Body ownership and kinaesthetic illusions: Dissociated bodily experiences for distinct levels of body consciousness?

Seeing an embodied humanoid avatar move its arms can induce in the observer the illusion that its own (static) arms are moving accordingly, the kinematic signals emanating from this avatar thus being considered like those from the biological body. Here, we investigated the causal relationship between these kinaesthetic illusions and the illusion of body ownership, manipulated through visuomotor synchronisation. The results of two experiments revealed that the sense of body ownership over an avatar seen from a first-person perspective was intimately linked to visuomotor synchrony. This was not the case for kinaesthetic illusions indicating that when superimposed on the biological body, the avatar is inevitably treated at the sensorimotor level as one's own body, whether consciously considered as such or not. The question of whether these two bodily experiences (body ownership and kinaesthetic illusion) are underpinned by distinct representations, the body image, and the body schema, is discussed.

The role of identity priming on the (unconscious) bodily self-attribution.

It has been recently demonstrated that hand stimuli presented in a first-, with respect to a third-, person perspective were prioritized before awareness independently from their identity (i.e., self, or other). This pattern would represent an unconscious advantage for self-related bodily stimuli rooted in spatial perspective. To deeper investigate the role of identity, we employed a breaking-Continuous Flash Suppression paradigm in which a self- or other-hand presented in first- or third-person perspective was displayed after a conscious identity-related prime (i.e., self or other face). We replicated the unconscious advantage of the first-person perspective but, crucially, we reported that within the first-person perspective, other-hand stimuli preceded by other-face priming slowed down the conscious access with respect to the other conditions. These findings demonstrate that a top-down conscious identity context modulates the unconscious self-attribution of bodily stimuli. Within a predictive processing framework, we suggest that, by adding ambiguous information, the prime forces a prediction update that slows conscious access.

Bodily self-recognition in patients with pathological embodiment.

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.

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.

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

Sensory attenuation (i.e., the phenomenon whereby self-produced sensations are perceived as less intense compared to externally occurring ones) is among the neurocognitive processes that help distinguishing ourselves from others. It is thought to be rooted in the motor system (e.g., related to motor intention and prediction), while the role of body awareness, which necessarily accompanies any voluntary movement, in this phenomenon is largely unknown. To fill this gap, here we compared the perceived intensity, somatosensory evoked potentials, and alpha-band desynchronization for self-generated, other-generated, and embodied-fake-hand-generated somatosensory stimuli. We showed that sensory attenuation triggered by the own hand and by the embodied fake hand had the same behavioral and neurophysiological signatures (reduced subjective intensity, reduced of N140 and P200 SEP components and post-stimulus alpha-band desynchronization). Therefore, signals subserving body ownership influenced attenuation of somatosensory stimuli, possibly in a postdictive manner. This indicates that body ownership is crucial for distinguishing the source of the perceived sensations.

Using Body Ownership to Modulate the Motor System in Stroke Patients.

Recent findings suggest that body ownership can activate the motor system in the absence of movement execution. Here, we investigated whether such a process promotes motor recovery in stroke patients. A group of patients with left-hemisphere damage (N = 12) and chronic motor deficits completed an immersive virtual reality training (three sessions of 15 min each week for 11 weeks). Patients sat still and either experienced (first-person perspective) or did not experience (third-person perspective) illusory ownership over the body of a standing virtual avatar. After the training, in which the avatar walked around a virtual environment, only patients who experienced the illusion improved gait and balance. We argue that representing the virtual body as their own allowed patients to access motor functioning and promoted motor recovery. This procedure might be integrated with rehabilitative approaches centered on motor execution. These findings also have an impact on the knowledge of the motor system in general.

Musical expertise affects the sense of agency: Intentional binding in expert pianists.

Intentional Binding (IB), a subjective compression of the time interval between a voluntary action and its consequence, is an implicit measure of the sense of agency (the feeling of controlling one's own actions and their outcomes). The sense of agency is influenced by experience, e.g. learning, development, or learned contingency. The present study aimed at analyzing expertise - an expert competence acquired through experience alone - as a possible means to affect the sense of agency. We compared performance of expert pianists and non-musicians within the IB paradigm with two types of sensory outcome - a piano note and an electronic sound. Pianists showed significantly greater outcome binding and composite binding for both types of stimuli. Therefore, musical expertise might influence the sense of agency, possibly due to continuous exposure to action-outcome associations during the musical training. Additionally, such effect might extend beyond the specific expertise to the other types of auditory outcomes.

Do movements contribute to sense of body ownership? Rubber hand illusion in expert pianists.

Currently, it is still debated whether, how and to what extent movements contribute to the sense of body ownership (i.e., the feeling that one's body belongs to oneself). To answer this question, here we examined if a prolonged increase of the amount of movements affects body ownership. Specifically, we administered the rubber hand illusion paradigm within a natural condition of long-term motor practice, namely in expert pianists. We compared the illusory effects of both static (visuotactile stimulation) and dynamic (active/passive movements) versions of that paradigm in a group of expert pianists and a group of non-musicians. The illusion was measured behaviorally (proprioceptive drift) and subjectively (questionnaire). Our results showed that pianists were significantly less susceptible to any type of the illusion, compared to the non-musicians. Moreover, they did not experience the illusion in general (presenting neither the proprioceptive drift, nor the subjective feeling of ownership). These findings suggest that the increased amount of motor-related afferent and efferent signals does affect the construction and the coherence of body ownership, thus showing the role of movements in this process.

On my right or on your left? Spontaneous spatial perspective taking in blind people.

Spatial perspective taking is a human ability that permits to assume another person's spatial viewpoint. Data show that spatial perspective taking might arise even spontaneously by the mere presence of another person in the environment. We investigated whether this phenomenon is observable also in blind people. Blind and blindfolded sighted participants explored a tridimensional tactile map and memorized the localization of different landmarks. Then, after the presentation of sounds coming from three landmarks-positioned on the right, on the left, and in front-participants had to indicate the reciprocal position of the two lateral landmarks. Results showed that when the sound coming from the frontal landmark suggested the presence of a speaking (voice) or moving person (footsteps), several blind and sighted people adopted this person's perspective. These findings suggest that auditory stimuli can trigger spontaneous spatial perspective taking in sighted as well as in blind people.

"As long as that is my hand, that willed action is mine": Timing of agency triggered by body ownership.

We investigated whether agency triggered by body ownership shares similar temporal constraints with agency induced by actual movements. We compared agency over the movements of the own hand, a fake hand and an embodied fake hand when they pressed a button delivering a stimulus to the participant's body after 500, 1000 or 2000 ms. In the first two delays, the movement of the embodied fake hand was misattributed to the participant's own will and the stimulus intensity was attenuated, as it happened when the own hand delivered the stimulus. With the longest delay, the movement of the embodied fake hand was neither misattributed to the participant's will nor the stimulus intensity was attenuated, as it happened when the fake non-embodied hand delivered the stimulus. By showing that illusory and veridical agency arise under similar temporal constraints, we further demonstrated that body ownership per se acts upon agency attribution.

Sensing the body, representing the body: Evidence from a neurologically based delusion of body ownership.

Humans experience their own body as unitary and monolithic in nature. However, recent findings in cognitive neuroscience seem to suggest that body awareness has a complex and multifaceted structure that can be dissociated in several subcomponents, possibly underpinned by different brain circuits. In the present paper, we focus on a recently reported neuropsychological disorder of body ownership in which patients misattribute to themselves someone else's arm and its movements. As first, we briefly review the clinical and functional features of this disorder. Secondly, we attempt to explain the nature of the delusion and to gain new hints regarding the mechanisms subserving the construction and the maintenance of the sense of body ownership in the intact brain functioning.

Temporal perception in joint action: This is MY action.

Here we investigated the temporal perception of self- and other-generated actions during sequential joint actions. Participants judged the perceived time of two events, the first triggered by the participant and the second by another agent, during a cooperative or competitive interaction, or by an unspecified mechanical cause. Results showed that participants perceived self-generated events as shifted earlier in time (anticipation temporal judgment bias) and non-self-generated events as shifted later in time (repulsion temporal judgment bias). This latter effect was observed independently from the kind of cause (i.e., agentive or mechanical) or interaction (i.e., cooperative or competitive). We suggest that this might represent a mental process which allows discriminating events that cannot plausibly be linked to one's own action. When an event immediately follows a self-generated one, temporal judgment biases operate as self-serving biases in order to separate self-generated events from events of another physical causality.

The Role of Perspective in Mental Time Travel.

Recent years have seen accumulating evidence for the proposition that people process time by mapping it onto a linear spatial representation and automatically "project" themselves on an imagined mental time line. Here, we ask whether people can adopt the temporal perspective of another person when travelling through time. To elucidate similarities and differences between time travelling from one's own perspective or from the perspective of another person, we asked participants to mentally project themselves or someone else (i.e., a coexperimenter) to different time points. Three basic properties of mental time travel were manipulated: temporal location (i.e., where in time the travel originates: past, present, and future), motion direction (either backwards or forwards), and temporal duration (i.e., the distance to travel: one, three, or five years). We found that time travels originating in the present lasted longer in the self- than in the other-perspective. Moreover, for self-perspective, but not for other-perspective, time was differently scaled depending on where in time the travel originated. In contrast, when considering the direction and the duration of time travelling, no dissimilarities between the self- and the other-perspective emerged. These results suggest that self- and other-projection, despite some differences, share important similarities in structure.

Drawing lines while imagining circles: Neural basis of the bimanual coupling effect during motor execution and motor imagery.

When people simultaneously draw lines with one hand and circles with the other hand, both trajectories tend to assume an oval shape, showing that hand motor programs interact (the so-called "bimanual coupling effect"). The aim of the present study was to investigate how motor parameters (drawing trajectories) and the related brain activity vary during bimanual movements both in real execution and in motor imagery tasks. In the 'Real' modality, subjects performed right hand movements (lines) and, simultaneously, Congruent (lines) or Non-congruent (circles) left hand movements. In the 'Imagery' modality, subjects performed only right hand movements (lines) and, simultaneously, imagined Congruent (lines) or Non-congruent (circles) left hand movements. Behavioral results showed a similar interference of both the real and the imagined circles on the actually executed lines, suggesting that the coupling effect also pertains to motor imagery. Neuroimaging results showed that a prefrontal-parietal network, mostly involving the pre-Supplementary Motor Area (pre-SMA) and the posterior parietal cortex (PPC), was significantly more active in Non-congruent than in Congruent conditions, irrespective of task (Real or Imagery). The data also confirmed specific roles of the right superior parietal lobe (SPL) in mediating spatial interference, and of the left PPC in motor imagery. Collectively, these findings suggest that real and imagined Non-congruent movements activate common circuits related to the intentional and predictive operation generating bimanual coupling, in which the pre-SMA and the PPC play a crucial role.

Effects of hunger and calorie content on visual awareness of food stimuli.

Calorie content and hunger are two fundamental cues acting upon the processing of visually presented food items. However, whether and to which extent they affect visual awareness is still an open question. Here, high- and low-calorie food images administered to hungry or satiated participants were confronted in a breaking-Continuous Flash Suppression paradigm (Experiment 1), measuring the time required to access to visual awareness, and in a Binocular Rivalry paradigm (Experiment 2), quantifying the dominance time in visual awareness. Experiment 1 showed that high-calorie food accessed faster visual awareness, but mostly in satiated participants. Experiment 2 indicated that high-calorie food dominated longer visual awareness, regardless the degree of hunger. We argued that the unconscious advantage (Experiment 1) would represent a default state of the visual system towards highest-energy nutrients, yet the advantage is lost in hunger so to be tuned towards an increased need for any nutritional category. On the other hand, the conscious advantage of high-calorie food (Experiment 2) would represent a conscious perceptual and attentional bias towards highest energy-dense food useful for the actual detection of these stimuli in the environment.

Dissociations and similarities in motor intention and motor awareness: the case of anosognosia for hemiplegia and motor neglect.

OBJECTIVES: To confront motor awareness in anosognosia for hemiplegia (AHP), where paralyzed patients deny their motor impairment, and in motor neglect (MN), where non-paralyzed patients behave as if they were paretic. METHODS: Eight right-brain-damaged-patients, 4 hemiplegic (2 with and 2 without AHP) and 4 non-hemiplegic (2 with only perceptual-neglect and 2 with also MN) were evaluated with a bimanual motor battery, before and after examiner's reinforcement to use the contralesional limb. The requested bimanual movements could be either symmetric or asymmetric, either intransitive or transitive (with/without objects). We compared the examiner's evaluation of patients' performance with the patients' self-evaluation of their own motor capability (explicit knowledge). We also evaluated the presence/absence of compensatory unimanual strategies that, if present, suggests implicit knowledge of the motor deficit. RESULTS: We found significant differences between conditions only in MN patients, whose performance was better after the examiner's reinforcement than before it, during symmetric than asymmetric movements and during intransitive than transitive movements. As for motor awareness, we found a lack of explicit and implicit knowledge in both AHP and MN patients. CONCLUSION: Although different in terms of motor intention and motor planning, AHP and MN are both characterised by anosognosia for the motor impairment.

'Moving' a paralysed hand: bimanual coupling effect in patients with anosognosia for hemiplegia.

Selective neurological impairments can shed light on different aspects of motor cognition. Brain-damaged patients with anosognosia for hemiplegia deny their motor deficit and believe they can still move the paralysed limb. Here we study, for the first time, if the anomalous subjective experience that their affected hand can still move, may have objective consequences that constrain movement execution with the opposite, intact hand. Using a bimanual motor task, in which anosognosic patients were asked to simultaneously trace out lines with their unaffected hand and circles with their paralysed hand, we found that the trajectories of the intact hand were influenced by the requested movement of the paralysed hand, with the intact hand tending to assume an oval trajectory (bimanual coupling effect). This effect was comparable to that of a group of healthy subjects who actually moved both hands. By contrast, brain-damaged patients with motor neglect or actual hemiplegia but no anosognosia did not show this bimanual constraint. We suggest that anosognosic patients may have intact motor intentionality and planning for the plegic hand. Rather than being merely an inexplicable confabulation, anosognosia for the plegic hand can produce objective constraints on what the intact hand does.

Spatial perspective and identity in visual awareness of the bodily self-other distinction.

Spatial perspective and identity of visual bodily stimuli are two key cues for the self-other distinction. However, how they emerge into visual awareness is largely unknown. Here, self- or other-hands presented in first- or third-person perspective were compared in a breaking-Continuous Flash Suppression paradigm (Experiment 1) measuring the time the stimuli need to access visual awareness, and in a Binocular Rivalry paradigm (Experiment 2), measuring predominance in perceptual awareness. Results showed that, irrespectively of identity, first-person perspective speeded up the access, whereas the third-person one increased the dominance. We suggest that the effect of first-person perspective represents an unconscious prioritization of an egocentric body coding important for visuomotor control. On the other hand, the effect of third-person perspective indicates a conscious advantage of an allocentric body representation fundamental for detecting the presence of another intentional agent. Summarizing, the emergence of self-other distinction into visual awareness would strongly depend on the interplay between spatial perspectives, with an inverse prioritization before and after conscious perception. On the other hand, identity features might rely on post-perceptual processes.

Behavioral and neurophysiological indices of the racial bias modulation after virtual embodiment in other-race body.

Racial bias-nonconscious behavioral inclinations against people of other ethnic groups-heavily contributes to inequality and discrimination. Immersive virtual reality (IVR) can reduce implicit racial bias through the feeling of owning (embodying) a virtual body of a different "race"; however, it has been demonstrated only behaviorally for the implicit attitudes. Here, we investigated the implicit (racial IAT) and the neurophysiological (the N400 component of the event-related potentials for verbal stimuli that violated negative racial stereotypes) correlates of the embodiment-induced reduction of the implicit racial bias. After embodying a Black avatar, Caucasian participants had reduced implicit racial bias (IAT) but both groups showed the typical N400. This is the first evidence to suggest that virtual embodiment affects the evaluative component of the implicit biases but not the neurophysiological index of their cognitive component (i.e., stereotyping). This can inform interventions that promote inclusivity through the implicit/indirect procedures, such as embodiment.

How time gets spatial: factors determining the stability and instability of the mental time line.

Left-to-right readers classify faster past events with motor responses on the left side of space and future events with responses on the right side. This suggests a left-to-right spatial organization in the mental representation of time. Here, we show that the significance and reliability of this representation are linked to the joint use of temporal and spatial codes in the task at hand. In a first unimanual Go/No-Go Implicit Association Test (IAT), attending selectively to "past" or to "future" words did not activate corresponding "left" or "right" spatial concepts and vice versa. In a second IAT, attending to both temporal (i.e., "past" and "future") words and spatial targets (i.e., "left" and "right") pointing arrows produced faster responses for congruent rather than incongruent combinations of temporal and spatial concepts in task instructions (e.g., congruent = "Go with past words and left-pointing arrows"; incongruent = "Go with past words and right-pointing arrows"). This effect increased markedly in a STEARC task where spatial codes defined the selection between "left-side" and "right-side" button presses that were associated with "past" and "future" words. Two control experiments showed only partial or unreliable space-time congruency effects when (a) participants attended to superordinate semantic codes that included both spatial "left"/"right" or temporal "past/future" subordinate codes; (b) a primary speeded response was assigned to one dimension (e.g., "past vs. future") and a nonspeeded one to the other dimension (e.g., "left" vs. "right"). These results help to define the conditions that trigger a stable and reliable spatial representation of time-related concepts.