Littérature scientifique sur le sujet « Visuo-tactile stimulation »

Previous studies have shown that illusory ownership over a mannequin’s body can be induced through synchronous visuo-tactile stimulation as well as through synchronous visuo-vestibular stimulation. The current study aimed to elucidate how three-way combinations of correlated visual, tactile and vestibular signals contribute to the senses of body ownership and self-motion. Visuo-tactile temporal congruence was manipulated by touching the mannequin’s body and the participant’s unseen real body on the trunk with a small object either synchronously or asynchronously. Visuo-vestibular temporal congruence was manipulated by synchronous or asynchronous presentation of a visual motion cue (the background rotating around the mannequin in one direction) and galvanic stimulation of the vestibular nerve generating a rotation sensation (in the same direction). The illusory experiences were quantified using a questionnaire; threat-evoked skin-conductance responses (SCRs) provided complementary indirect physiological evidence for the illusion. Ratings on the illusion questionnaire statement showed significant main effects of synchronous visuo-vestibular and synchronous visuo-tactile stimulations, suggesting that both of these pairs of bimodal correlations contribute to the ownership illusion. Interestingly, visuo-tactile synchrony dominated because synchronous visuo-tactile stimulation combined with asynchronous visuo-vestibular stimulation elicited a body ownership illusion of similar strength as when both bimodal combinations were synchronous. Moreover, both visuo-tactile and visuo-vestibular synchrony were associated with enhanced self-motion perception; self-motion sensations were even triggered when visuo-tactile synchrony was combined with visuo-vestibular asynchrony, suggesting that ownership enhanced the relevance of visual information as a self-motion cue. Finally, the SCR results suggest that synchronous stimulation of either modality pair led to a stronger illusion compared to the asynchronous conditions. Collectively, the results suggest that visuo-tactile temporal correlations have a stronger influence on body ownership than visuo-vestibular correlations and that ownership boosts self-motion perception. We present a Bayesian causal inference model that can explain how visuo-vestibular and visuo-tactile information are combined in multisensory own-body perception.

Research has shown that the ability to integrate complementary sensory inputs into a unique and coherent percept based on spatiotemporal coincidence can improve perceptual precision, namely multisensory integration. Despite the extensive research on multisensory integration, very little is known about the principal mechanisms responsible for the spatial interaction of multiple sensory stimuli. Furthermore, it is not clear whether the size of spatialized stimulation can affect unisensory and multisensory perception. The present study aims to unravel whether the stimulated area’s increase has a detrimental or beneficial effect on sensory threshold. Sixteen typical adults were asked to discriminate unimodal (visual, auditory, tactile), bimodal (audio-visual, audio-tactile, visuo-tactile) and trimodal (audio-visual-tactile) stimulation produced by one, two, three or four devices positioned on the forearm. Results related to unisensory conditions indicate that the increase of the stimulated area has a detrimental effect on auditory and tactile accuracy and visual reaction times, suggesting that the size of stimulated areas affects these perceptual stimulations. Concerning multisensory stimulation, our findings indicate that integrating auditory and tactile information improves sensory precision only when the stimulation area is augmented to four devices, suggesting that multisensory interaction is occurring for expanded spatial areas.

Bodily self-identification has shown to be easily altered through spatiotemporally congruent multimodal signals. While such manipulations are mostly studied through visuo-tactile or visuo-motor stimulation, here we investigated whether congruent visuo-olfactory cues might enhance illusory self-identification with an arbitrary object. Using virtual reality, healthy individuals saw a grapefruit from its supposed first-person perspective that was touched in synchrony with their own body. The touch attempted to replicate what was seen as softly squeezing the grapefruit. Crucially, when we additionally presented the smell of a grapefruit in synchrony with the squeezing, they self-identified stronger with the fruit than when they smelled strawberry.

Prior studies have identified an ‘enfacement effect’ in which participants incorporate another’s face into their self-face representation after observing that face touched repeatedly in synchrony with touch on their own face (Sforza et al., 2010; Tsakiris, 2008). The degree of self-face/other-face merging is positively correlated with participants’ trait-level empathy scores (Sforza et al., 2010) and affects judgments of the other’s personality (Paladino et al., 2010), suggesting that enfacement also modulates higher-order representations of ‘self’ and ‘other’ involved in social and emotional evaluations. To test this hypothesis, we varied not only whether visuo-tactile stimulation was synchronous or asynchronous but also whether the person being touched in the video displayed an emotional expression indicative of threat, either fear or anger. We hypothesized that participants would incorporate the faces of fearful others more than the faces of angry others after a shared visuo-tactile experience because of a potentially stronger representation of the sight of fear in somatosensory cortices compared to the sight of anger (Cardini et al., 2012). Instead, we found that the enfacement effect (i.e., greater self-face/other-face merging following synchronous compared to asynchronous visuo-tactile stimulation) was abolished if the other person displayed fear but remained if they expressed anger. This nonetheless suggests that enfacement operates on an evaluative self-representation as well as a physical one because the effect changes with the emotional content of the other’s face. Further research into the neural mechanism behind the enfacement effect is needed to determine why sight of fear diminishes it rather than enhancing it.

The neural correlates of exploration and cognitive mapping in blindness remain elusive. The role of visuo-spatial pathways in blind vs. sighted subjects is still under debate. In this preliminary study, we investigate, as a possible estimation of the activity in the visuo-spatial pathways, the EEG patterns of blind and blindfolded-sighted subjects during the active tactile construction of cognitive maps from virtual objects compared with rest and passive tactile stimulation. Ten blind and ten matched, blindfolded-sighted subjects participated in the study. Events were defined as moments when the finger was only stimulated (passive stimulation) or the contour of a virtual object was touched (during active exploration). Event-related spectral power and coherence perturbations were evaluated within the beta 1 band (14–18 Hz). They were then related to a subjective cognitive-load estimation required by the explorations [namely, perceived levels of difficulty (PLD)]. We found complementary cues for sensory substitution and spatial processing in both groups: both blind and sighted subjects showed, while exploring, late power decreases and early power increases, potentially associated with motor programming and touch, respectively. The latter involved occipital areas only for blind subjects (long-term plasticity) and only during active exploration, thus supporting tactile-to-visual sensory substitution. In both groups, coherences emerged among the fronto-central, centro-parietal, and occipito-temporal derivations associated with visuo-spatial processing. This seems in accordance with mental map construction involving spatial processing, sensory-motor processing, and working memory. The observed involvement of the occipital regions suggests that a substitution process also occurs in sighted subjects. Only during explorations did coherence correlate positively with PLD for both groups and in derivations, which can be related to visuo-spatial processing, supporting the existence of supramodal spatial processing independently of vision capabilities.

Body image disturbances (BIDs) have been widely studied using virtual reality (VR) devices that induce a full body illusion (FBI) and allow manipulation of the individual’s perceptual and affective experiences of the body. This study aimed to assess whether the induction of the FBI over a virtual body would produce changes in body-related anxiety and BIDs using a new whole-body visuo-tactile stimulation procedure. Fifty non-clinical participants were randomly assigned to synchronous or asynchronous visuo-tactile groups. During the pre-assessment, all participants filled in BIDs and body-anxiety questionnaires. Then, they were embodied into two virtual bodies (VBs): firstly, with their real measurements, and secondly, with a larger-size body. Body image disturbances, body anxiety, fear of gaining weight, and FBI levels were assessed after exposure to each avatar. All participants in both conditions showed higher levels of BIDs and body anxiety after owning the larger-size VB than after owning the real-size VB (p < 0.05). The synchronous visuo-tactile group had higher scores, although the differences did not reach statistical significance. This study provides evidence of the usefulness of this new embodiment-based technique to induce changes in BIDs or body anxiety in a non-clinical sample, being suitable for use in future body image interventions.

Sense of body ownership is an immediate and distinct experience of one’s body as belonging to oneself. While it is well-recognized that ownership feelings emerge from the integration of visual and somatosensory signals, the principles upon which they are integrated are still intensely debated. Here, we used the rubber hand illusion (RHI) to examine how the interplay of visual, tactile, and proprioceptive signals is governed depending on their spatiotemporal properties. For this purpose, the RHI was elicited in different conditions varying with respect to the extent of visuo-proprioceptive divergence (i.e., the distance between the real and fake hands) and differing in terms of the availability and spatiotemporal complexity of tactile stimulation (none, simple, or complex). We expected that the attenuating effect of distance on illusion strength will be more pronounced in the absence of touch (when proprioception gains relatively higher importance) and absent in the presence of complex tactile signals. Additionally, we hypothesized that participants with greater proprioceptive acuity—assessed using an elbow joint position discrimination task—will be less susceptible to the illusion, but only under the conditions of limited tactile stimulation. In line with our prediction, RHI was attenuated at the farthest distance only when tactile information was absent or simplified, but the attenuation was effectively prevented by the use of complex tactile stimulation—in this case, RHI was comparably vivid at both distances. However, passive proprioceptive acuity was not related to RHI strength in either of the conditions. The results indicate that complex-structured tactile signals can override the influence of proprioceptive signals in body attribution processes. These findings extend our understanding of body ownership by showing that it is primarily determined by informative cues from the most relevant sensory domains, rather than mere accumulation of multisensory evidence.

Spatial navigation within a real 3-D maze was investigated to study space perception on the sole basis of tactile information transmitted by means of a ‘tactile vision substitution system' (TVSS) allowing the conversion of optical images—collected by a micro camera—into ‘tactile images’ via a matrix in contact with the skin. The development of such a device is based on concepts of cerebral and functional plasticity, enabling subjective reproduction of visual images from tactile data processing. Blindfolded sighted subjects had to remotely control the movements of a robot on which the TVSS camera was mounted. Once familiarised with the cues in the maze, the subjects were given two exploration sessions. Performance was analysed according to an objective point of view (exploration time, discrimination capacity), as well as a subjective one (speech). The task was successfully carried out from the very first session. As the subjects took a different path during each navigation, a gradual improvement in performance (discrimination and exploration time) was noted, generating a phenomenon of learning. Moreover, subjective analysis revealed an evolution of the spatialisation process towards distal attribution. Finally, some emotional expressions seemed to reflect the genesis of ‘qualia’ (emotional qualities of stimulation).

The physical boundaries of our body do not define what we perceive as self. This malleable representation arises from the neural integration of sensory information coming from the environment. Manipulating the visual and haptic cues produces changes in body perception, inducing the Full Body Illusion (FBI), a vastly used approach to exploring humans’ perception. After pioneering FBI demonstrations, issues arose regarding its setup, using experimenter-based touch and pre-recorded videos. Moreover, its outcome measures are based mainly on subjective reports, leading to biased results, or on heterogeneous objective ones giving poor consensus on their validity. To address these limitations, we developed and tested a multisensory platform allowing highly controlled experimental conditions, thanks to the leveraged use of innovative technologies: Virtual Reality (VR) and Transcutaneous Electrical Nerve Stimulation (TENS). This enabled a high spatial and temporal precision of the visual and haptic cues, efficiently eliciting FBI. While it matched the classic approach in subjective measures, our setup resulted also in significant results for all objective measurements. Importantly, FBI was elicited when all 4 limbs were multimodally stimulated but also in a single limb condition. Our results behoove the adoption of a comprehensive set of measures, introducing a new neuroscientific platform to investigate body representations.

Le proprietà della corteccia somato-sensoriale secondaria (SII) sono state largamente discusse in molteplici studi sia nella scimmia, sia nell’uomo, suggerendo che quest’area assolva funzioni di alto livello nel processamento dello stimolo tattile, quali, ad esempio, l’apprendimento o la memoria. Recentemente, alcuni studi su scimmia hanno evidenziato che, oltre agli stimoli somato-sensoriali, SII risponde anche alla stimolazione dello spazio peri-personale, all’esecuzione di azioni, alla vista di oggetti in movimento ed all’osservazione di azioni, candidando SII ad essere un’area complessa, non limitata a sole funzioni somato-sensoriali. Partendo dallo studio delle risposte di SII agli stimoli tattili, lo scopo di questa tesi è di investigare la risposta di quest’area a stimoli complessi, con particolare attenzione a task di integrazione visuo-tattile e all’osservazione di azioni nell’uomo. Con queste finalità, gli esperimenti presentati sono stati condotti mediante elettroencefalografia stereotassica (stereo-EEG) su pazienti epilettici farmaco-resistenti, impiantati come parte della loro valutazione pre-chirurgica. In una prima fase, sono stati studiati la distribuzione spaziale ed il profilo temporale delle risposte intra-corticali alla stimolazione del nervo mediano controlaterale ed ipsilaterale. I risultati ottenuti indicano che mentre la corteccia somato-sensoriale primaria (SI), il giro precentrale ed il solco intra-parietale rispondono solo alla stimolazione controlaterale, la corteccia somato-sensoriale secondaria e l’insula posteriore sono attivate bilateralmente. Inoltre, queste ultime sono caratterizzate da una risposta tonica e duratura nel tempo. Questa potrebbe rappresentare un meccanismo di ritenzione temporale dell’informazione tattile ed essere l’espressione di funzioni di alto livello quali appunto la memoria e l’apprendimento degli stimoli. Nella seconda sezione della tesi, per testare il possibile coinvolgimento dell’opercolo parietale nell’integrazione visuo-tattile, la stimolazione del nervo mediano controlaterale è stata somministrata congiuntamente ad una stimolazione visiva (i.e. flash). I risultati ottenuti evidenziano un aumento in ampiezza della componente tonica, se comparato alla sola stimolazione tattile, localizzato nell’insula posteriore e nelle porzioni più rostrali dell’opercolo parietale mentre SII mostra un comportamento del tutto inalterato. Tuttavia, tenendo in considerazione che studi su primati non umani riportano risposte visiva in SII a stimoli biologici, sono necessarie ulteriori indagini per comprendere quale tipologia di stimolazione determina l’attivazione di quest’area. Infine, la terza parte della tesi mostra le risposte intra-corticali di SI e SII ad un task motorio che include compiti di afferramento e manipolazione di oggetti, e all’osservazione delle stesse azioni eseguite da un altro individuo. I risultati evidenziano un’attivazione bilaterale di SII, sia durante l’esecuzione sia durante l’osservazione di azioni, con un profilo temporale sincrono. Al contrario SI è attiva solo durante l’esecuzione: l’input a SI durante l’osservazione non ha dunque una natura somato-sensoriale ma piuttosto deve essere sostenuto da un circuito visuo-motorio capace di operare in maniera simultanea. In conclusione, questa tesi dimostra il ruolo cruciale di SII non solo nel processamento degli stimoli tattili ma anche nell’integrazione di stimoli visuo-motori.
The somatosensory properties of the second somatosensory cortex (SII) have been largely described by many studies in both monkeys and humans, suggesting for this area a high-order role in tactile stimulation processing with functions including tactile learning and memory. More interestingly, recent studies on monkeys showed that beyond somatosensory stimuli, SII responds to a wider number of stimuli including peripersonal space stimulation, active movements, observation of objects displacement and action observation. Taking into account these results, SII is a candidate to be more than just a somatosensory area. Starting from its somatosensory properties, this thesis aims to disentangle the role of SII in more complex tasks with particular attention to visuo-tactile integration and action observation in humans. To this purpose, the experiments presented in this thesis are carried with stereotactic electroencephalography (stereo-EEG) on drug-resistant epileptic patients to take advantage of its high temporal and spatial resolution. Firstly, I investigated the spatial distribution and the temporal profile of the intracortical responses to both contralateral and ipsilateral median nerve stimulation. Results indicated that while the primary somatosensory area, precentral gyrus and intra-parietal sulcus respond only to the contralateral stimulation, the secondary somatosensory cortex and posterior insula are activated bilaterally. Furthermore, these regions exhibit a tonic long-lasting temporal profile, which might represent a mechanism of temporal retention of the tactile information, and thus be the signature of high-level somatosensory functions such as tactile memory and awareness. In a second stage of the thesis, to test the possible involvement of parietal operculum in visuo-tactile integration, we administered to patients contralateral median nerve stimulation jointly with visual stimulation (i.e. flash) to about 100 drug-resistant epileptic patients. Results underline an enhancement of the tonic components relative to tactile stimulation only, limited to posterior insula and to the rostral areas of parietal operculum, with SII maintaining an unaltered behavior. Considering previous findings in non-human primates, which reported visual responses in SII in response to biological stimuli, further researches are needed to understand which threshold in the stimulus might determine the eventual activation of this area. With this aim, the third part of this thesis presents the intracortical responses of both SI and SII to a motor task requiring reaching, grasping and manipulation, as well as to the observation of the same actions performed by another individual. The results obtained highlighted that SII activates bilaterally, both during the execution and the observation of actions, with a synchronous temporal profile. Conversely, SI activates only during the execution, leading to the conclusion that the input to SII during the observation condition has not a somatosensory nature, but rather that it is sustained by visuo-motor circuits operating simultaneously. Taking together all the evidence, this thesis demonstrates the pivotal role of SII not only in somatosensory functions, as largely reported in literature, but also in the integration of visuo-motor stimuli.

In the last decade, research on visual perception of the human body remarkably increased, especially following the discovery of the Extrastriate Body Area, an occipito-temporal region selectively involved in body-processing (Downing et al., 2001). However, an intriguing issue is to what extent a specific kind of body representation in the brain is devoted to the knowledge of the bodily self. Research on the visual recognition of the self-body, in particular, is still scarce, especially if compared to the extensive body of research devoted to the recognition of self-face. In the present thesis, a systematic investigation of unexplored aspects of self-body and self-face recognition is presented, with particular focus on the one side, on the possible neural correlates, and on the other, on the variables of personality that may play a role in these cognitive functions. Recent work in neuroscience indicates a superiority in the visual processing of one’s own than other people’s body-parts (Frassinetti et al., 2008). Specifically, subjects show higher accuracy when asked to match pictures depicting their own compared to unfamiliar body-parts, the so-called “self advantage”. It remains to be established, however, which cortical regions are involved in this phenomenon. To this aim, in experiments reported in Chapter 2 the causal role of cortical regions specifically involved in body-parts processing (i.e., the right Extrastriate Body Area) and in self-face recognition (i.e., the right Inferior Parietal Lobule) was investigated by means of Transcranial Magnetic Stimulation. The results did not allow definitive conclusions regarding the role of the cortical areas under investigation for self body-parts recognition; nonetheless, behavioural data seem to suggest that the self-recognition ability is not as universal as generally believed. In particular, the strength of the “self advantage” showed a large degree of variability across participants. Therefore, the contribution presented in Chapter 3 was aimed at finding some possible determinants that modulate the self-body advantage. Namely, it was examined whether the self-body recognition ability is modulated by implicit and explicit self-esteem, relying upon studies linking the physical self and self-esteem. Two studies were conducted using paradigms assessing covert (Experiment 3) or overt (Experiment 4) self-body recognition (i.e., the matching-to-sample used in previous studies and a new-developed paradigm of overt recognition). Results revealed that the self-body recognition ability is qualified by individual differences in self-esteem, and especially implicit self-esteem, measured with the Implicit Association Test, a widely used procedure for measuring strengths of automatic associations between concepts (Greenwald et al., 1998). Moreover, considering the two studies together, only the implicit self-esteem showed incremental validity in predicting the ability to recognize self body-parts. The results are discussed in terms of the role of individual differences such as implicit self-esteem for cognitive functions such as self-body recognition. Finally, a study (Experiment 5) was conducted to better address whether self-esteem and other personality traits with strong interpersonal value (i.e., empathy) also correlate with the strength and stability of self-representation. Self-face representation was recently found to be less stable than believed in the past (Tsakiris, 2008). Our findings reveal that higher level of implicit self-esteem correspond to lower susceptibility to the “enfacement” illusion, measured in terms of incorporation of other people’s facial features in the self-face representation following synchronous visuo-tactile stimulation in a mirror-like setting. Moreover, the Perspective Taking component of empathy was found to correlate with the introspective experience of the illusion. All in all, the present contribution bridges recent research in the cognitive neurosciences and social cognition and points toward a complex interplay among cognitive and personality factors in the domain of self-recognition.

Haptics is the science of merging tactile sensation with computer applications, thereby enabling users to receive feedback they can feel (in addition to auditory and visual cues). Multimodal environments where visual, auditory and haptic stimuli are present convey information more efficiently since the user manipulates and experiences the environment through multiple sensory channels. The availability of haptic systems enables the augmentation of traditional instruction with interactive interfaces offering enhanced motivation and intellectual stimulation. Although the haptic devices have not made large in roads into education, we believe that the potential for revolutionary change now exists due to the recent availability of both the hardware and software components. This paper brings into discussion some of the most relevant technological issues involving haptic systems in education. One of these issues is choosing the suitable haptic hardware, API or framework for developing a visuo-haptic e-Learning system. The decision is based on several criteria such as multimodal resources needed by the software system, compatibility with haptic devices, dynamic configuration of the scene, and so on. Another issue is related to the software system reactivity at the user actions. The immediate haptic feedback from virtual models, together with the synchronisation of haptic and visual cues generated by computer to its users are essential for enhancing the user's learning path. Providing realistic visuo-haptic models as much as it is possible is also another problem that the development of the haptic e-learning system should deal with. Such models help to obtain accurate training scenarios developed for the teaching, for example, of medical protocols, or chemical or physical processes.