Precise tactile localization on the human fingernail.

Fingernails are specialized features of the primate hand, which are believed to contribute to manual dexterity. The sensorimotor functions of fingernails, however, remain poorly understood. This study investigates the ability of humans to precisely localize touches applied to the fingernail plate. Nine different locations on the fingernail were touched and participants judged the location by clicking a mouse cursor on a photograph of their finger. Performance in this condition was compared with stimuli applied to the skin of the fingertip. The results showed that participants are able to localize touch on the fingernails at substantially higher than chance levels. Moreover, the precision of this ability is not appreciably lower than that of the fingertips. These results show that the fingernail is a highly sensitive sensory organ, which is capable of providing rich spatial information about tactile stimuli.

Improved tactile speech perception and noise robustness using audio-to-tactile sensory substitution with amplitude envelope expansion.

Recent advances in haptic technology could allow haptic hearing aids, which convert audio to tactile stimulation, to become viable for supporting people with hearing loss. A tactile vocoder strategy for audio-to-tactile conversion, which exploits these advances, has recently shown significant promise. In this strategy, the amplitude envelope is extracted from several audio frequency bands and used to modulate the amplitude of a set of vibro-tactile tones. The vocoder strategy allows good consonant discrimination, but vowel discrimination is poor and the strategy is susceptible to background noise. In the current study, we assessed whether multi-band amplitude envelope expansion can effectively enhance critical vowel features, such as formants, and improve speech extraction from noise. In 32 participants with normal touch perception, tactile-only phoneme discrimination with and without envelope expansion was assessed both in quiet and in background noise. Envelope expansion improved performance in quiet by 10.3% for vowels and by 5.9% for consonants. In noise, envelope expansion improved overall phoneme discrimination by 9.6%, with no difference in benefit between consonants and vowels. The tactile vocoder with envelope expansion can be deployed in real-time on a compact device and could substantially improve clinical outcomes for a new generation of haptic hearing aids.

Local field potential sharp waves with diversified impact on cortical neuronal encoding of haptic input.

Cortical sensory processing is greatly impacted by internally generated activity. But controlling for that activity is difficult since the thalamocortical network is a high-dimensional system with rapid state changes. Therefore, to unwind the cortical computational architecture there is a need for physiological 'landmarks' that can be used as frames of reference for computational state. Here we use a waveshape transform method to identify conspicuous local field potential sharp waves (LFP-SPWs) in the somatosensory cortex (S1). LFP-SPW events triggered short-lasting but massive neuronal activation in all recorded neurons with a subset of neurons initiating their activation up to 20 ms before the LFP-SPW onset. In contrast, LFP-SPWs differentially impacted the neuronal spike responses to ensuing tactile inputs, depressing the tactile responses in some neurons and enhancing them in others. When LFP-SPWs coactivated with more distant cortical surface (ECoG)-SPWs, suggesting an involvement of these SPWs in global cortical signaling, the impact of the LFP-SPW on the neuronal tactile response could change substantially, including inverting its impact to the opposite. These cortical SPWs shared many signal fingerprint characteristics as reported for hippocampal SPWs and may be a biomarker for a particular type of state change that is possibly shared byboth hippocampus and neocortex.

Co-coding of head and whisker movements by both VPM and POm thalamic neurons.

Rodents continuously move their heads and whiskers in a coordinated manner while perceiving objects through whisker-touch. Studies in head-fixed rodents showed that the ventroposterior medial (VPM) and posterior medial (POm) thalamic nuclei code for whisker kinematics, with POm involvement reduced in awake animals. To examine VPM and POm involvement in coding head and whisker kinematics in awake, head-free conditions, we recorded thalamic neuronal activity and tracked head and whisker movements in male mice exploring an open arena. Using optogenetic tagging, we found that in freely moving mice, both nuclei equally coded whisker kinematics and robustly coded head kinematics. The fraction of neurons coding head kinematics increased after whisker trimming, ruling out whisker-mediated coding. Optogenetic activation of thalamic neurons evoked overt kinematic changes and increased the fraction of neurons leading changes in head kinematics. Our data suggest that VPM and POm integrate head and whisker information and can influence head kinematics during tactile perception.

Confrontation with others' emotions changes bodily resonance differently in those with low and high levels of depersonalization.

We present novel research on the cortical dynamics of atypical perceptual and emotional processing in people with symptoms of depersonalization-derealization disorder (DP-DR). We used electroencephalography (EEG)/event-related potentials (ERPs) to delineate the early perceptual mechanisms underlying emotional face recognition and mirror touch in adults with low and high levels of DP-DR symptoms (low-DP and high-DP groups). Face-sensitive visual N170 showed markedly less differentiation for emotional versus neutral face-voice stimuli in the high- than in the low-DP group. This effect was related to self-reported bodily symptoms like disembodiment. Emotional face-voice primes altered mirror touch at somatosensory cortical components P45 and P100 differently in the two groups. In the high-DP group, mirror touch occurred only when seeing touch after being confronted with angry face-voice primes. Mirror touch in the low-DP group, however, was unaffected by preceding emotions. Modulation of mirror touch following angry others was related to symptoms of self-other confusion. Results suggest that others' negative emotions affect somatosensory processes in those with an altered sense of bodily self. Our findings are in line with the idea that disconnecting from one's body and self (core symptom of DP-DR) may be a defence mechanism to protect from the threat of negative feelings, which may be exacerbated through self-other confusion. This article is part of the theme issue 'Sensing and feeling: an integrative approach to sensory processing and emotional experience'.

Primary somatosensory cortical processing in tactile communication.

Touch is an essential form of non-verbal communication. While language and its neural basis are widely studied, tactile communication is less well understood. We used fMRI and multivariate pattern analyses in pairs of emotionally close adults to examine the neural basis of human-to-human tactile communication. In each pair, a participant was designated either as sender or as receiver. The sender was instructed to communicate specific messages by touching only the arm of the receiver, who was inside the scanner. The receiver then identified the message based on the touch expression alone. We designed two multivariate decoder algorithms-one based on the sender's intent (sender-decoder), and another based on the receiver's response (receiver-decoder). We identified several brain areas that significantly predicted behavioural accuracy of the receiver. Regarding our a priori region of interest, the receiver's primary somatosensory cortex (S1), both decoders were able to accurately differentiate the messages based on neural activity patterns here. The receiver-decoder, which relied on the receivers' interpretations of the touch expressions, outperformed the sender-decoder, which relied on the sender's intent. Our results identified a network of brain areas involved in human-to-human tactile communication and supported the notion of non-sensory factors being represented in S1. This article is part of the theme issue 'Sensing and feeling: an integrative approach to sensory processing and emotional experience'.

Biomimetic bimodal haptic perception using triboelectric effect.

Multimodal haptic perception is essential for enhancing perceptual experiences in augmented reality applications. To date, several artificial tactile interfaces have been developed to perceive pressure and precontact signals, while simultaneously detecting object type and softness with quantified modulus still remains challenging. Here, inspired by the campaniform sensilla on insect antennae, we proposed a hemispherical bimodal intelligent tactile sensor (BITS) array using the triboelectric effect. The system is capable of softness identification, modulus quantification, and material type recognition. In principle, due to the varied deformability of materials, the BITS generates unique triboelectric output fingerprints when in contact with the tested object. Furthermore, owing to the different electron affinities, the BITS array can accurately recognize material type (99.4% accuracy), facilitating softness recognition (100% accuracy) and modulus quantification. It is promising that the BITS based on the triboelectric effect has the potential to be miniaturized to provide real-time accurate haptic information as an artificial antenna toward applications of human-machine integration.

Two-Point Discrimination for Upper Extremity and Face in Healthy Young Adults: A Cross-Sectional Study.

BACKGROUND AND AIM: The threshold values of two-point discrimination (TPD) provide a numerical measure of tactile acuity. Normal reference values are needed to decide whether sensory variability is within normal sensorial limits. The study aimed to determine the upper extremity and face threshold values in healthy young adults. MATERIALS AND METHODS: Static TPD thresholds of 67 healthy young adults aged 18-35 years were assessed. Eight skin areas in the face and upper extremity on the dominant side were assessed using a "method of limits" approach with an aesthesiometer. Differences between genders were examined with the Mann-Whitney U test. The Spearman correlation analysis investigated the relationship between age and TPD measurements. RESULTS: TPD values ranged between 4.66 and 19.16 mm and 1.33-68.66 mm in the face and upper extremity, respectively, in the participants with a mean age of 23.83 ± 4.66 years. Fingertips and the area over the lateral mandibula showed the greatest sensitivity. The threshold values of TPD showed both interindividual and intraindividual variability. There was no statistical difference in the TPD values according to gender in any of the measured areas, and there was no relationship between age and TPD test values. CONCLUSIONS: The threshold values of TPD have clinical applicability in various diseases affecting the sensation of the upper extremity and/or face. These data may help the detection of early sensory loss.

A review of assessment methods for measuring individual differences in oral somatosensory perception.

While taste and smell perception have been thoroughly investigated, our understanding of oral somatosensory perception remains limited. Further, assessing and measuring individual differences in oral somatosensory perception pose notable challenges. This review aimed to evaluate the existing methods to assess oral somatosensory perception by examining and comparing the strengths and limitations of each method. The review highlighted the lack of standardized assessment methods and the various procedures within each method. Tactile sensitivity can be assessed using several methods, but each method measures different tactile dimensions. Further investigations are needed to confirm its correlation with texture sensitivity. In addition, measuring a single textural attribute may not provide an overall representation of texture sensitivity. Thermal sensitivity can be evaluated using thermal-change detection or temperature discrimination tests. The chemesthetic sensitivity tests involve either localized or whole-mouth stimulation tests. The choice of an appropriate method for assessing oral somatosensory sensitivity depends on several factors, including the specific research objectives and the target population. Each method has its unique intended purpose, strengths, and limitations, so no universally superior approach exists. To overcome some of the limitations associated with certain methods, the review offers alternative or complementary approaches that could be considered. Researchers can enhance the comprehensive assessment of oral somatosensory sensitivity by carefully selecting and potentially combining methods. In addition, a standardized protocol remains necessary for each method.

Early attentional processing and cortical remapping strategies of tactile stimuli in adults with an early and late-onset visual impairment: A cross-sectional study.

Neuroplastic changes appear in people with visual impairment (VI) and they show greater tactile abilities. Improvements in performance could be associated with the development of enhanced early attentional processes based on neuroplasticity. Currently, the various early attentional and cortical remapping strategies that are utilized by people with early (EB) and late-onset blindness (LB) remain unclear. Thus, more research is required to develop effective rehabilitation programs and substitution devices. Our objective was to explore the differences in spatial tactile brain processing in adults with EB, LB and a sighted control group (CG). In this cross-sectional study 27 participants with VI were categorized into EB (n = 14) and LB (n = 13) groups. They were then compared with a CG (n = 15). A vibrotactile device and event-related potentials (ERPs) were utilized while participants performed a spatial tactile line recognition task. The P100 latency and cortical areas of maximal activity were analyzed during the task. The three groups had no statistical differences in P100 latency (p>0.05). All subjects showed significant activation in the right superior frontal areas. Only individuals with VI activated the left superior frontal regions. In EB subjects, a higher activation was found in the mid-frontal and occipital areas. A higher activation of the mid-frontal, anterior cingulate cortex and orbitofrontal zones was observed in LB participants. Compared to the CG, LB individuals showed greater activity in the left orbitofrontal zone, while EB exhibited greater activity in the right superior parietal cortex. The EB had greater activity in the left orbitofrontal region compared to the LB. People with VI may not have faster early attentional processing. EB subjects activate the occipital lobe and right superior parietal cortex during tactile stimulation because of an early lack of visual stimuli and a multimodal information processing. In individuals with LB and EB the orbitofrontal area is activated, suggesting greater emotional processing.

Recognition of facial expressions of emotions in tactile drawings by blind children, children with low vision and sighted children.

In the context of blindness, studies on the recognition of facial expressions of emotions by touch are essential to define the compensatory touch abilities and to create adapted tools on emotions. This study is the first to examine the effect of visual experience in the recognition of tactile drawings of facial expressions of emotions by children with different visual experiences. To this end, we compared the recognition rates of tactile drawings of emotions between blind children, children with low vision and sighted children aged 6-12 years. Results revealed no effect of visual experience on recognition rates. However, an effect of emotions and an interaction effect between emotions and visual experience were found. Indeed, while all children had a low average recognition rate, the drawings of fear, anger and disgust were particularly poorly recognized. Moreover, sighted children were significantly better at recognizing the drawings of surprise and sadness than the blind children who only showed high recognition rates for joy. The results of this study support the importance of developing emotion tools that can be understood by children with different visual experiences.

Simultaneous cortical, subcortical, and brainstem mapping of sensory activation.

Nonpainful tactile sensory stimuli are processed in the cortex, subcortex, and brainstem. Recent functional magnetic resonance imaging studies have highlighted the value of whole-brain, systems-level investigation for examining sensory processing. However, whole-brain functional magnetic resonance imaging studies are uncommon, in part due to challenges with signal to noise when studying the brainstem. Furthermore, differentiation of small sensory brainstem structures such as the cuneate and gracile nuclei necessitates high-resolution imaging. To address this gap in systems-level sensory investigation, we employed a whole-brain, multi-echo functional magnetic resonance imaging acquisition at 3T with multi-echo independent component analysis denoising and brainstem-specific modeling to enable detection of activation across the entire sensory system. In healthy participants, we examined patterns of activity in response to nonpainful brushing of the right hand, left hand, and right foot (n = 10 per location), and found the expected lateralization, with distinct cortical and subcortical responses for upper and lower limb stimulation. At the brainstem level, we differentiated the adjacent cuneate and gracile nuclei, corresponding to hand and foot stimulation respectively. Our findings demonstrate that simultaneous cortical, subcortical, and brainstem mapping at 3T could be a key tool to understand the sensory system in both healthy individuals and clinical cohorts with sensory deficits.

Contribution of external reference frame to tactile localization.

The purpose of the present study was to elucidate whether an external reference frame contributes to tactile localization in blindfolded healthy humans. In a session, the right forearm was passively moved until the elbow finally reached to the target angle, and participants reached the left index finger to the right middle fingertip. The locus of the right middle fingertip indicated by the participants deviated in the direction of the elbow extension when vibration was provided to the biceps brachii muscle during the passive movement. This finding indicates that proprioception contributes to the identification of the spatial coordinate of the specific body part in an external reference frame. In another session, the tactile stimulus was provided to the dorsal of the right hand during the passive movement, and the participants reached the left index finger to the spatial locus at which the tactile stimulus was provided. Vibration to the biceps brachii muscle did not change the perceived locus of the tactile stimulus indicated by the left index finger. This finding indicates that an external reference frame does not contribute to tactile localization during the passive movement. Humans may estimate the spatial coordinate of the tactile stimulus based on the time between the movement onset and the time at which the tactile stimulus is provided.

Experiencing an Elongated Limb in Virtual Reality Modifies the Tactile Distance Perception of the Corresponding Real Limb.

In measurement, a reference frame is needed to compare the measured object to something already known. This raises the neuroscientific question of which reference frame is used by humans when exploring the environment. Previous studies suggested that, in touch, the body employed as measuring tool also serves as reference frame. Indeed, an artificial modification of the perceived dimensions of the body changes the tactile perception of external object dimensions. However, it is unknown if such a change in tactile perception would occur when the body schema is modified through the illusion of owning a limb altered in size. Therefore, employing a virtual hand illusion paradigm with an elongated forearm of different lengths, we systematically tested the subjective perception of distance between two points [tactile distance perception (TDP) task] on the corresponding real forearm following the illusion. Thus, the TDP task is used as a proxy to gauge changes in the body schema. Embodiment of the virtual arm was found significantly greater after the synchronous visuotactile stimulation condition compared with the asynchronous one, and the forearm elongation significantly increased the TDP. However, we did not find any link between the visuotactile-induced ownership over the elongated arm and TDP variation, suggesting that vision plays the main role in the modification of the body schema. Additionally, significant effect of elongation found on TDP but not on proprioception suggests that these are affected differently by body schema modifications. These findings confirm the body schema malleability and its role as a reference frame in touch.

Humans flexibly use visual priors to optimize their haptic exploratory behavior.

Humans can use prior information to optimize their haptic exploratory behavior. Here, we investigated the usage of visual priors, which mechanisms enable their usage, and how the usage is affected by information quality. Participants explored different grating textures and discriminated their spatial frequency. Visual priors on texture orientation were given each trial, with qualities randomly varying from high to no informational value. Adjustments of initial exploratory movement direction orthogonal to the textures' orientation served as an indicator of prior usage. Participants indeed used visual priors; the more so the higher the priors' quality (Experiment 1). Higher task demands did not increase the direct usage of visual priors (Experiment 2), but possibly fostered the establishment of adjustment behavior. In Experiment 3, we decreased the proportion of high-quality priors presented during the session, hereby reducing the contingency between high-quality priors and haptic information. In consequence, even priors of high quality ceased to evoke movement adjustments. We conclude that the establishment of adjustment behavior results from a rather implicit contingency learning. Overall, it became evident that humans can autonomously learn to use rather abstract visual priors to optimize haptic exploration, with the learning process and direct usage substantially depending on the priors' quality.

Interleaved Multi-Contact Peripheral Nerve Stimulation to Enhance Reproduction of Tactile Sensation: A Computational Modeling Study.

Peripheral nerve stimulation (PNS) is an effective means to elicit sensation for rehabilitation of people with loss of a limb or limb function. While most current PNS paradigms deliver current through single electrode contacts to elicit each tactile percept, multi-contact extraneural electrodes offer the opportunity to deliver PNS with groups of contacts individually or simultaneously. Multi-contact PNS strategies could be advantageous in developing biomimetic PNS paradigms to recreate the natural neural activity during touch, because they may be able to selectively recruit multiple distinct neural populations. We used computational models and optimization approaches to develop a novel biomimetic PNS paradigm that uses interleaved multi-contact (IMC) PNS to approximate the critical neural coding properties underlying touch. The IMC paradigm combines field shaping, in which two contacts are active simultaneously, with pulse-by-pulse contact and parameter variations throughout the touch stimulus. We show in simulation that IMC PNS results in better neural code mimicry than single contact PNS created with the same optimization techniques, and that field steering via two-contact IMC PNS results in better neural code mimicry than one-contact IMC PNS. We also show that IMC PNS results in better neural code mimicry than existing PNS paradigms, including prior biomimetic PNS. Future clinical studies will determine if the IMC paradigm can improve the naturalness and usefulness of sensory feedback for those with neurological disorders.

High frequency heart rate variability is associated with sensitivity to affective touch.

C-tactile afferents (CTs) are a class of unmyelinated, mechanosensitive nerve fibre that respond optimally to skin temperature, slow moving touch typical of a caress. They are hypothesised to signal the rewarding value of affiliative tactile interactions. While CT firing frequency is positively correlated with subjective ratings of touch pleasantness, trait differences in sensitivity to the specific hedonic value of CT targeted touch have been reported. Inter-individual differences in vagally mediated, high frequency heart rate variability (HF-HRV) have been linked to variation in visual social cognition. Thus, the aim of the present study was to examine the relationship between resting state HF-HRV and sensitivity to socially relevant CT targeted touch. 58 healthy participants first had a 5-minute electrocardiogram. They then rated the pleasantness of 5 randomly presented velocities of robotically delivered touch. Three velocities fell within (1, 3, 10 cm/s) and two outside (0.3, 30 cm/s) the CT optimal range. Each velocity was delivered twice. On a group level, affective touch ratings were described by a negative quadratic function, with CT optimal velocities rated as more pleasant than slower and faster speeds. Simple regression analysis confirmed participants' HF-HRV was significantly predicted by the quadratic curve fit of their touch ratings, with higher HF-HRV associated with a better quadratic fit. These findings indicate that, in line with previous observations that higher HF-HRV is associated with enhanced sensitivity to visual social cues, trait differences in autonomic control could account for previously reported individual differences in CT sensitivity.

Spatiotemporal dynamics of cortical somatosensory network in typically developing children.

Sense of touch is essential for our interactions with external objects and fine control of hand actions. Despite extensive research on human somatosensory processing, it is still elusive how involved brain regions interact as a dynamic network in processing tactile information. Few studies probed temporal dynamics of somatosensory information flow and reported inconsistent results. Here, we examined cortical somatosensory processing through magnetic source imaging and cortico-cortical coupling dynamics. We recorded magnetoencephalography signals from typically developing children during unilateral pneumatic stimulation. Neural activities underlying somatosensory evoked fields were mapped with dynamic statistical parametric mapping, assessed with spatiotemporal activation analysis, and modeled by Granger causality. Unilateral pneumatic stimulation evoked prominent and consistent activations in the contralateral primary and secondary somatosensory areas but weaker and less consistent activations in the ipsilateral primary and secondary somatosensory areas. Activations in the contralateral primary motor cortex and supramarginal gyrus were also consistently observed. Spatiotemporal activation and Granger causality analysis revealed initial serial information flow from contralateral primary to supramarginal gyrus, contralateral primary motor cortex, and contralateral secondary and later dynamic and parallel information flows between the consistently activated contralateral cortical areas. Our study reveals the spatiotemporal dynamics of cortical somatosensory processing in the normal developing brain.

Conductive block copolymer elastomers and psychophysical thresholding for accurate haptic effects.

Electrotactile stimulus is a form of sensory substitution in which an electrical signal is perceived as a mechanical sensation. The electrotactile effect could, in principle, recapitulate a range of tactile experience by selective activation of nerve endings. However, the method has been plagued by inconsistency, galvanic reactions, pain and desensitization, and unwanted stimulation of nontactile nerves. Here, we describe how a soft conductive block copolymer, a stretchable layout, and concentric electrodes, along with psychophysical thresholding, can circumvent these shortcomings. These purpose-designed materials, device layouts, and calibration techniques make it possible to generate accurate and reproducible sensations across a cohort of 10 human participants and to do so at ultralow currents (≥6 microamperes) without pain or desensitization. This material, form factor, and psychophysical approach could be useful for haptic devices and as a tool for activation of the peripheral nervous system.

How material sensory properties and individual differences influence the haptic aesthetic appeal of visually presented stimuli.

Touch plays a crucial role for humans. Despite its centrality in sensory experiences, the field of haptic aesthetics is underexplored. So far, existing research has revealed that preferences in the haptic domain are related to stimulus properties and the Gestalt laws of grouping. Additionally, haptic aesthetics is influenced by top-down processes, e.g., stimulus familiarity, and is likely to be modulated by personality and expertise. To further our understanding of these influences on haptic aesthetic appraisal, the current study investigated the imagined haptic aesthetic appeal of visually presented material surfaces, considering the role of haptic expertise, Need for touch, personality traits. The results revealed a positive influence of familiarity, simplicity, smoothness, warmth, lightness, dryness, slipperiness and a negative influence of complexity on individuals' aesthetic responses. While the study failed to support the predicted influence of Need for touch and haptic expertise on aesthetic responses, results did reveal an influence of openness to experience, conscientiousness and neuroticism. Despite the limitations related to the indirect stimuli presentation (vision only), the findings contribute to the relatively unexplored role of bottom-up and top-down features in haptic aesthetics that might be incorporated into the design of consumers' products to better meet their preferences.