EXPRESS: Vicarious Pain is an Outcome of Atypical Body Ownership: Evidence from the Rubber Hand Illusion and Enfacement Illusion

Some people report localised pain on their body when seeing other people in pain (sensory-localised vicarious pain responders). In this study we assess whether this is related to atypical computations of body ownership which, in paradigms such as the Rubber Hand Illusion (RHI), can be conceptualised as a Bayesian inference as to whether multiple sources of sensory information (visual, somatosensory) belong together on a single body (one’s own) or are distributed across several bodies (vision=other, somatosensory=self). According to this model, computations of body ownership depend on the degree (and precision) of sensory evidence, rather than synchrony per se. Sensory-localised vicarious pain responders exhibit the RHI following synchronous stroking and – unusually – also after asynchronous stroking. Importantly, this occurs only in asynchronous conditions in which the stroking is predictable (alternating) rather than unpredictable (random). There was no evidence that their bottom-up proprioceptive signals are less precise, suggesting individual differences in the top-down weighting of sensory evidence. Finally, the Enfacement illusion (EI) was also employed as a conceptually-related bodily illusion paradigm that involves a completely different response judgment (based on vision rather than proprioception). Sensory-localised responders show a comparable pattern on this task after synchronous and asynchronous stroking. This is consistent with the idea that they have top-down (prior) differences in the way body ownership is inferred that transcends the exact judgment being made (visual or proprioceptive).

Gut markers of bodily self-consciousness

Bodily self-consciousness, the state of mind that allows humans to be aware of their own body, forms the backdrop for almost every human experience, yet its underpinnings remain elusive. Here we use an ingestible, minimally invasive capsule and surface electrogastrography to probe if gut physiology correlates with bodily self-consciousness during a virtual bodily illusion. We discover that specific patterns of stomach and large intestine activity (temperature, pressure, pH, and gastric peak frequency) covary with specific facets of bodily self-consciousness (feelings of body ownership, agency, location, and disembodiment). Furthermore, we show that the link between gastro-intestinal parameters and bodily self-consciousness depends also on individual levels of interoception. These results reveal a deep visceral pathway to the self-conscious perception of ourselves as embodied beings.

'Embreathment' illusion reveals hierarchical influence of respiratory, visual and spatial signals on corporeal awareness

Recent theories posit that physiological signals contribute to corporeal awareness – the basic feeling that one has a body (body ownership) which acts according to one’s will (body agency) and occupies a specific position (body location). However, these signals are notoriously difficult to manipulate. Using immersive virtual reality, we found that an ecological mapping of real respiratory patterns onto a virtual body led to illusory changes of corporeal awareness. This new bodily illusion, called ‘embreathment’, revealed that breathing uniquely influences corporeal awareness over and above other bodily cues. In particular, breathing turned out to be almost as important as visual appearance for inducing body ownership, and more important than any other cue for body agency. By showing that respiratory, visual and spatial signals exert an interoception-mediated, specific, and weighted influence on the fundamental feeling that one is an embodied agent, we pave the way for a comprehensive hierarchical model of corporeal awareness.

The Bodily Illusion in Adverse Conditions: Virtual Arm Ownership During Visuomotor Mismatch

Classically, body ownership illusions are triggered by cross-modal synchronous stimulations, and hampered by multisensory inconsistencies. Nonetheless, the boundaries of such illusions have been proven to be highly plastic. In this immersive virtual reality study, we explored whether it is possible to induce a sense of body ownership over a virtual body part during visuomotor inconsistencies, with or without the aid of concomitant visuo-tactile stimulations. From a first-person perspective, participants watched a virtual tube moving or an avatar’s arm moving, with or without concomitant synchronous visuo-tactile stimulations on their hand. Three different virtual arm/tube speeds were also investigated, while all participants kept their real arms still. The subjective reports show that synchronous visuo-tactile stimulations effectively counteract the effect of visuomotor inconsistencies, but at slow arm movements, a feeling of body ownership might be successfully induced even without concomitant multisensory correspondences. Possible therapeutical implications of these findings are discussed.

On the Syndrome of the “Spare Limb”: One Case

We describe a case of a brain-damaged patient who had a peculiar bodily illusion which could not be labelled an hallucination but seemed somatognosically and phenomenologically similar to the phantom limb without amputation. The patient, who showed left hemiplegia, felt a third upper limb (without seeing it) which he himself defined as “spare.” The spare limb was not deformed; it could be moved and controlled by the patient, and there was no sensation of pain. The patient did not show psychopathological or cognitive disorders. A possible interpretation of the phenomenon is as a “phantom movement” of the paralysed limb: the mental representation of the movement of the limb was dissociated from the bodily representation of his own limb and so was still present in his consciousness despite the paralysis.