Body ownership and kinaesthetic illusions: Dissociated bodily experiences for distinct levels of body consciousness?

Seeing an embodied humanoid avatar move its arms can induce in the observer the illusion that its own (static) arms are moving accordingly, the kinematic signals emanating from this avatar thus being considered like those from the biological body. Here, we investigated the causal relationship between these kinaesthetic illusions and the illusion of body ownership, manipulated through visuomotor synchronisation. The results of two experiments revealed that the sense of body ownership over an avatar seen from a first-person perspective was intimately linked to visuomotor synchrony. This was not the case for kinaesthetic illusions indicating that when superimposed on the biological body, the avatar is inevitably treated at the sensorimotor level as one's own body, whether consciously considered as such or not. The question of whether these two bodily experiences (body ownership and kinaesthetic illusion) are underpinned by distinct representations, the body image, and the body schema, is discussed.

Impact of an overweight body representation in virtual reality on locomotion in a motor imagery task.

Virtual reality immersion enables a person to embody avatars that strongly deviate from his/her biological body. Interestingly, the person's expectations about the embodied avatar lead to congruous behavior, phenomenon referred to as the Proteus effect. The objective of the present study was to investigate, in virtual reality, the relationship between body-shape representation and expected physical abilities in a locomotor imagery task, in the context of overweight avatar embodiment. Given the negative stereotypes concerning overweight people's physical abilities, we expected overweight avatar embodiment to have a negative impact on performance in the locomotor imagery task. Thirty-five healthy-weight participants, with a body mass index between 16.5 and 30 at the time of the experiment or in the past, embodied both a healthy-weight avatar and an overweight avatar on two different experimental sessions while performing the imagery task (walking four different distances on two different slopes). In accordance with our hypothesis, participants took longer to perform the locomotor imagery task when embodying an overweight avatar than when embodying a healthy-weight one (the "avatar effect")-especially so when the distance to be covered was long. We conclude that, as has already been reported for people with anorexia nervosa, considering one's own body to be fatter than it really is leads to congruent weight-related behavior.

The impact of embodying an "elderly" body avatar on motor imagery.

When an individual embodies an avatar, the latter's characteristics or stereotype can change the individual's behavior and attitudes; this is known as the Proteus effect. Here, we looked at whether the embodiment of an avatar resembling an elderly adult (seen from a first-person perspective and facing a virtual mirror) changed mentally represented physical activity in a motor imagery task performed by young adult participants (N = 52). To ensure that the impact of embodiment of an elderly avatar on the motor imagery task was not influenced by a potentially confounded stereotype assimilation effect (due to the mere presence of an avatar), a "young" avatar and an "elderly" avatar were always present together in the virtual environment-even though only one (the self-avatar) was embodied at a given time. We found that it took longer for the participants to perform the motor imagery task with the elderly self-avatar than with the young self-avatar. The more negative the participant's beliefs about motor activity in the elderly, the greater the observed effect of the avatar on motor imagery performance. We conclude that knowledge about the characteristics of an embodied avatar can modify the subject's level of mentally represented physical activity.

Inter- and Intra-Rater Reliability of the Visual Vertical in Subacute Stroke.

BACKGROUND AND PURPOSE: Visual vertical (VV) has been used increasingly as a routine clinical assessment to identify alteration of verticality perception as a possible cause of postural disorders after stroke. This study aims to determine whether the reliability of VV is sufficient to support a wide clinical use in neurorehabilitation for monitoring of patients with stroke. METHODS: Twenty patients with subacute stroke in neurorehabilitation unit were tested after a first and unique hemispheric stroke. To evaluate the inter-rater reliability, VV was assessed the same day by 2 examiners whose degrees of expertise differed. The second examiner repeated the test the next day to investigate intrarater reliability. VV orientation (mean, primary criterion) and uncertainty (SD, secondary criterion) were calculated for 10 trials. Their reliability was quantified by the intraclass correlation coefficient, Bland-Altman plots, and the minimal detectable change. The concordance between 2 examiners was quantified by Cohen's κ coefficients (κ). RESULTS: About VV orientation, inter- and intrarater reliability were excellent (intraclass correlation coefficient, 0.979 and 0.982). The Bland-Altman plots and the minimal detectable change revealed a difference inferior to 2° between 2 tests. The concordance between 2 assessments for the diagnosis of abnormal VV orientation was absolute for the same examiner (κ=1; P<0.05) and excellent between 2 examiners (κ=0.92; P<0.05). As for VV uncertainty the intrarater reliability was satisfactory (intraclass correlation coefficient, 0.836) but the inter-rater reliability was poor (intraclass correlation coefficient, 0.211). CONCLUSIONS: The orientation of the VV is a highly reliable criterion, which may be used both in research and in routine clinical practice.

UniPseudo: A universal pseudoword generator.

Pseudowords are letter strings that look like words but are not words. They are used in psycholinguistic research, particularly in tasks such as lexical decision. In this context, it is essential that the pseudowords respect the orthographic statistics of the target language. Pseudowords that violate them would be too easy to reject in a lexical decision and would not enforce word recognition on real words. We propose a new pseudoword generator, UniPseudo, using an algorithm based on Markov chains of orthographic n-grams. It generates pseudowords from a customizable database, which allows one to control the characteristics of the items. It can produce pseudowords in any language, in orthographic or phonological form. It is possible to generate pseudowords with specific characteristics, such as frequency of letters, bigrams, trigrams, or quadrigrams, number of syllables, frequency of biphones, and number of morphemes. Thus, from a list of words composed of verbs, nouns, adjectives, or adverbs, UniPseudo can create pseudowords resembling verbs, nouns, adjectives, or adverbs in any language using an alphabetic or syllabic system.

Differing effects of prosaccades and antisaccades on postural stability.

The goal of the study was to examine the effect of different types of eye movements on postural stability. Ten healthy young adults (25 ± 3 years) participated in the study. Postural control was measured by the TechnoConcept© platform and recorded in Standard Romberg and Tandem Romberg conditions while participants performed five oculomotor tasks: two fixation tasks (central fixation cross, without and with distractors), two prosaccade tasks toward peripheral targets displayed 4° to the left or to the right of the fixation cross (reactive saccades induced by a gap 0 ms paradigm and voluntary saccades induced by an overlap 600 ms paradigm) and one antisaccade task (voluntary saccade made in the opposite direction of the visual target). The surface, the length, and the mean speed of the center of pressure were analyzed. We found that saccadic eye movements improved postural stability with respect to the fixation tasks. Furthermore, antisaccades were found to decrease postural stability compared to prosaccades (reactive as well as voluntary saccades). This result is in line with the U-shaped nonlinear model described by Lacour et al. (Neurophysiol Clin 38:411-421, 2008), showing that a secondary task performed during a postural task could increase (prosaccade task) or decrease (antisacade task) postural stability depending on its complexity. We suggest that the different degree of attentional resources needed for performing prosaccade or antisaccade tasks are, most likely, responsible for the different effect on postural control.

Involvement of visual signals in kinaesthesia: A virtual reality study.

Body movements are invariably accompanied by various proprioceptive, visual, tactile and/or motor signals. It is therefore difficult to completely dissociate these various signals from each other in order to study their specific involvement in the perception of movement (kinaesthesia). Here, we manipulated visual motion signals in a virtual reality display by using a humanoid avatar. The visual signals of movement could therefore be manipulated freely, relative to the participant's actual movement or lack of movement. After an embodiment phase in which the avatar's movements were coupled to the participant's voluntary movements, kinaesthetic illusions were evoked by moving the avatar's right forearm (flexion or extension) while the participant's right arm remained static. The avatar's left forearm was hidden from view. In parallel, somaesthetic signals could be masked by agonist-antagonist co-vibration or be amplified (by agonist vibration only or antagonist vibration only) so that the real impact of visual cues of movement in kinaesthesia could be studied. In a study of 24 participants, masking the somaesthetic signals (which otherwise provide signals indicating that the arm is static) was associated with a greater intensity and shorter latency of the visually evoked illusions. These results confirm the importance of carefully considering somaesthetic signals when assessing the contribution of vision to kinaesthesia. The use of a combination of virtual reality and somaesthetic signal manipulation might be of clinical value.

Humans use internal models to construct and update a sense of verticality.

Internal models serve sensory processing, sensorimotor integration and motor control. They could be a way to construct and update a sense of verticality, by combining vestibular and somatosensory graviception. We tested this hypothesis by investigating self-orientation relative to gravity in 39 normal subjects and in subjects with various somatosensory losses showing either a complete deafferentation of trunk and lower limbs (14 paraplegic patients after complete traumatic spinal cord injury) or a gradient in the degree of a hemibody sensory loss (23 hemiplegic patients after stroke). We asked subjects to estimate, in the dark, the direction of the Earth vertical in two postural conditions-upright and at lateral whole body tilt. For upright conditions, verticality estimates were not different from the direction of the Earth vertical in normal (0.24° ± 1; P = 0.42) and paraplegic subjects (0.87° ± 0.9; P = 0.14). The within-subject variability was much greater in hemiplegic than in normal subjects (2.05° ± 1.15 versus 1.06° ± 0.4; P < 0.01) and greater in paraplegic than in normal subjects (1.13° ± 0.4 versus 0.72° ± 0.4; P < 0.01). These findings indicate that, even if vestibular graviception is intact, somaesthetic graviception contributes to the sense of verticality, leading to a more robust judgement about the direction of verticality when vestibular and somaesthetic graviception yield congruent information. As expected, when normal subjects were tilted, their verticality estimates were biased in the direction of the body tilt (5.55° ± 3.9). This normal modulation of verticality perception (Aubert effect), was preserved in hemiplegics on the side of the normoaesthetic hemibody (ipsilesional) (6.09° ± 6.3), and abolished both in paraplegics (1.06° ± 2.5) and in hemiplegics (0.04° ± 6.7) on the side of hypoaesthetic hemibody (contralesional). This incongruence did not exist in deafferented paraplegics who exclusively used vestibular graviception with a similar efficacy no matter what the lateral body position. The Aubert effect was not an on-off phenomenon since the degree of hemiplegics' somatosensory loss correlated with the modulation of verticality perception when they were tilted to the side of hypoaesthetic hemibody (r = -0.55; P < 0.01). The analysis of anatomical correlates showed that the Aubert effect required the integrity of the posterolateral thalamus. This study reveals the existence of a synthesis of vestibular and somaesthetic graviception for which the posterolateral thalamus plays a major role. This corresponds to a primary property of internal models and yields the neural bases of the Aubert effect. We conclude that humans construct and update internal models of verticality in which somatosensory information plays an important role.

Multisensory integration of visual cues from first- to third-person perspective avatars in the perception of self-motion.

In the perception of self-motion, visual cues originating from an embodied humanoid avatar seen from a first-person perspective (1st-PP) are processed in the same way as those originating from a person's own body. Here, we sought to determine whether the user's and avatar's bodies in virtual reality have to be colocalized for this visual integration. In Experiment 1, participants saw a whole-body avatar in a virtual mirror facing them. The mirror perspective could be supplemented with a fully visible 1st-PP avatar or a suggested one (with the arms hidden by a virtual board). In Experiment 2, the avatar was viewed from the mirror perspective or a third-person perspective (3rd-PP) rotated 90° left or right. During an initial embodiment phase in both experiments, the avatar's forearms faithfully reproduced the participant's real movements. Next, kinaesthetic illusions were induced on the static right arm from the vision of passive displacements of the avatar's arms enhanced by passive displacement of the participant's left arm. Results showed that this manipulation elicited kinaesthetic illusions regardless of the avatar's perspective in Experiments 1 and 2. However, illusions were more likely to occur when the mirror perspective was supplemented with the view of the 1st-PP avatar's body than with the mirror perspective only (Experiment 1), just as they are more likely to occur in the latter condition than with the 3rd-PP (Experiment 2). Our results show that colocalization of the user's and avatar's bodies is an important, but not essential, factor in visual integration for self-motion perception.

The black superiority effect: Black is taller than gray.

A novel illusion entitled "the letter height superiority effect" has been demonstrated. This shows that letters are perceived as being taller than pseudoletters, while in reality their objective sizes are identical. An explanation of this illusion has been proposed in the framework of the Interactive Activation Model. Indeed, we postulated that the more a feature is activated, the taller a stimulus is perceived as being. The objective of the current study was to test this postulate by manipulating feature activation through signal-to-noise ratio. We presented gray stimuli (low signal-to-noise ratio) or black ones (high signal-to-noise ratio). In a first experiment, participants judged the size of pairs of either letters or pseudoletters presented as black or gray. In a second experiment we presented pairs consisting of a letter and a pseudoletter, of identical or different colors. In a third experiment, we presented pairs of letters or pseudoletters of identical or different colors by block to test the possible effect of previous exposure on perceptual judgments. The results showed that for identical objective size, participants perceive black stimuli to be taller than gray ones and that the effects of the nature of the stimuli and their color are cumulative. The results also indicated that the effects were not due to previous exposure to color or sizes. These results confirm the Interactive Activation Model as a credible explanation for the letter height superiority effect.

Functional properties of extended body representations in the context of kinesthesia.

A person's internal representation of his/her body is not fixed. It can be substantially modified by neurological injuries and can also be extended (in healthy participants) to incorporate objects that have a corporeal appearance (such as fake body segments, e.g. a rubber hand), virtual whole bodies (e.g. avatars), and even objects that do not have a corporeal appearance (e.g. tools). Here, we report data from patients and healthy participants that emphasize the flexible nature of body representation and question the extent to which incorporated objects have the same functional properties as biological body parts. Our data shed new light by highlighting the involvement of visual motion information from incorporated objects (rubber hands, full body avatars and hand-held tools) in the perception of one's own movement (kinesthesia). On the basis of these findings, we argue that incorporated objects can be treated as body parts, especially when kinesthesia is involved.

From Embodiment of a Point-Light Display in Virtual Reality to Perception of One's Own Movements.

Humans can recognize living organisms and understand their actions solely on the basis of a small animated set of well-positioned points of light, i.e. by recognizing biological motion. Our aim was to determine whether this type of recognition and integration also occurs during the perception of one's own movements. The participants (60 females) were immersed with a virtual reality headset in a virtual environment, either dark or illuminated, in which they could see a humanoid avatar from a first-person perspective. The avatar's forearms were either realistic or represented by three points of light. Embodiment was successfully achieved through a 1-min period during which either the realistic or point-light avatar's forearms faithfully reproduced voluntary flexion-extension movements. Then, the "virtual mirror paradigm" was used to evoke kinesthetic illusions. In this paradigm, a passive flexion-extension of the participant's left arm was coupled with the movements of the avatar's forearms. This combined visuo-proprioceptive stimulation, was compared with unimodal stimulation (either visual or proprioceptive stimulation only). We found that combined visuo-proprioceptive stimulation with realistic avatars evoked more vivid kinesthetic illusions of a moving right forearm than unimodal stimulations, regardless of whether the virtual environment was dark or illuminated. Kinesthetic illusions also occurred with point-light avatars, albeit less frequently and a little less intense, and only when the visual environment was optimal for slow motion detection of the point-light display (lit environment). We conclude that kinesthesia does not require visual access to an elaborate representation of a body segment. Access to biological movement can be sufficient.

Are rotations in perceived visual vertical and body axis after stroke caused by the same mechanism?

BACKGROUND AND PURPOSE: The aim of this study was to investigate whether allocentric and egocentric coordinate systems are congruently biased after hemisphere stroke, which would suggest a single underlying mechanism. METHODS: The perception of the long body axis (LBA), an egocentric reference, and that of the subjective visual vertical (SVV), an allocentric reference, was assessed in both the upright position and with 30 degrees lateral body tilts in 15 patients with a hemisphere stroke and 12 control subjects. RESULTS: In control subjects, estimates were accurate in upright but rotated in tilted positions (LBA 7 degrees +/-6 degrees overestimation and SVV 8.8 degrees +/-7.8 degrees toward the body). In patients, SVV (-4.4 degrees +/-4.6 degrees ) and LBA (-4.8 degrees +/-5.3 degrees ) were congruent in upright positions and when patients were ipsilesionally tilted (1.5 degrees +/-7 degrees and 1.9 degrees +/-7 degrees , respectively). In contrast, SVV and LBA were dissociated when the body was tilted to the contralesional side with overestimation of the LBA (-9.2 degrees +/-4.6 degrees ) but no effect on SVV (-4.1 degrees +/-6.4 degrees ). CONCLUSIONS: Because rotations in egocentric and allocentric reference systems found after stroke are differently modulated by lateral tilts, they are not due to a single underlying mechanism. However, they share common bases and can be simultaneously reduced by ipsilesional body tilt. Differences in the way somesthetic information is integrated may explain the differences in LBA and SVV.

The respective contributions of visual and proprioceptive afferents to the mirror illusion in virtual reality.

The reflection of passive arm displacement in a mirror is a powerful means of inducing a kinaesthetic illusion in the static arm hidden behind the mirror. Our recent research findings suggest that this illusion is not solely visual in origin but results from the combination of visual and proprioceptive signals from the two arms. To determine the respective contributions of visual and proprioceptive signals to this illusion, we reproduced the mirror paradigm in virtual reality. As in the physical version of the mirror paradigm, one of the participant's arms (the left arm, in our study) could be flexed or extended passively. This movement was combined with displacements of the avatar's left and right forearms, as viewed in a first-person perspective through a virtual reality headset. In order to distinguish between visual and proprioceptive contributions, two unimodal conditions were applied separately: displacement of the avatar's forearms in the absence of physical displacement of the left arm (the visual condition), and displacement of the left forearm while the avatar's forearms were masked (the proprioceptive condition). Of the 34 female participants included in the study, 28 experienced a kinaesthetic mirror illusion in their static (right) arm. The strength of the illusion (expressed in terms of speed and duration) evoked by the bimodal condition was much higher than that observed in either of the two unimodal conditions. Our present results confirm that the involvement of visual signals in the mirror illusion-often considered as a prototypic visual illusion-has been overstated. The mirror illusion also involves non-visual signals (bilateral proprioceptive-somaesthetic signals, in fact) that interact with the visual signals and strengthen the kinaesthetic effect.

Internal model of gravity influences configural body processing.

Human bodies are processed by a configural processing mechanism. Evidence supporting this claim is the body inversion effect, in which inversion impairs recognition of bodies more than other objects. Biomechanical configuration, as well as both visual and embodied expertise, has been demonstrated to play an important role in this effect. Nevertheless, the important factor of body inversion effect may also be linked to gravity orientation since gravity is one of the most fundamental constraints of our biology, behavior, and perception on Earth. The visual presentation of an inverted body in a typical body inversion paradigm turns the observed body upside down but also inverts the implicit direction of visual gravity in the scene. The orientation of visual gravity is then in conflict with the direction of actual gravity and may influence configural processing. To test this hypothesis, we dissociated the orientations of the body and of visual gravity by manipulating body posture. In a pretest we showed that it was possible to turn an avatar upside down (inversion relative to retinal coordinates) without inverting the orientation of visual gravity when the avatar stands on his/her hands. We compared the inversion effect in typical conditions (with gravity conflict when the avatar is upside down) to the inversion effect in conditions with no conflict between visual and physical gravity. The results of our experiment revealed that the inversion effect, as measured by both error rate and reaction time, was strongly reduced when there was no gravity conflict. Our results suggest that when an observed body is upside down (inversion relative to participants' retinal coordinates) but the orientation of visual gravity is not, configural processing of bodies might still be possible. In this paper, we discuss the implications of an internal model of gravity in the configural processing of observed bodies.

The letter height superiority illusion.

Letters are identified better when they are embedded within words rather than within pseudowords, a phenomenon known as the word superiority effect (Reicher in Journal of Experimental Psychology, 81, 275-280, 1969). This effect is, inter alia, accounted for by the interactive-activation model (McClelland & Rumelhart in Psychological Review, 88, 375-407, 1981) through feedback from word to letter nodes. In this study, we investigated whether overactivation of features could lead to perceptual bias, wherein letters would be perceived as being taller than pseudoletters, or words would be perceived as being taller than pseudowords. In two experiments, we investigated the effects of letter and lexical status on the perception of size. Participants who had to compare the heights of letters and pseudoletters, or of words and pseudowords, indeed perceived the former stimuli as being taller than the latter. Possible alternative interpretations of this height superiority effect for letters and words are discussed.

Maintaining Trunk and Head Upright Optimizes Visual Vertical Measurement After Stroke.

BACKGROUND: Visual vertical (VV) measurement provides information about spatial cognition and is now part of postural disorders assessment. Guidelines for clinical VV measurement after stroke remain to be established, especially regarding the orientation settings for patients who do not sit upright. OBJECTIVES: We analyzed the need to control body orientation while patients estimate the VV. METHODS: VV orientation and variability were assessed in 20 controls and 36 subacute patients undergoing rehabilitation after a first hemisphere stroke, in 3 settings: body not maintained (trunk and head free), partially maintained (trunk maintained, head free), or maintained (trunk and head). VV was analyzed as a function of trunk and head tilt, also quantified. RESULTS: Trunk and head orientations were independent. The ability to sit independently was affected by a tilted trunk. The setting had a strong effect on VV orientation and variability in patients with contralesional trunk tilt (n = 11; trunk orientation -18.4 ± 11.7°). The contralesional VV bias was severe and consistent under partially maintained (-8.4 ± 5.2°) and maintained (-7.8 ± 3.5°) settings, whereas various individual behaviors reduced the mean bias under the nonmaintained setting (-3.6 ± 9.3°, P < .05). VV variability was lower under the maintained (1.5 ± 0.2°) than nonmaintained (3.7 ± 0.4°, P < .001) and partially maintained (3.6 ± 0.2°, P < .001) settings. In contrast, setting had no effect in patients with satisfactory postural control in sitting. CONCLUSION: Subject setting improves VV measurement in stroke patients with postural disorders. Maintaining the trunk upright enhances the validity of VV orientation, and maintaining the head upright enhances the validity of within-subject variability. Measuring VV without any body maintaining is valid in patients with satisfactory balance abilities.

Postural control system influences intrinsic alerting state.

OBJECTIVE: Numerous studies using dual-task paradigms (postural and cognitive) have shown that postural control requires cognitive resources. However, the influence of postural control on attention components has never been directly addressed. METHOD: Using the attention network test (ANT), which assesses specifically each of the 3 components of attention-alertness, orientation, and executive control-within a single paradigm, we investigated the effect of postural balance demand on these 3 components. Forty-two participants completed the ANT in 3 postural conditions: (a) supine, a very stable position; (b) sitting on a chair, an intermediate position; and (c) standing with feet lined up heel to toe, a very instable position known as the Romberg position. RESULTS: Our results revealed that the difficulty of postural control does modulate alerting in such a way that it improves with the level of instability of the position. Regarding the orienting and executive control components of attention, performance was not different when participants were standing upright or seated, whereas in the supine position, performance dropped. CONCLUSIONS: The strong and specific interaction between postural control and the alerting system suggests that these mechanisms may share parts of the underlying neural circuits. We discuss the possible implication of the locus coeruleus, known to be involved in both postural balance and alerting. Also, our findings concerning orienting and executive control systems suggest that supine posture could have a specific effect on cognitive activities. These effects are discussed in terms of particularities resulting from the supine position.

The influence of action on episodic memory: a virtual reality study.

A range of empirical findings suggest that active learning is important for memory. However, few studies have focused on the mechanisms underlying this enactment effect in episodic memory using complex environments. Research using virtual reality has yielded inconsistent results. We postulated that the effect of action depends on the degree of interaction with the environment and freedom in the planning of an itinerary. To test these hypotheses, we disentangled the interaction and planning components of action to investigate whether each enhances factual and spatial memory. Seventy-two participants (36 male and 36 female) explored a virtual town in one of three experimental conditions: (a) a passive condition where participants were immersed as passenger of the car (no interaction, no planning); (b) a planning-only condition (the subject chose the itinerary but did not drive the car); (c) an interaction-only condition (the subject drove the car but the itinerary was fixed). We found that itinerary choice and motor control both enhanced spatial memory, while factual memory was impaired by online motor control. The role of action in memory is discussed.

Where are my hands? Influence of limb posture on tactile extinction.

OBJECTIVE: Tactile localization on the skin involves both a somatotopic and a postural schema (body-schema) representation. The present study determines the extent to which body posture influences tactile perception in right-brain-damaged patients. METHOD: In a first set of experiments, patients were asked to detect single tactile stimulation delivered to their left or right hands or to both hands simultaneously (double stimulation) in different arm postures. Only patients who had no difficulty localizing single and double tactile stimulations when their hands were placed in anatomic position were tested. Participant's hands were crossed, one over the other, and the tactile stimuli were delivered either to the hand (beyond the crossing point, Experiment 1) or to the forearm (before the crossing point, Experiment 2). In Experiment 3, the left hand was placed in the right hemispace and the right hand in the left hemispace without crossing over (opposite condition). In a second set of experiments, patients were asked to detect stimulation delivered to the forefinger. The fingers were crossed, one over the other at the level of the middle phalanx, and stimuli were delivered either beyond or before the crossing point. RESULTS: In all experimental conditions, control participants performed at ceiling. We observed a left-hand tactile extinction on double stimulation in the crossed condition. CONCLUSIONS: These results suggest that tactile stimuli can be encoded based on multiple specific body-part representations rather than on an integrated body-schema representation.