Separate multisensory integration processes for ownership and localization of body parts

Audio visual stimuli based out of body illusion

Previous studies have reported that bodily self-consciousness could be altered so that one's body was perceived in extra-personal space. However, whether this could be induced without tactile stimuli has not been investigated. We investigated whether out-of-body illusion could be induced via synchronized audio-visual stimuli, in which auditory stimuli were used instead of tactile stimuli. We conducted an experiment in which a sounding bell was moved in front of the participant, and synchronously, a non-sounding bell was moved in front of a camera that captured the image and projected on a head-mounted device. We expected the participants to experience that the sound came from the non-sounding bell in the video and they were in the camera's position. Results from the questionnaires conducted after the experiment revealed that items related to out-of-body illusion were significantly enhanced in the synchronized conditions. Furthermore, participants reported a similarly strong out-of-body illusion for both the synchronized audio-visual and tactile-visual stimuli. This study demonstrated that out-of-body illusion could also be induced by synchronized audio-visual stimuli, which was a novel finding.

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.

Multiple representations of the body schema for the same body part

Accurate motor control depends on maps of the body in the brain, called the body schema. Disorders of the body schema cause motor deficits. Although we often execute actions with different motor systems such as the eye and hand, how the body schema operates during such actions is unknown. In this study, participants simultaneously directed eye and hand movements to the same body part. These two movements were found to be guided by different body maps. This finding demonstrates multiple motor system–specific representations of the body schema, suggesting that the choice of motor system toward one’s body can determine which of the brain’s body maps is observed. This may offer a new way to visualize patients’ body schema.

Purposeful motor actions depend on the brain’s representation of the body, called the body schema, and disorders of the body schema have been reported to show motor deficits. The body schema has been assumed for almost a century to be a common body representation supporting all types of motor actions, and previous studies have considered only a single motor action. Although we often execute multiple motor actions, how the body schema operates during such actions is unknown. To address this issue, I developed a technique to measure the body schema during multiple motor actions. Participants made simultaneous eye and reach movements to the same location of 10 landmarks on their hand. By analyzing the internal configuration of the locations of these points for each of the eye and reach movements, I produced maps of the mental representation of hand shape. Despite these two movements being simultaneously directed to the same bodily location, the resulting hand map (i.e., a part of the body schema) was much more distorted for reach movements than for eye movements. Furthermore, the weighting of visual and proprioceptive bodily cues to build up this part of the body schema differed for each effector. These results demonstrate that the body schema is organized as multiple effector-specific body representations. I propose that the choice of effector toward one’s body can determine which body representation in the brain is observed and that this visualization approach may offer a new way to understand patients’ body schema.

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.

Effect of Avatar Appearance on Detection Thresholds for Remapped Hand Movements

Hand interaction techniques in virtual reality often exploit visual dominance over proprioception to remap physical hand movements onto different virtual movements. However, when the offset between virtual and physical hands increases, the remapped virtual hand movements are hardly self-attributed, and the users become aware of the remapping. Interestingly, the sense of self-attribution of a body is called the sense of body ownership (SoBO) in the field of psychology, and the realistic the avatar, the stronger is the SoBO. Hence, we hypothesized that realistic avatars (i.e., human hands) can foster self-attribution of the remapped movements better than abstract avatars (i.e., spherical pointers), thus making the remapping less noticeable. In this article, we present an experiment in which participants repeatedly executed reaching movements with their right hand while different amounts of horizontal shifts were applied. We measured the remapping detection thresholds for each combination of shift directions (left or right) and avatar appearances (realistic or abstract). The results show that realistic avatars increased the detection threshold (i.e., lowered sensitivity) by 31.3 percent than the abstract avatars when the leftward shift was applied (i.e., when the hand moved in the direction away from the body-midline). In addition, the proprioceptive drift (i.e., the displacement of self-localization toward an avatar) was larger with realistic avatars for leftward shifts, indicating that visual information was given greater preference during visuo-proprioceptive integration in realistic avatars. Our findings quantifiably show that realistic avatars can make remapping less noticeable for larger mismatches between virtual and physical movements and can potentially improve a wide variety of hand-remapping techniques without changing the mapping itself.

Awareness of voluntary action, rather than body ownership, improves motor control

Awareness of the body is essential for accurate motor control. However, how this awareness influences motor control is poorly understood. The awareness of the body includes awareness of visible body parts as one’s own (sense of body ownership) and awareness of voluntary actions over that visible body part (sense of agency). Here, I show that sense of agency over a visible hand improves the initiation of movement, regardless of sense of body ownership. The present study combined the moving rubber hand illusion, which allows experimental manipulation of agency and body ownership, and the finger-tracking paradigm, which allows behavioral quantification of motor control by the ability to coordinate eye with hand movements. This eye–hand coordination requires awareness of the hand to track the hand with the eye. I found that eye–hand coordination is improved when participants experience a sense of agency over a tracked artificial hand, regardless of their sense of body ownership. This improvement was selective for the initiation, but not maintenance, of eye–hand coordination. These results reveal that the prospective experience of explicit sense of agency improves motor control, suggesting that artificial manipulation of prospective agency may be beneficial to rehabilitation and sports training techniques.

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.

An Investigation of Lower Limb Representations Underlying Vision, Touch, and Proprioception in Body Integrity Identity Disorder

Individuals with Body Integrity Identity Disorder (BIID) have a (non-psychotic) longstanding desire to amputate or paralyze one or more fully-functioning limbs, often the legs. This desire presumably arises from experiencing a mismatch between one's perceived mental image of the body and the physical structural and/or functional boundaries of the body itself. While neuroimaging studies suggest a disturbed body representation network in individuals with BIID, few behavioral studies have looked at the manifestation of this disrupted lower limb representations in this population. Specifically, people with BIID feel like they are overcomplete in their current body. Perhaps sensory input, processed normally on and about the limb, cannot communicate with a higher-order model of the leg in the brain (which might be underdeveloped). We asked individuals who desire paralysis or amputation of the lower legs (and a group of age- and sex-matched controls) to make explicit and implicit judgments about the size and shape of their legs while relying on vision, touch, and proprioception. We hypothesized that BIID participants would mis-estimate the size of their affected leg(s) more than the same leg of controls. Using a multiple single-case analysis, we found no global differences in lower limb representations between BIID participants and controls. Thus, while people with BIID feel that part of the body is foreign, they can still make normal sensory-guided implicit and explicit judgments about the limb. Moreover, these results suggest that BIID is not a body image disorder, per se, and that an examination of leg representation does not uncover the disturbed bodily experience that individuals with BIID have.