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BACKGROUND/OBJECTIVES: With the rising ubiquity of digital media and screens in everyday life, toddlers are increasingly exposed to different screens from an early age (i.e., television, computer, tablet, phone). However, few studies have examined the effect of these screens on toddlers' perceptual development. Since tactile exploration skills are necessary for environmental discovery and overall development, the current research investigates the links between screen-use habits and the tactile exploration skills (with visual control) of 6- to 36-month-old toddlers. METHODS: The study involved observing the interactions of 135 toddlers with various objects and assessing the complexity of their visuo-tactile exploration strategies through two original experimental tasks. Data concerning screen habits and other relevant factors, such as socio-economic level, were collected using a parental questionnaire. RESULTS: Toddlers with greater screen exposure time demonstrated weaker tactile exploration skills and employed less age-appropriate exploration strategies. Socio-economic factors and parental engagement in alternative activities significantly influenced these developmental outcomes. CONCLUSIONS: These findings emphasize the importance of reducing screen time and promoting interactive co-viewing and alternative activities to mitigate the negative effects of screen exposure. Further longitudinal research is needed to determine the long-term impacts of early screen exposure on tactile exploration and overall psychological development.

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Introduction: We report a very unique clinical presentation of a patient who complained, after a left parietal brain damage, about feeling tactile stimulations on his right upper limb without being able to localize them. Methods: Using a single case study approach, we report three experiments relying on several custom-made tasks to explore the different levels of somatosensory information processing, ranging from somato-sensation to somato-representation. Results: Our results showed a preserved ability to localize tactile stimuli applied on the right upper limb when using pointing responses while the ability to localize was less efficient when having to name the stimulated part (akin Numbsense). When the stimuli were applied on more distal locations (i.e., on the hand and on fingers), the number of correct responses decreased significantly independently of the modality of response. Finally, when visually presented with a stimulus delivered on the hand of an examiner in synchrony with the stimulation on the hidden hand of the patient, responses were largely influenced by the visual information available. Altogether, the convergence of these different customized tasks revealed an absence of autotopagnosia for motor responses for the right upper limb, associated with altered abilities to discriminate stimulus applied on distal and restricted/closer zones in the hand. Discussion: The somato-representation of our patient seemed to significantly rely on visual information, leading to striking deficits to localize tactile stimuli when vision and somesthesic afferences are discordant. This case report offers a clinical illustration of pathological imbalance between vision and somesthesia. Implications of these troubles in somato-representation on higher cognitive level processes are discussed.

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Self-motion through an environment induces various sensory signals, i.e., visual, vestibular, auditory, or tactile. Numerous studies have investigated the role of visual and vestibular stimulation for the perception of self-motion direction (heading). Here, we investigated the rarely considered interaction of visual and tactile stimuli in heading perception. Participants were presented optic flow simulating forward self-motion across a horizontal ground plane (visual), airflow toward the participants' forehead (tactile), or both. In separate blocks of trials, participants indicated perceived heading from unimodal visual or tactile or bimodal sensory signals. In bimodal trials, presented headings were either spatially congruent or incongruent with a maximum offset between visual and tactile heading of 30°. To investigate the reference frame in which visuo-tactile heading is encoded, we varied head and eye orientation during presentation of the stimuli. Visual and tactile stimuli were designed to achieve comparable precision of heading reports between modalities. Nevertheless, in bimodal trials heading perception was dominated by the visual stimulus. A change of head orientation had no significant effect on perceived heading, whereas, surprisingly, a change in eye orientation affected tactile heading perception. Overall, we conclude that tactile flow is more important to heading perception than previously thought.NEW & NOTEWORTHY We investigated heading perception from visual-only (optic flow), tactile-only (tactile flow), or bimodal self-motion stimuli in different conditions varying in head and eye position. Overall, heading perception was body or world centered and non-Bayes optimal and revealed a centripetal bias. Although being visually dominated, tactile flow revealed a significant influence during bimodal heading perception.

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The sense of body ownership builds on proper multisensory integration mechanisms. The Rubber-Hand Illusion (RHI) paradigm exploits a visuo-tactile multisensory conflict to induce illusory body ownership toward a fake hand, assessed by multidimensional subjective ratings and univocal objective measurements. Considering the controversy as to whether viewing the rubber hand is necessary or not to induce the illusion, we investigated the effects of targeted manipulations of visibility on subjective and objective aspects of the RHI. To this aim, we collected questionnaire and proprioceptive drift data from thirty participants receiving visuo-tactile stimulation in a setup that allowed for increasing and decreasing the visibility (illumination) of the rubber hand. We found that specific subjective ratings (Movement and Loss of Ownership) were sensitive to the interaction between rubber hand's visibility and illusory ownership. The interaction was not significant for the Embodiment subjective component and for the objective one (proprioceptive drift). Since different degrees of visibility did not differentially affect the RHI, these findings highlight that relatively abrupt changes in the visibility of the rubber hand can differentially impact subjective versus objective components of body ownership. This understanding may be critical for neuroscientific theories on the relationship between multisensory integration and body consciousness.

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Early models of multisensory integration posited that cross-modal signals only converged in higher-order association cortices and that vision automatically dominates. However, recent studies have challenged this view. In this study, the significance of the alignment of motion axes and spatial alignment across visual and tactile stimuli, as well as the effect of hand visibility on visuo-tactile interactions were examined. Using binocular rivalry, opposed motions were presented to each eye and participants were required to track the perceived visual direction. A tactile motion that was either a leftward or rightward sweep across the fingerpad was intermittently presented. Results showed that tactile effects on visual percepts were dependent on the alignment of motion axes: rivalry between up/down visual motions was not modulated at all by left/right tactile motion. On the other hand, visual percepts could be altered by tactile motion signals when both modalities shared a common axis of motion: a tactile stimulus could maintain the dominance duration of a congruent visual stimulus and shorten its suppression period. The effects were also conditional on the spatial alignment of the visual and tactile stimuli, being eliminated when the tactile device was displaced 15 cm away to the right of the visual stimulus. In contrast, visibility of the hand touching the tactile stimulus facilitated congruent switches relative to a visual-only baseline but did not present a significant advantage overall. In sum, these results show a low-level sensory interaction that is conditional on visual and tactile stimuli sharing a common motion axis and location in space.

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To investigate whether individual differences in Empathy predict the characteristics of Peripersonal Space (PPS) representations, we asked participants to complete the IRI questionnaire and a visuo-tactile crossmodal congruency task (CCT) as an index of PPS. In the CCT, they responded to the elevation of a tactile target while ignoring a visual distractor presented at the same (i.e. congruent) or different (i.e. incongruent) elevation. The target-distractor distance was also manipulated in depth, with visual distractors randomly presented at near, middle or far locations (0 cm, 25 cm or 50 cm). The near and middle crossmodal congruency effects (CCE) were inversely related to participants' scores on the Empathic Concern sub-scale (EC). Furthermore, the slope of participants' CCE across locations was related to EC scores, with flatter slopes for higher EC individuals. Thus, higher EC individuals showed reduced visuo-tactile integration responses within PPS and a reduced differentiation between PPS and extra-personal space (EPS).

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We investigated the characteristic features of perception in octopuses by examining multisensory information from an object simulating prey, which provided different visual and tactile stimuli. In experiments, we presented plain-body octopus with four kinds of models, namely, the Lifelike crab, the Embedded crab, the Translucent crab, and the Black cuboid. These models contain different amounts of visual and tactile information that a crab originally contains: the Lifelike crab resembles a crab both visually and tactilely, the Embedded crab resembles a crab visually but provides different tactile information, the Translucent crab provides tactile information of a crab but contains less visual information, and the Black cuboid lacks both visual and tactile information of a crab. Among these four models, octopuses contacted most with the Lifelike crab, which was similar to their behavior with a crab. Indeed, octopuses were fastest to contact the Lifelike crab and had the longest duration of contacting it among the four models. Octopuses contacted the Embedded crab more than the Translucent crab, both of which had contrasting visuo-tactile information compared to that of a crab. Quickness of octopuses to contact and duration of contact with the Embedded crab were more similar to those with the Lifelike crab than to those with the Translucent crab. Furthermore, octopuses contacted the Black cuboid least among the models. These results suggest that octopuses compositely detect both visual and tactile information in order to perceive an object. Furthermore, octopuses possess the potential priority either for visual or tactile information, by which they process the target object.

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Although various recognizing abilities have been revealed for octopuses, they predominantly deal with only a few species. Therefore, cognition diversity among other octopus species that have been overlooked needs to be investigated. We investigated whether plain-body octopus can learn a symbolic stimulus, for the reason that this octopus is abundant around Okinawa Island with a complex coral community landscape. Attention was paid to whether an octopus can learn a stimulus based solely on visual information without previous experience of learning it tactilely as well as visually. Furthermore, we examined whether different sensory inputs affect learning in octopuses. First, we tested whether octopuses can be conditioned to three different stimuli (object, picture, and video of a white cross). Octopuses that were presented an object or a picture could learn to touch them. However, octopuses that were presented a video could not learn to touch the stimulus. Second, we showed a video to octopuses that had already learned about an object or a picture to investigate whether the octopuses, having experienced a target using visual and tactile senses, can recognize a video of the target based solely on visual information. Octopuses could learn to touch the video. When a conditioned stimulus and a novel stimulus were simultaneously presented on a computer screen, an octopus that had learned an object more often selected the conditioned stimulus when compared with an octopus that had experienced only a picture. These findings suggest that octopuses use multisensory information to recognize a specific object.

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Peripersonal space (PPS) is the space immediately surrounding the body, conceptualised as a sensory-motor interface between body and environment. PPS size differs between individuals and contexts, with intrapersonal traits and states, as well as social factors having a determining role on the size of PPS. Testosterone plays an important role in regulating social-motivational behaviour and is known to enhance dominance motivation in an implicit and unconscious manner. We investigated whether the dominance-enhancing effects of testosterone reflect as changes in the representation of PPS in a within-subjects testosterone administration study in women (N = 19). Participants performed a visuo-tactile integration task in a mixed-reality setup. Results indicated that the administration of testosterone caused a significant enlargement of participants' PPS, suggesting that testosterone caused participants to implicitly appropriate a larger space as their own. These findings suggest that the dominance-enhancing effects of testosterone reflect at the level of sensory-motor processing in PPS.

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Sensitivity to the temporal coherence of visual and tactile signals increases perceptual reliability and is evident during infancy. However, it is not clear how, or whether, bidirectional visuotactile interactions change across childhood. Furthermore, no study has explored whether viewing a body modulates how children perceive visuotactile sequences of events. Here, children aged 5-7 years (n = 19), 8 and 9 years (n = 21), and 10-12 years (n = 24) and adults (n = 20) discriminated the number of target events (one or two) in a task-relevant modality (touch or vision) and ignored distractors (one or two) in the opposing modality. While participants performed the task, an image of either a hand or an object was presented. Children aged 5-7 years and 8 and 9 years showed larger crossmodal interference from visual distractors when discriminating tactile targets than the converse. Across age groups, this was strongest when two visual distractors were presented with one tactile target, implying a "fission-like" crossmodal effect (perceiving one event as two events). There was no influence of visual context (viewing a hand or non-hand image) on visuotactile interactions for any age group. Our results suggest robust interference from discontinuous visual information on tactile discrimination of sequences of events during early and middle childhood. These findings are discussed with respect to age-related changes in sensory dominance, selective attention, and multisensory processing.

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Slow, gentle stimulation of hairy skin is generally accompanied by hedonic sensations. This phenomenon, also known as (positive) affective touch, is likely to be the basis of affiliative interactions with conspecifics by promoting inter-individual bindings. Previous studies on healthy humans have demonstrated that affective touch can remarkably impact behavior. For instance, by administering the Rubber Hand Illusion (RHI) paradigm, the embodiment of a fake hand enhances after a slow, affective touch compared to a fast, neutral touch. However, results coming from this area are not univocal. In addition, there are no clues in the existing literature on the relationship between affective touch and the space around our body. To overcome these lacks, we carried out two separate experiments where participants underwent a RHI paradigm (Experiment 1) and a Visuo-Tactile Interaction task (Experiment 2), designed to tap into body representation and peripersonal space processing, respectively. In both experiments, an affective touch (CT-optimal, 3 cm/s) and neutral touch (CT-suboptimal, 18 cm/s) were delivered by the experimenter on the dorsal side of participants' hand through a "skin to skin" contact. In Experiment 1, we did not find any modulation of body representation-not at behavioral nor at a physiological level-by affective touch. In Experiment 2, no visuo-tactile spatial modulation emerged depending upon the pleasantness of the touch received. These null findings are interpreted in the light of the current scientific context where the real nature of affective touch is often misguided, and they offer the possibility to pave the way for understanding the real effects of affective touch on body/space representation.

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Vision of the body has been reported to improve tactile acuity even when vision is not informative about the actual tactile stimulation. However, it is currently unclear whether this effect is limited to body parts such as hand, forearm or foot that can be normally viewed, or it also generalizes to body locations, such as the shoulder, that are rarely before our own eyes. In this study, subjects consecutively performed a detection threshold task and a numerosity judgment task of tactile stimuli on the shoulder. Meanwhile, they watched either a real-time video showing their shoulder or simply a fixation cross as control condition. We show that non-informative vision improves tactile numerosity judgment which might involve tactile acuity, but not tactile sensitivity. Furthermore, the improvement in tactile accuracy modulated by vision seems to be due to an enhanced ability in discriminating the number of adjacent active electrodes. These results are consistent with the view that bimodal visuotactile neurons sharp tactile receptive fields in an early somatosensory map, probably via top-down modulation of lateral inhibition.

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Peripersonal Space (PPS) is the multisensory space immediately surrounding our body. Visual and tactile stimuli here are promptly processed, since their interaction gradually strengthens as the distance between visual stimulus and the body decreases. Recently, a modified version of the Temporal Order Judgment (TOJ) task was proposed to assess PPS based on the spatial congruence between somatosensory and visual stimuli. Here, we used this paradigm to explore how a temporary vs a permanent state of anxiety can alter PPS. Indeed, previous research showed that PPS boundaries are not fixed, but they can be enlarged by contingent factors (i.e. emotional features). Participants performed the TOJ paradigm twice, just before and after completing an anxiety-inducing task (experimental breathing condition) or a neutral one (control breathing condition), while their trait and state anxiety levels were repeatedly measured. We found that the pattern of visuo-tactile integration in PPS changes in the very opposite way following the two breathing tasks for participants with high levels of temporary anxiety, by strengthening and weakening its power after the experimental and control conditions, respectively. On the contrary, both the breathing tasks are capable of reducing the cross-modal interplay as compared to baseline for high trait-anxious participants, who show an overall stronger visuo-tactile integration inside the PPS than low trait anxious individuals. These results are discussed in the light of the double dissociation between orienting and alerting attentional network over-functioning, reported in state anxiety participants, and impoverished prefrontal attentional control shown by trait anxiety individuals.

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Previous research showed that full body ownership illusions in virtual reality (VR) can be robustly induced by providing congruent visual stimulation, and that congruent tactile experiences provide a dispensable extension to an already established phenomenon. Here we show that visuo-tactile congruency indeed does not add to already high measures for body ownership on explicit measures, but does modulate movement behavior when walking in the laboratory. Specifically, participants who took ownership over a more corpulent virtual body with intact visuo-tactile congruency increased safety distances towards the laboratory's walls compared to participants who experienced the same illusion with deteriorated visuo-tactile congruency. This effect is in line with the body schema more readily adapting to a more corpulent body after receiving congruent tactile information. We conclude that the action-oriented, unconscious body schema relies more heavily on tactile information compared to more explicit aspects of body ownership.

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Joint attention refers to the coordinated attention between social partners to an object of shared interest, usually involving shared gaze toward the object. In the laboratory, however, joint attention is often investigated using computerized gaze cueing tasks that do not allow shared gaze. Instead, these computerized tasks require the participant to maintain fixation on the virtual partner's face, while the partner gazes to the left or right. Here we designed a modified gaze cueing task that better simulates a natural joint attention episode by allowing shared gaze, while still maintaining tight experimental control. In our computerized task the participant's gaze and the gaze of a virtual partner were manipulated independently, resulting in shared or unshared gaze. Following each gaze shift of the virtual partner a touch stimulus was delivered on one of the cheeks of the participant. We analyzed behavioral and neural (electro-encephalography) responses to the touch. Faster reaction-times and stronger lateralization of alpha power were observed when the touched cheek was in a jointly attended hemispace compared with a singly attended or unattended hemispace. Importantly, these effects were unique to joint attention and could not be explained as the additive effects of own gaze and gaze cue direction. Underlining its social nature, we found that the behavioral effect was absent when we repeated our experiment with nonsocial cues (arrows) instead of gaze cues. Furthermore, when we compared trustworthy with untrustworthy virtual partners (trustworthiness judgements based on facial appearance) we found the effect only for trustworthy and not for untrustworthy virtual partners. We conclude that joint attention based on shared gaze influences attentional orienting such that cross-modal sensory processing at the jointly attended location is facilitated, particularly when the partner is trustworthy. This indicates that social interactions and trustworthiness judgements affect cortical and behavioral responses to sensory information.

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The neural representation of multisensory space near the body is modulated by the active use of long tools in non-human primates. Here, we investigated whether the electrophysiological correlates of visuo-tactile integration in near and far space were modulated by active tool use in healthy humans. Participants responded to a tactile target delivered to one hand while an irrelevant visual stimulus was presented ipsilaterally in near or far space. This crossmodal task was performed after the use of either short or long tools. Crucially, the P100 components elicited by visuo-tactile stimuli was enhanced on far as compared to near space trials after the use of long tools, while no such difference was present after short tool use. Thus, we found increased neural responses in brain areas encoding tactile stimuli to the body when visual stimuli were presented close to the tip of the tool after long tool use. This increased visuo-tactile integration on far space trials following the use of long tools might indicate a transient remapping of multisensory space. We speculate that performing voluntary actions with long tools strengthens the representation of sensory information arising within portions of space (i.e. the hand and the tip of the tool) that are most functionally relevant to one's behavioural goals.

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The peripersonal space (PPS) is the space surrounding our body, represented in a multisensory fashion by integrating stimuli of different modalities. Recently, it has been demonstrated that PPS is emotionally connoted, being sensitive to the different affective valence of the stimuli located inside it. However, how visuo-tactile interactions can be spatially shaped by intrinsic or acquired valence of stimuli is not clear. To investigate this, we conducted three experiments in which participants performed a visuo-tactile interaction task, while the intrinsic valence (Exp. 1 and 2) or the learned valence (Exp. 3) of visual stimuli was manipulated. Participants were asked to respond as fast as possible to a tactile stimulus that was delivered while a visual stimulus was approaching (Exp.1 and 3) or receding (Exp.2) from the hand. Touch was synchronized with different distances of the visual stimulus from the hand. We found that both the expectancy of stimulus and the distance of the visual one from the hand impact RTs to tactile targets. Crucially, we found that spatial modulation was also influenced by stimulus valence, but only for the approaching and not the receding stimuli. At far distances, neutral stimuli yielded overall slower RTs than intrinsically positive or negative stimuli (Experiment 1), while no modulation was exerted by the level of conditioning (Experiment 3). At near distances, response to touches accompanied by looming neutral stimuli became as fast as that occurring with positive and negative ones. Stimulus valence did not interact with the expectancy of a tactile stimulus (Experiment 2). Overall, these findings support the vision that visuo-tactile interactions can be dynamically modulated by the valence of looming visual stimuli when these are located at longer distances from the body. When closer to it, all stimuli acquire saliency, regardless of their intrinsic or acquired valence, due to their proximity, and then relevance, to the body. Overall, a view of PPS as a gradient modulating visuo-tactile integration, also based on stimulus valence, is discussed.

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It is widely accepted that the integration of visual and tactile information is a necessity to induce ownership over a rubber hand. This idea has recently been challenged by Ferri et al. (Proc R Soc B 280:1-7, 2013), as they found that sense of ownership was evident by mere expectation of touch. In our study, we aimed to further investigate this finding, by studying whether the mere potential for touch yields a sense of ownership similar in magnitude to that resulting from actually being touched. We conducted two experiments. In the first experiment, our set-up was the classical horizontal set-up (similar to Botvinick and Cohen, Nature 391:756, 1998). Sixty-three individuals were included and performed the classical conditions (synchronous, asynchronous), an approached but not touched (potential for touch), and a 'visual only' condition. In the second experiment, we controlled for differences between the current set-up and the vertical set-up used by Ferri et al. (Proc R Soc B 280:1-7, 2013). Fifteen individuals were included and performed a synchronous and various approaching conditions [i.e., vertical approach, horizontal approach, and a control approach (no hands)]. In our first experiment, we found that approaching the rubber hand neither induced a larger proprioceptive drift nor a stronger subjective sense of ownership than asynchronous stimulation did. Generally, our participants gained most sense of ownership in the synchronous condition, followed by the visual only condition. When using a vertical set-up (second experiment), we confirmed previous suggestions that tactile expectation was able to induce embodiment over a foreign hand, similar in magnitude to actual touch, but only when the real and rubber hand were aligned on the vertical axis, thus along the trajectory of the approaching stimulus. These results indicate that our brain uses bottom-up sensory information, as well as top-down predictions for building a representation of our body.

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Previous studies have examined the neural correlates for crossmodal paired-associate (PA) memory and the temporal dynamics of its formation. However, the neural dynamics for feedback processing of crossmodal PA learning remain unclear. To examine this process, we recorded event-related scalp electrical potentials for PA learning of unimodal visual-visual pairs and crossmodal visual-tactile pairs when participants performed unimodal and crossmodal tasks. We examined event-related potentials (ERPs) after the onset of feedback in the tasks for three effects: feedback type (positive feedback vs. negative feedback), learning (as the learning progressed) and the task modality (crossmodal vs. unimodal). The results were as follows: (1) feedback type: the amplitude of P300 decreased with incorrect trials and the P400/N400 complex was only present in incorrect trials; (2) learning: progressive positive voltage shifts in frontal recording sites and negative voltage shifts in central and posterior recording sites were identified as learning proceeded; and (3) task modality: compared with the unimodal PA learning task, positive voltage shifts in frontal sites and negative voltage shifts in posterior sites were found in the crossmodal PA learning task. To sum up, these results shed light on cortical excitability related to feedback processing of crossmodal PA learning.

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Tracking one's own body is essential for environmental interaction, and involves integrating multisensory cues with stored information about the body's typical features. Exactly how multisensory information is integrated in own-body perception is still unclear. For example, Ide and Hidaka (Exp Brain Res 228:43-50, 2013) found that participants made less precise visuo-tactile temporal order judgments (TOJ) when viewing hands in a plausible orientation (upright; typical for one's own hand) compared to an implausible orientation (rotated 180°). This suggests that viewing one's own body relaxes the precision for perceived visuo-tactile synchrony. In contrast, visuo-proprioceptive research shows improvements for multisensory temporal perception near one's own body in asynchrony detection tasks, implying an increase in precision. Hence, it is unclear whether viewed hand orientation generally modulates the ability to detect small asynchronies between vision and touch, or if this effect is specific to TOJ tasks. We investigated whether viewed hand orientation affects detection of visuo-tactile asynchrony. In two experiments, participants viewed model hands in anatomically plausible or implausible orientations. In one experiment, we stroked the hands to induce the rubber hand illusion. Participants were asked to detect short delays (40-280 ms) between vision (an LED flash on the model hand) and touch (a tap to fingertip of the participant's hidden hand) in a two-interval forced-choice task. Bayesian analyses show that our data provide strong evidence that viewed hand orientation does not affect visuo-tactile asynchrony detection. This study suggests the mechanisms for fine-grained time perception differ between visuo-tactile and visuo-proprioceptive contexts.

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