A Better Touch: C-tactile Fibers Related Activity is Associated to Pain Reduction During Temporal Summation of Second Pain

Neural response associated with the modulation of temporal summation of second pain by affective touch

Temporal summation of second pain (TSSP) is a phenomenon that has clinical relevance but insights into its functioning are limited. Lately, 'affective touch' (AT) has been shown to have pain relieving properties but only one study has investigated its effects on TSSP and the neural underpinnings of such interaction are unknown. In the present EEG study, thirty-six healthy participants went through three conditions where a TSSP protocol was applied in concomitance with no touch (NoT), discriminative touch (DT) and AT. A fourth no-pain no-touch condition acted as a baseline. Measures of attention during the four conditions and of pleasantness during the touch conditions were also recorded. Pain ratings were significantly lower only during the AT condition. The neural response during NoT, compared to the baseline, brought about a temporal decrease in power at delta and theta frequencies and a fronto-central increase mainly in the alpha rhythm. Adding AT to TSSP yielded, compared to NoT, a decrease in delta, theta and beta bands in midline regions at both central (Cz) and parietal (Pz) and also of gamma at Pz. Notably, DT was not associated with significant changes compared to pain alone (NoT), but a specific marked difference was found between AT and DT with the former showing a significant decrease in beta frequencies localized at Pz. While TSSP seems to be characterized by a modulation mostly of the lower frequencies, adding AT to TSSP brings a clear depression of all the major frequency bands. Additionally, the parietal beta reduction may be a biomarker of AT. Future studies can examine if such brain response can help finding a suitable intervention for TSSP-related chronic pain conditions. PERSPECTIVE: This study consolidates the idea that AT can lower pain in a TSSP paradigm and shows what are the brain (EEG) responses associated with both TSSP and TSSP modulation by AT. Given that TSSP is linked to central sensitization and that it can be used as an experimental model for chronic pain, our results pave the way for further studies into the neural mechanisms of AT-led analgesia, which can lead to future effective treatments.

Impact of Gentle Touch Stimulation Combined with Advanced Sensory Stimulation in Patients in a Minimally Conscious State: A Quasi-Randomized Clinical Trial

Touch, particularly affective touch mediated by C-tactile fibers, plays a key role in emotional regulation and therapeutic interventions. However, tactile stimulation is underutilized in sensory stimulation (SS) protocols for brain injury patients, despite its potential to enhance consciousness and promote recovery through neural and autonomic regulation. Tools like the Neurowave enable advanced multisensory stimulation, including audio-visual and emotional inputs, but lack tactile components. Integrating gentle touch stimulation with such systems could further enhance neuroplasticity, improve heart rate regulation, and support recovery in patients with disorders of consciousness. In this study, twenty patients affected by minimally conscious state (MCS) were divided into two groups: an experimental group (EG n.10) and a control group (CG n.10). Both groups underwent standard neurorehabilitation, including conventional physiotherapy and speech therapy. The key difference was in the type of sensory stimulation. The EG received advanced sensory stimulation with the Neurowave system (which provides audio-visual and emotional sensory stimulation) in addition to gentle touch stimulation. The CG received conventional sensory stimulation without the Neurowave and neutral gentle touch stimulation. Each patient was evaluated by a multidisciplinary rehabilitation team, using clinical scales such as coma recovery scale-revised (CSR-R), level of cognitive functioning (LCF), before (T0) and after (T1) treatment. Additionally, heart rate (HR) and neurophysiological outcomes (P300) were also recorded for both groups (EG and CG). The MANOVA model revealed a significant interaction effect between group and phase on P300 latency (F (1, 18) = 10.23, p < 0.001, η2 = 0.09), indicating that the intervention involving gentle touch stimulation significantly influenced the P300 latency in the EG. The findings of this study contribute to our understanding of the therapeutic potential of emotional multisensory stimulation, which also includes gentle touch stimulation, in MCS rehabilitation. By demonstrating significant effects on both neurophysiological and functional measures, our results support the integration of tactile interventions into comprehensive neurorehabilitation programs.

Different ways to reach the same goal: Are the analgesic effects of different types of tactile stimuli similar?

BACKGROUND

Tactile-induced analgesia (TIA) is a phenomenon in which different types of tactile stimulation alleviate pain via different mechanisms including empathy. As TIA plays an essential role in therapeutic situations and clinical conditions, it is crucial to determine whether specific tactile stimulations confer distinct benefits.

METHODS

Fifty-two subjects (31 females; 21-47 years) were exposed to four distinct experimental conditions involving three types of touch provided within a simulated supportive therapeutic setting. First, a contact heat (70 s) at a pain intensity of 60/100 Numerical Pain Scale (NPS) was applied to the forearm. The pain stimulus was then given simultaneously with vibration, gentle stroking, or handholding in random order. Pain ratings were reported at 5 and 65 s of each stimulation. Given the role of empathy in TIA, we also assessed the levels of empathy experienced by the subjects and the experimenter.

RESULTS

Handholding and vibration conditions were associated with a more rapid decrease in pain ratings compared to pain-alone (B values: handholding = -150.94 vs. pain-alone = -99.38, p = 0.01; vibration = -163.54 vs. pain-alone = -99.38, p < 0.001). Higher levels of the experimenter's empathy toward the subjects were associated with greater pain alleviation during vibration vs. pain-alone condition (B values: vibration = -56.42 vs. B pain-alone = -9.57, p = 0.04).

CONCLUSIONS

Vibration's potent analgesic effects may be attributed to its multi-channel analgesic mechanisms, including the therapist's empathy toward the participant. On the other hand, for handholding establishing an empathic interaction should be considered in a therapeutic setting to enhance its analgesic efficacy.

SIGNIFICANCE

This article explores the effectiveness of touch-based pain relief methods and their association with empathetic therapeutic interactions. The study emphasizes the significance of positive therapeutic interactions in facilitating tactile-induced analgesia.

WATSU (Water-Shiatsu) increases body awareness and improves pain and mood: A randomised controlled study

BACKGROUND AND PURPOSE

WATSU (Water-Shiatsu) is a body-oriented therapy in warm-water pool with passive movements, massage and breathing techniques. The present study investigated the short-term effect of a single session of WATSU on body awareness.

MATERIALS AND METHODS

In a randomized-controlled study 60 volunteers (18-65 years) were assigned to a 20-min intervention of WATSU or to a 20-min lecture on WATSU. Immediately before and after the intervention they had to answer questionnaires. Primary endpoint of the investigation was the change of the total score of the non-verbal Awareness Body Chart (ABC). Secondary endpoints were the changes of the scores of the 14 single body parts of the ABC, of the Visual Analogue Pain Scale, of a German self-perception questionnaire (KEKS) and changes in mood scored with a German questionnaire on mood (Bf-SR). To compare groups, Mann-Whitney U tests were used.

RESULTS

The WATSU group had a significantly higher increase in the ABC total score in comparison to the lecture group (WATSU: 0.3 (IQR 0 - 0.8), lecture: 0.1 (IQR -0.2 - 0.2), p = 0.014). In the KEKS no group differences were seen. In WATSU a significantly stronger improvement of pain (Visual Analogue Pain Scale: WATSU: -15 (IQR -29 to -8), lecture: -4 (IQR -9 - 2), p < 0.001) and of mood (Bf-SR: WATSU: -6 (IQR -14 to -2), lecture: -1 (IQR -5 - 1), p = 0.003) could be observed.

CONCLUSION

This study showed that WATSU immediately increases body awareness and improves pain and mood.

The effects of C-tactile stimulation on temporal summation of second pain: A study of the central and peripheral neural correlates

Affective touch is mediated by specialized receptors sensitive to gentle and slow touch called C-tactile afferents (CT). The activation of these receptors has shown promise in reducing subjective pain ratings, however, how this type of touch can affect central sensitization processes is poorly studied. This work aimed to investigate if affective touch is able to modulate pain sensitization and its electrophysiological correlates during Temporal Summation of Second Pain (TSSP), a phenomenon characterized by an increase in pain perception due to repeated noxious stimuli. Thirty-seven participants underwent a TSSP protocol involving three conditions: TSSP alone, TSSP during vibrotactile stimulation, and TSSP during CT stimulation (administered with a brush mounted in a robot arm). We measured subjective pain ratings, electroencephalographic (N2-P2 complex) and electrocardiographic activity during these conditions. Participants reported a significantly lower increase of pain during CT stimulation compared to vibrotactile stimulation, but not to TSSP alone. In addition, TSSP was reduced when administered in the ipsilateral arm compared to the other somatosensory stimulation. Subjective reports of attention towards painful stimuli, amplitude of the N2-P2 complex, and heart rate were also reduced during CT stimulation. Conclusion: Our results indicated that the activation of CT receptors may reduce sensitization compared to other types of somatosensory stimulation, which is possibly related to the reduction of the attention devoted to nociceptive stimulation. Our results suggest that activation of CT receptors may alleviate the occurrence of central pain sensitization.

Insecure Attachment, Oxytocinergic System and C-Tactile Fibers: An Integrative and Translational Pathophysiological Model of Fibromyalgia and Central Sensitivity Syndromes

Although the pathophysiology of fibromyalgia syndrome has been better understood in recent decades, a unified model of its pathogenesis and an effective therapeutic approach are still far from being realized. The main aim of this article will be to delve into the fundamental mechanisms of the pathophysiology of fibromyalgia conceptualized as stress intolerance syndrome. Using the biopsychosocial model of chronic pain syndromes, we will describe the potential role of the attachment system, C-tactile fibers, and oxytocinergic system dysfunction in the pathophysiology of fibromyalgia syndrome and other central sensitivity syndromes. At the end of the article, the therapeutic implications of this new global and translational pathophysiological model will be briefly discussed.

A systematic review and multivariate meta-analysis of the physical and mental health benefits of touch interventions

Receiving touch is of critical importance, as many studies have shown that touch promotes mental and physical well-being. We conducted a pre-registered (PROSPERO: CRD42022304281) systematic review and multilevel meta-analysis encompassing 137 studies in the meta-analysis and 75 additional studies in the systematic review (n = 12,966 individuals, search via Google Scholar, PubMed and Web of Science until 1 October 2022) to identify critical factors moderating touch intervention efficacy. Included studies always featured a touch versus no touch control intervention with diverse health outcomes as dependent variables. Risk of bias was assessed via small study, randomization, sequencing, performance and attrition bias. Touch interventions were especially effective in regulating cortisol levels (Hedges' g = 0.78, 95% confidence interval (CI) 0.24 to 1.31) and increasing weight (0.65, 95% CI 0.37 to 0.94) in newborns as well as in reducing pain (0.69, 95% CI 0.48 to 0.89), feelings of depression (0.59, 95% CI 0.40 to 0.78) and state (0.64, 95% CI 0.44 to 0.84) or trait anxiety (0.59, 95% CI 0.40 to 0.77) for adults. Comparing touch interventions involving objects or robots resulted in similar physical (0.56, 95% CI 0.24 to 0.88 versus 0.51, 95% CI 0.38 to 0.64) but lower mental health benefits (0.34, 95% CI 0.19 to 0.49 versus 0.58, 95% CI 0.43 to 0.73). Adult clinical cohorts profited more strongly in mental health domains compared with healthy individuals (0.63, 95% CI 0.46 to 0.80 versus 0.37, 95% CI 0.20 to 0.55). We found no difference in health benefits in adults when comparing touch applied by a familiar person or a health care professional (0.51, 95% CI 0.29 to 0.73 versus 0.50, 95% CI 0.38 to 0.61), but parental touch was more beneficial in newborns (0.69, 95% CI 0.50 to 0.88 versus 0.39, 95% CI 0.18 to 0.61). Small but significant small study bias and the impossibility to blind experimental conditions need to be considered. Leveraging factors that influence touch intervention efficacy will help maximize the benefits of future interventions and focus research in this field.

Spatial factors influencing the pain-ameliorating effect of CT-optimal touch: a comparative study for modulating temporal summation of second pain

Recent studies show that CT-optimal touch, gentle slow stroking of the skin, can reduce pain. However, much is unknown regarding the factors influencing its pain-ameliorating effect, such as tactile attention and touch application site. The current study investigates in 36 healthy individuals, whether CT-optimal touch can reduce temporal summation of second pain (TSSP) compared to CT non-optimal touch and tapping the skin. TSSP refers to activation of the C-nociceptors; by stimulating these fibers a burning and/or tingling sensation can be elicited. All participants underwent three conditions on both the contralateral and ipsilateral side of pain induction. The results show that tapping the skin did not reduce TSSP, meaning that pain reduction through touch cannot be explained by tactile attention effects. CT non-optimal touch only reduced TSSP when applied on the ipsilateral side. Importantly, CT-optimal touch effectively reduced TSSP when applied on the contralateral or ipsilateral side. Furthermore, CT-optimal touch was more effective in reducing TSSP compared to CT non-optimal touch and Tapping. This study shows that that CT-optimal touch can reduce TSSP and this effect appears to be independent of touch application site, which is highly relevant for implementing CT-optimal touch as a treatment.

Topography and relationship-specific social touching in individuals displaying body image disturbances

Interpersonal touch is intimately related to the emotional bond between the touch giver and the touch receiver. Which bodily regions we touch in those individuals in our social network is relationship specific. Perception of interpersonal touch is altered in psychiatric disorders characterised by body image disturbances (BIDs). Here, we examined whether the 'imagined' experience of social touch in individuals with BIDs is body topography- and relationship-specific. By using an interactive media mobile App, the Virtual Touch Toolkit, high versus low levels of BIDs participants completed heatmaps of full-body virtual avatars, to indicate the body regions they find soothing/unpleasant to be touched by a loved one versus an acquaintance. Self-reports of interoceptive awareness and dysmorphic concerns were also measured. Overall, imagined touch was rated as the most soothing when received from a loved one, and also when this was delivered to 'social' body regions. The importance of the social relationship for the imagined tactile interactions was particularly evident for the high levels of BIDs group, with greater problems with interoceptive awareness predicting higher soothing touch ratings when this was received by a loved one. Despite the evidence that imagined bodily contacts between meaningful people is the most pleasant for socially acceptable bodily regions, our findings may suggest a greater sensitivity to relation-specific bodily patterns of social touch particularly in the high level of BIDs group. Heightened interoceptive awareness may also play a key role in this experience of bodily affective contacts. Future research for body-oriented therapy for BIDs is encouraged to systematically probe the efficacy of imagined social touch interaction protocols which use more plausible, ecological, scenarios where touch is delivered by loved ones and to socially acceptable bodily regions.

What are C-tactile afferents and how do they relate to "affective touch"?

Since their initial discovery in cats, low-threshold C-fiber mechanoreceptors have become a central interest of scientists studying the affective aspects of touch. Their pursuit in humans, here termed C-tactile (CT) afferents, has led to the establishment of a research field referred to as "affective touch", which is differentiated from "discriminative touch". Presently, we review these developments based on an automated semantic analysis of more than 1000 published abstracts as well as empirical evidence and the solicited opinions of leading experts in the field. Our review provides a historical perspective and update of CT research, it reflects on the meaning of "affective touch", and discusses how current insights challenge established views on the relation between CTs and affective touch. We conclude that CTs support gentle, affective touch, but that not every affective touch experience relies on CTs or must necessarily be pleasant. Moreover, we speculate that currently underappreciated aspects of CT signaling will prove relevant for the manner in which these unique fibers support how humans connect both physically and emotionally.

Therapeutic touch and therapeutic alliance in pediatric care and neonatology: An active inference framework

Therapeutic affective touch has been recognized as essential for survival, nurturing supportive interpersonal interactions, accelerating recovery-including reducing hospitalisations, and promoting overall health and building robust therapeutic alliances. Through the lens of active inference, we present an integrative model, combining therapeutic touch and communication, to achieve biobehavioural synchrony. This model speaks to how the brain develops a generative model required for recovery, developing successful therapeutic alliances, and regulating allostasis within paediatric manual therapy. We apply active inference to explain the neurophysiological and behavioural mechanisms that underwrite the development and maintenance of synchronous relationships through touch. This paper foregrounds the crucial role of therapeutic touch in developing a solid therapeutic alliance, the clinical effectiveness of paediatric care, and triadic synchrony between health care practitioner, caregiver, and infant in a variety of clinical situations. We start by providing a brief overview of the significance and clinical role of touch in the development of social interactions in infants; facilitating a positive therapeutic alliance and restoring homeostasis through touch to allow a more efficient process of allostatic regulation. Moreover, we explain the role of CT tactile afferents in achieving positive clinical outcomes and updating prior beliefs. We then discuss how touch is implemented in treatment sessions to promote cooperative interactions in the clinic and facilitate theory of mind. This underwrites biobehavioural synchrony, epistemic trust, empathy, and the resolution of uncertainty. The ensuing framework is underpinned by a critical application of the active inference framework to the fields of pediatrics and neonatology.

Social Touch Reduces Pain Perception—An fMRI Study of Cortical Mechanisms

Unmyelinated low-threshold mechanoreceptors (C-tactile, CT) in the human skin are important for signaling information about hedonic aspects of touch. We have previously reported that CT-targeted brush stroking by means of a robot reduces experimental mechanical pain. To improve the ecological validity of the stimulation, we developed standardized human–human touch gestures for signaling attention and calming. The attention gesture is characterized by tapping of the skin and is perceived as neither pleasant nor unpleasant, i.e., neutral. The calming gesture is characterized by slow stroking of the skin and is perceived as moderately to very pleasant. Furthermore, the attention (tapping) gesture is ineffective, whereas the calming (stroking) gesture is effective in activating CT-afferents. We conducted an fMRI study (n = 32) and capitalized on the previous development of touch gestures. We also developed an MR compatible stimulator for high-precision mechanical pain stimulation of the thenar region of the hand. Skin-to-skin touching (stroking or tapping) was applied and was followed by low and high pain. When the stroking gesture preceded pain, the pain was rated as less intense. When the tapping gesture preceded the pain, the pain was rated as more intense. Individual pain perception related to insula activation, but the activation was not higher for stroking than for tapping in any brain area during the stimulation period. However, during the evaluation period, stronger activation in the periaqueductal gray matter was observed after calming touch compared to after tapping touch. This finding invites speculation that human–human gentle skin stroking, effective in activating CT-afferents, reduced pain through neural processes involving CT-afferents and the descending pain pathway.

Interaction Matters: The Effect of Touching the Social Robot PARO on Pain and Stress is Stronger When Turned ON vs. OFF

Social touch between humans, as well as between humans and animals, was previously found to reduce pain and stress. We previously reported that touching a social robot can also induce a reduction in pain ratings. However, it is unclear if the effect that touching a robot has on pain perception is due to its appearance and its pleasant touch, or due to its ability to socially interact with humans. In the current experiment, we aimed to assess the contribution of the interactive quality to pain perception. We assessed the effect of touching the social robot PARO on mild and strong pain ratings and on stress perception, on a total of 60 healthy young participants. The robot either interacted with participants (ON group, n = 30) or was turned off (OFF group, n = 30). Touching the robot induced a decrease in mild pain ratings (compared to baseline) only in the ON group while strong pain ratings decreased similarly in both the ON and the OFF groups. The decrease in mild pain ratings in the ON group was significantly greater in participants with a higher positive perception of the interaction with PARO. We conclude that part of the effect that touching the robot has on pain stems from its interactive features.

Understanding sex differences in affective touch: Sensory pleasantness, social comfort, and precursive experiences

Although previous research revealed sex differences in affective touch, the implicated processes and the manner in which men and women differ have been left uncertain. Here we addressed this issue in two studies examining sensory pleasure, interpersonal comfort, and touch motivators. Study 1 comprised a series of lab-based experiments in which a robot stroked 214 participants (half female) at five different velocities modulating the activity of C-tactile afferents thought to support tactile pleasantness. Average pleasantness ratings followed velocity with the typical inverted u-shape similarly in both sexes. In Study 2, 260 participants (half female) completed an online survey. Here, women were more likely than men to express touch comfort with less familiar or unknown individuals, had a greater preference for touch with other women, and felt more comfortable giving and receiving touch to the forearm. Additionally, when describing how their own experiences might motivate others to touch them affectively, women produced more negative descriptions than men. Together, these results show that, while the sexes compare in a touch's sensory pleasantness, they differ in their preceding affective experiences and how they value touch at a higher-order social level. This agrees with extant research on negative affect and stress and suggests that affective touch may be a more relevant coping mechanism for women than for men.

Evaluation of short-term effects of three passive aquatic interventions on chronic non-specific low back pain: Study protocol for a randomized cross-over clinical trial

Background

Low back pain (LBP) is among the most common physical ailments and its chronic manifestation is a leading cause for disability worldwide. LBP is not attributable to a known diagnosis in 85% of the cases and therefore called chronic non-specific LBP (cnLBP). Passive immersion in warm water is commonly claimed to reduce muscular tension and pain, but not yet sufficiently investigated with regard to cnLBP. The current study compares three passive aquatic interventions regarding their effects on cnLBP: floating (resting in a supine immersed position on flotation devices), WATSU (a passive hands-on treatment, in which a practitioner stands in warm water, gently moving and massaging the client), and a Spa session.

Methods

In this randomized cross-over clinical trial, all 24 adult participants with cnLBP will undergo the three interventions in balanced order with a washout-period of at least two weeks in between. Assessments will take place at baseline and follow-up of study and immediately before and after each intervention. Assessments cover the primary outcome self-reported current pain (Visual Analog Scale, range: 0–100 mm), other self-report questionnaires (addressing, e.g., personality traits or -states), and physiological parameters (e.g., measurement of spinal range of motion).

Discussion

The study adds estimates of intervention-specific effect-sizes of widespread passive aquatic interventions to cnLBP. The study also points to potential underlying pain-reducing mechanisms.

Trial registration

The protocol was approved by the Ethics Committee of the Canton Bern (ProjectID: 2018–00461). Trial registration is intended at ClinicalTrials.gov.

Social touch experience in different contexts: A review

Social touch is increasingly utilized in a variety of psychological interventions, ranging from parent-child interventions to psychotherapeutic treatments. Less attention has been paid, however, to findings that exposure to social touch may not necessarily evoke positive or pleasant responses. Social touch can convey different emotions from love and gratitude to harassment and envy, and persons' preferences to touch and be touched do not necessarily match with each other. This review of altogether 99 original studies focuses on how contextual factors modify target person's behavioral and brain responses to social touch. The review shows that experience of social touch is strongly modified by a variety of toucher-related and situational factors: for example, toucher's facial expressions, physical attractiveness, relationship status, group membership, and touched person's psychological distress. At the neural level, contextual factors modify processing of social touch from early perceptual processing to reflective cognitive evaluation. Based on the review, we present implications for using social touch in behavioral and neuroscientific research designs.

Widespread Pressure Delivered by a Weighted Blanket Reduces Chronic Pain: A Randomized Controlled Trial

Pleasant sensation is an underexplored avenue for modulation of chronic pain. Deeper pressure is perceived as pleasant and calming, and can improve sleep. Although pressure can reduce acute pain, its effect on chronic pain is poorly characterized. The current remote, double-blind, randomized controlled trial tested the hypothesis that wearing a heavy weighted blanket - providing widespread pressure to the body - relative to a light weighted blanket would reduce ratings of chronic pain, mediated by improvements in anxiety and sleep. Ninety-four adults with chronic pain were randomized to wear a 15-lb. (heavy) or 5-lb. (light) weighted blanket during a brief trial and overnight for one week. Measures of anxiety and chronic pain were collected pre- and post-intervention, and ratings of pain intensity, anxiety, and sleep were collected daily. After controlling for expectations and trait anxiety, the heavy weighted blanket produced significantly greater reductions in broad perceptions of chronic pain than the light weighted blanket (Cohen's f = .19, CI [-1.97, -.91]). This effect was stronger in individuals with high trait anxiety (P = .02). However, weighted blankets did not alter pain intensity ratings. Pain reductions were not mediated by anxiety or sleep. Given that the heavy weighted blanket was associated with greater modulation of affective versus sensory aspects of chronic pain, we propose that the observed reductions are due to interoceptive and social/affective effects of deeper pressure. Overall, we demonstrate that widespread pressure from a weighted blanket can reduce the severity of chronic pain, offering an accessible, home-based tool for chronic pain. The study purpose, targeted condition, study design, and primary and secondary outcomes were pre-registered in ClinicalTrials.gov (NCT04447885: "Weighted Blankets and Chronic Pain"). Perspective: This randomized-controlled trial showed that a 15-lb weighted blanket produced significantly greater reductions in broad perceptions of chronic pain relative to a 5-lb weighted blanket, particularly in highly anxious individuals. These findings are relevant to patients and providers seeking home-based, nondrug therapies for chronic pain relief.