C-tactile afferent stimulating touch carries a positive affective value

Abstract

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.

Maternal caregiving and DNA methylation in human infants and children: Systematic review

“…Among different maternal caregiving displays, maternal touch appears to be consistently associated with significant changes in the epigenetic status of genes involved in behavioral and stress regulation (eg, NR3C1 , SLC6A4 , OXTR ). It should be noted that maternal touch presents individual differences in the amount and quality across different species and a specific quality of maternal touch with infants and children, namely affectionate touch, is present in different animals as well as in humans and it appears to be physiologically underpinned by specific sensory fibers, namely the C‐tactile fibers . C‐tactile fibers activate specifically in response to gentle and affectionate maternal touch—like the one observed in a subgroup of the studies included in the present systematic review (eg, stroking and sensitive touch).…”

Dopaminergic and opioidergic regulation of implicit hedonic facial reactions during anticipation and consumption of social and nonsocial rewards

“…Recently, we have shown [27] that in healthy human adults the anticipation and consumption of food rewards results in the relaxation of the main frowning muscle corrugator supercilii (CS), and that the experience of social touch rewards elicits activation of the main smiling muscle, the zygomaticus major (ZM). The latter result emerged from explorative analyses, but others have also reported ZM contraction and CS relaxation in response to pleasant social touch [28][29][30]. Human adults thus relax the CS and activate the ZM during both the anticipation and the consumption of rewards, although differences between types of rewards may also exist.…”

“…Importantly, following Pavlovian conditioning, animals also show facial reactions to cues (thus before reward delivery) that they have learned to associate with the delivery of the unconditioned taste stimuli [26]. However, facial reactions to pleasant tastes and other types of reward are more subtle in adult humans, and have only started to be investigated using facial electromyography (EMG) [27][28][29][30][31]. Recently, we have shown [27] that in healthy human adults the anticipation and consumption of food rewards results in the relaxation of the main frowning muscle corrugator supercilii (CS), and that the experience of social touch rewards elicits activation of the main smiling muscle, the zygomaticus major (ZM).…”

The anticipation and perception of affective touch in women with and recovered from Anorexia Nervosa

“…heart rate), others such as noxious or gentle skin stimulation tend to also convey hedonic and motivational signals. Of interest here, 'affective touch', hereafter referring to a specialised tactile modality of gentle, stroking touch, thought to be (at least partly, see Marshall et al, 2019) mediated peripherally by unmyelinated, slowconducting C-tactile (CT) afferents (Löken et al, 2009) and to be centrally processed by an interoceptive pathway converging at the insular cortex (Morrison, 2016;Kirsch et al, 2020), has been associated with subjective sensations of skin pleasure (Löken et al, 2009) and related behavioural measures (Pawling et al, 2017).…”