C-low threshold mechanoafferent targeted dynamic touch modulates stress resilience in rats exposed to chronic mild stress.

Affiliative tactile interactions buffer social mammals against neurobiological and behavioral effects of stress. The aim of this study was to investigate the cutaneous mechanisms underlying such beneficial consequences of touch by determining whether daily stroking, specifically targeted to activate a velocity/force tuned class of low-threshold c-fiber mechanoreceptor (CLTM), confers resilience against established markers of chronic unpredictable mild stress (CMS). Adult male Sprague Dawley rats were exposed to 2 weeks of CMS. Throughout the CMS protocol, some rats were stroked daily, either at CLTM optimal velocity (5 cm/s) or outside the CLTM optimal range (30 cm/s). A third CMS exposed group did not receive any tactile stimulation. The effect of CMS on serum corticosterone levels, anxiety- and depressive-like behaviors in these three groups was assessed in comparison to a control group of non-CMS exposed rats. While stroking did not mitigate the effects of CMS on body weight gain, CLTM optimal velocity stroking did significantly reduce CMS-induced elevations in corticosterone following an acute forced-swim. Rats receiving CLTM optimal stroking also showed significantly fewer anxiety-like behaviors (elevated plus-maze) than the other CMS exposed rats. In terms of depressive-like behavior, whereas the same velocity-specific resilience was observed in a forced-swim test and social interaction test both groups of stroked rats spent significantly less time interacting than control rats, though they also spent significantly less time in the corner than non-stroked CMS rats. Together, these findings support the theory CLTMs play a functional role in regulating the physiological condition of the body.

Effects of Acute Tryptophan Depletion on Human Taste Perception.

Taste perception has been reported to vary with changes in affective state. Distortions of taste perception, including blunted recognition thresholds, intensity, and hedonic ratings have been identified in those suffering from depressive disorders. Serotonin is a key neurotransmitter implicated in the etiology of anxiety and depression; systemic and peripheral manipulations of serotonin signaling have previously been shown to modulate taste detection. However, the specific effects of central serotonin function on taste processing have not been widely investigated. Here, in a double-blind placebo-controlled study, acute tryptophan depletion was used to investigate the effect of reduced central serotonin function on taste perception. Twenty-five female participants aged 18-28 attended the laboratory on two occasions at least 1 week apart. On one visit, they received a tryptophan depleting drink and on the other, a control drink was administered. Approximately, 6 h after drink consumption, they completed a taste perception task which measured detection thresholds and supra-threshold perceptions of the intensity and pleasantness of four basic tastes (sweet, sour, bitter, and salt). While acutely reducing central levels of serotonin had no effect on the detection thresholds of sweet, bitter, or sour tastes, it significantly enhanced detection of salt. For supra-threshold stimuli, acutely reduced serotonin levels significantly enhanced the perceived intensity of both bitter and sour tastes and blunted pleasantness ratings of bitter quinine. These findings show manipulation of central serotonin levels can modulate taste perception and are consistent with previous reports that depletion of central serotonin levels enhances neural and behavioral responsiveness to aversive signals.

Autistic traits modulate cortical responses to affective but not discriminative touch.

The sense of touch is primarily considered a discriminative and exteroceptive sense, facilitating the detection, manipulation and exploration of objects, via an array of low-threshold mechanoreceptors and fast conducting A-beta (Aß) afferents. However, a class of unmyelinated, low-threshold mechanoreceptors identified in the hairy skin of mammals have been proposed to constitute a second, anatomically distinct system coding the affective qualities of touch. Unlike Aßs, which increase their firing rate linearly with the velocity of a stimulus moving across their receptive field, the response of these C-tactile afferents (CTs) is described by an inverted 'U' curve fit, responding optimally to a skin temperature stimulus moving at between 1 and 10 cm/s. Given the distinct velocity tuning of these fast and slow touch fibres, here we used event-related potentials to compare the time course of neural responses to 1st (fast) and 2nd (slow) touch systems. We identified a higher amplitude P300 in response to fast, Aß-targeted, versus slow CT-targeted, stroking touch. In contrast, we identified a previously described, C-fibre specific, ultra-late potential (ULP) associated with CT-targeted input. Of special note as regards the function of CTs is that the amplitude of the ULP was negatively correlated with self-reported levels of autistic traits, which is consistent with the hypothesized affective and social significance of this response. Taken together, these findings provide further support for distinct discriminative and affective touch systems and suggests the temporal resolution of EEG provides an as yet underutilized tool for exploring individual differences in response sensitivity to CT-targeted touch.

Superior Identification of Component Odors in a Mixture Is Linked to Autistic Traits in Children and Adults.

Most familiar odors are complex mixtures of volatile molecules, which the olfactory system automatically synthesizes into a perceptual whole. However, odors are rarely encountered in isolation; thus, the brain must also separate distinct odor objects from complex and variable backgrounds. In vision, autistic traits are associated with superior performance in tasks that require focus on the local features of a perceptual scene. The aim of the present study was to determine whether the same advantage was observed in the analysis of olfactory scenes. To do this, we compared the ability of 1) 40 young adults (aged 16-35) with high (n = 20) and low levels of autistic traits and 2) 20 children (aged 7-11), with (n = 10) and without an autism spectrum disorder diagnosis, to identify individual odor objects presented within odor mixtures. First, we used a 4-alternative forced choice task to confirm that both adults and children were able to reliably identify 8 blended fragrances, representing food-related odors, when presented individually. We then used the same forced choice format to test participants' ability to identify the odors when they were combined in either binary or ternary mixtures. Adults with high levels of autistic traits showed superior performance on binary but not ternary mixture trials, whereas children with an autism spectrum disorder diagnosis outperformed age-matched neurotypical peers, irrespective of mixture complexity. These findings indicate that the local processing advantages associated with high levels of autistic traits in visual tasks are also apparent in a task requiring analytical processing of odor mixtures.

The perception of affective touch in Parkinson's disease and its relation to small fibre neuropathy.

Affective touch sensation is conducted by a sub-class of C-fibres in hairy skin known as C-Tactile (CT) afferents. CT afferents respond maximally to gentle skin stroking at velocities between 1 and 10 cm/s. Parkinson's disease (PD) is characterised by markedly reduced cutaneous C-fibres. It is not known if affective touch perception is influenced by C-fibre density and if affective touch is impaired in PD compared to healthy controls. We predicted that perceived pleasantness to gentle stroking in PD would correlate with C-afferent density and that affective touch perception would be impaired in PD compared to healthy controls. Twenty-four PD patients and 27 control subjects rated the pleasantness of brush stroking at an optimum CT stimulation velocity (3 cm/s) and two sub-optimal velocities (0.3 and 30 cm/s). PD patients underwent quantification of C-fibre density using skin biopsies and corneal confocal microscopy. All participants rated a stroking velocity of 3 cm/s as the most pleasant with significantly lower ratings for 0.3 and 30 cm/s. There was a significant positive correlation between C-fibre density and pleasantness ratings at 3 and 30 cm/s but not 0.3 cm/s. Mean pleasantness ratings were consistently higher in PD patients compared to control subjects across all three velocities. This study shows that perceived pleasantness to gentle touch correlates significantly with C-fibre density in PD. The higher perceived pleasantness in PD patients compared to controls suggests central sensitisation to peripheral inputs, which may have been enhanced by dopamine therapy.

Effects of acute tryptophan depletion on central processing of CT-targeted and discriminatory touch in humans.

C-tactile afferents (CTs) are slowly conducting nerve fibres, present only in hairy skin. They are optimally activated by slow, gentle stroking touch, such as those experienced during a caress. CT stimulation activates affective processing brain regions, alluding to their role in affective touch perception. We tested a theory that CT-activating touch engages the pro-social functions of serotonin, by determining whether reducing serotonin, through acute tryptophan depletion, diminishes subjective pleasantness and affective brain responses to gentle touch. A tryptophan depleting amino acid drink was administered to 16 healthy females, with a further 14 receiving a control drink. After 4 h, participants underwent an fMRI scan, during which time CT-innervated forearm skin and CT non-innervated finger skin was stroked with three brushes of differing texture, at CT-optimal force and velocity. Pleasantness ratings were obtained post scanning. The control group showed a greater response in ipsilateral orbitofrontal cortex to CT-activating forearm touch compared to touch to the finger where CTs are absent. This differential response was not present in the tryptophan depleted group. This interaction effect was significant. In addition, control participants showed a differential primary somatosensory cortex response to brush texture applied to the finger, a purely discriminatory touch response, which was not observed in the tryptophan depleted group. This interaction effect was also significant. Pleasantness ratings were similar across treatment groups. These results implicate serotonin in the differentiation between CT-activating and purely discriminatory touch responses. Such effects could contribute to some of the social abnormalities seen in psychiatric disorders associated with abnormal serotonin function.

Olfactory processing: detection of rapid changes.

Changes in the olfactory environment have a rather poor chance of being detected. Aim of the present study was to determine, whether the same (cued) or different (uncued) odors can generally be detected at short inter stimulus intervals (ISI) below 2.5 s. Furthermore we investigated, whether inhibition of return, an attentional phenomenon facilitating the detection of new stimuli at longer ISI, is present in the domain of olfaction. Thirteen normosmic people (3 men, 10 women; age range 19-27 years; mean age 23 years) participated. Stimulation was performed using air-dilution olfactometry with 2 odors: phenylethylalcohol and hydrogen disulfide. Reaction time to target stimuli was assessed in cued and uncued conditions at ISIs of 1, 1.5, 2, and 2.5 s. There was a significant main effect of ISI, indicating that odors presented only 1 s apart are missed frequently. Uncued presentation facilitated detection at short ISIs, implying that changes of the olfactory environment are detected better than presentation of the same odor again. Effects in relation to "olfactory inhibition of return," on the other hand, are not supported by our results. This suggests that attention works different for the olfactory system compared with the visual and auditory systems.

High frequency heart rate variability is associated with sensitivity to affective touch.

C-tactile afferents (CTs) are a class of unmyelinated, mechanosensitive nerve fibre that respond optimally to skin temperature, slow moving touch typical of a caress. They are hypothesised to signal the rewarding value of affiliative tactile interactions. While CT firing frequency is positively correlated with subjective ratings of touch pleasantness, trait differences in sensitivity to the specific hedonic value of CT targeted touch have been reported. Inter-individual differences in vagally mediated, high frequency heart rate variability (HF-HRV) have been linked to variation in visual social cognition. Thus, the aim of the present study was to examine the relationship between resting state HF-HRV and sensitivity to socially relevant CT targeted touch. 58 healthy participants first had a 5-minute electrocardiogram. They then rated the pleasantness of 5 randomly presented velocities of robotically delivered touch. Three velocities fell within (1, 3, 10 cm/s) and two outside (0.3, 30 cm/s) the CT optimal range. Each velocity was delivered twice. On a group level, affective touch ratings were described by a negative quadratic function, with CT optimal velocities rated as more pleasant than slower and faster speeds. Simple regression analysis confirmed participants' HF-HRV was significantly predicted by the quadratic curve fit of their touch ratings, with higher HF-HRV associated with a better quadratic fit. These findings indicate that, in line with previous observations that higher HF-HRV is associated with enhanced sensitivity to visual social cues, trait differences in autonomic control could account for previously reported individual differences in CT sensitivity.

Congenital Anosmia and Facial Emotion Recognition.

Major functions of the olfactory system include guiding ingestion and avoidance of environmental hazards. People with anosmia report reliance on others, for example to check the edibility of food, as their primary coping strategy. Facial expressions are a major source of non-verbal social information that can be used to guide approach and avoidance behaviour. Thus, it is of interest to explore whether a life-long absence of the sense of smell heightens sensitivity to others' facial emotions, particularly those depicting threat. In the present, online study 28 people with congenital anosmia (mean age 43.46) and 24 people reporting no olfactory dysfunction (mean age 42.75) completed a facial emotion recognition task whereby emotionally neutral faces (6 different identities) morphed, over 40 stages, to express one of 5 basic emotions: anger, disgust, fear, happiness, or sadness. Results showed that, while the groups did not differ in their ability to identify the final, full-strength emotional expressions, nor in the accuracy of their first response, the congenital anosmia group successfully identified the emotions at significantly lower intensity (i.e. an earlier stage of the morph) than the control group. Exploratory analysis showed this main effect was primarily driven by an advantage in detecting anger and disgust. These findings indicate the absence of a functioning sense of smell during development leads to compensatory changes in visual, social cognition. Future work should explore the neural and behavioural basis for this advantage.

Affective touch and regulation of stress responses.

Much has been documented on the association between stress and health. Both direct and indirect pathways have been identified and explored extensively, helping us understand trajectories from healthy individuals to reductions in well-being, and development of preclinical and disease states. Some of these pathways are well established within the field; physiology, affect regulation, and social relationships. The purpose of this review is to push beyond what is known separately about these pathways and provide a means to integrate them using one common mechanism. We propose that social touch, specifically affective touch, may be the missing active ingredient fundamental to our understanding of how close relationships contribute to stress and health. We provide empirical evidence detailing how affective touch is fundamental to the development of our stress systems, critical to the development of attachment bonds and subsequent social relationships across the life course. We will also explore how we can use this in applied contexts and incorporate it into existing interventions.

Evaluation of a longitudinal digital citizen science initiative to understand the impact of culture on Indigenous youth mental health: Findings from a quasi-experimental qualitative study.

BACKGROUND: Indigenous youth in settler nations are susceptible to poor mental health due to complex intergenerational systemic inequities. Research has shown benefits of cultural connectedness for improving mental health; however, there are few studies which have evaluated the impact of culturally relevant mental health interventions, particularly among Indigenous youth. The purpose of this study is to assess the impact of a culturally-responsive, land-based, active living initiative on the mental health of Indigenous youth. METHODS: This quasi-experimental qualitative study is part of Smart Indigenous Youth (SIY), a mixed-methods 5-year longitudinal digital citizen science initiative. SIY embeds culturally responsive, land-based active living programs into the curricula of high schools in rural Indigenous communities in the western Canadian province of Saskatchewan. In year-1 (Winter 2019), 76 Indigenous youth citizen scientists (13-18 years) from 2 schools participated in the study. At the beginning of the term, each school initiated separate 4-month land-based active living programs specific to their culture, community, geography, and language (Cree and Saulteaux). Before and after the term, focus groups were conducted with the 2 Youth Citizen Scientist Councils, which included students from both participating schools. This study includes data from focus groups of one participating school, with 11 youth citizen scientists (5 boys, 6 girls). Focus group data were transcribed and analyzed by two independent reviewers using Nvivo to identify themes and subthemes. Both reviewers discussed their thematic analysis to reach consensus about final findings. RESULTS: Baseline focus group analyses (before land-based programming) revealed themes demonstrating the importance of Indigenous culture, identity, history, and language. Youth emphasized the impact of loss of language and culture, the importance of being a helper, and the necessity of intergenerational knowledge transfer. Follow-up focus group analyses (post land-based programming) indicated that cultural school programming led to students expressing positive mental health benefits, increased interest in ceremonies, increased participation in physical activity, and greater knowledge of culture, identity, and ceremonial protocol. CONCLUSIONS: This novel qualitative quasi-experimental study offers a window into the future of upstream interventions in partnership with Indigenous communities, where Indigenous youth can be engaged in real-time via their digital devices, while participating in culturally-sensitive, land-based school programming that promotes culture, identity, and mental health.

Hold me or stroke me? Individual differences in static and dynamic affective touch.

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.

Social distancing with chronic pain during COVID-19: A cross-sectional correlational analysis.

BACKGROUND: Understanding of the role social factors play in chronic pain is growing, with more adaptive and satisfying social relationships helping pain management. During the COVID-19 pandemic, social distancing measures facilitated a naturalistic study of how changes to social interaction affected chronic pain intensity. METHODS: In a cross-sectional correlational design, questionnaire data was collected over a 38-day period during the March 2020 COVID-19 lockdown, individuals with chronic pain were asked about their current pain experience as well as notable social factors which might relate to pain. RESULTS: Multiple regression analysis revealed social satisfaction significantly predicted pain experience, with a reduction in social participation during COVID-19 lockdowns increasing pain disability, and increased social satisfaction associated with decreasing pain intensity. CONCLUSIONS: While pain management often focuses on the functional aspects of pain alleviation, these findings suggest psychological aspects of socialising satisfaction also impact pain experience. Pain management strategies should consider ways to increase social satisfaction in individuals with chronic pain, perhaps by facilitating socialisation in the home using remote communication methods similar to those which became popular during the COVID-19 lockdown.

Acute tryptophan depletion alters affective touch perception.

RATIONALE: Affiliative tactile interactions help regulate physiological arousal and confer resilience to acute and chronic stress. C-tactile afferents (CTs) are a population of unmyelinated, low threshold mechanosensitive cutaneous nerve fibres which respond optimally to a low force stimulus, moving at between 1 and 10 cm/s. As CT firing frequencies correlate positively with subjective ratings of touch pleasantness, they are hypothesised to form the first stage of encoding affiliative tactile interactions. Serotonin is a key modulator of social responses with known effects on bonding. OBJECTIVES: The aim of the present study was to determine the effect of acutely lowering central serotonin levels on perceptions of CT-targeted affective touch. METHODS: In a double blind, placebo-controlled design, the effect of acute tryptophan depletion (ATD) on 25 female participants' ratings of directly and vicariously experienced touch was investigated. Psychophysical techniques were used to deliver dynamic tactile stimuli; some velocities were targeted to optimally activate CTs (1-10 cm/s), whereas other, faster and slower strokes fell outside the CT optimal range. Discriminative tactile function, cold pain threshold and tolerance were also measured. RESULTS: ATD significantly increased pleasantness ratings of both directly and vicariously experienced affective touch, increasing discrimination of the specific hedonic value of CT targeted velocities. While ATD had no effect on either tactile or cold pain thresholds, there was a trend for reduced tolerance to cold pain. CONCLUSIONS: These findings are consistent with previous reports that depletion of central serotonin levels modulates neural and behavioural responsiveness to appetitive sensory signals.

Differential contributions of the primate ventrolateral prefrontal and orbitofrontal cortex to serial reversal learning.

The discrimination reversal paradigm is commonly used to measure a subject's ability to adapt their behavior according to changes in stimulus-reward contingencies. Human functional neuroimaging studies have demonstrated activations in the lateral orbitofrontal cortex (OFC) and the inferior frontal gyrus (IFG) in subjects performing this task. Excitotoxic lesions of analogous regions in marmosets have revealed, however, that although the OFC is indeed critical for reversal learning, ventrolateral prefrontal cortex (VLPFC) (analogous to IFG) is not, contributing instead to higher order processing, such as that required in attentional set-shifting and strategy transfer. One major difference between the marmoset and human studies has been the level of training subjects received in reversal learning, being far greater in the latter. Since exposure to repeated contingency changes, as occurs in serial reversal learning, is likely to trigger the development of higher order, rule-based strategies, we hypothesized a critical role of the marmoset VLPFC in performance of a serial reversal learning paradigm. After extensive training in reversal learning, marmosets received an excitotoxic lesion of the VLPFC, OFC, or a sham control procedure. In agreement with our prediction, postsurgery, VLPFC lesioned animals were impaired in performing a series of discrimination reversals, but only when novel visual stimuli were introduced. In contrast, OFC lesioned animals were impaired regardless of whether the visual stimuli were the same or different from those used during presurgery training. Together, these data demonstrate the heterogeneous but interrelated involvement of primate OFC and VLPFC in the performance of serial reversal learning.

Children's vicarious ratings of social touch are tuned to the velocity but not the location of a caress.

Affective sharing is a bottom-up process involving automatic processing of sensory inputs that facilitate vicarious experience of another's emotional state. It is grounded directly in the prior experiences of the perceiver. In adults, vicarious ratings of affective touch match the known velocity tuning and hypothesised anatomical distribution of C-tactile afferents (CT), a subclass of C-fibre which respond preferentially to low force/velocity stroking touch, typically perceived as pleasant. Given the centrality of touch to early nurturing interactions, here we examined whether primary school aged children's vicarious ratings of affective touch show the same anatomical and velocity specific patterns reported in adults. Forty-four children aged between 8 and 11 (mean age 9, 24 male) rated a sequence of video clips depicting one individual being touched by another on 5 different upper-body sites (palm, dorsal forearm, ventral forearm, upper-arm and back) at 3 different velocities (static, CT optimal, slow stroking and non-CT optimal, fast stroking). Immediately after viewing each clip, participants were asked to rate how pleasant they perceived the touch to be. While children rated the CT optimal velocity significantly higher than static or non-CT optimal touch, unlike adults their ratings did not vary across skin sites. This difference may reflect the fact children's ratings are grounded in bottom-up affective resonance while adults also draw on top-down cognitive evaluation of the broader social context when rating the stimuli.

Vicarious ratings of social touch the effect of age and autistic traits.

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.

Response disengagement on a spatial self-ordered sequencing task: effects of regionally selective excitotoxic lesions and serotonin depletion within the prefrontal cortex.

Prefrontal cortex (PFC) is critical for self-ordered response sequencing. Patients with frontal lobe damage are impaired on response sequencing tasks, and increased blood flow has been reported in ventrolateral and dorsolateral PFC in subjects performing such tasks. Previously, we have shown that large excitotoxic lesions of the lateral PFC (LPFC) and orbitofrontal cortex FC (OFC), but not global prefrontal dopamine depletion, markedly impaired marmoset performance on a spatial self-ordered sequencing task (SSOST). To determine whether LPFC or OFC was responsible for the previously observed impairments and whether the underlying neural mechanism was modulated by serotonin, the present study compared the effects of selective LPFC and OFC excitotoxic lesions and 5,7-DHT-induced PFC serotonin depletions in marmosets on SSOST performance. Severe and long-lasting impairments in SSOST performance, including robust perseverative responding, followed LPFC but not OFC lesions. The deficit was ameliorated by task manipulations that precluded perseveration. Depletions of serotonin within LPFC and OFC had no effect, despite impairing performance on a visual discrimination reversal task, thus providing further evidence for differential monaminergic regulation of prefrontal function. In the light of the proposed attentional control functions of ventrolateral PFC and the failure of LPFC-lesioned animals to disengage from the immediately preceding response, it is proposed that this deficit may be due to a failure to attend to and register that a response has been made and thus should not be repeated. However, 5-HT does not appear to be implicated in this response inhibitory capacity.

Childhood Adversity and Affective Touch Perception: A Comparison of United Kingdom Care Leavers and Non-care Leavers.

In the United Kingdom, the most common reasons for a child to come under the care of social services are neglect and abuse. Such early childhood adversity is a risk factor for social-isolation and poor mental health in adulthood. Touch is a key channel for nurturing interactions, and previous studies have shown links between early somatosensory input, experience dependent neural plasticity, and later life emotional functioning. The aim of the present study was to test the relationship between childhood neglect/abuse and later life experiences, attitudes, and hedonic ratings of affective touch. Here, affective touch is defined as low force, dynamic touch which C-Tactile afferents (CTs) respond optimally to. We hypothesized that a childhood lacking in early nurturing tactile stimulation would be associated with reduced sensitivity to socially relevant affective touch in adulthood. To test this, 19 care leavers (average 9.32 ± 3.70 years in foster care) and 32 non-care leavers were recruited through opportunity sampling (mean age = 21.25 ± 1.74 years). Participants completed a range of psychophysical somatosensory tests. First, they rated the pleasantness of CT-optimal (3 cm/s) and non-CT-optimal (0.3 and 30 cm/s) stroking touch applied to their forearm, both robotically and by an experimenter. They also made vicarious ratings of the anticipated pleasantness of social tactile interactions depicted in a series of videos. Finally, they filled in the Childhood Trauma Questionnaire (CTQ) and the Touch Experiences and Attitudes Questionnaire (TEAQ). As expected, care leavers reported significantly higher levels of childhood trauma than the control group. They also reported significantly lower levels of positive childhood touch compared to non-care leavers, but their attitudes and experiences of current intimate and affiliative touch did not differ. Across all psychophysical tests, care leavers showed specific reduction in sensitivity to the affective value of CT targeted 3 cm/s touch. The results of this study support the hypothesis that a lack of nurturing touch in early developmental periods leads to blunted sensitivity to the specific social value of affective touch. Future research should investigate the neural and physiological mechanisms underlying the observed effect.

Effects of MDMA on attention to positive social cues and pleasantness of affective touch.

The psychostimulant drug ±3,4-methylenedioxymethamphetamine (MDMA) reportedly produces distinctive feelings of empathy and closeness with others. MDMA increases social behavior in animal models and has shown promise in psychiatric disorders, such as autism spectrum disorder (ASD) and post-traumatic stress disorder (PTSD). How it produces these prosocial effects is not known. This behavioral and psychophysiological study examined the effects of MDMA, compared with the prototypical stimulant methamphetamine (MA), on two measures of social behavior in healthy young adults: (i) responses to socially relevant, "affective" touch, and (ii) visual attention to emotional faces. Men and women (N = 36) attended four sessions in which they received MDMA (0.75 or 1.5 mg/kg), MA (20 mg), or a placebo in randomized order under double-blind conditions. Responses to experienced and observed affective touch (i.e., being touched or watching others being touched) were assessed using facial electromyography (EMG), a proxy of affective state. Responses to emotional faces were assessed using electrooculography (EOG) in a measure of attentional bias. Subjective ratings were also included. We hypothesized that MDMA, but not MA, would enhance the ratings of pleasantness and psychophysiological responses to affective touch and increase attentional bias toward positive facial expressions. Consistent with this, we found that MDMA, but not MA, selectively enhanced ratings of pleasantness of experienced affective touch. Neither drug altered the ratings of pleasantness of observed touch. On the EOG measure of attentional bias, MDMA, but not MA, increased attention toward happy faces. These results provide new evidence that MDMA can enhance the experience of positive social interactions; in this case, pleasantness of physical touch and attentional bias toward positive facial expressions. The findings are consistent with evidence that the prosocial effects are unique to MDMA relative to another stimulant. Understanding the behavioral and neurobiological processes underlying the distinctive social effects of MDMA is a key step to developing the drug for psychiatric disorders.