Two hands are better than one: a new assessment method and a new interpretation of the non-visual illusion of self-touch

A simple experimental paradigm creates the powerful illusion that one is touching one’s own hand even when the two hands are separated by 15 cm. The participant uses her right hand to administer stimulation to a prosthetic hand while the Examiner provides identical stimulation to the participant’s receptive left hand. Change in felt position of the receptive hand toward the prosthetic hand has previously led to the interpretation that the participant experiences self-touch at the location of the prosthetic hand, and experiences a sense of ownership of the prosthetic hand. Our results argue against this interpretation. We assessed change in felt position of the participant’s receptive hand but we also assessed change in felt position of the participant’s administering hand. Change in felt position of the administering hand was significantly greater than change in felt position of the receptive hand. Implications for theories of ownership are discussed.     

Touching my face with my supernumerary hand: a cheeky illusion

A self-touch paradigm elicits a surprising illusion. With the participant's eyes closed, the examiner guides the participant's right index finger to administer strokes and taps to the right side of the participant's face. At the same time, the examiner strokes and taps the corresponding location on the left side of the participant's face. Although the participant administered touch to only the right side of the face, this paradigm elicited the illusion of self-touch to both sides of the face, and the illusion often implicated a third, disconnected or disembodied, hand. We propose an explanation, and draw parallels with the phenomenon of supernumerary phantom limb.     

Roughness perception during the rubber hand illusion

Watching a rubber hand being stroked by a paintbrush while feeling identical stroking of one’s own occluded hand can create a compelling illusion that the seen hand becomes part of one’s own body. It has been suggested that this so-called rubber hand illusion (RHI) does not simply reflect a bottom–up multisensory integration process but that the illusion is also modulated by top–down, cognitive factors. Here we investigated for the first time whether the conceptual interpretation of the sensory quality of the visuotactile stimulation in terms of roughness can influence the occurrence of the illusion and vice versa, whether the presence of the RHI can modulate the perceived sensory quality of a given tactile stimulus (i.e., in terms of roughness). We used a classical RHI paradigm in which participants watched a rubber hand being stroked by either a piece of soft or rough fabric while they received synchronous or asynchronous tactile stimulation that was either congruent or incongruent with respect to the sensory quality of the material touching the rubber hand. (In)congruencies between the visual and tactile stimulation did neither affect the RHI on an implicit level nor on an explicit level, and the experience of the RHI in turn did not cause any modulations of the felt sensory quality of touch on participant’s own hand. These findings first suggest that the RHI seems to be resistant to top–down knowledge in terms of a conceptual interpretation of tactile sensations. Second, they argue against the hypothesis that participants own hand tends to disappear during the illusion and that the rubber hand actively replaces it.     

Spatial limits on the nonvisual self-touch illusion and the visual rubber hand illusion: Subjective experience of the illusion and proprioceptive drift

The nonvisual self-touch rubber hand paradigm elicits the compelling illusion that one is touching one’s own hand even though the two hands are not in contact. In four experiments, we investigated spatial limits of distance (15 cm, 30 cm, 45 cm, 60 cm) and alignment (0°, 90° anti-clockwise) on the nonvisual self-touch illusion and the well-known visual rubber hand illusion. Common procedures (synchronous and asynchronous stimulation administered for 60 s with the prosthetic hand at body midline) and common assessment methods were used. Subjective experience of the illusion was assessed by agreement ratings for statements on a questionnaire and time of illusion onset. The nonvisual self-touch illusion was diminished though never abolished by distance and alignment manipulations, whereas the visual rubber hand illusion was more robust against these manipulations. We assessed proprioceptive drift, and implications of a double dissociation between subjective experience of the illusion and proprioceptive drift are discussed.     

Touching my left elbow: the anatomical structure of the body affects the illusion of self-touch

A self-touch paradigm is used to create the illusion that one is touching one's own left elbow when one is actually touching the examiner's arm. Our new self-touch illusion is sensitive to the anatomical structure of the body: you can touch your left elbow with your right index finger but not with your left index finger. Illusion onset was faster and illusion ratings were higher when participants administered touch using the plausible right index finger compared with the implausible left index finger.     

我观故我在？— 从橡胶手错觉对自我身体所有权的探讨

橡胶手错觉是一种能将非自我的肢体感知为自我的肢体的反应。继发现橡胶手错觉现象后,研究者通过操纵自变量以及观测不同的因变量,得到了大量新的研究结果。橡胶手错觉的出现与强度受到时间、空间和手姿势的影响。橡胶手错觉的产生机制为单纯的多感觉整合或多感觉整合与身体表征共同作用的结果。未来研究侧重于被试取样、研究策略以及医学领域中瘫痪患者的认知神经康复和截肢病人的假肢控制方面。     

Visual capture of touch: out-of-the-body experiences with rubber gloves

When the apparent visual location of a body part conflicts with its veridical location, vision can dominate proprioception and kinesthesia. In this article, we show that vision can capture tactile localization. Participants discriminated the location of vibrotactile stimuli (upper, at the index finger, vs. lower, at the thumb), while ignoring distractor lights that could independently be upper or lower. Such tactile discriminations were slowed when the distractor light was incongruent with the tactile target (e.g., an upper light during lower touch) rather than congruent, especially when the lights appeared near the stimulated hand. The hands were occluded under a table, with all distractor lights above the table. The effect of the distractor lights increased when rubber hands were placed on the table, 'holding' the distractor lights, but only when the rubber hands were spatially aligned with the participant's own hands. In this aligned situation, participants were more likely to report the illusion of feeling touch at the rubber hands. Such visual capture of touch appears cognitively impenetrable.     

The rubber hand illusion: sensitivity and reference frame for body ownership

When subjects view stimulation of a rubber hand while feeling congruent stimulation of their own hand, they may come to feel that the rubber hand is part of their own body. This illusion of body ownership is termed 'Rubber Hand Illusion' (RHI). We investigated sensitivity of RHI to spatial mismatches between visual and somatic experience. We compared the effects of spatial mismatch between the stimulation of the two hands, and equivalent mismatches between the postures of the two hands. We created the mismatch either by adjusting stimulation or posture of the subject's hand, or, in a separate group of subjects, by adjusting stimulation or posture of the rubber hand. The matching processes underlying body ownership were asymmetrical. The illusion survived small changes in the subject's hand posture, but disappeared when the same posture transformations were applied to the rubber hand. Mismatch between the stimulation delivered to the subject's hand and the rubber hand abolished the illusion. The combination of these two situations is of particular interest. When the subject's hand posture was slightly different from the rubber hand posture, the RHI remained as long as stimulation of the two hands was congruent in a hand-centred spatial reference frame, even though the altered posture of the subject's hand meant that stimulation was incongruent in external space. Conversely, the RHI was reduced when the stimulation was incongruent in hand-centred space but congruent in external space. We conclude that the visual-tactile correlation that causes the RHI is computed within a hand-centred frame of reference, which is updated with changes in body posture. Current sensory evidence about what is 'me' is interpreted with respect to a prior mental body representation.     

Seeing the body distorts tactile size perception

Vision of the body modulates somatosensation, even when entirely non-informative about stimulation. For example, seeing the body increases tactile spatial acuity, but reduces acute pain. While previous results demonstrate that vision of the body modulates somatosensory sensitivity, it is unknown whether vision also affects metric properties of touch, and if so how. This study investigated how non-informative vision of the body modulates tactile size perception. We used the mirror box illusion to induce the illusion that participants were directly seeing their stimulated left hand, though they actually saw their reflected right hand. We manipulated whether participants: (a) had the illusion of directly seeing their stimulated left hand, (b) had the illusion of seeing a non-body object at the same location, or (c) looked directly at their non-stimulated right-hand. Participants made verbal estimates of the perceived distance between two tactile stimuli presented simultaneously to the dorsum of the left hand, either 20, 30, or 40 mm apart. Vision of the body significantly reduced the perceived size of touch, compared to vision of the object or of the contralateral hand. In contrast, no apparent changes of perceived hand size were found. These results show that seeing the body distorts tactile size perception.     

Vision of a pictorial hand modulates visual-tactile interactions

The participants in this study discriminated the position of tactile target stimuli presented at the tip or the base of the forefinger of one of the participants’ hands, while ignoring visual distractor stimuli. The visual distractor stimuli were presented from two circles on a display aligned with the tactile targets in Experiment 1 or orthogonal to them in Experiment 2. Tactile discrimination performance was slower and less accurate when the visual distractor stimuli were presented from incongruent locations relative to the tactile target stimuli (e.g., tactile target at the base of the finger with top visual distractor) highlighting a cross-modal congruency effect. We examined whether the presence and orientation of a simple line drawing of a hand, which was superimposed on the visual distractor stimuli, would modulate the cross-modal congruency effects. When the tactile targets and the visual distractors were spatially aligned, the modulatory effects of the hand picture were small (Experiment 1). However, when they were spatially misaligned, the effects were much larger, and the direction of the cross-modal congruency effects changed in accordance with the orientation of the picture of the hand, as if the hand picture corresponded to the participants’ own stimulated hand (Experiment 2). The results suggest that the two-dimensional picture of a hand can modulate processes maintaining our internal body representation. We also observed that the cross-modal congruency effects were influenced by the postures of the stimulated and the responding hands. These results reveal the complex nature of spatial interactions among vision, touch, and proprioception.     

Sensitivity to auditory‐tactile colocation in early infancy

An ability to detect the common location of multisensory stimulation is essential for us to perceive a coherent environment, to represent the interface between the body and the external world, and to act on sensory information. Regarding the tactile environment “at hand”, we need to represent somatosensory stimuli impinging on the skin surface in the same spatial reference frame as distal stimuli, such as those transduced by vision and audition. Across two experiments we investigated whether 6‐ (n = 14; Experiment 1) and 4‐month‐old (n = 14; Experiment 2) infants were sensitive to the colocation of tactile and auditory signals delivered to the hands. We recorded infants’ visual preferences for spatially congruent and incongruent auditory‐tactile events delivered to their hands. At 6 months, infants looked longer toward incongruent stimuli, whilst at 4 months infants looked longer toward congruent stimuli. Thus, even from 4 months of age, infants are sensitive to the colocation of simultaneously presented auditory and tactile stimuli. We conclude that 4‐ and 6‐month‐old infants can represent auditory and tactile stimuli in a common spatial frame of reference. We explain the age‐wise shift in infants’ preferences from congruent to incongruent in terms of an increased preference for novel crossmodal spatial relations based on the accumulation of experience. A comparison of looking preferences across the congruent and incongruent conditions with a unisensory control condition indicates that the ability to perceive auditory‐tactile colocation is based on a crossmodal rather than a supramodal spatial code by 6 months of age at least.     

Application of the rubber hand illusion paradigm: comparison between upper and lower limbs

The “rubber hand illusion (RHI)” is a perceptual illusion, which allows the integration of artificial limbs into the body representation of a person by means of combined visual and tactile stimulation. The illusion has been frequently replicated but always concerning the upper limbs. The present study verified an analog illusion that can be called the “rubber foot illusion” (RFI). In a conjoint experiment using both a rubber hand and a rubber foot, brushstrokes were applied to the respective real and rubber limb placed alongside the real one. However, only the artificial limb’s handling was visible. The brushstrokes were given either synchronously, with a delay of ±0.5 s, or without tactile stimulation of the real limb. Questionnaire data and the proprioceptive drift towards the rubber limb (determined by calling on the subjects to show where they locate their unseen limb) defined the illusion strength. Results revealed that the illusion was induced in both limbs with comparable strength, but only in the synchronous condition.     

Prosthesis embodiment and attenuation of prosthetic touch in upper limb amputees – A proof-of-concept study

Sensory attenuation of self-touch, that is, the perceptual reduction of self-generated tactile stimuli, is considered a neurocognitive basis for self-other distinction. However, whether this effect can also be found in upper limb amputees using a prosthesis is unknown. Thirteen participants were asked to touch their foot sole with a) their intact hand (self-touch), b) their prosthesis (prosthesis-touch), or c) let it be touched by another person (other-touch). Intensity of touch was assessed with a questionnaire. In addition, prosthesis embodiment was assessed in nine participants. Self-touch as well as prosthesis-touch was characterized by significant perceptual attenuation compared to other-touch, while self- and prosthesis-touch did not differ. The more embodied the prosthesis was, the more similar was its elicited touch perception to actual self-touch. These findings – although preliminary – suggest that perceptually embodied prostheses can be represented as an actual limb by the users’ sensorimotor system.     

Having a body versus moving your body: How agency structures body-ownership

We investigated how motor agency in the voluntary control of body movement influences body awareness. In the Rubber Hand Illusion (RHI), synchronous tactile stimulation of a rubber hand and the participant's hand leads to a feeling of the rubber hand being incorporated in the participant's own body. One quantifiable behavioural correlate of the illusion is an induced shift in the perceived location of the participant's hand towards the rubber hand. Previous studies showed that the induced changes in body awareness are local and fragmented: the proprioceptive drift is largely restricted to the stimulated finger. In the present study, we investigated whether active and passive movements, rather than tactile stimulation, would lead to similarly fragmented body awareness. Participants watched a projected image of their hand under three conditions: active finger movement, passive finger movement, and tactile stimulation. Visual feedback was either synchronous or asynchronous with respect to stimulation of the hand. A significant overall RHI, defined as greater drifts following synchronous than asynchronous stimulation, was found in all cases. However, the distribution of the RHI across stimulated and non-stimulated fingers depended on the kind of stimulation. Localised proprioceptive drifts, specific to the stimulated finger, were found for tactile and passive stimulation. Conversely, during active movement of a single digit, the proprioceptive drifts were not localised to that digit, but were spread across the whole hand. Whereas a purely proprioceptive sense of body-ownership is local and fragmented, the motor sense of agency integrates distinct body-parts into a coherent, unified awareness of the body.     

Spatial specificity of tactile enhancement during reaching

Tactile signals on a hand that serves as movement goal are enhanced during movement planning and execution. Here, we examined how spatially specific tactile enhancement is when humans reach to their own static hand. Participants discriminated two brief and simultaneously presented tactile stimuli: a comparison stimulus on the left thumb or little finger from a reference stimulus on the sternum. Tactile stimuli were presented either during right-hand reaching towards the left thumb or little finger or while holding both hands still (baseline). Consistent with our previous findings, stimuli on the left hand were perceived stronger during movement than baseline. However, tactile enhancement was not stronger when the stimuli were presented on the digit that served as reach target, thus the perception across the whole hand was uniformly enhanced. In experiment 2, we also presented stimuli on the upper arm in half of the trials to reduce the expectation of the stimulus location. Tactile stimuli on the target hand, but not on the upper arm, were generally enhanced, supporting the idea of a spatial gradient of tactile enhancement. Overall, our findings argue for low spatial specificity of tactile enhancement at movement-relevant body parts, here the target hand.     

The influence of exploration mode, orientation, and configuration on the haptic Müller-Lyer illusion

We studied the impact of manner of exploration, orientation, spatial position, and configuration on the haptic Müller-Lyer illusion. Blindfolded sighted subjects felt raised-line Müller-Lyer and control stimuli. The stimuli were felt by tracing with the index finger, free exploration, grasping with the index finger and thumb, or by measuring with the use of any two or more fingers. For haptic judgments of extent a sliding tangible ruler was used. The illusion was present in all exploration conditions, with overestimation of the wings-out compared to wings-in stimuli. Tracing with the index finger reduced the magnitude of the illusion. However, tracing and grasping induced an overall underestimation of size. The illusion was greatly attenuated when stimuli were felt with the index fingers of both hands. Illusory misperception was not altered by the position in space of the Müller-Lyer stimuli. No effects of changes in the thickness of the line shaft were found, but there were effects of the length of the wing endings for the smaller, 5.1 cm stimuli. The theoretical and practical implications of the results are discussed.     

Spatial constraints on visual-tactile cross-modal distractor congruency effects

Across three experiments, participants made speeded elevation discrimination responses to vibrotactile targets presented to the thumb (held in a lower position) or the index finger (upper position) of either hand, while simultaneously trying to ignore visual distractors presented independently from either the same or a different elevation. Performance on the vibrotactile elevation discrimination task was slower and less accurate when the visual distractor was incongruent with the elevation of the vibrotactile target (e.g., a lower light during the presentation of an upper vibrotactile target to the index finger) than when they were congruent, showing that people cannot completely ignore vision when selectively attending to vibrotactile information. We investigated the attentional, temporal, and spatial modulation of these cross-modal congruency effects by manipulating the direction of endogenous tactile spatial attention, the stimulus onset asynchrony between target and distractor, and the spatial separation between the vibrotactile target, any visual distractors, and the participant’s two hands within and across hemifields. Our results provide new insights into the spatiotemporal modulation of crossmodal congruency effects and highlight the utility of this paradigm for investigating the contributions of visual, tactile, and proprioceptive inputs to the multisensory representation of peripersonal space.     

Metrical congruency and kinematic familiarity facilitate temporal binding between musical and dance rhythms

Although music and dance are often experienced simultaneously, it is unclear what modulates their perceptual integration. This study investigated how two factors related to music–dance correspondences influenced audiovisual binding of their rhythms: the metrical match between the music and dance, and the kinematic familiarity of the dance movement. Participants watched a point-light figure dancing synchronously to a triple-meter rhythm that they heard in parallel, whereby the dance communicated a triple (congruent) or a duple (incongruent) visual meter. The movement was either the participant’s own or that of another participant. Participants attended to both streams while detecting a temporal perturbation in the auditory beat. The results showed lower sensitivity to the auditory deviant when the visual dance was metrically congruent to the auditory rhythm and when the movement was the participant’s own. This indicated stronger audiovisual binding and a more coherent bimodal rhythm in these conditions, thus making a slight auditory deviant less noticeable. Moreover, binding in the metrically incongruent condition involving self-generated visual stimuli was correlated with self-recognition of the movement, suggesting that action simulation mediates the perceived coherence between one’s own movement and a mismatching auditory rhythm. Overall, the mechanisms of rhythm perception and action simulation could inform the perceived compatibility between music and dance, thus modulating the temporal integration of these audiovisual stimuli.     

On the inability to ignore touch when responding to vision in the crossmodal congruency task

We investigated the extent to which people can selectively ignore distracting vibrotactile information when performing a visual task. In Experiment 1, participants made speeded elevation discrimination responses (up vs. down) to a series of visual targets presented from one of two eccentricities on either side of central fixation, while simultaneously trying to ignore task-irrelevant vibrotactile distractors presented independently to the finger (up) vs. thumb (down) of either hand. Participants responded significantly more slowly, and somewhat less accurately, when the elevation of the vibrotactile distractor was incongruent with that of the visual target than when they were presented from the same (i.e., congruent) elevation. This crossmodal congruency effect was significantly larger when the visual and tactile stimuli appeared on the same side of space than when they appeared on different sides, although the relative eccentricity of the two stimuli within the hemifield (i.e., same vs. different) had little effect on performance. In Experiment 2, participants who crossed their hands over the midline showed a very different pattern of crossmodal congruency effects to participants who adopted an uncrossed hands posture. Our results suggest that both the relative external location and the initial hemispheric projection of the target and distractor stimuli contribute jointly to determining the magnitude of the crossmodal congruency effect when participants have to respond to vision and ignore touch.     

Part‐based representations of the body in early childhood: evidence from perceived distortions of tactile space across limb boundaries

Studies show that touch in adults is referenced to a representation of the body that is structured topologically according to body parts; the perceived distance between two stimuli crossing over a body part boundary is elongated relative to the perceived distance between two stimuli presented within one body part category. Here we investigate this influence of body parts on tactile space perception in children of 5, 6 and 7 years of age. We presented children with pairs of tactile stimuli on the left hand/arm, either within the hand, within the forearm, or over the wrist. With their eyes closed children were asked to adjust the distance between the thumb and forefinger of their right hand to represent the felt distance between the two tactile stimuli. Like adults, the children perceived the distance between two stimuli that cross the body part boundary to be further apart than those that were presented within the hand or arm. They also perceive tactile distance to be greater on the hand than the arm which is the first observation of Weber's illusion in young children. We propose that a topological mode of body representation is particularly advantageous during early life given that body part categories remain constant while the metric proportions of the body change substantially as the child grows.