Higher-order cognitive factors affect subjective but not proprioceptive aspects of self-representation in the rubber hand illusion

Embodiment in the enfacement illusion is mediated by self-other overlap

The enfacement illusion is a facial version of the rubber hand illusion, in which participants experience tactile stimulation of their own faces synchronously with the observation of the same stimulation applied to another's face. In previous studies, participants have reported experiencing an illusory embodiment of the other's face following synchronous compared to asynchronous stimulation. In a series of three experiments, we addressed the following three questions: (i) how does similarity between the self and the other, operationalized here as being of the same or different gender to the other, impact the experience of embodiment in the enfacement illusion; (ii) does the experience of embodiment result from alterations to the self-concept; and (iii) is susceptibility to the experience of embodiment associated with interoceptive processing, i.e. perception of the internal state of the body? Results indicate that embodiment is facilitated by the similarity between the self and the other and is mediated by the incorporation of the other into the self-concept, but sensitivity to one's own internal states does not impact upon embodiment within the enfacement illusion. This article is part of the theme issue 'Minds in movement: embodied cognition in the age of artificial intelligence'.

The correlation between proprioceptive drift and subjective embodiment during the rubber hand illusion: A meta-analytic approach

In the rubber hand illusion (RHI), participants see a fake hand touched synchronously with their real hand, which is hidden from view. The three-way interaction between vision, touch, and proprioception induces the sensation that the dummy hand belongs to oneself (i.e., subjective embodiment) and the illusory displacement of the real hand towards the fake one (i.e., proprioceptive drift). In the literature, there are mixed results (some positive and some null) regarding the existence of a relationship between subjective embodiment and proprioceptive drift. We conducted a Bayesian meta-analysis to tackle this issue quantitatively. Evidence strongly favours the presence of a correlation between subjective embodiment and proprioceptive drift, supporting the model proposed by Botvinick and Cohen in 1998. However, the correlation is around .35, a value suggesting that the two indices capture different facets of the RHI. This result clarifies the association between the illusory effects produced by the RHI and may be helpful for designing studies having appropriate statistical power.

Measuring embodiment: A review of methods for prosthetic devices

The development of neural interfaces to provide improved control and somatosensory feedback from prosthetic limbs has initiated a new ability to probe the various dimensions of embodiment. Scientists in the field of neuroprosthetics require dependable measures of ownership, body representation, and agency to quantify the sense of embodiment felt by patients for their prosthetic limbs. These measures are critical to perform generalizable experiments and compare the utility of the new technologies being developed. Here, we review outcome measures used in the literature to evaluate the senses of ownership, body-representation, and agency. We categorize these existing measures based on the fundamental psychometric property measured and whether it is a behavioral or physiological measure. We present arguments for the efficacy and pitfalls of each measure to guide better experimental designs and future outcome measure development. The purpose of this review is to aid prosthesis researchers and technology developers in understanding the concept of embodiment and selecting metrics to assess embodiment in their research. Advances in the ability to measure the embodiment of prosthetic devices have far-reaching implications in the improvement of prosthetic limbs as well as promoting a broader understanding of ourselves as embodied agents.

Body transfer illusions in the schizophrenia spectrum: a systematic review

Schizophrenia has been viewed as a disorder of the self. Accordingly, the question arises if and how senses of ownership and agency are impaired in schizophrenia. To address this question, several body transfer illusions (BTIs) have been investigated in schizophrenia patients and other schizophrenia spectrum (SCZ-S) populations. The objective of the study was to systematically review the current evidence from BTIs in the SCZ-S. A systematic literature search in PubMed and CENTRAL (search date: February 12, 2022) was conducted on BTI studies carried out in SCZ-S populations. Studies were included if they were published in English after peer review, reported original research data, related to the SCZ-S, and used a BTI as its study method. Conference papers, study protocols, and reviews were excluded. For each included BTI study, various study characteristics and outcomes were retrieved, and a risk-of-bias score was calculated based on six study quality criteria. K = 40 studies were identified, of which k = 20 studies met the eligibility criteria. For BTI paradigms using visuotactile stimulation, most studies found elevated sense of ownership ratings in SCZ-S populations compared to healthy controls (HC). Implicit illusion measures (e.g., proprioceptive drift), in turn, did not generally indicate elevated embodiment levels in SCZ-S populations. Likewise, no consistent group differences emerged between SCZ-S populations and HC with respect to BTI paradigms using visuomotor stimulation. Furthermore, BTI vividness was found to correlate significantly with core symptoms of schizophrenia and various subclinical characteristics related to the SCZ-S. In line with the self-disturbance hypothesis, SCZ-S populations appear to be affected by aberrations in bodily self-awareness. Review registration: PROSPERO (identifier: CRD42022287960).

EXPRESS: The Rubber Hand Illusion: Top-Down Attention Modulates Embodiment

The Rubber Hand Illusion (RHI) creates distortions of body ownership through multimodal integration of somatosensory and visual inputs. This illusion largely rests on bottom-up (automatic multisensory and perceptual integration) mechanisms. However, the relative contribution from top-down factors, such as controlled processes involving attentional regulation, remains unclear. Following previous work that highlights the putative influence of higher-order cognition in the RHI, we aimed to further examine how modulations of working memory load and task instructions—two conditions engaging top-down cognitive processes—influence the experience of the RHI, as indexed by a number of psychometric dimensions. Relying on exploratory factor analysis for assessing this phenomenology within the RHI, our results confirm the influence of higher-order, top-down mental processes. Whereas task instruction strongly modulated embodiment of the rubber hand, cognitive load altered the affective dimension of the RHI. Our findings corroborate that top-down processes shape the phenomenology of the RHI and herald new ways to improve experimental control over the RHI.

Attenuation of sensory processing in the primary somatosensory cortex during rubber hand illusion

The neural representation of the body is easily altered by the integration of multiple sensory signals in the brain. The “rubber hand illusion” (RHI) is one of the most popular experimental paradigms to investigate this phenomenon. During this illusion, a feeling of ownership of the rubber hand is created. Some studies have shown that somatosensory processing in the brain is attenuated when RHI occurs. However, it is unknown where attenuation of somatosensory processing occurs. Here, we show that somatosensory processing is attenuated in the primary somatosensory cortex. We found that the earliest response of somatosensory evoked potentials, which is thought to originate from the primary somatosensory cortex, was attenuated during RHI. Furthermore, this attenuation was observed before the occurrence of the illusion. Our results suggest that attenuation of sensory processing in the primary somatosensory cortex is one of the factors influencing the occurrence of the RHI.

Dissociation of proprioceptive drift and feelings of ownership in the somatic rubber hand illusion

The sense of self is a complex phenomenon, comprising various sensations of bodily self-consciousness. Interestingly, the experience of possessing a body - 'embodiment' - and locating the body within space may be modulated by the Rubber Hand Illusion (RHI). Measures of the RHI include proprioceptive drift (PD), the extent to which the hand is mis-localised towards the rubber hand, and subjective questionnaires. Although these measures often correlate, research from the visual RHI suggests that they reflect separate underlying processes. We investigated whether increasing the duration of tactile stimulation would affect PD and questionnaires differently during the somatic RHI. Participants experienced 30 s, 2 min, or 5 min of synchronous or asynchronous tactile stimulation. Increasing duration affected only PD, with increased drift following 5 min vs 30 s of stimulation. Our findings suggest that PD and questionnaires are not proxies for one another, but reflect separate underlying processes of the somatic RHI.

Multisensory integration underlying body-ownership experiences in schizophrenia and offspring of patients: a study using the rubber hand illusion paradigm

BACKGROUND

Schizophrenia is a disorder of basic self-disturbance. Evidence suggests that people with schizophrenia may have aberrant experiences of body ownership: they may feel that they are not the subject of their own body experiences. However, little is known about the development of such disturbances.

METHODS

Using a rubber hand illusion paradigm, we assessed body ownership in patients with schizophrenia (n = 54), healthy controls (n = 56), children/adolescents at increased familial risk of developing schizophrenia (n = 24) or mood disorders (n = 33), and children/adolescents without this risk (n = 18). In this paradigm, a rubber hand (visible) and a participant’s real hand (invisible) were stroked synchronously and asynchronously; we then measured subjective illusory experiences and proprioceptive drift.

RESULTS

All groups showed the expected effect of the rubber hand illusion: stronger proprioceptive drift and increased subjective illusory experiences after synchronous versus asynchronous stroking. The effect of synchronicity on subjective experiences was significantly weaker in patients with schizophrenia than in healthy controls, and subjective ratings were positively correlated with delusions in patients. We found no significant differences between children/adolescents with and without increased familial risk.

LIMITATIONS

Large individual differences raised questions for future research.

CONCLUSION

We found subtle disturbances in body-ownership experiences in patients with schizophrenia, which were associated with delusions. We found no evidence for impairments in children/adolescents at increased familial risk of developing schizophrenia or a mood disorder. Longitudinal data might reveal whether impairments in body ownership are predictive of psychosis onset.

Rubber hand illusion modulates the influences of somatosensory and parietal inputs to the motor cortex

The rubber hand illusion (RHI) paradigm experimentally produces an illusion of rubber hand ownership and arm shift by simultaneously stroking a rubber hand in view and a participant’s visually occluded hand. It involves visual, tactile, and proprioceptive multisensory integration and activates multisensory areas in the brain, including the posterior parietal cortex (PPC). Multisensory inputs are transformed into outputs for motor control in association areas such as PPC. A behavioral study reported decreased motor performance after RHI. However, it remains unclear whether RHI modifies the interactions between sensory and motor systems and between PPC and the primary motor cortex (M1). We used transcranial magnetic stimulation (TMS) and examined the functional connections from the primary somatosensory and association cortices to M1 and from PPC to M1 during RHI. In experiment 1, short-latency afferent inhibition (SAI) and long-latency afferent inhibition (LAI) were measured before and immediately after a synchronous (RHI) or an asynchronous (control) condition. In experiment 2, PPC-M1 interaction was measured using two coils. We found that SAI and LAI were reduced in the synchronous condition compared with baseline, suggesting that RHI decreased somatosensory processing in the primary sensory and the association cortices projecting to M1. We also found that greater inhibitory PPC-M1 interaction was associated with stronger RHI assessed by questionnaire. Our findings suggest that RHI modulates both the early and late stages of processing of tactile afferent, which leads to altered M1 excitability by reducing the gain of somatosensory afferents to resolve conflicts among multisensory inputs.

NEW & NOTEWORTHY Perception of one’s own body parts involves integrating different sensory information and is important for motor control. We found decreased effects of cutaneous stimulation on motor cortical excitability during rubber hand illusion (RHI), which may reflect decreased gain of tactile input to resolve multisensory conflicts. RHI strength correlated with the degree of inhibitory posterior parietal cortex-motor cortex interaction, indicating that parietal-motor connection is involved in resolving sensory conflicts and body ownership during RHI.

Handedness modulates proprioceptive drift in the rubber hand illusion

Preference for use of either the left or right hand (‘handedness’) has been linked with modulations of perception and sensory processing—both of space and the body. Here we ask whether multisensory integration of bodily information also varies as a function of handedness. We created a spatial disparity between visual and somatosensory hand position information using the rubber hand illusion, and use the magnitude of illusory shifts in hand position (proprioceptive ‘drift’) as a tool to probe the weighted integration of multisensory information. First, we found drift was significantly reduced when the illusion was performed on the dominant vs. non-dominant hand. We suggest increased manual dexterity of the dominant hand causes greater representational stability and thus an increased resistance to bias by the illusion induction. Second, drift was generally greatest when the hand was in its habitual action space (i.e., near the shoulder of origin), compared to when it laterally displaced towards, or across the midline. This linear effect, however, was only significant for the dominant hand—in both left- and right-handed groups. Thus, our results reveal patterns of habitual hand action modulate drift both within a hand (drift varies with proximity to action space), and between hands (differences in drift between the dominant and non-dominant hands). In contrast, we were unable to find conclusive evidence to support, or contradict, an overall difference between left- and right-handers in susceptibility to RHI drift (i.e., total drift, collapsed across hand positions). In sum, our results provide evidence that patterns of daily activity—and the subsequent patterns of sensory input—shape multisensory integration across space.

On the relation between body ownership and sense of agency: A link at the level of sensory-related signals

The relation between sense of body ownership and sense of agency is still highly debated. Here we investigated in a large sample of healthy participants the associations between several implicit and explicit indexes of the two senses. Specifically, we examined the correlations between proprioceptive shift (implicit measure) and questionnaire on the subjective experience of ownership (explicit measure) within the rubber hand illusion paradigm (body ownership), and intentional binding (implicit measure), attenuation of the intensity of auditory outcomes of actions (implicit measure) and questionnaire on the subjective experience of authorship (explicit measure) within the Libet's clock paradigm (sense of agency). Our results showed that proprioceptive shift was positively correlated with the attenuation of auditory outcomes. No significant correlations were found between the explicit measures of the two senses. We argue that the individual spatiotemporal constraints subserving the integration of sensory-related signals (implicit signature) would be common to both senses, whereas their subjective experience (explicit signature) would rely on additional processes specific for any given sense.

The second me: Seeing the real body during humanoid robot embodiment produces an illusion of bi-location

Whole-body embodiment studies have shown that synchronized multi-sensory cues can trick a healthy human mind to perceive self-location outside the bodily borders, producing an illusion that resembles an out-of-body experience (OBE). But can a healthy mind also perceive the sense of self in more than one body at the same time? To answer this question, we created a novel artificial reduplication of one's body using a humanoid robot embodiment system. We first enabled individuals to embody the humanoid robot by providing them with audio-visual feedback and control of the robot head movements and walk, and then explored the self-location and self-identification perceived by them when they observed themselves through the embodied robot. Our results reveal that, when individuals are exposed to the humanoid body reduplication, they experience an illusion that strongly resembles heautoscopy, suggesting that a healthy human mind is able to bi-locate in two different bodies simultaneously.