Short-term visual deprivation boosts the flexibility of body representation

Are blind individuals immune to bodily illusions? Somatic rubber hand illusion in the blind revisited

Multisensory awareness of one's own body relies on the integration of signals from various sensory modalities such as vision, touch, and proprioception. But how do blind individuals perceive their bodies without visual cues, and does the brain of a blind person integrate bodily senses differently from a sighted person? To address this question, we aimed to replicate the only two previous studies on this topic, which claimed that blind individuals do not experience the somatic rubber hand illusion, a bodily illusion triggered by the integration of correlated tactile and proprioceptive signals from the two hands. We used a larger sample size than the previous studies and added Bayesian analyses to examine statistical evidence in favor of the lack of an illusion effect. Moreover, we employed tests to investigate whether enhanced tactile acuity and cardiac interoceptive accuracy in blind individuals could also explain the weaker illusion. We tested 36 blind individuals and 36 age- and sex-matched sighted volunteers. The results show that blind individuals do not experience the somatic rubber hand illusion based on questionnaire ratings and behavioral measures that assessed changes in hand position sense toward the location of the rubber hand. This conclusion is supported by Bayesian evidence in favor of the null hypothesis. The findings confirm that blind individuals do not experience the somatic rubber hand illusion, indicating that lack of visual experience leads to permanent changes in multisensory bodily perception. In summary, our study suggests that changes in multisensory integration of tactile and proprioceptive signals may explain why blind individuals are "immune" to the nonvisual version of the rubber hand illusion.

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.

Tactile temporal order judgment during rubber hand illusion: Distinct modulation of the point of subjective simultaneity and temporal resolution

During the rubber hand illusion (RHI), individuals feel a fake hand as their own (ownership) and a perceived position of their real hand shifts toward the fake hand (proprioceptive drift; PD), which represents updating of multisensory hand representations. Bimanual tactile temporal order judgment (TOJ) includes processes of localizing tactile stimuli in space, for which multisensory hand representations are essential. According to the common processes, we examined tactile TOJ performance during the RHI and non-RHI. Temporal resolution (TR) as TOJ accuracy worsened during the non-RHI compared to the RHI. Additionally, a significant correlation between TR and PD was observed only in the non-RHI condition. However, the point of subjective simultaneity (PSS), which offers relative weighting of tactile inputs from the right and left hands, was correlated with illusory hand ownership. These results suggest that PSS and TR from tactile TOJ during RHI relate to self-attribution and localization of the hand, respectively.

Variations of Sensorimotor Representation (Structure): The Functional Interplay between Object Features and Goal-Directed Grasping Actions

This study investigated the structure of sensorimotor representations during goal-directed grasping actions and explored their relationship with object features. Sixteen 3D-printed spheres that varied in size (i.e., a diameter of 20 mm, 40 mm, 60 mm, 80 mm) and weight (i.e., 40 g, 52 g, 76 g, 91 g) were used as experimental stimuli. The Structural Dimensional Analysis of Mental Representation (SDA-M) method was used to assess the sensorimotor representation structure during grasping. Participants were instructed in each trial to weigh, lift, or transport sets of two different spheres and to judge the similarity of the objects’ features, taking into account the executed grasping movement. Each participant performed a total of 240 trials, and object presentation was randomized. The results suggest that the functional interplay between object features and goal-directed actions accounts for the significant variations in the structure of sensorimotor representations after grasping. Specifically, the relevance of the perceived objects’ size and weight is closely interrelated to the grasping task demands and movement dynamics of the executed action. Our results suggest that distinct sensorimotor representations support individual grasping actions according to top-down influences modulated by motor intentions, functional task demands, and task-relevant object features.

Analysing the Influence of Selected Eye Dysfunctions on Palpation Abilities of Massage Therapists

Introduction: Palpation arises controversy in the field of physiotherapy. On the one hand, this method provides a lot of valuable information. On the other, issues related to its reliability still exist. Additionally, questions about factors affecting palpation and users’ dispositions should be taken into consideration. Research objectives: The main aim of the study was to determine the impact of visual impairment on the palpatory skills of massage therapists. Material and methods: The research involved 58 participants divided into 3 groups. The first group consisted of 23, visually-impaired massage therapists, the second one was made up of 22 massage therapists and the control group consisted of 13 people unrelated to massage. The subjects took the hair test, the weight test and the measurement of Static Two-Point Discrimination within the fingertips I-III, thenar and hypothenar. Results: The analysis does not show any significant differences between the first and second group. Statistics reveal differences between the group of massage practitioners (group 1 and 2) and the control group. Visual impairment was not the most influential factor in the study. Conclusions: The level of palpation skills may be modified in both visually-impaired and healthy people. However, mechanisms that allow these modifications remain unclear. On the basis of the research, visual impairment is not considered as the only factor affecting palpation skills. Experience in palpation may well be an equally important factor.

Implicit Body Representation of the Hand Enlarged by Repetitive Peripheral Magnetic Stimulation within the Boundary of a Real Hand

Deafferentation induced by local anesthesia causes a larger perceived area than the real area of the mouth, which, in the perspective of body representation, belongs to implicit body representation. In this study, we applied repetitive peripheral magnetic stimulation (rPMS) on the motor branch of the radial nerve of participants’ non-dominant-side forearm to induce extension movements of wrist and fingers. This intervention was supposed to increase proprioception to the brain and had an enlargement effect on implicit body representation of the hand in our hypothesis. A total of 39 participants were randomly allocated to the real rPMS group (n = 19) or the sham rPMS group (n = 20). Implicit representation of the hand was measured by a simplified paradigm based on the proposal of Longo and Haggard that depicted perceived locations of fingertips and metacarpophalangeal joints of participants’ occluded hand, in which they showed that implicit body representation of the hand was smaller than the real hand. We compare the main effect of real rPMS vs. sham rPMS and its interaction effect with time by setting four timepoints—before stimulation, right after stimulation, 10 min after stimulation and 20 min after stimulation—to demonstrate the possible short-lasting effect. Results showed that real rPMS had a short-lasting enlargement effect on implicit representation of the hand in general, which was significant especially on the ulnar side of fingers. What is more, the enlarged implicit body representation of the hand was still within the boundary of a real hand, which might indicate the identification role of a real body part.

Limits of Cross-modal Plasticity? Short-term Visual Deprivation Does Not Enhance Cardiac Interoception, Thermosensation, or Tactile Spatial Acuity

In the present study, we investigated the effect of short-term visual deprivation on discriminative touch, cardiac interoception, and thermosensation by asking 64 healthy volunteers to perform four behavioral tasks. The experimental group contained 32 subjects who were blindfolded and kept in complete darkness for 110minutes, while the control group consisted of 32 volunteers who were not blindfolded but were otherwise kept under identical experimental conditions. Both groups performed the required tasks three times: before and directly after deprivation (or control) and after an additional washout period of 40minutes, in which all participants were exposed to normal light conditions. Our results showed that short-term visual deprivation had no effect on any of the senses tested. This finding suggests that short-term visual deprivation does not modulate basic bodily senses and extends this principle beyond tactile processing to the interoceptive modalities of cardiac and thermal sensations.

Predictability of Delayed Visual Feedback Under Rubber Hand Illusion Modulates Localization but Not Ownership of the Hand

The rubber hand illusion (RHI) is a perceptual illusion, whereby a fake hand is recognized as one’s own hand when a fake hand and felt real hand are stroked synchronously. RHI strength is mainly assessed using a questionnaire rating and proprioceptive drift (PD). PD is characterized by the proprioceptively sensed location of the participant’s own hand shifting toward the location of the fake hand after RHI. However, the relationship between the two measures of hand ownership and location remains controversial due to mixed findings: some studies report correlations between them, while others show that they are independent. Here, we demonstrated significant PD without RHI using delayed visual feedback. In this RHI study, video images of the fake hand were delivered to the subjects, and four delay intervals of visual feedback (80, 280, 480, and 680ms) were introduced. In four of six conditions, the delay interval was fixed throughout the condition. In the other two conditions, four delays were delivered in a predetermined order (i.e., serial condition; higher predictability) or in a pseudo-random order (i.e., random condition; low predictability). For the four conditions with a fixed delay, the questionnaire ratings and PD declined significantly when the delay interval exceeded circa 300ms. In both the serial and random conditions, no illusory ownership of the fake hand was reported in the questionnaire. In contrast, greater PD was found in the random condition but not in the serial condition. Our findings suggest that hand ownership and localization are caused by distinct multisensory integration processes.

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.

The rubber hand illusion in hypnosis provides new insights into the sense of body ownership

Body ownership can be experimentally investigated with the rubber hand illusion (RHI), in which watching a rubber hand stroked synchronously with one’s own hidden hand induces a feeling of ownership over the rubber hand. The aim of this study was to investigate response to the RHI in high (N = 21) and low (N = 19) hypnotizable individuals in normal waking state and in hypnosis. Response to the RHI was measured via a question on the illusory feeling of ownership and with proprioceptive drift. The Highs expressed an overall feeling of more ownership over the rubber hand in both the normal waking state and hypnosis, although both groups gave higher ownership scores after synchronous than after asynchronous stroking and the difference between conditions was similar across groups. Conversely, the proprioceptive drift appeared to be differentially modulated by hypnosis and hypnotic suggestibility: it was increased in the Highs and decreased in the Lows after hypnosis induction. These findings hint at an interplay between hypnotic suggestibility and hypnosis in modulating response to the RHI. The selective breakdown of proprioceptive drift among the Lows suggests resistance to recalibrate one’s own limb in hypnosis.

Subjective embodiment during the rubber hand illusion predicts severity of premonitory sensations and tics in Tourette Syndrome

In Tourette Syndrome, the expression of tics and commonly preceding premonitory sensations is associated with perturbed subjective feelings of self-control and agency. We compared responses to the Rubber Hand Illusion in 23 adults with TS and 22 controls. Both TS and control participants reported equivalent subjective embodiment of the artificial hand: feelings of ownership, location, and agency were greater during synchronous visuo-tactile stimulation, compared to asynchronous. However, individuals with TS did not manifest greater proprioceptive drift, an objective marker of embodiment observed in controls. An 'embodiment prediction error' index of the difference between subjective embodiment and objective proprioceptive drift correlated with severity of premonitory sensations. Feelings of ownership also correlated with premonitory sensation severity, and feelings of agency with tic severity. These findings suggest that subjective bodily ownership, as measured by the rubber hand illusion, contributes to susceptibility to the premonitory sensations that may be a precipitating factor in tics.

Psychedelics, Meditation, and Self-Consciousness

In recent years, the scientific study of meditation and psychedelic drugs has seen remarkable developments. The increased focus on meditation in cognitive neuroscience has led to a cross-cultural classification of standard meditation styles validated by functional and structural neuroanatomical data. Meanwhile, the renaissance of psychedelic research has shed light on the neurophysiology of altered states of consciousness induced by classical psychedelics, such as psilocybin and LSD, whose effects are mainly mediated by agonism of serotonin receptors. Few attempts have been made at bridging these two domains of inquiry, despite intriguing evidence of overlap between the phenomenology and neurophysiology of meditation practice and psychedelic states. In particular, many contemplative traditions explicitly aim at dissolving the sense of self by eliciting altered states of consciousness through meditation, while classical psychedelics are known to produce significant disruptions of self-consciousness, a phenomenon known as drug-induced ego dissolution. In this article, we discuss available evidence regarding convergences and differences between phenomenological and neurophysiological data on meditation practice and psychedelic drug-induced states, with a particular emphasis on alterations of self-experience. While both meditation and psychedelics may disrupt self-consciousness and underlying neural processes, we emphasize that neither meditation nor psychedelic states can be conceived as simple, uniform categories. Moreover, we suggest that there are important phenomenological differences even between conscious states described as experiences of self-loss. As a result, we propose that self-consciousness may be best construed as a multidimensional construct, and that "self-loss," far from being an unequivocal phenomenon, can take several forms. Indeed, various aspects of self-consciousness, including narrative aspects linked to autobiographical memory, self-related thoughts and mental time travel, and embodied aspects rooted in multisensory processes, may be differently affected by psychedelics and meditation practices. Finally, we consider long-term outcomes of experiences of self-loss induced by meditation and psychedelics on individual traits and prosocial behavior. We call for caution regarding the problematic conflation of temporary states of self-loss with "selflessness" as a behavioral or social trait, although there is preliminary evidence that correlations between short-term experiences of self-loss and long-term trait alterations may exist.