The rubber hand illusion in microgravity and water immersion.

Our body has evolved in terrestrial gravity and altered gravitational conditions may affect the sense of body ownership (SBO). By means of the rubber hand illusion (RHI), we investigated the SBO during water immersion and parabolic flights, where unconventional gravity is experienced. Our results show that unconventional gravity conditions remodulate the relative weights of visual, proprioceptive, and vestibular inputs favoring vision, thus inducing an increased RHI susceptibility.

What pathological embodiment/disembodiment tell us about body representations.

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.

Immersive virtual reality reveals that visuo-proprioceptive discrepancy enlarges the hand-centred peripersonal space.

Vision and proprioception, informing the system about the body position in space, seem crucial in defining the boundary of the peripersonal space (PPS). What happens to the PPS representation when a conflict between vision and proprioception arises? We capitalize on the Immersive Virtual Reality to dissociate vision and proprioception by presenting the participants' 3D hand image in congruent/incongruent positions with respect to the participants' real hand. To measure the hand-centred PPS, we exploit multisensory integration occurring when visual stimuli are delivered simultaneously with tactile stimuli applied to a body district; i.e., visual enhancement of touch (VET). Participants are instructed to respond to tactile stimuli while ignoring visual stimuli (red LED), which can appear either near to or far from the hand receiving tactile (electrical) stimuli. The results show that, when vision and proprioception are congruent (i.e., real and virtual hand coincide), a space-dependent modulation of the VET effect occurs (with faster responses when visual stimuli are near to than far from the stimulated hand). Contrarily, when vision and proprioception are incongruent (i.e., a discrepancy between real and virtual hand is present), a comparable VET effect is observed when visual stimuli occur near to the real hand and when they occur far from it, but close to the virtual hand. These findings, also confirmed by the independent estimate of a Bayesian Causal Inference model, suggest that, when the visuo-proprioceptive discrepancy makes the coding of the hand position less precise, the hand-centred PPS is enlarged, likely to optimize reactions to external events.

Agent-dependent modulation of corticospinal excitability during painful transcutaneous electrical stimulation.

Pain has an inhibitory effect on the corticospinal excitability that has been interpreted as an evolutionary mechanism, directed to down-regulate cortical activity in order to facilitate rapid protective spinal reflexes. Here, we focused on the link between defensive mechanisms and motor system and we asked whether voluntary actions can modulate the corticospinal excitability during painful stimulations. To this aim, we manipulated the volition-related aspects of our paradigm by comparing conditions in which either the participant (self-generated action) or the experimenter (other-generated action) pressed the button to deliver painful high-intensity transcutaneous electric shocks to the right digit V. MEPs to TMS were recorded from the FDI and APB muscles of the stimulated hand. A compelling agent-dependent modulation of the corticospinal excitability was found, showing, in self-generated compared to other-generated actions, a significantly lower inhibitory effect, as measured by greater MEP amplitude. This finding suggests a top-down modulation of volitional actions on defensive mechanisms, promoting the view that predictive information from the motor system attenuates the responses to the foreseeable adverse events generated by one's own actions as compared to unpredictable events generated by someone else's actions.

The sense of body-ownership gates cross-modal improvement of tactile extinction in brain-damaged patients.

When body ownership is impaired after brain-damage, the capacity to discriminate between one's own and others' body-parts is lost. Delusional body-ownership has been recently described in patients who misidentify someone else's limb as their own (pathological embodiment) whenever it is positioned in a body congruent position. This delusion can be frequently associated with somatosensory and attentional deficits. Here, we leveraged the phenomenon of tactile extinction, as this clinical sign can be substantially ameliorated when contralesional touches are combined with proximal visual stimulation. Is body ownership a necessary prerequisite to modulate cross-modal processing and thus reducing tactile extinction? Fourteen patients with tactile extinction (TE+) took part in the study: eight of them with pathological embodiment (E+, experimental group) and six of them without pathological embodiment (E-, control group). In two different paradigms, differing for the nature of visuo-tactile stimuli, bilateral tactile stimulation of the patients' hands was combined with visual stimuli occurring on A) their own contralesional (affected) hand, B) the examiner's hand (embodied in E+), or C) a neutral object. In both groups, visual stimuli proximal to the own hand significantly improved contralesional tactile detection, while visual stimuli occurring on the neutral object did not. Crucially, only in E+TE+ patients did visual stimuli on the examiner's (embodied) hand improve contralesional tactile detection. This finding shows that cross-modal visuo-tactile integration is conditional to body-ownership, so that it ameliorates tactile extinction when visual stimuli occur on what is believed to be one's own body. From a clinical point of view, this study suggests that the effectiveness of cross-modal rehabilitative intervention can benefit from a careful evaluation of the patients' sense of body-ownership, so often impaired after brain-damage.

Bodily ownership modulation in defensive responses: physiological evidence in brain-damaged patients with pathological embodiment of other's body parts.

Do conscious beliefs about the body affect defensive mechanisms within the body? To answer this question we took advantage from a monothematic delusion of bodily ownership, in which brain-damaged patients misidentify alien limbs as their own. We investigated whether the delusional belief that an alien hand is their own hand modulates a subcortical defensive response, such as the hand-blink reflex. The blink, dramatically increases when the threated hand is inside the defensive peripersonal-space of the face. In our between-subjects design, including patients and controls, the threat was brought near the face either by the own hand or by another person's hand. Our results show an ownership-dependent modulation of the defensive response. In controls, as well as in the patients' intact-side, the response enhancement is significantly greater when the threat was brought near the face by the own than by the alien hand. Crucially, in the patients' affected-side (where the pathological embodiment occurs), the alien (embodied) hand elicited a response enhancement comparable to that found when the threat is brought near the face by the real hand. These findings suggest the existence of a mutual interaction between our conscious beliefs about the body and the physiological mechanisms within the body.

Interpersonal interactions and empathy modulate perception of threat and defensive responses.

The defensive peripersonal space (DPPS) is a vital "safety margin" surrounding the body. When a threatening stimulus is delivered inside the DPPS, subcortical defensive responses like the hand-blink reflex (HBR) are adjusted depending on the perceived threat content. In three experiments, we explored whether and how defensive responses are affected by the interpersonal interaction within the DPPS of the face. In Experiment 1, we found that the HBR is enhanced when the threat is brought close to the face not only by one's own stimulated hand, but also by another person's hand, although to a significantly lesser extent. In Experiments 2 and 3, we found that the HBR is also enhanced when the hand of the participant enters the DPPS of another individual, either in egocentric or in allocentric perspective. This enhancement is larger in participants with strong empathic tendency when the other individual is in a third person perspective. These results indicate that interpersonal interactions shape perception of threat and defensive responses. These effects are particularly evident in individuals with greater tendency to having empathic concern to other people.