Facilitated Processing of Visual Stimuli Associated with the Body

Topographic mapping of the sensorimotor qualities of empathic reactivity: A psychophysiological study in people with spinal cord injuries

The experience of empathy for pain is underpinned by sensorimotor and affective dimensions which, although interconnected, are at least in part behaviorally and neurally distinct. Spinal cord injuries (SCI) induce a massive, below-lesion level, sensorimotor body-brain disconnection. This condition may make it possible to test whether sensorimotor deprivation alters specific dimensions of empathic reactivity to observed pain. To explore this issue, we asked SCI people with paraplegia and healthy controls to observe videos of painful or neutral stimuli administered to a hand (intact) or a foot (deafferented). The stimuli were displayed by means of a virtual reality set-up and seen from a first person (1PP) or third person (3PP) visual perspective. A number of measures were recorded ranging from explicit behaviors like explicit verbal reports on the videos, to implicit measures of muscular activity (like EMG from the corrugator and zygomatic muscles that may represent a proxy of sensorimotor empathy) and of autonomic reactivity (like the electrodermal response and Respiratory Sinus Arrhythmia that may represent a general proxy of affective empathy). While no across group differences in explicit verbal reports about the pain stimuli were found, SCI people exhibited reduced facial muscle reactivity to the stimuli applied to the foot (but not the hand) seen from the 1PP. Tellingly, the corrugator activity correlated with SCI participants' neuropathic pain. There were no across group differences in autonomic reactivity suggesting that SCI lesions may affect sensorimotor dimensions connected to empathy for pain.

Valence moderates the effect of stimulus-hand proximity on conflict processing and gaze-cueing

An effective interaction with the environment requires adaptation of one's own behaviour to environmental demands. We do so by using cues from our environment and relating these cues to our body to predict the outcomes of events. The recent literature on embodied cognition suggests that task-relevant stimuli, presented near the hands, receive more attentional capacity and are processed differently than stimuli, presented spatially more distant to our body. It has also been proposed that near-hand processing is beneficial to conflict resolution. In the current study, we tested the assumption of an attentional bias towards the near hand space in the context of our previous work by combining a cueing paradigm (allocation of visual attention) with a conflict processing paradigm (Simon task) in the near vs far hand space. In addition, the relevance of processing was manipulated by using affective (angry vs neutral smileys) gaze cues (i.e., varying the valence of the cues). Our results indicate that (a) the interaction of valence × cue congruency × hand proximity was significant, indicating that the cueing effect was larger for negative valence in the proximal condition. (b) The interaction of valence × Simon compatibility × stimulus-hand proximity interaction was significant, indicating that for negative valence processing, the Simon effect was smaller in the proximal than in the distal stimulus-hand condition. This effect was at least numerically but not significantly reversed in the neutral valence condition. (c) Overall, cue congruency, indicating the correct vs incorrect attention allocation to the target stimulus onset, did not reveal any effect on Simon compatibility × stimulus-hand proximity. Our results suggest that valence, the allocation of attention, and conflict, seem to be decisive factors determining the direction and strength of hand proximity effects.

Measuring embodiment: A review of methods for prosthetic devices

The development of neural interfaces to provide improved control and somatosensory feedback from prosthetic limbs has initiated a new ability to probe the various dimensions of embodiment. Scientists in the field of neuroprosthetics require dependable measures of ownership, body representation, and agency to quantify the sense of embodiment felt by patients for their prosthetic limbs. These measures are critical to perform generalizable experiments and compare the utility of the new technologies being developed. Here, we review outcome measures used in the literature to evaluate the senses of ownership, body-representation, and agency. We categorize these existing measures based on the fundamental psychometric property measured and whether it is a behavioral or physiological measure. We present arguments for the efficacy and pitfalls of each measure to guide better experimental designs and future outcome measure development. The purpose of this review is to aid prosthesis researchers and technology developers in understanding the concept of embodiment and selecting metrics to assess embodiment in their research. Advances in the ability to measure the embodiment of prosthetic devices have far-reaching implications in the improvement of prosthetic limbs as well as promoting a broader understanding of ourselves as embodied agents.

Relationship Between Body-Specific Attention to a Paretic Limb and Real-World Arm Use in Stroke Patients: A Longitudinal Study

Learned nonuse is a major problem in upper limb (UL) rehabilitation after stroke. Among the various factors that contribute to learned nonuse, recent studies have focused on body representation of the paretic limb in the brain. We previously developed a method to measure body-specific attention, as a marker of body representation of the paretic limb and revealed a decline in body-specific attention to the paretic limb in chronic stroke patients by a cross-sectional study. However, longitudinal changes in body-specific attention and paretic arm use in daily life (real-world arm use) from the onset to the chronic phase, and their relationship, remain unknown. Here, in a longitudinal, prospective, observational study, we sought to elucidate the longitudinal changes in body-specific attention to the paretic limb and real-world arm use, and their relationship, by using accelerometers and psychophysical methods, respectively, in 25 patients with subacute stroke. Measurements were taken at baseline (TBL), 2 weeks (T2w), 1 month (T1M), 2 months (T2M), and 6 months (T6M) after enrollment. UL function was measured using the Fugl-Meyer Assessment (FMA) and Action Research Arm Test (ARAT). Real-world arm use was measured using accelerometers on both wrists. Body-specific attention was measured using a visual detection task. The UL function and real-world arm use improved up to T6M. Longitudinal changes in body-specific attention were most remarkable at T1M. Changes in body-specific attention up to T1M correlated positively with changes in real-world arm use up to T6M, and from T1M to T6M, and the latter more strongly correlated with changes in real-world arm use. Changes in real-world arm use up to T2M correlated positively with changes in FMA up to T2M and T6M. No correlation was found between body-specific attention and FMA scores. Thus, these results suggest that improved body-specific attention to the paretic limb during the early phase contributes to increasing long-term real-world arm use and that increased real-world use is associated with the recovery of UL function. Our results may contribute to the development of rehabilitation strategies to enhance adaptive changes in body representation in the brain and increase real-world arm use after stroke.

Body-Specific Attention to the Hands and Feet in Healthy Adults

To execute the intended movement, the brain directs attention, called body-specific attention, to the body to obtain information useful for movement. Body-specific attention to the hands has been examined but not to the feet. We aimed to confirm the existence of body-specific attention to the hands and feet, and examine its relation to motor and sensory functions from a behavioral perspective. The study included two groups of 27 right-handed and right-footed healthy adults, respectively. Visual detection tasks were used to measure body-specific attention. We measured reaction times to visual stimuli on or off the self-body and calculated the index of body-specific attention score to subtract the reaction time on self-body from that off one. Participants were classified into low and high attention groups based on each left and right body-specific attention index. For motor functions, Experiment 1 comprised handgrip strength and ball-rotation tasks for the hands, and Experiment 2 comprised toe grip strength involved in postural control for the feet. For sensory functions, the tactile thresholds of the hands and feet were measured. The results showed that, in both hands, the reaction time to visual stimuli on the hand was significantly lesser than that offhand. In the foot, this facilitation effect was observed in the right foot but not the left, which showed the correlation between body-specific attention and the normalized toe gripping force, suggesting that body-specific attention affected postural control. In the hand, the number of rotations of the ball was higher in the high than in the low attention group, regardless of the elaboration exercise difficulty or the left or right hand. However, this relation was not observed in the handgripping task. Thus, body-specific attention to the hand is an important component of elaborate movements. The tactile threshold was higher in the high than in the low attention group, regardless of the side in hand and foot. The results suggested that more body-specific attention is directed to the limbs with lower tactile abilities, supporting the sensory information reaching the brain. Therefore, we suggested that body-specific attention regulates the sensory information to help motor control.

Expertise in Tool Use Promotes Tool Embodiment

Body representations are known to be dynamically modulated or extended through tool use. Here, we review findings that demonstrate the importance of a user's tool experience or expertise for successful tool embodiment. Examining expert tool users, such as individuals who use tools in professional sports, people who use chopsticks at every meal, or spinal injury patients who use a wheelchair daily, offers new insights into the role of expertise in tool embodiment: Not only does tool embodiment differ between novices and experts, but experts may experience enhanced changes to their body representation when interacting with their own, personal tool. The findings reviewed herein reveal the importance of assessing tool skill in future studies of tool embodiment.

Beyond one's body parts: Remote object movement with sense of agency involuntarily biases spatial attention

Humans' own body postures and/or actions bias visual attention. This study examined whether an object controlled remotely with a sense of agency (SoA) can bias attentional allocation, even if the mappings of body movement and object movement do not correspond (i.e., with a less extended body representation). In the experiments, participants shifted a circle toward an instructed direction via keypress. On some trials, the task changed to identification of visual characters instead of the circle shift. Strength of SoA was manipulated based on the probability that the circle actually moved to the participant's intended location, with high probability producing a strong SoA. Results showed that visual identification performance became higher for appearance of the target at the intended location than at the unintended location, but only when observers felt a strong SoA. This suggests that the object's movement attracts attention when the observer feels a strong SoA over its control.

Time-dependent decline of body-specific attention to the paretic limb in chronic stroke patients

OBJECTIVE

To examine whether reduced body-specific attention to a paretic limb is found in chronic stroke patients in a time-dependent manner.

METHODS

Twenty-one patients with chronic hemiparesis (10 left and 11 right hemiparesis) after subcortical stroke and 18 age-matched healthy controls were recruited in this study. Standard neuropsychological examinations showed no clear evidence of spatial neglect in any patient. In order to quantitatively measure spatial attention to the paretic hand, a visual detection task for detecting a target appearing on the surface of either a paretic or dummy hand was used. This task can measure the body facilitation effect, which makes faster detection of a target on the body compared with one far from the body.

RESULTS

In stroke patients, there was no difference in the reaction time for a visual target between the paretic and the dummy hands, while the healthy participants showed faster detection for the visual target on the real hand than on the dummy one. The index of the body facilitation effect, subtracting the reaction time for the target-on-paretic hand from that for the target-on-dummy one, was correlated with the duration since onset and with finger function test on the Stroke Impairment Assessment Set.

CONCLUSIONS

The reduction of the body facilitation effect in the paretic limb suggests the decline of body-specific attention to the paretic one in patients with chronic hemiparesis. This decline of body-specific attention, leading to neglect for the paretic limb, will be one of the most serious problems for rehabilitation based on use-dependent plasticity.

Individuals with pronounced schizotypal traits are particularly successful in tickling themselves

We assessed self-tickling sensations in a group of participants high in schizotypal traits (n=27) and group of participants low in schizotypal traits (n=27). The groups were formed by screening a pool of 397 students for extreme scores in the French version of the Schizotypal Personality Questionnaire. As observed in a previous study involving psychiatric people with auditory hallucinations and/or passivity experiences our results showed that self-applied tactile stimulations are felt to be more ticklish by healthy individuals high in schizotypal traits. In contrast, there were no significant intergroup differences in the mean tickle rating in the externally-produced tickling condition. Furthermore, more successful self-tickling was associated with more frequent self-reports of unusual perceptual experiences (such as supernatural experiences) and passivity experiences in particular (such as a feeling of being under the control of an outside force or power).

Hand posture and cognitive control: The congruency sequence effect is reduced near the hands

According to several recent articles, attentional processing seems to be modulated by the proximity of one's own hand to a stimulus. Weidler and Abrams (Psychonomic Bulletin & Review, 21, 462-469, 2014) found a significant reduction of the Eriksen flanker effect when the stimuli were presented close to the participants' hands. They interpreted this as evidence for stronger cognitive control near the hands. Using a near-by hands manipulation intended to vary distance while keeping posture of the hands constant, we found a modulation of the congruency sequence effect (CSE or Gratton effect), i.e., a larger flanker effect following incompatible trials than following compatible trials. The CSE was eliminated near the hands. Though we did not find a reduction in the flanker effect itself (which might be the result of using a near-by hand manipulation somewhat different from that of Weidler and Abrams), this result can be considered to be compatible with Weidler and Abrams' original hypothesis if the congruency sequence effect is interpreted in terms of cognitive control.

That's not quite me: limb ownership encoding in the brain

With congruent stimulation of one's limb together with a fake counterpart, an illusory self-attribution of the fake limb can be induced. Such illusions have brought profound insights into the cognitive and neuronal mechanisms underlying temporary changes in body representation, but to put them in perspective, they need to be compared with ownership as experienced for one's real body. We used functional magnetic resonance imaging (fMRI) to compare the neuronal correlates of touch under different degrees of body ownership. Participants' left and right arms were stimulated either alone or together with a fake counterpart while this stimulation was synchronous, ambiguous or asynchronous. Synchronous stimulation induced illusory fake arm ownership, but the brain still differentiated between touch to one's real arm and to an illusory 'owned' arm: the degree of arm ownership was encoded positively by activity in the ventromedial prefrontal cortex and lateral occipitotemporal cortex and negatively in the temporoparietal cortex. Conversely, the ventral premotor cortex responded more strongly to synchronous stimulation compared with asynchronous stimulation and with real arm only stimulation. These results offer new insights into the differential representation of the real body vs a body that is temporarily self-attributed following the resolution of multisensory conflict.

The multisensory body revealed through its cast shadows

One key issue when conceiving the body as a multisensory object is how the cognitive system integrates visible instances of the self and other bodies with one's own somatosensory processing, to achieve self-recognition and body ownership. Recent research has strongly suggested that shadows cast by our own body have a special status for cognitive processing, directing attention to the body in a fast and highly specific manner. The aim of the present article is to review the most recent scientific contributions addressing how body shadows affect both sensory/perceptual and attentional processes. The review examines three main points: (1) body shadows as a special window to investigate the construction of multisensory body perception; (2) experimental paradigms and related findings; (3) open questions and future trajectories. The reviewed literature suggests that shadows cast by one's own body promote binding between personal and extrapersonal space and elicit automatic orienting of attention toward the body-part casting the shadow. Future research should address whether the effects exerted by body shadows are similar to those observed when observers are exposed to other visual instances of their body. The results will further clarify the processes underlying the merging of vision and somatosensation when creating body representations.

Network activity underlying the illusory self-attribution of a dummy arm

Neuroimaging has demonstrated that the illusory self-attribution of body parts engages frontal and intraparietal brain areas, and recent evidence further suggests an involvement of visual body-selective regions in the occipitotemporal cortex. However, little is known about the principles of information exchange within this network. Here, using automated congruent versus incongruent visuotactile stimulation of distinct anatomical locations on the participant's right arm and a realistic dummy counterpart in an fMRI scanner, we induced an illusory self-attribution of the dummy arm. The illusion consistently activated a left-hemispheric network comprising ventral premotor cortex (PMv), intraparietal sulcus (IPS), and body-selective regions of the lateral occipitotemporal cortex (LOC). Importantly, during the illusion, the functional coupling of the PMv and the IPS with the LOC increased substantially, and dynamic causal modeling revealed a significant enhancement of connections from the LOC and the secondary somatosensory cortex to the IPS. These results comply with the idea that the brain's inference mechanisms rely on the hierarchical propagation of prediction error. During illusory self-attribution, unpredicted ambiguous sensory input about one's body configuration may result in the generation of such prediction errors in visual and somatosensory areas, which may be conveyed to parietal integrative areas.

Multisensory integration and child neurodevelopment

A considerable number of cognitive processes depend on the integration of multisensory information. The brain integrates this information, providing a complete representation of our surrounding world and giving us the ability to react optimally to the environment. Infancy is a period of great changes in brain structure and function that are reflected by the increase of processing capacities of the developing child. However, it is unclear if the optimal use of multisensory information is present early in childhood or develops only later, with experience. The first part of this review has focused on the typical development of multisensory integration (MSI). We have described the two hypotheses on the developmental process of MSI in neurotypical infants and children, and have introduced MSI and its neuroanatomic correlates. The second section has discussed the neurodevelopmental trajectory of MSI in cognitively-challenged infants and children. A few studies have brought to light various difficulties to integrate sensory information in children with a neurodevelopmental disorder. Consequently, we have exposed certain possible neurophysiological relationships between MSI deficits and neurodevelopmental disorders, especially dyslexia and attention deficit disorder with/without hyperactivity.

Reduced Temporal Fusion in Near-Hand Space

Object-substitution masking (OSM) is thought to reflect a failure of object individuation. That is, a briefly presented target surrounded by four dots is perceptually fused with the four-dot mask when the mask is visible after the target has disappeared, thereby obscuring the visibility of the target. If OSM depends on the inability to temporally segregate objects, then increasing the temporal precision of the visual system should reduce OSM. In the study reported here, we manipulated temporal precision by varying the proximity of participants’ hands to visual stimuli, because stimuli in near-hand space have been found to enjoy enhanced attentional processing, and attention is known to speed visual processing. Hand placement was indeed found to affect OSM: Placing participants’ hands near the visual stimuli reduced the magnitude of the masking. This finding demonstrates that object individuation can be facilitated by increasing the temporal resolution of vision via increasing the proximity of visual stimuli to the hands.

Preference for orientations commonly viewed for one's own hand in the anterior intraparietal cortex

Brain regions in the intraparietal and the premotor cortices selectively process visual and multisensory events near the hands (peri-hand space). Visual information from the hand itself modulates this processing potentially because it is used to estimate the location of one’s own body and the surrounding space. In humans specific occipitotemporal areas process visual information of specific body parts such as hands. Here we used an fMRI block-design to investigate if anterior intraparietal and ventral premotor ‘peri-hand areas’ exhibit selective responses to viewing images of hands and viewing specific hand orientations. Furthermore, we investigated if the occipitotemporal ‘hand area’ is sensitive to viewed hand orientation. Our findings demonstrate increased BOLD responses in the left anterior intraparietal area when participants viewed hands and feet as compared to faces and objects. Anterior intraparietal and also occipitotemporal areas in the left hemisphere exhibited response preferences for viewing right hands with orientations commonly viewed for one’s own hand as compared to uncommon own hand orientations. Our results indicate that both anterior intraparietal and occipitotemporal areas encode visual limb-specific shape and orientation information.

Agency and control for the integration of a virtual tool into the peripersonal space

Our representation of the peripersonal space is tied to our representation of our bodies. This representation appears to be flexible and it can be updated to include the space in which tools work, particularly when the tool is actively used. One indicator of this update is the increased efficiency with which sensory events near the tool are processed. In the present study we examined the role of visuomotor control in extending peripersonal space to a common virtual tool-a computer mouse cursor. In particular, after participants were exposed to different spatial mappings between movements of the mouse cursor and movements of their hand, participants' performance in a motion-onset detection task was measured, with the mouse cursor as the stimulus. When participants, during exposure, had the ability to move the cursor efficiently and accurately (familiar hand-cursor mapping), they detected motion-onset targets more quickly than when they could not move the cursor at all during exposure (no hand-cursor mapping). Importantly, reversing the spatial correspondence between the movements of the hand and the cursor (unfamiliar hand-cursor mapping) during exposure, which was thought to preserve the ability to move the cursor (ie agency) while weakening the ability to make the movements efficiently and accurately (ie control), eliminated the detection-facilitation effect. These results provide evidence for the possible extension of peripersonal space to frequently used objects in the virtual domain. Importantly, these extensions seem to depend on the participant's knowledge of the dynamic spatial mapping between the acting limb and the visible virtual tool.

Sense of agency primes manual motor responses

Perceiving the body influences how we perceive and respond to stimuli in the world. We investigated the respective effects of different components of bodily representation--the senses of ownership and agency--on responses to simple visual stimuli. Participants viewed a video image of their hand on a computer monitor presented either in real time, or with a systematic delay. Blocks began with an induction period in which the index finger was (i) brushed, (ii) passively moved, or (iii) actively moved by the participant. Subjective reports showed that the sense of ownership over the seen hand emerged with synchronous video, regardless of the type of induction, whereas the sense of agency over the hand emerged only following synchronous video with active movement. Following induction, participants responded as quickly as possible to the onset of visual stimuli near the hand by pressing a button with their other hand. Reaction time was significantly speeded when participants had a sense of agency over their seen hand. This effect was eliminated when participants responded vocally, suggesting that it reflects priming of manual responses, rather than enhanced stimulus detection. These results suggest that vision of one's own hand-and, specifically, the sense of agency over that hand-primes manual motor responses.

From maps to form to space: touch and the body schema

Evidence from patients has shown that primary somatosensory representations are plastic, dynamically changing in response to central or peripheral alterations, as well as experience. Furthermore, recent research has also demonstrated that altering body posture results in changes in the perceived sensation and localization of tactile stimuli. Using evidence from behavioral studies with brain-damaged and healthy subjects, as well as functional imaging, we propose that the traditional concept of the body schema should be divided into three components. First are primary somatosensory representations, which are representations of the skin surface that are typically somatotopically organized, and have been shown to change dynamically due to peripheral (usage, amputation, deafferentation) or central (lesion) modifications. Second, we argue for a mapping from a primary somatosensory representation to a secondary representation of body size and shape (body form representation). Finally, we review evidence for a third set of representations that encodes limb position and is used to represent the location of tactile stimuli relative to the subject using external, non-somatotopic reference frames (postural representations).

Visual processing and the bodily self

The 'body schema' has traditionally been defined as a passively updated, proprioceptive representation of the body. However, recent work has suggested that body representations are more complex and flexible than previously thought. They may integrate current perceptual information from all sensory modalities, and can be extended to incorporate indirect representations of the body and functional portions of tools. In the present study, we investigate the source of a facilitatory effect of viewing the body on speeded visual discrimination reaction times. Participants responded to identical visual stimuli that varied only in their context: being presented on the participant's own body, on the experimenter's body, or in a neutral context. The stimuli were filmed and viewed in real-time on a projector screen. Careful controls for attention, biological saliency, and attribution confirmed that the facilitatory effect depends critically on participants attributing the context to a real body. An intermediate effect was observed when the stimuli were presented on another person's body, suggesting that the effect of viewing one's own body might represent a conjunction of an interpersonal body effect and an egocentric effect.

The selective effect of the image of a hand on visuotactile interactions as assessed by performance on the crossmodal congruency task

Seeing one's own body (either directly or indirectly) can influence visuotactile crossmodal interactions. Previously, it has been shown that even viewing a simple line drawing of a hand can also modulate such crossmodal interactions, as if viewing the picture of a hand somehow primes the representation of one's own hand. However, factors other than the sight of a symbolic picture of a hand may have modulated the crossmodal interactions reported in previous research. In the present study, we examined the crossmodal modulatory effects of viewing five different visual images (photograph of a hand, line drawing of a hand, line drawing of a car, an U-shape, and an ellipse) on tactile performance. Participants made speeded discrimination responses regarding the location of brief vibrotactile targets presented to either the tip or base of their left index finger, while trying to ignore visual distractors presented to either the left or right of central fixation. We compared the visuotactile congruency effects elicited when the five different visual images were presented superimposed over the visual distractors. Participants' tactile discrimination performance was modulated to a significantly greater extent by viewing the photograph of a hand than when viewing the outline drawing of a hand. No such crossmodal congruency effects were reported in any of the other conditions. These results therefore suggest that visuotactile interactions are specifically modulated by the image of the hand rather than just by any simple orientation cues that may be provided by the image of a hand.

Self-attributed body-shadows modulate tactile attention

Our body-shadows are special stimuli in the visual world. They often have anatomical resemblance with our own body-parts and move as our body moves, with spatio-temporal correlation. Here, we show that self-attributed body-shadows cue attention to the body-part they refer to, rather than the location they occupy. Using speeded spatial discrimination for tactile or visual targets at the hands, or for visual targets delivered near the hand-shadows, we demonstrate that mere viewing of task-irrelevant shadows can selectively facilitate tactile discrimination at the body-part casting the shadow (Experiment 1). In addition, such facilitation only develops through time for cast-shadows that have no resemblance with the body-part, but move in spatio-temporal correlation with it (Experiment 2). Conversely, facilitation fades away rapidly for shadow-like images that resemble the stimulated body-part, but are in fact static pictures (Experiment 3). Thus, recognising oneself as the owner of a shadow affects distribution of tactile attention.

Multisensory numerosity judgments for visual and tactile stimuli

To date, numerosity judgments have been studied only under conditions of unimodal stimulus presentation. It is therefore unclear whether the same limitations on correctly reporting the number of unimodal visual or tactile stimuli presented in a display might be expected under conditions in which participants have to count stimuli presented simultaneously in two or more different sensory modalities. In Experiment 1, we investigated numerosity judgments using both unimodal and bimodal displays consisting of one to six vibrotactile stimuli (presented over the body surface) and one to six visual stimuli (seen on the body via mirror reflection). Participants had to count the number of stimuli regardless of their modality of presentation. Bimodal numerosity judgments were significantly less accurate than predicted on the basis of an independent modality-specific resources account, thus showing that numerosity judgments might rely on a unitary amodal system instead. The results of a second experiment demonstrated that divided attention costs could not account for the poor performance in the bimodal conditions of Experiment 1. We discuss these results in relation to current theories of cross-modal integration and to the cognitive resources and/or common higher order spatial representations possibly accessed by both visual and tactile stimuli.

Can vision of the body ameliorate impaired somatosensory function?

Viewing the body is reported to improve tactile acuity [Kennett, S., Taylor-Clarke, M., & Haggard, P. (2001). Non-informative vision improves the spatial resolution of touch in humans. Current Biology, 11, 1188-1191]. The aim of the present study was to investigate whether this effect might be useful in improving somatosensory deficits of brain damaged patients. To support this proposal, we firstly tested the hypothesis that vision might modulate tactile performance when tactile information is limited. Thirty-two healthy subjects performed a two points discrimination task (2PDT) in three conditions: looking at their stimulated forearm, at a neutral object or at a rubber foot. The results showed that the effectiveness of visual enhancement of touch varies as a function of subjects' tactile acuity. Moreover, the accuracy in 2PDT was higher when viewing their arm only in subjects with lower tactile sensitivity. To directly demonstrate that viewing the body might ameliorate tactile deficits, the same experiment was conducted on 10 brain damaged patients suffering a reduced somatosensory sensitivity. An amelioration of the performance was found in viewing arm condition. These findings suggest that the interaction between different sensory modalities might be effective in ameliorating deficits in single modalities.