different distances, (next to the subject and next to the partner). Considering previous literature's evidence that integration of two different stimuli (e.g. a tactile and an auditory stimulus) is faster in near space compared to far space, we predicted that a cooperative interaction would have extended the peripersonal space of the musicians towards their partner, facilitating reaction times to bimodal stimuli in both spaces. Surprisingly, we obtained complementary results in terms of an increase of reaction times to tactile-auditory near stimuli, but only following the uncooperative condition. We interpret this finding as a suppression of the subject's peripersonal space or as a withdrawal from the uncooperative partner. Subjective reports and correlations between these reports and reaction times comply with that interpretation. Finally, we determined an overall better multisensory integration competence in musicians compared to non-musicians tested in the same task.

Recently, a monothematic delusion of body ownership due to brain damage (i.e., the embodiment of someone else's body part within the patient's sensorimotor system) has been extensively investigated. Here we aimed at defining in-depth the clinical features and the neural correlates of the delusion. Ninety-six stroke patients in a sub-acute or chronic phase of the illness were assessed with a full ad-hoc protocol to evaluate the embodiment of an alien arm under different conditions. A sub-group of seventy-five hemiplegic patients was also evaluated for the embodiment of the movements of the alien arm. Fifty-five patients were studied to identify the neural bases of the delusion by means of voxel-based lesion-symptom mapping approach. Our results show that, in forty percent of the whole sample, simply viewing the alien arm triggered the delusion, but only if it was a real human arm and that was seen from a 1st person perspective in an anatomically-correct position. In the hemiplegic sub-group, the presence of the embodiment of the alien arm was always accompanied by the embodiment of its passive and active movements. Furthermore, the delusion was significantly associated to primary proprioceptive deficits and to damages of the corona radiata and the superior longitudinal fasciculus. To conclude, we show that the pathological embodiment of an alien arm is well-characterized by recurrent and specific features and might be explained as a disconnection deficit, mainly involving white matter tracts. The proposed exhaustive protocol can be successfully employed to assess stroke-induced disorders of body awareness, unveiling even their more undetectable or covert clinical forms.

In the last decade, a considerable amount of studies investigated different neuropsychological syndromes related to the disorder of body awareness. In this paper, we shall review neuropsychological evidence of pathological embodiment/disembodiment conditions with the aim of describing the major common features, and the complementary characteristics, that may suggest the structure and function of a shared body representation. In particular, we shall first discuss experimental studies conducted on a bizarre disorder of body ownership we recently described [i.e., brain-damaged patients claiming that another person's hand belongs to them (Pathological Embodiment, PE)]. Then complementary syndromes, with an apparent opposite attitude with respect to the PE (i.e., somatoparaphrenia and xenomelia, which is part of the Body Integrity Identity Disorders) will be also considered. We shall discuss the behavioural similarities/differences between these complementary disturbances also referring to already existing conceptual knowledge and proposals about body representation.

A challenge for neuroscience is to understand the conscious and unconscious processes underlying construction of willed actions. We investigated the neural substrate of human motor awareness during awake brain surgery. In a first experiment, awake patients performed a voluntary hand motor task and verbally monitored their real-time performance, while different brain areas were transiently impaired by direct electrical stimulation (DES). In a second experiment, awake patients retrospectively reported their motor performance after DES. Based on anatomo-clinical evidence from motor awareness disorders following brain damage, the premotor cortex (PMC) was selected as a target area and the primary somatosensory cortex (S1) as a control area. In both experiments, DES on both PMC and S1 interrupted movement execution, but only DES on PMC dramatically altered the patients' motor awareness, making them unconscious of the motor arrest. These findings endorse PMC as a crucial hub in the anatomo-functional network of human motor awareness.

Transcranial Magnetic Stimulation (TMS) may affect attentional processing when applied to the right posterior parietal cortex (PPC) of healthy participants in line with neuropsychological and neuroimaging evidence on the neural bases of this cognitive function. Specifically, the application of TMS to right PPC induces a rightward attentional bias on line length estimation in healthy participants (i.e., neglect-like bias), mimicking the rightward bias shown by patients with unilateral spatial neglect after damage of the right PPC. With the present study, we investigated whether right PPC might play a crucial role in attentional processing of illusory depth perception, given the evidence that a rightward bias may be observed in patients with neglect during perception of the Necker Cube (NC). To this end, we investigated the effects of low-frequency rTMS applied to the right or left PPC on attentional disambiguation of the NC in two groups of healthy participants. To control for the effectiveness of TMS on visuospatial attention, rTMS effects were also assessed on a frequently used line length estimation (i.e., the Landmark Task or LT). Both groups also received sham stimulation. RTMS of the right or left PPC did not affect NC perception. On the other hand, rTMS of the right PPC (but not left PPC) induces neglect-like bias on the LT, in line with previous studies. These findings confirm that right PPC is involved in deployment of spatial attention on line length estimation. Interestingly, they suggest that this brain region does not critically contribute to deployment of visuospatial attention during attentional disambiguation of the Necker Cube. Future investigations, targeting different areas of fronto-parietal circuits, are necessary to further explore the neuro-functional bases of attentional contribution to illusory depth perception.

Despite the fact that any successful achievement of willed actions necessarily entails the sense of body ownership (the feeling of owning the moving body parts), it is still unclear how this happens. To address this issue at both behavioral and neural levels, we capitalized on sensory attenuation (SA) phenomenon (a self-generated stimulus is perceived as less intense than an identical externally generated stimulus). We compared the intensity of somatosensory stimuli produced by one's own intended movements and by movements of an embodied fake hand. Then, we investigated if in these two conditions SA was equally affected by interfering with the activity of the supplementary motor area (SMA; known to be related to motor intention and SA) using single-pulse transcranial magnetic stimulation. We showed that ownership of the fake hand triggered attenuation of somatosensory stimuli generated by its movements that were comparable to the attenuation of self-generated stimuli. Furthermore, disrupting the SMA eliminated the SA effect regardless of whether it was triggered by actual participant's movements or by illusory ownership. Our findings suggest that SA triggered by body ownership relies, at least in part, on the activation of the same brain structures as SA triggered by motor-related signals.