Psychophysiology and motivated emotion: testing the affective touch hypothesis of C-tactile afferent function

Brain oscillatory dynamics during discriminative vs CT-optimal touch

The affective dimension of touch is conveyed by low-threshold mechanoreceptors known as C-Tactile (CT) afferents. Literature has shown that the stimulation of these fibers appears to have an important modulatory function in neural oscillations. However, much remains to be explored in this field. This study aims to provide background knowledge about the brain oscillatory dynamics and spatial field distributions of CT stimulation, by comparing the brain's spectral power and the microstates to affective (stroking) vs discriminative touch (vibration) conditions. Thirty-four healthy participants (18 female) received tactile stimulation with a cosmetic brush at CT-optimal speeds or vibrotactile stimulation (at around 200 Hz) on the left forearm's dorsum. They evaluated the pleasantness and intensity ratings of the stimulation while an electroencephalogram was recorded. Stroking stimulation was rated as more pleasant than the vibrotactile stimuli, with no significant differences in the intensity ratings. Power spectral density results revealed reduced power in alpha/mu and beta bands in central/Rolandic areas for the stroking condition compared to the vibration condition. Microstates analysis showed a reduced prevalence of class A and an increased prevalence of classes B and D during stroking. These findings indicate that CT-tuned stroking increased sensorimotor cortical excitability and engaged greater attentional resources, suggesting that this form of touch may be a prioritised type of information.

Positive and negative touch differentially modulate metacognitive memory judgements for emotional stimuli

Touch plays a crucial role in providing humans with information from the external environment and can be perceived by humans as positively or negatively valenced. It is well documented that touch can differentially influence social functions, but very little is known about how touch can modulate (meta)cognition. Utilizing a within-subject design, participants were exposed to (a) positive, (b) negative, and (c) no touch, alongside encoding of emotionally valenced (positive and negative) images. After a 20-minute delay, participants completed a Yes/No recognition task to investigate how touch influenced memory-related decision components (e.g. criterion, confidence). Results showed that, compared to the control condition, both positive and negative touch were associated with overall lower confidence ratings, a less liberal response bias and slower response times. Interestingly, for correct recognitions, only negative touch (vs. no touch) led to inappropriately lower confidence and slower response times while both confidence and response time remained unaltered in positive touch. Our findings provide the first evidence that positive and negative touch differentially interact with metacognitive memory-related decisions. Altered metacognitive judgements after being touched could have significant consequences in applied settings, such as situations of eyewitness testimony.

Pupil dilation responds to the intrinsic social characteristics of affective touch

Affective Touch is characterized by both emotional and arousing dimensions that rely on specific features of a gentle human caress. In this study, we investigated whether and how both the nature of the touching effector (Human hand vs. Artificial hand) and touch type (Dynamic vs. Static) influenced the participants' pupil dilation and their subjective experience during tactile stimulation. We observed that when participants received a dynamic touch, their pupil dilation increased more when the touch was produced by a human compared to an artificial hand. This discrimination was not present for static touch. Also, dynamic touch given by a human hand invoked a supralinear enhancement of pupil dilation indicating that the combination of these two features induced a stronger autonomic activation than the summed effects of each separately. Moreover, this specific type of touch was perceived as the most pleasant compared to all other tactile stimulations. Overall, our results suggest that pupil dilation could reflect the pleasant experience of human-to-human tactile interactions, supporting the notion that the autonomic nervous system is responsive to the emotional and hedonic aspects associated with Affective Touch as a part of a complex and holistic social experience, rather than solely reacting to its low-level sensory properties.

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.

The space between us: The effect of perceived threat on discomfort distance and perceived pleasantness of interpersonal vicarious touch

The space we keep between ourselves and others allows us either to engage in close shared experiences or to distance ourselves for safety. Focusing primarily on the latter, previous studies have identified a link between interpersonal boundaries and perceived threat, perceptual discrimination including pain perception as well as how we move and behave as a result. Although interpersonal distancing has been studied in a range of contexts, a mechanistic way of how such spatial behaviour might alter how we perceive affective touch has yet to be investigated. Here we probe the effect of perceived threat of COVID-19 on interpersonal boundary preferences and perceived pleasantness of vicarious affective touch. Our results demonstrate that increased perceived threat from COVID-19 is associated with larger boundaries of discomfort distance. Moreover, we show a positive association between perceived threat and pleasantness of vicarious touch coming from a member of the household, but no association with outsider touch. Importantly, rather than focusing on the purely "positive" and prosocial functions of affective touch, these results bolster a novel perspective that socially-relevant cues guide both approach and avoidance behaviours.

Characterizing affiliative touch in humans and its role in advancing haptic design

An emerging view in cognitive neuroscience holds that the extraction of emotional relevance from sensory experience extends beyond the centralized appraisal of sensation in associative brain regions, including frontal and medial-temporal cortices. This view holds that sensory information can be emotionally valenced from the point of contact with the world. This view is supported by recent research characterizing the human affiliative touch system, which carries signals of soft, stroking touch to the central nervous system and is mediated by dedicated C-tactile afferent receptors. This basic scientific research on the human affiliative touch system is informed by, and informs, technology design for communicating and regulating emotion through touch. Here, we review recent research on the basic biology and cognitive neuroscience of affiliative touch, its regulatory effects across the lifespan, and the factors that modulate it. We further review recent work on the design of haptic technologies, devices that stimulate the affiliative touch system, such as wearable technologies that apply the sensation of soft stroking or other skin-to-skin contact, to promote physiological regulation. We then point to future directions in interdisciplinary research aimed at both furthering scientific understanding and application of haptic technology for health and wellbeing.

Changes in food cue reactivity through affective and nonaffective touch: An event-related potential study

Affective touch (gentle/slow brushing of the skin) can facilitate the allocation of processing resources to simultaneously present stimuli from different modalities. The present event-related potential (ERP) study investigated whether affective touch can enhance attention to visual cues of healthy food. Female participants (n = 117) were randomly assigned to three different groups that either received affective touch, nonaffective touch (fast brushing of the skin), or no touch during the presentation of pictures of healthy food (fruits and vegetables) and non-food. Electrocortical markers of motivated attention (frontal/parietal P300, late positive potential: LPP) and reported appetite for the depicted food items were compared between the three groups. Nonaffective touch was associated with reduced amplitudes of the frontal P300/LPP (300-1000 ms) for food pictures indexing reduced motivated attention. Affective touch did not influence food cue reactivity (P300/LPP, appetite). Effects of affective touch may be restricted to specific stimuli, e.g. those with social relevance.

Atypical Response to Affective Touch in Children with Autism: Multi-Parametric Exploration of the Autonomic System

This study aimed at evaluating the autonomic response to pleasant affective touch in children with Autism Spectrum Disorders (ASD) and age-matched typically developing (TD) peers, thanks to multiple autonomic nervous system (ANS) parameters and by contrasting CT (C-tactile fibers) high- vs. low-density territory stimulations. We measured pupil diameter, skin conductance, and heart rate during gentle stroking of two skin territories (CT high- and low-density, respectively, forearm and palm of the hand) in thirty 6-12-year-old TD children and twenty ASD children. TD children showed an increase in pupil diameter and skin conductance associated with a heart rate deceleration in response to tactile stimulations at the two locations. Only the pupil was influenced by the stimulated location, with a later dilation peak following CT low-density territory stimulation. Globally, ASD children exhibited reduced autonomic responses, as well as different ANS baseline values compared to TD children. These atypical ANS responses to pleasant touch in ASD children were not specific to CT-fiber stimulation. Overall, these results point towards both basal autonomic dysregulation and lower tactile autonomic evoked responses in ASD, possibly reflecting lower arousal and related to social disengagement.

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.