C-tactile Research Articles

Dynamics of Affective Habituation to Touch Differ on the Group and Individual Level

Slow stroking touch activates C-tactile (CT) fibres in the human skin and is typically described as pleasant. Over a longer duration of stroking, affective habituation sets in, indicated by a reduction in rated pleasantness. However, it is not clear whether the group level effect is represented on an individual level. We analyse the stability of individual ratings of slow stroking touch and their relationship to behavioural and physiological measures.Forty-eight participants (23 women) were repeatedly stroked with a velocity maximally activating CT fibres. The perceived pleasantness was rated on a visual analogue scale. In order to examine the stability of pleasantness ratings, the experiment was repeated after a couple of days. During the experiment, electrocardiogram (ECG) and facial-electromyography (EMG) data were recorded.On the group-level, previous results of affective habituation to touch were replicated and stable across sessions. On the individual level, however, less than half of the participants showed a significant reduction of pleasantness in the course of the experiment. Moreover, the remaining participants showed either no change, random rating behaviour or even an increase in pleasantness ratings during the course of the experiment.The individual response patterns were variable across sessions but stable above the chance level. Furthermore, the response patterns could not be explicitly associated with any of the behavioural or physiological measures. Our findings indicate a lack of group-to-individual generalizability for affective habituation to touch. The variability of rating patterns over time indicates that they are not conclusively determined by stable individual characteristics. Future research investigating touch should favour a more individual approach to the more commonly applied group analysis.

Evidence for sparse C-tactile afferent innervation of glabrous human hand skin.

C-tactile (CT) afferents were long-believed to be lacking in humans, but these were subsequently shown to densely innervate the face and arm skin, and to a lesser extent the leg. Their firing frequency to stroking touch at different velocities has been correlated with ratings of tactile pleasantness. CT afferents were thought to be absent in human glabrous skin; however, tactile pleasantness can be perceived across the whole body, including glabrous hand skin. We used microneurography to investigate mechanoreceptive afferents in the glabrous skin of the human hand, during median and radial nerve recordings. We describe CTs found in the glabrous skin, with characteristics comparable with those in hairy arm skin, and detail recordings from three such afferents. CTs were infrequently encountered in the glabrous skin and we estimate that the ratio of recorded CTs relative to myelinated mechanoreceptors (1:80) corresponds to an absolute innervation density of around seven times lower than in hairy skin. This sparse innervation sheds light on discrepancies between psychophysical findings of touch perception on glabrous skin and hairy skin, although the role of these CT afferents in the glabrous skin remains subject to future work.NEW & NOTEWORTHY Human touch is encoded by low-threshold mechanoreceptors, including myelinated Aβ afferents and unmyelinated C-tactile (CT) afferents. CTs are abundant in hairy skin and are thought to code gentle, stroking touch that signals positive affective interactions. CTs have never been described in human glabrous skin, yet we show evidence of their existence on the hand, albeit at a relatively low density. Glabrous skin CTs may provide modulatory reinforcement of gentle tactile interactions during touch using the hands.

The Anticipation and Perception of Affective Touch in Women with and Recovered from Anorexia Nervosa

Disruptions in reward processing and anhedonia have long been observed in Anorexia Nervosa (AN). Interoceptive deficits have also been observed in AN, including reduced tactile pleasure. However, the extent to which this tactile anhedonia is specifically liked to an impairment in a specialised, interoceptive C-tactile system originating at the periphery, or a more top-down mechanism in the processing of tactile pleasantness remains debated. Here, we investigated differences between patients with and recovered from AN (RAN) and healthy controls (HC) in the perception of pleasantness of touch delivered in a CT-optimal versus a CT-non-optimal manner, and in their top-down, anticipatory beliefs about the perceived pleasantness of touch. To this end, we measured the anticipated pleasantness of various materials touching the skin and the perceived pleasantness of light, dynamic touch applied to the forearm of 27 women with AN, 24 women who have recovered and 30 HCs using C Tactile (CT) afferents-optimal (slow) and non-optimal (fast) velocities. Our results showed that both clinical groups anticipated tactile experiences and rated delivered tactile stimuli as less pleasant than HCs, but the latter difference was not related to the CT optimality of the stimulation. Instead, differences in the perception of CT-optimal touch were predicted by differences in top-down beliefs, alexithymia and interoceptive sensibility. Thus, tactile anhedonia in AN might persist as a trait even after otherwise successful recovery of AN and it is not linked to a bottom-up interoceptive deficit in the CT system, but rather to a learned, defective top-down anticipation of tactile pleasantness.

Pleasantness Only?

. When gently stroked with velocities between 0.1 and 30 cm/s, participants typically rate velocities around 3 cm/s as most pleasant, and the ratings follow an inverted u-shape. This pleasantness curve correlates often, but not always, with the firing rate of unmyelinated C-tactile (CT) afferents, leading to the notion that CT afferents code for the hedonic or emotional aspect of gentle touch. However, there is also evidence that CT firing does not necessarily equal pleasantness, and the range of attributes that CT afferents code for is not known. Here, participants were stroked with different velocities assumed to activate CT afferents to a different extent while they rated the touch on several sensory and emotional attributes. We expected an inverted u-shaped rating curve for pleasantness and other emotional attributes, but not for sensory attributes. Inverted u-shaped rating patterns were found for the emotional attributes “pleasant” and “not burdensome,” but also for the sensory attribute “rough.” CT-directed stimulation is thus not only experienced as hedonic. The sensations arising from CTs together with all other types of mechanoreceptors might be centrally integrated into a percept that represents those aspects which are most salient for the stimulation at hand.

Disorganized Attachment pattern affects the perception of Affective Touch

Touch, such as affective caress, can be interpreted as being pleasant. The emotional valence that is assigned to touch is related to certain bottom-up factors, such as the optimal activation of C-tactile (CT) afferents. Tactile processing with a hedonic or emotional component has been defined as affective touch—a component that CT fibers are likely to convey. Tactile deficiencies are frequent in the psychiatric population but also in healthy people with disorganized attachment; accordingly, it is likely that affective difficulties in adults with disorganized attachment are reflected in altered perception of affective touch. To test this hypothesis, we combined methods from clinical psychology, psychophysics, and neuroimaging. We found that people with a history of traumatic parental bonds and a disorganized attachment pattern perceive a “caress-like” stimulus as being unpleasant, whereas participants with organized attachment consider the same tactile stimulation to be pleasant. Further, unlike in organized adults, the responses of disorganized adults to CT and non-CT stimulation activated limbic and paralimbic structures in a fight-or-flight manner, suggesting that early experiences with parental deficiencies shape the physiological responses of peripheral CT fibers and central nervous networks.

Anhedonia to Gentle Touch in Fibromyalgia: Normal Sensory Processing but Abnormal Evaluation

Social touch is important for interpersonal interaction. Gentle touch and slow brushing are typically perceived as pleasant, the degree of pleasantness is linked to the activity of the C-tactile (CT) fibers, a class of unmyelinated nerves in the skin. The inability to experience pleasure in general is called anhedonia, a common phenomenon in the chronic pain condition fibromyalgia. Here, we studied the perception and cortical processing of gentle touch in a well-characterized cohort of fibromyalgia. Patients and controls participated in functional brain imaging while receiving tactile stimuli (brushing) on the forearm. They were asked to provide ratings of pleasantness of the tactile stimulus and ongoing pain. We found high distress, pain catastrophizing, and insomnia, and a low perceived state of health in fibromyalgia. Further, patients rated both slow (CT-optimal) and fast (CT-suboptimal) brushing as less pleasant than healthy participants. While there was no difference in brain activity during touch, patients showed deactivation in the right posterior insula (contralateral to the stimulated arm) during pleasantness rating and activation during pain rating. The opposite pattern was observed in healthy participants. Voxel-based morphometry analysis revealed reduced grey matter density in patients, in the bilateral hippocampus and anterior insula. Our results suggest anhedonia to gentle touch in fibromyalgia with intact early-stage sensory processing but dysfunctional evaluative processing. These findings contribute to our understanding of the mechanisms underlying anhedonia in fibromyalgia.

Social touch alters newborn monkey behavior

In humans, infants respond positively to slow, gentle stroking—processed by C-tactile (CT) nerve fibers—by showing reductions in stress and increases in eye contact, smiling, and positive vocalizations. More frequent maternal touch is linked to greater activity and connectivity strength in social brain regions, and increases children’s attention to and learning of faces. It has been theorized that touch may prime children for social interactions and set them on a path towards healthy social cognitive development. However, less is known about the effects of touch on young infants’ psychological development, especially in the newborn period, a highly sensitive period of transition with rapid growth in sensory and social processing. It remains untested whether newborns can distinguish CT-targeted touch from other types of touch, or whether there are benefits of touch for newborns’ social, emotional, or cognitive development. In the present study, we experimentally investigated the acute effects of touch in newborn monkeys, a common model for human social development. Rhesus macaques (Macaca mulatta), like humans, are highly social, have complex mother-infant interactions with frequent body contact for the first weeks of life, making them an excellent model of infant sociality. Infant monkeys in the present study were reared in a neonatal nursery, enabling control over their early environment, including all caregiver interactions. One-week-old macaque infants (N = 27) participated in three 5-minute counter-balanced caregiver interactions, all with mutual gaze: stroking head and shoulders (CT-targeted touch), stroking palms of hands and soles of feet (Non-CT touch), or no stroking (No-touch). Immediately following the interaction, infants watched social and nonsocial videos and picture arrays including faces and objects, while we tracked their visual attention with remote eye tracking. We found that, during the caregiver interactions, infants behaved differently while being touched compared to the no-touch condition, irrespective of the body part touched. Most notably, in both touch conditions, infants exhibited fewer stress-related behaviors—self-scratching, locomotion, and contact time with a comfort object—compared to when they were not touched. Following CT-targeted touch, infants were faster to orient to the picture arrays compared to the other interaction conditions, suggesting CT-targeted touch may activate or prime infants’ attentional orienting system. In the No-touch condition infants attended longer to the nonsocial compared to the social video, possibly reflecting a baseline preference for nonsocial stimuli. In contrast, in both touch conditions, infants’ looked equally to the social and nonsocial videos, suggesting that touch may influence the types of visual stimuli that hold infants’ attention. Collectively, our results reveal that newborn macaques responded positively to touch, and touch appeared to influence some aspects of their subsequent attention, although we found limited evidence that these effects are mediated by CT fibers. These findings suggest that newborn touch may broadly support infants’ psychological development, and may have early evolutionary roots, shared across primates. This study illustrates the unique insight offered by nonhuman primates for exploring early infant social touch, revealing that touch may positively affect emotional and attentional development as early as the newborn period.

Infants Autonomic Cardio- Respiratory Responses to Nurturing Stroking Touch Delivered by the Mother or the Father.

The building of physiological self-regulation during bonding is a crucial developmental process based on early cardio-respiratory maturation. The mother’s role as a facilitator of this physiological maturation has been evidenced and recognized in many respects. Research in fathers, however, remains sparse which may be due to the belief that bonding is a physiological behavior reserved for a mother’s maternal instinct. In the current study we compared the impact of paternal and maternal nurturing stroking touch on infants’ physiological self-regulation in terms of respiratory sinus arrhythmia (RSA). We compared the impact of a 3-min stroking period (STROKING) with a pre-baseline (PRE-STROKING) and post-baseline (POST-STROKING) of 25 mothers and 25 fathers (unrelated to one another) on their infants, aged 4–16 weeks. We registered infant electrocardiogram (ECG) and respiration to calculate infant RR-interval (RRI), respiration rate (fR) and (respiratory corrected) RSA (RSAcorr). Based on video-recordings, we analyzed the stroking speed. Infants’ RSAcorr significantly increased during and after stroking, no matter whether touch was delivered by fathers or mothers. This effect was mediated by both heart rate (HR) and respiration. However, respiratory mediation occurred later when delivered by fathers than by mothers. Both mothers’ and fathers’ stroking speed occurred within the optimal stimulation range of c-tactile (CT) afferents, a particular class of cutaneous unmyelinated, low-threshold mechano-sensitive nerves hypothesized to be involved in inter-personal bonding. The discussion builds on the idea to mitigate fathers’ doubts about their paternal capabilities and proposes a research agenda regarding the further examination of the role of nurturing touch and its underlying mechanisms within the development of infants’ physiological self-regulation. Finally, the importance of respiratory measurements in infant physiological research is emphasized.

Spatio-temporal profile of brain activity during gentle touch investigated with magnetoencephalography

Positive affective touch plays a central role in social and inter-personal interactions. Low-threshold mechanoreceptive afferents, including slowly-conducting C-tactile (CT) afferents found in hairy skin, transmit such signals from gentle touch to the brain. Tactile signals are processed, in part, by the posterior insula, where it is the thought to be the primary target for CTs. We used magnetoencephalography (MEG) to assess brain activity evoked by gentle, naturalistic stroking touch on the arm delivered by a new MEG-compatible brush robot. We aimed to use high temporal resolution MEG to allow us to distinguish between brain responses from fast-conducting Aβ and slowly-conducting CT afferents. Brush strokes were delivered to the left upper arm and left forearm of 15 healthy participants. We hypothesized that late brain responses, due to slow CT afference, would appear with a time shift between the two different locations on the arm. Our results show that gentle touch rapidly activated somatosensory, motor, and cingulate regions within the first 100 ms of skin contact, which was driven by fast-conducting mechanoreceptive afference, and that these responses were sustained during touch. Peak latencies in the posterior insula were shifted as a function of stimulus location and temporally-separate posterior insula activations were induced by Aβ and CT afference that may modulate the emotional processing of gentle touch on hairy skin. We conclude that the detailed information regarding temporal and spatial brain activity from MEG provides new insights into the central processing of gentle, naturalistic touch, which is thought to underpin affective tactile interactions.

Are the antidepressive effects of massage therapy mediated by restoration of impaired interoceptive functioning? A novel hypothetical mechanism

Interoception is an individual person’s sense of the physiological condition of his/her entire body. Recent research has shown that depression is associated with impaired interoceptive accuracy. Treatments that can improve disturbed interoception are scarce in clinical practice and could complement established therapies. Accumulating evidence suggests that massage therapy significantly alleviates symptoms of depression. However, the mechanisms underlying these effects have remained unclear. We are going to propose a novel mechanism linking these antidepressive effects to a massage-induced modulation of interoceptive states. Particularly affective massage therapy applies slow, rhythmic, and caress-like touch that stimulates C tactile (CT) afferents in the non-glabrous skin. CT mediated touch elicits responses in interoceptive brain areas (e.g. the insular cortex) that have been associated with abnormal interoceptive representations in depressed subjects. Thus, we hypothesize that antidepressive effects of massage therapy are mediated by restoration of the impaired interoceptive functioning through stimulation of CT afferents or related interoceptive structures. If our proposed mechanism is valid, massage is probably one of the most ancient interoceptive treatments.

Does touch matter? The impact of stroking versus non-stroking maternal touch on cardio-respiratory processes in mothers and infants

The beneficial effects of touch in development were already observed in different types of skin-to-skin care. In the current study, we aimed at studying potential underlying mechanisms of these effects in terms of parasympatho-inhibitory regulation. We examined the specific impact of affective maternal stroking versus non-stroking touch on the cardio-respiration of both mothers and infants in terms of respiratory sinus arrhythmia (RSA). We compared a 3-min TOUCH PERIOD (stroking or non-stroking touch) with a baseline before (PRE-TOUCH) and after (POST-TOUCH) in 45 dyads (24 stroking/21 non-stroking touch) with infants aged 4–16 weeks. We registered mother-infant ECG, respiration and made video-recordings. We calculated RR-interval (RRI), respiration rate (fR) and (respiratory corrected) RSA and analyzed stroking mean velocity rate (MVR) of the mothers. ANOVA-tests showed a significant different impact on infants' respiratory corrected RSA of stroking touch (increase) versus non-stroking touch (decrease). Further, during and after stroking touch, RRI significantly increased whereas fR significantly decreased. Non-stroking touch had no significant impact on infants' RRI and fR. In the mothers, RRI significantly decreased and fR significantly increased during the TOUCH PERIOD. The mothers' MVR occurred within the range of 1–10 cm/s matching with the optimal afferent stimulation range of a particular class of cutaneous unmyelinated, low-threshold mechano-sensitive nerves, named c-tactile (CT) afferents. We suggest CT afferents to be the a potential missing link between the processing of affective touch and the development of physiological and emotional self-regulation. The results are discussed with regard to the potential role of CT afferents within the building of early self-regulation as part of a multisensory intuitive parenting system and the importance to respect this ecological context of an infant in research and clinical applications.

Assessment of experimental orofacial pain, pleasantness and unpleasantness via standardized psychophysical testing.

Somatosensory assessment within the orofacial region may be performed using highly standardized quantitative sensory testing (QST). However, the function of the C-tactile (CT) afferent, a nerve fibre linked to the perception of pleasant touch, is usually not evaluated. Furthermore, the perception of unpleasantness is also rarely assessed, a dimension not only limited to a painful experience. Therefore, the primary aim was to apply standardized QST stimuli as well as standardized pleasant stimuli and evaluate their potential capacity for evocation of perceived pain, pleasant and unpleasant sensations in the facial region. Twenty-one female participants underwent QST as per the protocol derived from the German Research Network on Neuropathic Pain. For the first time, two modified protocols were used to investigate stimuli for perceived pleasantness and unpleasantness. Thermal stimuli provided separate thresholds for each sensation. From certain mechanical stimuli (e.g., vibration), overlap between the perceived sensations of pleasantness and unpleasantness was identified. It was not possible to evoke only an unpleasant sensation without a painful contribution, and both these sensations increased significantly when utilizing an increasing pinprick force (p<0.011). Between dynamic stimuli, the brush was rated as significantly more pleasant than the cotton wool tip (p=0.015). A quadratic model provided the best fit for velocity against mean pleasantness ratings (R2 =0.62±0.08), supporting previous CT afferent literature to some extent. Stimuli were generally not isolated to one sensation, highlighting the multidimensional construct of stimulus perception and the need for scales to capture this. Thebattery of QST tests from the DFNS protocol has been modified to investigate pleasant and unpleasant sensations. This allows the evaluation of psychophysical properties across standardized dimensions to provide a thorough view of somatosensory function and to better understand the affective spectrum of somatosensory function.

Using Facial Electromyography to Assess Facial Muscle Reactions to Experienced and Observed Affective Touch in Humans.

"Affective" touch is believed to be processed in a manner distinct from discriminatory touch and to involve activation of C-tactile (CT) afferent fibers. Touch that optimally activates CT fibers is consistently rated as hedonically pleasant. Patient groups with impaired social-emotional functioning also show disordered affective touch ratings. However, relying on self-reported ratings of touch has many limitations, including recall bias and communication barriers. Here, we describe a methodological approach to study affective responses to touch via facial electromyography (EMG) that circumvents the reliance on self-report ratings. Facial EMG is an objective, quantitative, and non-invasive method to measure facial muscle activity indicative of affective responses. Responses can be assessed across healthy and patient populations without the need for verbal communication. Here, we provide two separate datasets demonstrating that CT-optimal and non-optimal touch elicit distinct facial muscle reactions. Moreover, facial EMG responses are consistent across stimulus modalities, e.g. tactile (experienced touch) and visual (observed touch). Finally, the temporal resolution of facial EMG can detect responses on timescales that supersede that of verbal reporting. Together, our data suggest that facial EMG is a suitable methodology for use in affective tactile research that can be used to supplement, or in some cases, supplant, existing measures.

Touch targeting C-tactile afferent fibers has a unique physiological pattern: A combined electrodermal and facial electromyography study

Caress-like touch is thought to rely on C-tactile (CT) fiber signaling. Here, the arousing and emotional effects of CT-optimal touch were assessed via participants’ skin conductance level (SCL), facial electromyography (EMG) responses and subjective ratings of pleasantness and intensity. Temporal facial EMG analysis was based on the conduction velocity of CT-fibers.45 healthy participants received CT-optimal touch, control touch and visual control input during 2-minute trials.CT optimal touch was significantly more pleasant and intense than control touch. In the initial 700 ms of stimulation there was no difference in facial EMG responses to touch. Between 700 and 6300 ms there was a significant reduction of corrugator activity in response to CT optimal touch only, possibly reflecting the affective value of CT optimal touch. Based on the temporal dynamics of the reduced corrugator activity, we suggest that CT fibers are involved in mediating a reduction in corrugator activity.

Touch-induced pupil size reflects stimulus intensity, not subjective pleasantness

Interpersonal touch is known to influence human communication and emotion. An important system for interpersonal touch is the C-tactile (CT) system, which is activated by a soft stroke on hairy skin with a velocity of 1–10 cms−1. This system been proposed to play a unique role in hedonic valence and emotion of touch. For other sensory modalities, hedonic processing has been associated with pupil dilation. However, it is unclear whether pupil dilation can be modulated by hedonic touch. The current study investigated in two experiments how pupil size reacts to both affective and non-affective stroking. Pupil-size data were obtained to investigate differences between stroking conditions. In addition, an adjusted version of the Touch Perception Task (TPT) was used to assess subjective touch pleasantness ratings. In Experiment 1, affective (3 cms−1) and non-affective (0.3 and 30 cms−1) stroking was applied to the dorsal side of the right hand. Results revealed that stroking velocity had a significant effect on TPT-item scores, showing higher that affective touch was rated as more pleasant compared to non-affective touch, thereby replicating the previous studies. Results, however, revealed no specific pupil dilation for the 3 cms−1 condition; instead, a logarithmic relation was found between pupil-size dilation and stroking velocity. This relation was confirmed in a second experiment. Furthermore, the palm of the hand was used as a control site for tactile stimulation, for which similar findings were obtained as for the dorsal side of the hand. In addition, skin conductance recordings showed a pattern of response to different stroking velocities similar to pupil dilation. These results suggest that pupil-size dilation does respond to tactile input, but that this response is related to arousal caused by changes in stimulus intensity (e.g., stroking velocity) rather than specific C-tactile stimulation.

Sensitivity to CT-optimal, Affective Touch Depends on Adult Attachment Style

Affective touch supports affiliative bonds and social cognition. In particular, gentle, stroking touch, which has recently been associated with the C Tactile (CT) system, is typically perceived as pleasant and prosocial. However, it remains unknown whether pre-existing models of social relating influence the perception of CT-optimal touch. In this study (N = 44 adults), we examined how individual differences in attachment styles relate to the perception of CT-optimal touch, as well as to a different modality of interoception, namely heartbeat perception. Using the gold-standard assessment of attachment (Adult Attachment Interview), we found that insecure attachment was associated with reduced pleasantness discrimination between CT-optimal vs. non-CT optimal touch. Acknowledging the different traditions in measuring attachment, we also used a well-validated self-report questionnaire that pertains to explicit representations of current close relationships. Using this measure, we found that higher scores in attachment anxiety (but not attachment avoidance) were associated with reduced pleasantness discrimination between CT-optimal vs. non-CT optimal touch. Attachment patterns (in both measures) were not related to cardiac perception accuracy. These results corroborate and extend previous literature on CT-optimal touch and its relation with affiliative bonds and social cognition. Given that attachment was not related to perceived cardiac accuracy, these findings point to the specificity of the relationship between CT-optimal touch and attachment.

Differential effects of radiant and mechanically applied thermal stimuli on human C-tactile afferent firing patterns.

C-tactile (CT) afferents respond to gentle tactile stimulation, but only a handful of studies in humans and animals have investigated whether their firing is modified by temperature. We describe the effects of radiant thermal stimuli, and of stationary and very slowly moving mechanothermal stimuli, on CT afferent responses. We find that CT afferents are primarily mechanoreceptors, as they fired little during radiant thermal stimuli, but they exhibited different patterns of firing during combined mechano-cool stimulation compared with warming. CTs fired optimally to gentle, very slowly moving, or stationary mechanothermal stimuli delivered at neutral temperature (~32°C, normal skin temperature), but they responded with fewer spikes (median 67% decrease) and at significantly lower rates (47% decrease) during warm (~42°C) tactile stimuli. During cool tactile stimuli (~18°C), their mean instantaneous firing frequency significantly decreased by 35%, but they often fired a barrage of afterdischarge spikes at a low frequency (~5 Hz) that outlasted the mechanical stimulus. These effects were observed under a variety of stimulus conditions, including during stationary and slowly moving touch (0.1 cm/s), and we complemented these tactile approaches using a combined electrical-thermal stimulation experiment where we found a suppression of spiking during warming. Overall, CT afferents are exquisitely sensitive to tactile events, and we show that their firing is modulated with touch temperatures above and below neutral skin temperature. Warm touch consistently decreased their propensity to fire, whereas cool touch produced lower firing rates but afterdischarge spiking. NEW & NOTEWORTHY C-tactile (CT) afferents are thought to underpin pleasant touch, and previous work has shown that they respond optimally to a slow caress delivered at typical (neutral) skin temperature. Here, we show that, although CTs are primarily mechanoreceptive afferents, they are modified by temperature: warm touch decreases their firing, whereas cool touch produces lower firing rates but long-lasting spiking, frequently seen as afterdischarges. This has implications for the encoding of affective sensory events in human skin.

Putting a good face on touch: Facial expression reflects the affective valence of caress-like touch across modalities

Touch plays a central role in interpersonal behavior, especially in its capacity to convey—and induce— changes in affect. Previous research has established that slow, caress-like stroking over the skin elicits positive subjective affective responses, with higher ratings of “pleasantness” compared to a faster-moving touch stimulus. Ratings of pleasantness are associated with increased activity of a distinct class of nerve fibers: C-tactile (CT) afferents. Here, we used facial electromyography (EMG) to determine if touch that optimally activates CT afferents also influences facial muscle activity believed to reflect changes in affect. We found that less pleasant, fast-moving stroking (30 cm/s) elicited robustly negative facial EMG responses, as indexed by stronger contraction of the corrugator muscle. In contrast, pleasant, slow-moving stroking (3 cm/s) that optimally activates CT afferents resulted in decreased negative facial affective responses, manifested as significant corrugator relaxation compared to fast stroking. Moreover, the facial tracking of affective valence during touch was supra-modal, with similar effects during both directly-experienced touch and viewing of touch videos. The results of this EMG study imply that touch that fails to optimally activate CT afferent produces a negative affective response, whereas pleasant, caress-like touch has not only subjective but expressive correlates, reflected in net positive affective changes in facial expression.

Slow touch targeting CT-fibres does not increase prosocial behaviour in economic laboratory tasks

Field studies have demonstrated that humans become more generous, helpful and compliant after having been touched by another person. Here, we explored whether these effects are larger for touch activating the C-tactile (CT) fibres, as it is ascribed a particular role in establishing and maintaining bonds and affiliative interactions. The role of CT-targeted and non-targeted touch on prosocial behaviour was investigated in three different experiments using a trust game and a task measuring individual differences in social value orientations (the SVO task). Whereas participants in general acted prosocially, there was no influence of CT-targeted touch on prosocial behaviour, both in comparison to non-CT-targeted control touch and visual (non-tactile) stimulation. The null findings were further corroborated by Bayesian statistics. Thus, under the controlled laboratory conditions employed, CT-targeted touch did not play a particular role in prosocial behaviour. This indicates that touch does not increase prosocial behaviour in the absence of meaningful social and psychological connotations. Any touch related effects on prosocial behaviour likely depends on the ecological validity of the situation.

Developmental Perspectives on Interpersonal Affective Touch

In the last decade, philosophy, neuroscience and psychology alike have paid increasing attention to the study of interpersonal affective touch, which refers to the emotional and motivational facets of tactile sensation. Some aspects of affective touch have been linked to a neurophysiologically specialised system, namely the C tactile (CT) system. While the role of this system for affiliation, social bonding and communication of emotions have been widely investigated, only recently researchers have started to focus on the potential role of interpersonal affective touch in acquiring awareness of the body as our own, i.e. as belonging to our psychological ‘self’. We review and discuss recent developmental and adult findings, pointing to the central role of interpersonal affective touch in body awareness and social cognition in health and disorders. We propose that interpersonal affective touch, as an interoceptive modality invested of a social nature, can uniquely contribute to the ongoing debate in philosophy about the primacy of the relational nature of the minimal self.