Social touch alters newborn monkey behavior

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.

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.

Affective touch in the context of development, oxytocin signaling, and autism

Touch represents one of our most important senses throughout life and particularly in the context of our social and emotional experiences. In this review, we draw on research on touch processing from both animal models and humans. Firstly, we briefly describe the cutaneous touch receptors and neural processing of both affective and discriminative touch. We then outline how our sense of touch develops and summarize increasing evidence demonstrating how essential early tactile stimulation is for the development of brain and behavior, with a particular focus on effects of tactile stimulation in infant animals and pediatric massage and Kangaroo care in human infants. Next, the potential mechanisms whereby early tactile stimulation influences both brain and behavioral development are discussed, focusing on its ability to promote neural plasticity changes and brain interhemispheric communication, development of social behavior and bonding, and reward sensitivity through modulation of growth factor, oxytocin, and opioid signaling. Finally, we consider the implications of evidence for atypical responses to touch in neurodevelopmental disorders such as autism spectrum disorder and discuss existing evidence and future priorities for establishing potential beneficial effects of interventions using massage or pharmacological treatments targeting oxytocin or other neurochemical systems.

Oxytocin but not naturally occurring variation in caregiver touch associates with infant social orienting

Caregiver touch is crucial for infants’ healthy development, but its role in shaping infant cognition has been relatively understudied. In particular, despite strong premises to hypothesize its function in directing infant attention to social information, little empirical evidence exists on the topic. In this study, we investigated the associations between naturally occurring variation in caregiver touch and infant social attention in a group of 6‐ to 13‐month‐old infants (n = 71). Additionally, we measured infant salivary oxytocin as a possible mediator of the effects of touch on infant social attention. The hypothesized effects were investigated both short term, with respect to touch observed during parent–infant interactions in the lab, and long term, with respect to parent‐reported patterns of everyday touching behaviors. We did not find evidence that caregiver touch predicts infant social attention or salivary oxytocin levels, short term or long term. However, we found that salivary oxytocin predicted infant preferential attention to faces relative to nonsocial objects, measured in an eye‐tracking task. Our findings confirm the involvement of oxytocin in social orienting in infancy, but raise questions regarding the possible environmental factors influencing the infant oxytocin system.

A meta-analytic study of the effects of early maternal separation on cognitive flexibility in rodent offspring

Adverse early life experiences, such as maternal separation, are associated with an increased risk for several mental health problems. Symptoms induced by maternal separation that mirror clinically relevant aspects of mental problems, such as cognitive inflexibility, open the possibility of testing putative therapeutics prior to clinical development. Although several animal (e.g., rodent) studies have evaluated the effects of early maternal separation on cognitive flexibility, no consistent conclusions have been drawn. To clarify this issue, in this study, a meta-analysis method was used to systematically explore the relationship between early maternal separation and cognitive flexibility in rodent offspring. Results indicate that early maternal separation could significantly impair cognitive flexibility in rodent offspring. Moderator analyses further showed that the relationship between early maternal separation and cognitive flexibility was not consistent in any case, but was moderated by variations in the experimental procedures, such as the deprivation levels, task characteristics, and rodent strains. These clarify the inconsistent effects of maternal separation on cognitive flexibility in rodents and help us better understand the association between early life adversity and cognitive development.

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Meta-analysis method was used to discuss the inconsistent effects of maternal separation on cognitive flexibility in rodent.

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Maternal separation was found to necessarily impair the cognitive flexibility in rodent.

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Variations in the experimental procedures moderated the relationship between maternal separation and cognitive flexibility.

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Further studies on environment-cognition associations in rodents should take experimental procedural factors into account.

Age-based variation in calf independence, social behavior and play in a captive population of African elephant calves

African elephant calves are highly social and their behavioral development depends heavily on interactions with other elephants. Evaluating early social behaviors offers important information that can inform management decisions and maximize individual- and population-level welfare. We use data collected from the population of elephants at the San Diego Zoo Safari Park in Escondido, CA to evaluate developmental trajectories of spatial independence and social behavior in nine elephant calves across a range of ages. As calves aged, the probability of being further from mothers also increased. Tactile interactions were common among calves, with all individuals either initiating or receiving physical touches from other elephants in a large proportion of focal scans. While the probability of initiating tactile interactions tended to decline with increases in calf age, the probability of receiving tactile interactions from other elephants remained invariant with regard to this variable. The social play was also common, occurring in a fifth of all focal scans. While there was evidence that social play tended to decline with increases in calf age, results suggest additional factors may be useful in characterizing patterns in play behavior at the individual level. Calves most frequently engaged in play with individuals of similar age but showed substantial variation in play partner choice. Results of this study suggest that maintaining groups of elephants in captivity with diverse age structure positively contribute to their healthy social development.

Social and affective touch in primates and its role in the evolution of social cohesion

Primates are long-lived, highly social mammals who maintain long-term social bonds and cohesive social groups through many affiliative mechanisms, foremost among them social touch. From birth through adulthood, social touch - primarily mutual grooming - creates and maintains relationships of trust and reliance, which are the basis for individual physical and emotional well-being and reproductive success. Because social touch helps to establish, maintain, and repair social alliances in primates, it contributes to the emotional stability of individuals and the cohesion of social groups. In these fundamental ways, thus, social touch supports the slow life histories of primates. The reinforcing neurochemistry of social touch insures that it is a pleasurable activity and this, in turn, makes it a behavioral commodity that can be traded between primates for desirable rewards such as protection against future aggression or opportunities to handle infants. Social touch is essential to normal primate development, and individuals deprived of social touch exhibit high levels of anxiety and lower fertility compared to those receiving regular social touch. Understanding the centrality of social touch to primate health and well-being throughout the lifespan provides the foundation for appreciating the importance of social touch in human life.

Human and monkey infant attention to dynamic social and nonsocial stimuli

The present study explored behavioral norms for infant social attention in typically developing human and nonhuman primate infants. We examined the normative development of attention to dynamic social and nonsocial stimuli longitudinally in macaques (Macaca mulatta) at 1, 3, and 5 months of age (N = 75) and humans at 2, 4, 6, 8, and 13 months of age (N = 69) using eye tracking. All infants viewed concurrently played silent videos-one social video and one nonsocial video. Both macaque and human infants were faster to look to the social than the nonsocial stimulus, and both species grew faster to orient to the social stimulus with age. Further, macaque infants' social attention increased linearly from 1 to 5 months. In contrast, human infants displayed a nonlinear pattern of social interest, with initially greater attention to the social stimulus, followed by a period of greater interest in the nonsocial stimulus, and then a rise in social interest from 6 to 13 months. Overall, human infants looked longer than macaque infants, suggesting humans have more sustained attention in the first year of life. These findings highlight potential species similarities and differences, and reflect a first step in establishing baseline patterns of early social attention development.