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The rewarding sensation of touch in affiliative interactions is hypothesized to be underpinned by a specialized system of nerve fibers called C-Tactile afferents (CTs), which respond optimally to slowly moving, gentle touch, typical of a caress. However, empirical evidence to support the theory that CTs encode socially relevant, rewarding tactile information in humans is currently limited. While in healthy participants, touch applied at CT optimal velocities (1-10cm/sec) is reliably rated as subjectively pleasant, neuronopathy patients lacking large myelinated afferents, but with intact C-fibres, report that the conscious sensation elicited by stimulation of CTs is rather vague. Given this weak perceptual impact the value of self-report measures for assessing the specific affective value of CT activating touch appears limited. Therefore, we combined subjective ratings of touch pleasantness with implicit measures of affective state (facial electromyography) and autonomic arousal (heart rate) to determine whether CT activation carries a positive affective value. We recorded the activity of two key emotion-relevant facial muscle sites (zygomaticus major-smile muscle, positive affect & corrugator supercilii-frown muscle, negative affect) while participants evaluated the pleasantness of experimenter administered stroking touch, delivered using a soft brush, at two velocities (CT optimal 3cm/sec & CT non-optimal 30cm/sec), on two skin sites (CT innervated forearm & non-CT innervated palm). On both sites, 3cm/sec stroking touch was rated as more pleasant and produced greater heart rate deceleration than 30cm/sec stimulation. However, neither self-report ratings nor heart rate responses discriminated stimulation on the CT innervated arm from stroking of the non-CT innervated palm. In contrast, significantly greater activation of the zygomaticus major (smiling muscle) was seen specifically to CT optimal, 3cm/sec, stroking on the forearm in comparison to all other stimuli. These results offer the first empirical evidence in humans that tactile stimulation that optimally activates CTs carries a positive affective valence that can be measured implicitly.

Unmyelinated C-Tactile (CT) fibres are activated by caress-like touch, eliciting a pleasant feeling that decreases for static and faster stroking. Previous studies documented this effect also for vicarious touch, hypothesising simulation mechanisms driving the perception and appreciation of observed interpersonal touch. Notably, less is known about appreciation of vicarious execution of touch, that is as referred to the one giving gentle touch. To address this issue, 53 healthy participants were asked to view and rate a series of videoclips displaying an individual being touched by another on hairy (i.e., hand dorsum) or glabrous (i.e., palm) skin sites, with touch being delivered at CT-optimal (5 cm/s) or non-CT optimal velocities (0 cm/s or 30 cm/s). Following the observation of each clip, participants were asked to rate self-referred desirability and model-referred pleasantness of vicarious touch for both executer (toucher-referred) and receiver (touchee-referred). Consistent with the CT fibres properties, for both self-referred desirability and model-referred pleasantness judgements of vicarious touch execution and reception, participants provided higher ratings for vicarious touch delivered at CT-optimal than other velocities, and when observed CT-optimal touch was delivered to the hand-dorsum compared to the palm. However, higher ratings were attributed to vicarious reception compared to execution of CT-optimal touch. Notably, individual differences in interoceptive trusting and attitude to interpersonal touch were positively correlated with, respectively, toucher- and touchee-related overall appraisal ratings of touch. These findings suggest that the appreciation of both toucher- and touchee-referred vicarious touch is specifically attuned to CT-optimal touch, even though they might rely on different neurocognitive mechanisms to understand affective information conveyed by interpersonal tactile interactions.

In tactile sensing, decoding the journey from afferent tactile signals to efferent motor commands is a significant challenge primarily due to the difficulty in capturing population-level afferent nerve signals during active touch. This study integrates a finite element hand model with a neural dynamic model by using microneurography data to predict neural responses based on contact biomechanics and membrane transduction dynamics. This research focuses specifically on tactile sensation and its direct translation into motor actions. Evaluations of muscle synergy during in -vivo experiments revealed transduction functions linking tactile signals and muscle activation. These functions suggest similar sensorimotor strategies for grasping influenced by object size and weight. The decoded transduction mechanism was validated by restoring human-like sensorimotor performance on a tendon-driven biomimetic hand. This research advances our understanding of translating tactile sensation into motor actions, offering valuable insights into prosthetic design, robotics, and the development of next-generation prosthetics with neuromorphic tactile feedback. The study explores the pathway from tactile signals to motor control using the integration of hand modeling and neural dynamics, enhancing understanding of sensorimotor mechanisms. These results aim to improve prosthetic design and robotic applications.

C-tactile afferents form a distinct channel that encodes pleasant tactile stimulation. Prevailing views indicate they project, as with other unmyelinated afferents, in lamina I-spinothalamic pathways. However, we found that spinothalamic ablation in humans, whilst profoundly impairing pain, temperature and itch, had no effect on pleasant touch perception. Only discriminative touch deficits were seen. These findings preclude privileged C-tactile-lamina I-spinothalamic projections and imply integrated hedonic and discriminative spinal processing from the body.

Low-threshold mechanosensory C-fibres, C-tactile afferents (CTs), respond optimally to sensations associated with a human caress. Additionally, CT-stimulation activates brain regions associated with processing affective states. This evidence has led to the social touch hypothesis, that CTs have a key role in encoding the affective properties of social touch. Thus, to date, the affective touch literature has focussed on gentle stroking touch. However, social touch interactions involve many touch types, including static, higher force touch such as hugging and holding. This study aimed to broaden our understanding of the social touch hypothesis by investigating relative preference for static vs dynamic touch and the influence of force on these preferences. Additionally, as recent literature has highlighted individual differences in CT-touch sensitivity, this study investigated the influence of affective touch experiences and attitudes, autistic traits, depressive symptomology and perceived stress on CT-touch sensitivity. Directly experienced, robotic touch responses were obtained through a lab-based study and vicarious touch responses through an online study where participants rated affective touch videos. Individual differences were determined by self-report questionnaire measures. In general, static touch was preferred over CT-non-optimal stroking touch, however, consistent with previous reports, CT-optimal stroking (velocity 1–10 cm/s) was rated most pleasant. However, static and CT-optimal vicarious touch were rated comparably for dorsal hand touch. For all velocities, 0.4N was preferred over 0.05N and 1.5N robotic touch. Participant dynamic touch quadratic terms were calculated for robotic and vicarious touch as a proxy CT-sensitivity measure. Attitudes to intimate touch significantly predict robotic and vicarious quadratic terms, as well as vicarious static dorsal hand touch ratings. Perceived stress negatively predicted robotic static touch ratings. This study has identified individual difference predictors of CT-touch sensitivity. Additionally, it has highlighted the context dependence of affective touch responses and the need to consider static, as well as dynamic affective touch.

Tactile sensitivities are common in Autism Spectrum Conditions (autism). Psychophysically, slow, gentle stroking touch is typically rated as more pleasant than faster or slower touch. Vicarious ratings of social touch results in a similar pattern of velocity dependent hedonic ratings as directly felt touch. Here we investigated whether adults and children’s vicarious ratings vary according to autism diagnosis and self-reported autistic traits. Adults’ scoring high on the AQ rated stroking touch on the palm as less pleasant than a Low AQ group. However, in contrast to our hypothesis, we did not find any effect of autism diagnosis on children’s touch ratings despite parental reports highlighting significant somatosensory sensitivities. These results are discussed in terms of underpinning sensory and cognitive factors.

Current understanding of human genital-brain interactions relates primarily to neuroendocrine and autonomic control, whereas interactions during sexual stimulation remain largely unexplored. Here we present a systematic approach towards identifying how the human brain encodes sensory genital information. Using a validated affective touch paradigm and functional magnetic resonance imaging, we found that hedonic responses to discriminatory versus affective tactile stimulation were distinctly different for both penile shaft and forearm. This suggests that, as with other body sites, genital skin contains small diameter mechanoreceptive nerve fibres that signal pleasant touch. In the brain, secondary somatosensory cortex (S2) distinguished between affective and discriminative touch for the penile shaft, but not for the forearm. Frenulum stimulation induced the greatest reports of subjective pleasure and led to the greatest deactivation of the default-mode network. This study represents a first pass at investigating, in humans, the relationship between innervation of genital surfaces, hedonic feelings, and brain mechanisms, in a systematic way.

C-tactile afferents are hypothesized to form a distinct peripheral channel that encodes the affective nature of touch. Prevailing views indicate they project, as with other unmyelinated afferents, in lamina I-spinothalamic pathways that relay homeostatically relevant information from the body toward cortical regions involved in interoceptive processing. However, in a recent study, we found that spinothalamic ablation in humans, while profoundly impairing the canonical spinothalamic modalities of pain, temperature, and itch, had no effect on benchmark psychophysical affective touch metrics. These novel findings appear to indicate that perceptual judgments about the affective nature of touch pleasantness do not depend on the integrity of the lamina I-spinothalamic tract. In this commentary, we further discuss the implications of these unexpected findings. Intuitively, they suggest that signaling of emotionally relevant C-tactile mediated touch occurs in an alternative ascending pathway. However, we also argue that the deficits seen following interruption of a putative C-tactile lamina I-spinothalamic relay might be barely perceptible—a feature that would underline the importance of the C-tactile afferent in neurodevelopment.

Abstract Psychodermatology is a subdiscipline of dermatology at the intersection of dermatology, psychiatry, and psychology. In dermatology clinical practice, patients may present with skin disease that affects their mental health, or skin disorders induced or worsened by psychological/psychiatric problems so there is a need for specialised education of dermatologists, as well as multidisciplinary teams, to achieve better management of these patients. Understanding the interaction between the central nervous system and the skin underlying psychocutaneous disorders could help identify alternative therapies that may improve patient well-being. The concept of pleasurable touch has received increasing attention following the discovery of C-tactile (CT) fibres. While afferent C-fibre stimulation is usually associated with pain, temperature, or itch, CT-fibres are stimulated optimally by a stimulus not in the nociceptor range but by a gentle, low-force stroking. As this affective touch may counteract unpleasurable sensations, such as pain and itch, and elicit positive feelings, the potential benefits of gentle touch and massage are interesting for dermatological, especially psychocutaneous, disorders. Here we provide an overview of the skin-brain connection to help understand the benefits of touch and massage, as illustrated with studies on atopic dermatitis and burns, as an adjunct to dermatological treatment for improving patient well-being and optimising treatment outcomes.

The present study examined the effects of raising both skin temperature and core temperature, separately and in combination, on perceptions of heat-related fatigue (alertness, contentment, calmness and thermal comfort), cardiovascular function and on objective measures of cognitive performance (reaction time and accuracy). Ten (six males) subjects had cognitive performance assessed in three conditions; at low skin and low core temperature (LL), at high skin and low core temperature (HL) and at high skin and high core temperatures (HH). In one trial, subjects had their head and neck cooled (HC); the other trial was a control (CON). Raising skin temperature increased heart rate and decreased perception of thermal comfort ( P  < 0.05), whereas raising both skin and core temperature decreased perception of heat-related fatigue ( P  < 0.05) and increased cardiovascular strain ( P  < 0.05) resulting in decrements in cognitive performance shown by faster reaction times ( P  < 0.05) and a loss of accuracy ( P  < 0.05). At high skin and core temperatures, cooling the head and neck improved feelings of heat-related fatigue ( P  < 0.05) and cardiovascular strain ( P  < 0.05), but had no effect on cognitive performance. In conclusion, the results of this study suggest that feelings of heat-related fatigue and cardiovascular strain can be attributed to a combination of elevated skin and core body temperature, whereas decrements in cognitive performance can be attributed to an elevated core temperature.