Proprioceptive uncertainty promotes the rubber hand illusion

The prediction-confirmation account of the sense of body ownership: Evidence from a rubber hand illusion paradigm

We investigated the contribution of multisensory predictions to body ownership, and beyond, to the integration of body-related signals. Contrary to the prevailing idea, according to which, to be integrated, cues necessarily have to be perceived simultaneously, we instead proposed the prediction-confirmation account. According to this account, a perceived cue can be integrated with a predicted cue as long as both signals are relatively simultaneous. To test this hypothesis, a standard rubber hand illusion (RHI) paradigm was used. In the first part of each trial, the illusion was induced while participants observed the rubber hand being touched with a paintbrush. In the subsequent part of the trial, (i) both rubber hand and the participant's real hand were stroked as before (i.e., visible/synchronous condition), (ii) the rubber hand was not stroke anymore (i.e., visible/tactile-only condition), or (iii) both rubber hand and the participant's real hand were synchronously stroked while the location where the rubber hand was touched was occulted (i.e., occulted/synchronous condition). However, in this latter condition, participants still perceived the approaching movement of the paintbrush. Thus, based on this visual cue, the participants can properly predict the timepoint at which the tactile cue should occur (i.e., visuotactile predictions). Our major finding was that compared with the visible/tactile-only condition, the occulted/synchronous condition did not exhibit a decrease of the RHI as in the visible/synchronous condition. This finding supports the prediction-confirmation account and suggests that this mechanism operates even in the standard version of the RHI.

BCI Toolbox: An open-source python package for the Bayesian causal inference model

Psychological and neuroscientific research over the past two decades has shown that the Bayesian causal inference (BCI) is a potential unifying theory that can account for a wide range of perceptual and sensorimotor processes in humans. Therefore, we introduce the BCI Toolbox, a statistical and analytical tool in Python, enabling researchers to conveniently perform quantitative modeling and analysis of behavioral data. Additionally, we describe the algorithm of the BCI model and test its stability and reliability via parameter recovery. The present BCI toolbox offers a robust platform for BCI model implementation as well as a hands-on tool for learning and understanding the model, facilitating its widespread use and enabling researchers to delve into the data to uncover underlying cognitive mechanisms.

Biases in hand perception are driven by somatosensory computations, not a distorted hand model

To sense and interact with objects in the environment, we effortlessly configure our fingertips at desired locations. It is therefore reasonable to assume that the underlying control mechanisms rely on accurate knowledge about the structure and spatial dimensions of our hand and fingers. This intuition, however, is challenged by years of research showing drastic biases in the perception of finger geometry.1,2,3,4,5 This perceptual bias has been taken as evidence that the brain's internal representation of the body's geometry is distorted,6 leading to an apparent paradox regarding the skillfulness of our actions.7 Here, we propose an alternative explanation of the biases in hand perception-they are the result of the Bayesian integration of noisy, but unbiased, somatosensory signals about finger geometry and posture. To address this hypothesis, we combined Bayesian reverse engineering with behavioral experimentation on joint and fingertip localization of the index finger. We modeled the Bayesian integration either in sensory or in space-based coordinates, showing that the latter model variant led to biases in finger perception despite accurate representation of finger length. Behavioral measures of joint and fingertip localization responses showed similar biases, which were well fitted by the space-based, but not the sensory-based, model variant. The space-based model variant also outperformed a distorted hand model with built-in geometric biases. In total, our results suggest that perceptual distortions of finger geometry do not reflect a distorted hand model but originate from near-optimal Bayesian inference on somatosensory signals.

Visuotactile integration in individuals with fibromyalgia

Our brain constantly integrates afferent information, such as visual and tactile information, to perceive the world around us. According to the maximum-likelihood estimation (MLE) model, imprecise information will be weighted less than precise, making the multisensory percept as precise as possible. Individuals with fibromyalgia (FM), a chronic pain syndrome, show alterations in the integration of tactile information. This could lead to a decrease in their weight in a multisensory percept or a general disruption of multisensory integration, making it less beneficial. To assess multisensory integration, 15 participants with FM and 18 pain-free controls performed a temporal-order judgment task in which they received pairs of sequential visual, tactile (unisensory conditions), or visuotactile (multisensory condition) stimulations on the index and the thumb of the non-dominant hand and had to determine which finger was stimulated first. The task enabled us to measure the precision and accuracy of the percept in each condition. Results indicate an increase in precision in the visuotactile condition compared to the unimodal conditions in controls only, although we found no intergroup differences. The observed visuotactile precision was correlated to the precision predicted by the MLE model in both groups, suggesting an optimal integration. Finally, the weights of the sensory information were not different between the groups; however, in the group with FM, higher pain intensity was associated with smaller tactile weight. This study shows no alterations of the visuotactile integration in individuals with FM, though pain may influence tactile weight in these participants.

Hierarchical and dynamic relationships between body part ownership and full-body ownership

What is the relationship between experiencing individual body parts and the whole body as one's own? We theorised that body part ownership is driven primarily by the perceptual binding of visual and somatosensory signals from specific body parts, whereas full-body ownership depends on a more global binding process based on multisensory information from several body segments. To examine this hypothesis, we used a bodily illusion and asked participants to rate illusory changes in ownership over five different parts of a mannequin's body and the mannequin as a whole, while we manipulated the synchrony or asynchrony of visual and tactile stimuli delivered to three different body parts. We found that body part ownership was driven primarily by local visuotactile synchrony and could be experienced relatively independently of full-body ownership. Full-body ownership depended on the number of synchronously stimulated parts in a nonlinear manner, with the strongest full-body ownership illusion occurring when all parts received synchronous stimulation. Additionally, full-body ownership influenced body part ownership for nonstimulated body parts, and skin conductance responses provided physiological evidence supporting an interaction between body part and full-body ownership. We conclude that body part and full-body ownership correspond to different processes and propose a hierarchical probabilistic model to explain the relationship between part and whole in the context of multisensory awareness of one's own body.

A computational account of the development and evolution of psychotic symptoms

The mechanisms of psychotic symptoms like hallucinations and delusions are often investigated in fully-formed illness, well after symptoms emerge. These investigations have yielded key insights, but are not well-positioned to reveal the dynamic forces underlying symptom formation itself. Understanding symptom development over time would allow us to identify steps in the pathophysiological process leading to psychosis, shifting the focus of psychiatric intervention from symptom alleviation to prevention. We propose a model for understanding the emergence of psychotic symptoms within the context of an adaptive, developing neural system. We will make the case for a pathophysiological process that begins with cortical hyperexcitability and bottom-up noise transmission, which engenders inappropriate belief formation via aberrant prediction error signaling. We will argue that this bottom-up noise drives learning about the (im)precision of new incoming sensory information because of diminished signal-to-noise ratio, causing an adaptive relative over-reliance on prior beliefs. This over-reliance on priors predisposes to hallucinations and covaries with hallucination severity. An over-reliance on priors may also lead to increased conviction in the beliefs generated by bottom-up noise and drive movement toward conversion to psychosis. We will identify predictions of our model at each stage, examine evidence to support or refute those predictions, and propose experiments that could falsify or help select between alternative elements of the overall model. Nesting computational abnormalities within longitudinal development allows us to account for hidden dynamics among the mechanisms driving symptom formation and to view established symptomatology as a point of equilibrium among competing biological forces.

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.

Texture congruence modulates perceptual bias but not sensitivity to visuotactile stimulation during the rubber hand illusion

The sense of body ownership is the feeling that one's body belongs to oneself. To study body ownership, researchers use bodily illusions, such as the rubber hand illusion (RHI), which involves experiencing a visible rubber hand as part of one's body when the rubber hand is stroked simultaneously with the hidden real hand. The RHI is based on a combination of vision, touch, and proprioceptive information following the principles of multisensory integration. It has been posited that texture incongruence between rubber hand and real hand weakens the RHI, but the underlying mechanisms remain poorly understood. To investigate this, we recently developed a novel psychophysical RHI paradigm. Based on fitting psychometric functions, we discovered the RHI resulted in shifts in the point of subjective equality when the rubber hand and the real hand were stroked with matching materials. We analysed these datasets further by using signal detection theory analysis, which distinguishes between the participants' sensitivity to visuotactile stimulation and the associated perceptual bias. We found that texture incongruence influences the RHI's perceptual bias but not its sensitivity to visuotactile stimulation. We observed that the texture congruence bias effect was the strongest in shorter visuotactile asynchronies (50-100 ms) and weaker in longer asynchronies (200 ms). These results suggest texture-related perceptual bias is most prominent when the illusion's sensitivity is at its lowest. Our findings shed light on the intricate interactions between top-down and bottom-up processes in body ownership, the links between body ownership and multisensory integration, and the impact of texture congruence on the RHI.

Active self-touch restores bodily proprioceptive spatial awareness following disruption by 'rubber hand illusion'

Bodily self-awareness relies on a constant integration of visual, tactile, proprioceptive, and motor signals. In the 'rubber hand illusion' (RHI), conflicting visuo-tactile stimuli lead to changes in self-awareness. It remains unclear whether other, somatic signals could compensate for the alterations in self-awareness caused by visual information about the body. Here, we used the RHI in combination with robot-mediated self-touch to systematically investigate the role of tactile, proprioceptive and motor signals in maintaining and restoring bodily self-awareness. Participants moved the handle of a leader robot with their right hand and simultaneously received corresponding tactile feedback on their left hand from a follower robot. This self-touch stimulation was performed either before or after the induction of a classical RHI. Across three experiments, active self-touch delivered after-but not before-the RHI, significantly reduced the proprioceptive drift caused by RHI, supporting a restorative role of active self-touch on bodily self-awareness. The effect was not present during involuntary self-touch. Unimodal control conditions confirmed that both tactile and motor components of self-touch were necessary to restore bodily self-awareness. We hypothesize that active self-touch transiently boosts the precision of proprioceptive representation of the touched body part, thus counteracting the visual capture effects that underlie the RHI.

Facilitation of imitative movement in patients with chronic hemiplegia triggered by illusory ownership

The sense of body ownership, the feeling that one's body belongs to oneself, is a crucial subjective conscious experience of one's body. Recent methodological advances regarding crossmodal illusions have provided novel insights into how multisensory interactions shape human perception and cognition, underpinning conscious experience, particularly alteration of body ownership. Moreover, in post-stroke rehabilitation, encouraging the use of the paretic limb in daily life is considered vital, as a settled sense of ownership and attentional engagement toward the paralyzed body part may promote increased frequency of its use and prevent learned nonuse. Therefore, in addition to traditional methods, novel interventions using neurorehabilitation techniques that induce self-body recognition are needed. This study investigated whether the illusory experience of a patient's ownership alterations of their paretic hand facilitates the enhancement in the range of motion of succeeding imitation movements. An experiment combining a modified version of the rubber hand illusion with imitation training was conducted with chronic hemiplegia. A larger imitation movement of the paretic hand was observed in the illusion-induced condition, indicating that the feeling of ownership toward the observed limb promotes the induction of intrinsic potential for motor performance. This training, using subjective experience, may help develop new post-stroke rehabilitation interventions.