Bodily Illusions Research Articles

Keeping in touch with one's self: multisensory mechanisms of self-consciousness.

BackgroundThe spatial unity between self and body can be disrupted by employing conflicting visual-somatosensory bodily input, thereby bringing neurological observations on bodily self-consciousness under scientific scrutiny. Here we designed a novel paradigm linking the study of bodily self-consciousness to the spatial representation of visuo-tactile stimuli by measuring crossmodal congruency effects (CCEs) for the full body.Methodology/Principal FindingsWe measured full body CCEs by attaching four vibrator-light pairs to the trunks (backs) of subjects who viewed their bodies from behind via a camera and a head mounted display (HMD). Subjects made speeded elevation (up/down) judgments of the tactile stimuli while ignoring light stimuli. To modulate self-identification for the seen body subjects were stroked on their backs with a stick and the felt stroking was either synchronous or asynchronous with the stroking that could be seen via the HMD.We found that (1) tactile stimuli were mislocalized towards the seen body (2) CCEs were modulated systematically during visual-somatosensory conflict when subjects viewed their body but not when they viewed a body-sized object, i.e. CCEs were larger during synchronous than during asynchronous stroking of the body and (3) these changes in the mapping of tactile stimuli were induced in the same experimental condition in which predictable changes in bodily self-consciousness occurred.Conclusions/SignificanceThese data reveal that systematic alterations in the mapping of tactile stimuli occur in a full body illusion and thus establish CCE magnitude as an online performance proxy for subjective changes in global bodily self-consciousness.

Rubber Hand Illusions and Size–Weight Illusions: Self-Representation Modulates Representation of External Objects

Bodily illusions offer an experimental method to investigate the origins and functional role of the sense of one's own body. Using the rubber hand illusion (RHI) we show that a representation of one's own body is implicitly used to calibrate perception of external objects. Twelve participants experienced the RHI while watching stimulation of a large or small glove simultaneously with stimulation of their own hand. They then grasped cylinders of identical size but varying weight. RHI with the large glove caused the cylinders to feel heavier. We suggest that an illusory increase in hand size made the subsequently grasped cylinder feel correspondingly small, evoking a size-weight illusion. Self-representation thus influenced exteroception. The sense of one's own body provides a fundamental reference for perception in general.

Specificity and Coherence of Body Representations

Bodily illusions differently affect body representations underlying perception and action. We investigated whether this task dependence reflects two distinct dimensions of embodiment: the sense of agency and the sense of the body as a coherent whole. In experiment 1 the sense of agency was manipulated by comparing active versus passive movements during the induction phase in a video rubber hand illusion (vRHI) setup. After induction, proprioceptive biases were measured both by perceptual judgments of hand position, as well as by measuring end-point accuracy of subjects' active pointing movements to an external object with the affected hand. The results showed, first, that the vRHI is largely perceptual: passive perceptual localisation judgments were altered, but end-point accuracy of active pointing responses with the affected hand to an external object was unaffected. Second, within the perceptual judgments, there was a novel congruence effect, such that perceptual biases were larger following passive induction of vRHI than following active induction. There was a trend for the converse effect for pointing responses, with larger pointing bias following active induction. In experiment 2, we used the traditional RHI to investigate the coherence of body representation by synchronous stimulation of either matching or mismatching fingers on the rubber hand and the participant's own hand. Stimulation of matching fingers induced a local proprioceptive bias for only the stimulated finger, but did not affect the perceived shape of the hand as a whole. In contrast, stimulation of spatially mismatching fingers eliminated the RHI entirely. The present results show that (i) the sense of agency during illusion induction has specific effects, depending on whether we represent our body for perception or to guide action, and (ii) representations of specific body parts can be altered without affecting perception of the spatial configuration of the body as a whole.

Clothing the homunculus

The concept of the homunculus, or `body in the brain', has been used by neuroscientists to explain bodily illusions such as the Shrinking Waist, the disappearance of limbs from conscious perception with stroke, and the sensations of phantom limbs. In this article, the author proposes that the homunculus can also provide a theoretical basis for the design of wearable interfaces and information systems. This idea is explored in a series of practice-led experiments with textile interfaces. The first experiment, called ZiZi the Affectionate Couch, is a couch that responds with a tactile `purring' when it is stroked. Visitors to exhibitions of the couch have commented on the positive effects this tactile responsiveness has had on autistic children in their care. The next experiment, called Scruffy Scallyscrap, is a prototype of an actively tactile textile, called a `taxtile', that co-locates tactile input and output in the same textile substrate. The most recent development is a `nerve extension button' designed to connect a taxtile to the tactile sensations of the wearer. Ten of these buttons have been sewn on the inside of a fashion concept garment called Fauxy the Fake Fur with Feelings. In further work, Fauxy will be used to explore what effects these nerve extensions might have on the bodily perceptions and behaviours of the wearer.

The role of the right temporo-parietal junction in maintaining a coherent sense of one's body

We constantly feel, see and move our body, and have no doubt that it is our own. The brain possesses a distinction between the body and the objects in the outside world. This distinction may be based on a process that monitors whether sensations, events and objects should be attributed to one's body or not. We controlled whether an external object was represented as part of the body or not, by experimentally inducing a bodily illusion using correlated visual and tactile stimulation. We then studied the role of right temporo-parietal junction (rTPJ) in the processing of multisensory events that may or may not be attributed to one's body. Disruption of rTPJ using transcranial magnetic stimulation (TMS) made the distinction between what may or may not be part of one's body on the basis of multisensory evidence more ambiguous, suggesting that the rTPJ is actively involved in maintaining a coherent sense of one's body, distinct from external, non-corporeal, objects.

Human Superior Parietal Lobule Is Involved in Somatic Perception of Bimanual Interaction With an External Object

The question of how the brain represents the spatial relationship between the own body and external objects is fundamental. Here we investigate the neural correlates of the somatic perception of bimanual interaction with an external object. A novel bodily illusion was used in conjunction with functional magnetic resonance imaging (fMRI). During fMRI scanning, seven blindfolded right-handed participants held a cylinder between the palms of the two hands while the tendon of the right wrist extensor muscle was vibrated. This elicited a kinesthetic illusion that the right hand was flexing and that the hand-held cylinder was shrinking from the right side. As controls, we vibrated the skin surface over the nearby bone beside the tendon or vibrated the tendon when the hands were not holding the object. Neither control condition elicited this illusion. The significance of the illusion was also confirmed in supplementary experiments outside the scanner on another 16 participants. The "bimanual shrinking-object illusion" activated anterior parts of the superior parietal lobule (SPL) bilaterally. This region has never been activated in previous studies on unimanual hand or hand-object illusion. The illusion also activated left-hemispheric brain structures including area 2 and inferior parietal lobule, an area related to illusory unimanual hand-object interaction between a vibrated hand and a touched object in our previous study. The anterior SPL seems to be involved in the somatic perception of bimanual interaction with an external object probably by computing the spatial relationship between the two hands and a hand-held object.

Specular Right-left Disorientation, Finger-agnosia, and Asomatognosia in Right Hemisphere Stroke

To search for specular disorders of body representations in right hemisphere stroke. Mirror self-misidentification, asomatognosia, and personal confabulation are similar to body illusions or changes in sensorial or sensorimotor perceptions generated by mirror in right brain damage patients with body image disorders. Prospective study. Ten consecutive right-handed patients (1) performed body part naming and localization tasks and (2) were examined for asomatognosia at the acute phase of stroke, then at least 3 months after stroke, under 3 test conditions: without a mirror, with a conventional mirror, and with an inverted mirror. Video recordings of the tests were analyzed to assess performance. Analysis of variance of the data confirmed that the interaction of mirror's conditions (specifically without a mirror vs. an inverted mirror) with subtest type was significant. The errors are symmetrically distributed. Asomatognosia was "reactivated" in 10 patients who experienced asomatognosia during the acute phase. No particular pattern characterized the clinical manifestations of asomatognosia. A causal conflict of sensorial input is proposed. The specific symptoms observed would suggest the existence of an incomplete specular Gerstmann syndrome and/or Anton Babinski syndrome. These results emphasize the role of specular input in the generation of body representations and self-awareness.

Sensory Neuroscience: From Skin to Object in the Somatosensory Cortex

Humans can perceive the shape of objects by touch alone, by extracting geometric features such as edges. Recently recorded responses of single neurons in the secondary somatosensory cortex of monkeys suggest how the brain integrates tactile shape information across different regions of skin and builds up a representation of tactile objects.

Bodily Illusions Modulate Tactile Perception

Touch differs from other exteroceptive senses in that the body itself forms part of the tactile percept. Interactions between proprioception and touch provide a powerful way to investigate the implicit body representation underlying touch. Here, we demonstrate that an intrinsic primary quality of a tactile object, for example its size, is directly affected by the perceived size of the body part touching it. We elicited proprioceptive illusions that the left index finger was either elongating or shrinking by vibrating the biceps or triceps tendon of the right arm while subjects grasped the tip of their left index finger. Subjects estimated the distance between two simultaneous tactile contacts on the left finger during tendon vibration. We found that tactile distances feel bigger when the touched body part feels elongated. Control tests showed that the modulation of touch was linked to the perceived index-finger size induced by tendon vibration. Vibrations that did not produce proprioceptive illusion had no effect on touch. Our results show that the perception of tactile objects is referenced to an implicit body representation and that proprioception contributes to this body representation. We also provide, for the first time, a quantitative, implicit measure of distortions of body size.

Vestibular caloric stimulation evokes phantom limb illusions in patients with paraplegia.

Prospective study. To determine the mechanisms of body illusions in paraplegia patients as compared with the amputee phantom phenomena. A vestibular caloric stimulation was performed in 10 consecutive patients with complete section of the spinal cord. Perception of body, before and after stimulation, was classed as illusion of a normal body (lower limbs with normal morphological, postural and kinetic characteristics perceived as before spinal injury), normal phantom (overly vivid perception of all or part of the lower limbs), deformed phantom (perception of all or part of the limbs below the injury level as abnormal in shape, posture, movement or even number), or painful phantom. After vestibular caloric stimulation, nine out of 10 patients stated their perception of body segments below the injury level had changed to normal phantoms or to deformed phantoms (morphological, postural or kinetic changes). Among the four patients who initially had painful limbs, two stated the stimulation greatly relieved their pain. The normal or deformed phantom evoked by vestibular stimulation would result from use of identity data or instantaneous data as is observed in amputees. Cerebral remapping following deafferentation could be the origin of the deformed phantoms. Illusions corresponding to phenomena perceived at the time of the accident corresponding to autobiographical engrammes do not appear to be evoked by vestibular stimulation, as is also the case in amputees.

Body dissatisfaction adjusted for weight: The body illusion index

The current study extends earlier observations that body dissatisfaction is positively associated with body weight for eating disorder patients and for nonpatient college women. An adjustment to the Eating Disorder Inventory (EDI) Body Dissatisfaction score (i.e., the “Body Illusion Index” or “Bll”) is proposed to statistically eliminate the effects of relative body weight on Body Dissatisfaction scores and to systematically increase Body Dissatisfaction scores at lower relative weights. The adjustment results in a mean adjusted body dissatisfaction score for the anorexia nervosa patients more closely resembling that for bulimia ner-vosa patients and differing significantly from normal weight college women. The “Body Illusion Index” addresses the different meanings and clinical significance that body dissatisfaction implies for individuals at different weights. © 1992 John Wiley & Sons, Inc.