Tactile shape discrimination for moving stimuli.

In this study, we explored spatial-temporal dependencies and their impact on the tactile perception of moving objects. Building on previous research linking visual perception and human movement, we examined if an imputed motion mechanism operates within the tactile modality. We focused on how biological coherence between space and time, characteristic of human movement, influences tactile perception. An experiment was designed wherein participants were stimulated on their right palm with tactile patterns, either ambiguous (incongruent conditions) or non-ambiguous (congruent conditions) relative to a biological motion law (two-thirds power law) and asked to report perceived shape and associated confidence. Our findings reveal that introducing ambiguous tactile patterns (1) significantly diminishes tactile discrimination performance, implying motor features of shape recognition in vision are also observed in the tactile modality, and (2) undermines participants' response confidence, uncovering the accessibility degree of information determining the tactile percept's conscious representation. Analysis based on the Hierarchical Drift Diffusion Model unveiled the sensitivity of the evidence accumulation process to the stimulus's informational ambiguity and provides insight into tactile perception as predictive dynamics for reducing uncertainty. These discoveries deepen our understanding of tactile perception mechanisms and underscore the criticality of predictions in sensory information processing.

Cortical activations associated with spatial remapping of finger touch using EEG.

The spatial coding of tactile information is functionally essential for touch-based shape perception and motor control. However, the spatiotemporal dynamics of how tactile information is remapped from the somatotopic reference frame in the primary somatosensory cortex to the spatiotopic reference frame remains unclear. This study investigated how hand position in space or posture influences cortical somatosensory processing. Twenty-two healthy subjects received electrical stimulation to the right thumb (D1) or little finger (D5) in three position conditions: palm down on right side of the body (baseline), hand crossing the body midline (effect of position), and palm up (effect of posture). Somatosensory-evoked potentials (SEPs) were recorded using electroencephalography. One early-, two mid-, and two late-latency neurophysiological components were identified for both fingers: P50, P1, N125, P200, and N250. D1 and D5 showed different cortical activation patterns: compared with baseline, the crossing condition showed significant clustering at P1 for D1, and at P50 and N125 for D5; the change in posture showed a significant cluster at N125 for D5. Clusters predominated at centro-parietal electrodes. These results suggest that tactile remapping of fingers after electrical stimulation occurs around 100-125 ms in the parietal cortex.

Stroking in early mother-infant exchanges: The role of maternal tactile biography and interoceptive sensibility.

Caress-like is a crucial component of caregiving and a key factor in mother-infant interactions. Mother's experience of touch during her own childhood (i.e., tactile biography) has been found to be related to maternal actual use of caress-like touch (i.e., stroking) during mother-infant exchanges. Evidence also suggests that maternal interoceptive sensibility (i.e., self-perceived sensitivity to inner-body sensations) might be related to sensitive caregiving abilities. However, further empirical investigation is needed to understand to what extent tactile biography and interoceptive sensibility have an impact on mothers' stroking when interacting with their infants. Using an online survey, this cross-sectional study explored the potential association between maternal tactile biography, interoceptive sensibility and use of touch for interaction with their own infants in a group of 377 Italian mothers (mean age = 33.29; SD = 4.79). We tested and compared a series of multivariate linear mediation models using maternal tactile biography as predictor, maternal use of affective touch as outcome variable and Multidimensional Assessment of Interoceptive Awareness (MAIA) subscale scores as mediators. We found that, if a mother had positive touch experiences in her own childhood, she may be more likely to use touch in a positive and nurturing way with her own infant (i.e., stroking). Furthermore, mothers' interoceptive sensibility in the form of attention regulation, self-regulation and body listening mediates the association between their past experiences of positive touch and their use of caress-like touch in mother-infant exchanges. This study highlights that maternal tactile biography is directly associated with mothers' use of caress-like touch and indirectly linked to it through the mediating role of interoceptive sensibility.

Towards sensory substitution and augmentation: Mapping visual distance to audio and tactile frequency.

Multimodal perception is the predominant means by which individuals experience and interact with the world. However, sensory dysfunction or loss can significantly impede this process. In such cases, cross-modality research offers valuable insight into how we can compensate for these sensory deficits through sensory substitution. Although sight and hearing are both used to estimate the distance to an object (e.g., by visual size and sound volume) and the perception of distance is an important element in navigation and guidance, it is not widely studied in cross-modal research. We investigate the relationship between audio and vibrotactile frequencies (in the ranges 47-2,764 Hz and 10-99 Hz, respectively) and distances uniformly distributed in the range 1-12 m. In our experiments participants mapped the distance (represented by an image of a model at that distance) to a frequency via adjusting a virtual tuning knob. The results revealed that the majority (more than 76%) of participants demonstrated a strong negative monotonic relationship between frequency and distance, across both vibrotactile (represented by a natural log function) and auditory domains (represented by an exponential function). However, a subgroup of participants showed the opposite positive linear relationship between frequency and distance. The strong cross-modal sensory correlation could contribute to the development of assistive robotic technologies and devices to augment human perception. This work provides the fundamental foundation for future assisted HRI applications where a mapping between distance and frequency is needed, for example for people with vision or hearing loss, drivers with loss of focus or response delay, doctors undertaking teleoperation surgery, and users in augmented reality (AR) or virtual reality (VR) environments.

Multisensory peripersonal space: Visual looming stimuli induce stronger response facilitation to tactile than auditory and visual stimulations.

Anticipating physical contact with objects in the environment is a key component of efficient motor performance. Peripersonal neurons are thought to play a determinant role in these predictions by enhancing responses to touch when combined with visual stimuli in peripersonal space (PPS). However, recent research challenges the idea that this visuo-tactile integration contributing to the prediction of tactile events occurs strictly in PPS. We hypothesised that enhanced sensory sensitivity in a multisensory context involves not only contact anticipation but also heightened attention towards near-body visual stimuli. To test this hypothesis, Experiment 1 required participants to respond promptly to tactile (probing contact anticipation) and auditory (probing enhanced attention) stimulations presented at different moments of the trajectory of a (social and non-social) looming visual stimulus. Reduction in reaction time as compared to a unisensory baseline was observed from an egocentric distance of around 2 m (inside and outside PPS) for all multisensory conditions and types of visual stimuli. Experiment 2 tested whether these facilitation effects still occur in the absence of a multisensory context, i.e., in a visuo-visual condition. Overall, facilitation effects induced by the looming visual stimulus were comparable in the three sensory modalities outside PPS but were more pronounced for the tactile modality inside PPS (84 cm from the body as estimated by a reachability judgement task). Considered together, the results suggest that facilitation effects induced by visual looming stimuli in multimodal sensory processing rely on the combination of attentional factors and contact anticipation depending on their distance from the body.

Novel Wearable Device for Mindful Sensorimotor Training: Integrating Motor Decoding and Somatosensory Stimulation for Neurorehabilitation.

Sensorimotor impairment is a prevalent condition requiring effective rehabilitation strategies. This study introduces a novel wearable device for Mindful Sensorimotor Training (MiSMT) designed for sensory and motor rehabilitation. Our MiSMT device combines motor training using myoelectric pattern recognition along sensory training using two tactile displays. This device offers a comprehensive solution, integrating electromyography and haptic feedback, lacking in existing devices. The device features eight electromyography channels, a rechargeable battery, and wireless Bluetooth or Wi-Fi connectivity for seamless communication with a computer or mobile device. Its flexible material allows for adaptability to various body parts, ensuring ease of use in diverse patients. The two tactile displays, with 16 electromagnetic actuators each, provide touch and vibration sensations up to 250 Hz. In this proof-of-concept study, we show improved two-point discrimination after 5 training sessions in participants with intact limbs (p=0.047). We also demonstrated successful acquisition, processing, and decoding of myoelectric signals in offline and online evaluations. In conclusion, the MiSMT device presents a promising tool for sensorimotor rehabilitation by combining motor execution and sensory training benefits. Further studies are required to assess its effectiveness in individuals with sensorimotor impairments. Integrating mindful sensory and motor training with innovative technology can enhance rehabilitation outcomes and improve the quality of life for those with sensorimotor impairments.

The impact of daily affective touch on cortisol levels in institutionalized & fostered children.

Institutionalized children are often deprived of affective touch. Such tactile deprivation often leads to constant stress, as measured by the levels of salivary cortisol. We report here the impact of an affective touch program, optimized to activate a specific population of unmyelinated mechanosensitive nerves in the skin called c-tactile afferents (CT) on stress resistance. Two populations of children (age 4-10) were recruited: (i) a cohort living in an orphanage and (ii) a fostered cohort. Both groups received the affective touch program daily for 10-15 min for 5-6 weeks. A cohort of age-matched children living in a family environment acted as a control group and did not receive any instructions for tactile stimulation. Salivary cortisol was collected at the beginning (T1) and at the end (T2) of the study in all three groups. For institutionalized and fostered children there was a significant improvement in the level of cortisol (p < 0.0001) between T1 and T2, which is manifested in the balancing cortisol levels: a decrease where it was elevated and an increase, where the critically low level testified to the distress of the child. Balancing cortisol levels is a process of recovery to normal values, which indicates the restoration of neurohumoral mechanisms of stress regulation. The effect of balancing cortisol levels was more pronounced in the group of fostered children compared to the group of orphanage children (p = 0.0326). The children in the control group had no significant differences.

How the inner repetition of a desired perception changes actual tactile perception.

Autosuggestion is a cognitive process where the inner repetition of a thought actively influences one's own perceptual state. In spite of its potential benefits for medical interventions, this technique has gained little scientific attention so far. Here, we took advantage of the known link between intensity and frequency perception in touch ('Békésy effect'). In three separate experiments, participants were asked to modulate the perceived intensity of vibrotactile stimuli at the fingertip through the inner reiteration of the thought that this perception feels very strong (Experiment 1, n = 19) or very weak (Experiments 2, n = 38, and 3, n = 20), while they were asked to report the perceived frequency. We show that the task to change the perceived intensity of a tactile stimulus via the inner reiteration of a thought modulates tactile frequency perception. This constitutes the first experimental demonstration that an experimental design that triggers autosuggestion alters participants' tactile perception using a response orthogonal to the suggested variable. We discuss whether this cognitive process could be used to influence the perception of pain in a clinical context.

Sensory Schwann cells set perceptual thresholds for touch and selectively regulate mechanical nociception.

Previous work identified nociceptive Schwann cells that can initiate pain. Consistent with the existence of inherently mechanosensitive sensory Schwann cells, we found that in mice, the mechanosensory function of almost all nociceptors, including those signaling fast pain, were dependent on sensory Schwann cells. In polymodal nociceptors, sensory Schwann cells signal mechanical, but not cold or heat pain. Terminal Schwann cells also surround mechanoreceptor nerve-endings within the Meissner's corpuscle and in hair follicle lanceolate endings that both signal vibrotactile touch. Within Meissner´s corpuscles, two molecularly and functionally distinct sensory Schwann cells positive for Sox10 and Sox2 differentially modulate rapidly adapting mechanoreceptor function. Using optogenetics we show that Meissner's corpuscle Schwann cells are necessary for the perception of low threshold vibrotactile stimuli. These results show that sensory Schwann cells within diverse glio-neural mechanosensory end-organs are sensors for mechanical pain as well as necessary for touch perception.

Event-related potential evidence for tactile orientation processing in the human brain.

It is well known that information on stimulus orientation plays an important role in sensory processing. However, the neural mechanisms underlying somatosensory orientation perception are poorly understood. Adaptation has been widely used as a tool for examining sensitivity to specific features of sensory stimuli. Using the adaptation paradigm, we measured event-related potentials (ERPs) in response to tactile orientation stimuli presented pseudo-randomly to the right-hand palm in trials with all the same or different orientations. Twenty participants were asked to count the tactile orientation stimuli. The results showed that the adaptation-related N60 component was observed around contralateral central-parietal areas, possibly indicating orientation processing in the somatosensory regions. Conversely, the adaptation-related N120 component was identified bilaterally across hemispheres, suggesting the involvement of the frontoparietal circuitry in further tactile orientation processing. P300 component was found across the whole brain in all conditions and was associated with task demands, such as attention and stimulus counting. These findings help provide an understanding of the mechanisms of tactile orientation processing in the human brain.

Toe stimulation improves tactile perception of the genitals.

The human body is represented in a topographic pattern in the primary somatosensory cortex (S1), and genital representation is displaced below the toe representation. However, the relationship between the representation of the genitals and toe in S1 remains unclear. In this study, tactile stimulation was applied to the big toe in healthy subjects to observe changes in tactile acuity in the unstimulated genital area, abdomen, and metacarpal dorsal. Then tactile stimulation was applied to the right abdomen and metacarpal dorsal to observe changes in tactile acuity in bilateral genitals. The results revealed that tactile stimulation of the big toe led to a reduction in the 2-point discrimination threshold (2PDT) not only in the stimulated big toe but also in the bilateral unstimulated genitals, whereas the bilateral abdomen and metacarpal dorsal threshold remained unchanged. On the other hand, tactile stimulation of the abdomen and metacarpal dorsal did not elicit 2-point discrimination threshold changes in the bilateral genitals. Cortical and subcortical mechanisms have been proposed to account for the findings. One explanation involves the intracortical interaction between 2 adjacent representations. Another possible explanation is that the information content of a specific body part is broadly distributed across the S1. Moreover, exploring the links between human behaviors and changes in the cerebral cortex is of significant importance.

Bi-Manual Sensory Discrimination: A Kinesthetic Study.

The ability of humans to perceive and differentiate kinesthetic sensory information significantly influences our daily activities and motor control. This study examines the impact of asynchronous bi-manual discrimination tasks compared to uni-manual discrimination tasks on kinesthetic perception. Our study aims to reveal the relationship between kinesthetic perception of haptic signals by examining perceptual thresholds in three different scenarios using (i) the dominant hand, (ii) the non-dominant hand, and (iii) both hands simultaneously to differentiate between two successive signals. Subjects exposed to force signals in these three situations conveyed their perceptions of alterations in signal magnitude after each trial. Subsequently, we applied psychometric functions to the collected responses to determine perceptual thresholds. Our results indicate a substantial difference in threshold values between bi-manual and uni-manual scenarios, with the bi-manual scenario exhibiting higher thresholds, indicating inferior perceptual ability when both hands are simultaneously utilized in two separate discrimination tasks. Furthermore, our investigation reveals distinct perception thresholds between the dominant and non-dominant hands, owing to differences in the perceptual capability of the two hands. These findings provide substantial insight into how the nature of tasks may alter kinesthetic perception, offering implications for the development of haptic interfaces in practical applications.

Hierarchical and dynamic relationships between body part ownership and full-body ownership.

What is the relationship between experiencing individual body parts and the whole body as one's own? We theorised that body part ownership is driven primarily by the perceptual binding of visual and somatosensory signals from specific body parts, whereas full-body ownership depends on a more global binding process based on multisensory information from several body segments. To examine this hypothesis, we used a bodily illusion and asked participants to rate illusory changes in ownership over five different parts of a mannequin's body and the mannequin as a whole, while we manipulated the synchrony or asynchrony of visual and tactile stimuli delivered to three different body parts. We found that body part ownership was driven primarily by local visuotactile synchrony and could be experienced relatively independently of full-body ownership. Full-body ownership depended on the number of synchronously stimulated parts in a nonlinear manner, with the strongest full-body ownership illusion occurring when all parts received synchronous stimulation. Additionally, full-body ownership influenced body part ownership for nonstimulated body parts, and skin conductance responses provided physiological evidence supporting an interaction between body part and full-body ownership. We conclude that body part and full-body ownership correspond to different processes and propose a hierarchical probabilistic model to explain the relationship between part and whole in the context of multisensory awareness of one's own body.

Multimodal Haptic Feedback for Virtual Collisions Combining Vibrotactile and Electrical Muscle Stimulation.

In this paper, we explore the effects of multimodal haptic feedback combining vibrotactile and electrical muscle stimulation (EMS) on expressing virtual collisions. We first present a wearable multimodal haptic device capable of generating both mechanical vibration and EMS stimuli. The two types of haptic stimulus are combined into a haptic rendering method that conveys improved virtual collision sensations. This multimodal rendering method highlights the strengths of each modality while compensating for mutual weaknesses. The multimodal rendering method is compared in subjective quality with two unimodal methods (vibration only and EMS only) by a user study. Experimental results demonstrate that our multimodal feedback method can elicit more realistic, enjoyable, expressive, and preferable user experiences.

Comparing the Perceived Intensity of Vibrotacitle Cues Scaled Based on Inherent Dynamic Range.

Wearable devices increasingly incorporate vibrotactile feedback notifications to users, which are limited by the frequency-dependent response characteristics of the low-cost actuators that they employ. To increase the range and type of information that can be conveyed to users via vibration feedback, it is crucial to understand user perception of vibration cue intensity across the narrow range of frequencies that these actuators operate. In this paper, we quantify user perception of vibration cues conveyed via a linear resonant actuator embedded in a bracelet interface using two psychophysical experiments. We also experimentally determine the frequency response characteristics of the wearable device. We then compare user perceived intensity of vibration cues delivered by the bracelet when the cues undergo frequency-specific amplitude modulation based on user perception compared to modulation based on the experimental or manufacturer-reported characterization of the actuator dynamic response. For applications in which designers rely on user perception of cue amplitudes across frequencies to be equivalent, it is recommended that a perceptual calibration experiment be conducted to determine appropriate modulation factors. For applications in which only relative perceived amplitudes are important, basing amplitude modulation factors on manufacturer data or experimentally determined dynamic response characteristics of the wearable device should be sufficient.

With hand on heart: A cardiac Rubber Hand Illusion.

Body illusions such as the Rubber Hand Illusion (RHI) have highlighted how multisensory integration underpins the sense of one's own body. Much of this research has focused on senses arising from outside the body (e.g. vision and touch), but sensations from within the body may also play a role. In a pre-registered study, participants completed a cardiac variation of the RHI, where taps to the finger occurred in or out of time with the heartbeat. We replicated the RHI effect, showing that synchronous but not asynchronous taps to the real and rubber hand increased sensations of embodiment over the rubber hand and caused a shift in the perceived hand location. However, there were no significant influences of cardiac timing on embodiment, nor did it interact with visuo-tactile synchrony. An exploratory analysis found a three-way interaction between synchrony, cardiac timing and interoceptive accuracy as measured by a heartbeat counting task, such that greater interoceptive accuracy was associated with lower embodiment ratings in the systole condition compared to diastole, but only during synchronous stimulation. Although our novel methodology successfully replicated the RHI, our findings suggest that the cooccurence of vision and touch with cardiac signals may make little contribution to the sense of one's body.

Neuromorphic hardware for somatosensory neuroprostheses.

In individuals with sensory-motor impairments, missing limb functions can be restored using neuroprosthetic devices that directly interface with the nervous system. However, restoring the natural tactile experience through electrical neural stimulation requires complex encoding strategies. Indeed, they are presently limited in effectively conveying or restoring tactile sensations by bandwidth constraints. Neuromorphic technology, which mimics the natural behavior of neurons and synapses, holds promise for replicating the encoding of natural touch, potentially informing neurostimulation design. In this perspective, we propose that incorporating neuromorphic technologies into neuroprostheses could be an effective approach for developing more natural human-machine interfaces, potentially leading to advancements in device performance, acceptability, and embeddability. We also highlight ongoing challenges and the required actions to facilitate the future integration of these advanced technologies.

Spatial factors influencing the pain-ameliorating effect of CT-optimal touch: a comparative study for modulating temporal summation of second pain.

Recent studies show that CT-optimal touch, gentle slow stroking of the skin, can reduce pain. However, much is unknown regarding the factors influencing its pain-ameliorating effect, such as tactile attention and touch application site. The current study investigates in 36 healthy individuals, whether CT-optimal touch can reduce temporal summation of second pain (TSSP) compared to CT non-optimal touch and tapping the skin. TSSP refers to activation of the C-nociceptors; by stimulating these fibers a burning and/or tingling sensation can be elicited. All participants underwent three conditions on both the contralateral and ipsilateral side of pain induction. The results show that tapping the skin did not reduce TSSP, meaning that pain reduction through touch cannot be explained by tactile attention effects. CT non-optimal touch only reduced TSSP when applied on the ipsilateral side. Importantly, CT-optimal touch effectively reduced TSSP when applied on the contralateral or ipsilateral side. Furthermore, CT-optimal touch was more effective in reducing TSSP compared to CT non-optimal touch and Tapping. This study shows that that CT-optimal touch can reduce TSSP and this effect appears to be independent of touch application site, which is highly relevant for implementing CT-optimal touch as a treatment.

Touch and look: The role of affective touch in promoting infants' attention towards complex visual scenes.

In a complex social environment, stimuli from different sensory modalities need to be integrated to decode communicative meanings. From very early in life, infants have to combine a multitude of sensory features with social and affective attributes. Of all senses, touch constitutes a privileged channel to carry affective-motivational meanings and foster social connection. In the present study, we investigate whether sharing sensory stimulation that varies for its affective value differentially affects infants' attention towards visual stimuli. 6 to 11-month-old infants (N = 42) were familiarized with two characters respectively matched with tactile (affective or non-affective) and auditory stimulation; then repeatedly exposed to scenes where the two characters moved towards target objects. Our results showed a main effect of stimulation (sound vs. touch) on looking times during familiarization, with longer looking times when sound is provided. During scenes presentation, a main effect of the type of touch (affective vs. non affective) emerged, with longer looking times in infants that previously experienced affective touch, suggesting that this sensory experience may critically engage the self and modulate infant attention. Overall, these findings suggest that while sound acts as attention getter, affective touch supports sustained attention towards complex visual scenes beyond the stimulation period itself.

The developmental timing of spinal touch processing alterations predicts behavioral changes in genetic mouse models of autism spectrum disorders.

Altered somatosensory reactivity is frequently observed among individuals with autism spectrum disorders (ASDs). Here, we report that although multiple mouse models of ASD exhibit aberrant somatosensory behaviors in adulthood, some models exhibit altered tactile reactivity as early as embryonic development, whereas in others, altered reactivity emerges later in life. Additionally, tactile overreactivity during neonatal development is associated with anxiety-like behaviors and social behavior deficits in adulthood, whereas tactile overreactivity that emerges later in life is not. The locus of circuit disruption dictates the timing of aberrant tactile behaviors, as altered feedback or presynaptic inhibition of peripheral mechanosensory neurons leads to abnormal tactile reactivity during neonatal development, whereas disruptions in feedforward inhibition in the spinal cord lead to touch reactivity alterations that manifest later in life. Thus, the developmental timing of aberrant touch processing can predict the manifestation of ASD-associated behaviors in mouse models, and differential timing of sensory disturbance onset may contribute to phenotypic diversity across individuals with ASD.