Pleasant touch sensations may begin with neural coding in the periphery by specific afferents. We found that during soft brush stroking, low-threshold unmyelinated mechanoreceptors (C-tactile), but not myelinated afferents, responded most vigorously at intermediate brushing velocities (1-10 cm s(-1)), which were perceived by subjects as being the most pleasant. Our results indicate that C-tactile afferents constitute a privileged peripheral pathway for pleasant tactile stimulation that is likely to signal affiliative social body contact.

In general, social neuroscience research tends to focus on visual and auditory channels as routes for social information. However, because the skin is the site of events and processes crucial to the way we think about, feel about, and interact with one another, touch can mediate social perceptions in various ways. This review situates cutaneous perception within a social neuroscience framework by discussing evidence for considering touch (and to some extent pain) as a channel for social information. Social information conveys features of individuals or their interactions that have potential bearing on future interactions, and attendant mental and emotional states. Here, we discuss evidence for an affective dimension of touch and explore its wider implications for the exchange of social information. We consider three important roles for this affective dimension of the cutaneous senses in the transmission and processing of social information: first, through affiliative behavior and communication; second, via affective processing in skin-brain pathways; and third, as a basis for intersubjective representation.

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Results obtained with functional magnetic resonance imaging show that both feeling a moderately painful pinprick stimulus to the fingertips and witnessing another person's hand undergo similar stimulation are associated with common activity in a pain-related area in the right dorsal anterior cingulate cortex (ACC). Common activity in response to noxious tactile and visual stimulation was restricted to the right inferior Brodmann's area 24b. These results suggest a shared neural substrate for felt and seen pain for aversive ecological events happening to strangers and in the absence of overt symbolic cues. In contrast to ACC 24b, the primary somatosensory cortex showed significant activations in response to both noxious and innocuous tactile, but not visual, stimuli. The different response patterns in the two areas are consistent with the ACC's role in coding the motivational-affective dimension of pain, which is associated with the preparation of behavioral responses to aversive events.

Background. Affective touch carries strong significance for social mammals, including humans. Gentle, dynamic touch of a kind that occurs during social interactions is preferentially encoded by a distinct neural pathway involving tactile C (CT) afferents, a type of unmyelinated afferent nerve found exclusively in hairy skin. CT afferents increase firing when the skin is stroked at a pleasant, caress-like speed of ϳ3 cm/s, and their discharge frequency correlates with the subjective hedonic experience of the caress. In humans, the posterior insula is a cortical target for CT afferents. Since the potential social relevance of affective touch extends to the touch interactions of others, we postulated that information from CT afferents in posterior insular cortex provides a basis for encoding observed caresses. Results. In two experiments, we exploited CT afferents' functionally unique tuning curve for stroking speed, demonstrating that a speed optimal for eliciting CT discharge (3 cm/s) also gives rise to higher BOLD responses in posterior insula than a nonoptimal speed (30 cm/s). When participants viewed videos of others' arms being stroked at CT-optimal versus -nonoptimal speeds, the posterior insula showed a similar response as to directly felt touch. Further, this region's response was specific for social interactions, showing no CT-related modulation for nonsocial dynamic-touch videos. Conclusions. These findings provide direct evidence for a functional relationship between CT signaling and processing in posterior insular cortex. Such selective tuning for CT-optimal signals in insula may allow recognition of the hedonic relevance of a merely observed caress.

Recent evidence from neurophysiology and human functional neuroimaging has given rise to the hypothesis that social, affective touch belongs to a distinct category of tactile experience. Such hedonic and rewarding touch is proposed to operate mainly in the domain of social interactions and relationships. Social touch may play a functional role in the physiological regulation of the body's responses to acute stressors and other short-term challenges. In this perspective, touch can Bbuffer^disadvantageous physiological effects of potentially inefficient or maladaptive responses. This review outlines the evidence for such a role, as well as the neural pathways that may support it. Direct evidence for touch as a physiological regulator is strongest for systems that underlie the maintenance of physical proximity to conspecifics in a variety of circumstances. For example, mammalian social physical contact involves social thermoregulatory processes like huddling and snuggling, which also rely on tactile thermosensory and somatosensory pathways. There is also good evidence that touch systems contribute to preventing social separation and facilitating the re-instatement of contact following social separation. Finally, prosocial touch, such as allogrooming and consolation, may utilize some of the same neural pathways as other, non-social means of stress regulation. Social touch may thus serve as part of a system for regulation of responses to acute stressors, Bextended^to include the physiological effects of social interactions.

We examined patients with a heritable disorder associated with a mutation affecting the nerve growth factor beta gene. Their condition has been classified as hereditary sensory and autonomic neuropathy type V. Carriers of the mutation show a reduction in density of thin and unmyelinated nerve fibres, including C afferents. A distinct type of unmyelinated, low-threshold mechanoreceptive C fibre, the C-tactile afferent, is present in hairy but not glabrous skin of humans and other mammals. They have been implicated in the coding of pleasant, hedonic touch of the kind that occurs in affiliative social interactions. We addressed the relationship between C fibre function and pleasant touch perception in 10 individuals from a unique population of mutation carriers in Sweden. We also investigated the effect of reduced C-fibre density on patients' evaluation of observed interpersonal touch (empathy). Results showed that patients perceived gentle, slow arm stroking, optimal for eliciting C-tactile afferent responses (1-10  cm/s), as less pleasant than did matched controls and also differed in their rating patterns across stimulation velocities. Further, patients' blood-oxygen-level-dependent responses in posterior insular cortex--a target for C afferents--were not modulated by stimulation optimal for activating C-tactile afferents. Hence, perception of the hedonic aspect of dynamic touch likely depends on C-tactile afferent density. Closely similar patterns between individuals' ratings of felt and seen touch suggest that appraisal of others' touch is anchored in one's own perceptual experience, whether typical or atypical.

Emotionally‐laden tactile stimulation—such as a caress on the skin or the feel of velvet—may represent a functionally distinct domain of touch, underpinned by specific cortical pathways. In order to determine whether, and to what extent, cortical functional neuroanatomy supports a distinction between affective and discriminative touch, an activation likelihood estimate (ALE) meta‐analysis was performed. This meta‐analysis statistically mapped reported functional magnetic resonance imaging (fMRI) activations from 17 published affective touch studies in which tactile stimulation was associated with positive subjective evaluation (n = 291, 34 experimental contrasts). A separate ALE meta‐analysis mapped regions most likely to be activated by tactile stimulation during detection and discrimination tasks (n = 1,075, 91 experimental contrasts). These meta‐analyses revealed dissociable regions for affective and discriminative touch, with posterior insula (PI) more likely to be activated for affective touch, and primary somatosensory cortices (SI) more likely to be activated for discriminative touch. Secondary somatosensory cortex had a high likelihood of engagement by both affective and discriminative touch. Further, meta‐analytic connectivity (MCAM) analyses investigated network‐level co‐activation likelihoods independent of task or stimulus, across a range of domains and paradigms. Affective‐related PI and discriminative‐related SI regions co‐activated with different networks, implicated in dissociable functions, but sharing somatosensory co‐activations. Taken together, these meta‐analytic findings suggest that affective and discriminative touch are dissociable both on the regional and network levels. However, their degree of shared activation likelihood in somatosensory cortices indicates that this dissociation reflects functional biases within tactile processing networks, rather than functionally and anatomically distinct pathways. Hum Brain Mapp 37:1308‐1320, 2016. © 2016 Wiley Periodicals, Inc.