Handling newborn monkeys alters later exploratory, cognitive, and social behaviors

Repeatability of measures of behavioral organization over two years in captive infant rhesus macaques, Macaca mulatta

Individual differences of infant temperament have been associated with future health outcomes that provide explanatory power beyond adult personality. Despite the importance of such a metric, our developmental understanding of personality-like traits is poor. Therefore, we examined whether young primates show consistency in personality traits throughout development. We replicated a Biobehavioral Assessment (BBA) at three time periods: 3-4 months, 1 year, and 2 years of age in 47 rhesus macaque (Macaca mulatta) subjects from large mixed-sex outdoor social housing units at the California National Primate Research Center. We report results for tests focused on responses and adaptation to the temporary separation and relocation, responses to a threatening stimulus, and ratings of overall temperament. We found consistently repeatable associations in measures of Emotionality; these associations were stronger in males, but also present in females, and broadly consistent between Years 1 and 2. We also explored whether behavioral responses to this experimental relocation might be influenced by their experience being relocated for other reasons (i.e., hospitalizations) as individuals' responses might be influenced by similar experiences to the BBA procedure. Only locomotion, during one of the assessments, was associated with past hospitalization events. Overall, repeatability in Emotionality-associated behaviors was evident across the 2 years, in both sexes. We did, however, find evidence of the emergence of sex differences via differentiated expression of behavioral responses during the BBA. We emphasize that there is likely contextual nuance in the use of these BBA factor-associated behaviors. Further research is required to determine whether and how shifts occur in underlying factor structure and the expression of associated behaviors.

Effect of gentle physical contact on behavioural indicators in sheep

The available literature provides ample evidence for the importance of gentle treatment and handling of farm animals for the establishment of relationships with humans. The present study is an attempt to verify the assumption that massage can be a tool for building sheep-human relationships and to determine the impact of this technique on the behaviour of lambs. In total, 40 3-week-old lambs (20 ewes of the synthetic prolific-meat line (BCP) and 20 Świniarka breed ewes) were assigned to two control groups (BCP, Świniarka) and two experimental groups (BCP, Świniarka). The lambs from the experimental groups received modified Shantala massage. During each session, the zoophysiotherapists massaged the chest, forelimbs, abdomen, hindlimbs, back, and face always in the same way and the same order. The massage was applied three times a day (at fixed times) for 21 consecutive days. Based on the behaviour manifested by the lambs during the massage sessions, a 2-point scale was developed for assessment of the animals from the research groups. After the massage, behavioural tests with the participation of a human, a dog, and a novel object were carried out for three consecutive days. Behaviours indicating curiosity (frequency of interaction, time of direct contact, latency) and fear (frequency of climbing and attempts to escape, duration of vocalisation) were assessed. The behavioural tests revealed significant differences in the behaviour of massaged and non-massaged lambs of both breeds, which confirms that the massage has a modifying effect on the behaviour of these animals. The influence of the applied technique was reflected by an increase in curiosity and a reduced level of fear.

Cognitive enrichment in a social setting: assessing the use of a novel food maze in sanctuary-housed chimpanzees

Foraging devices are effective enrichment tools for non-human primates, as they provide both cognitive and manipulative stimulation that may enhance these animals’ welfare. We assessed the behavioral effects of a novel tool-based enrichment on 14 chimpanzees (Pan troglodytes) housed at Fundació Mona (Girona, Spain). The device consisted of a vertical maze filled with food rewards, which chimpanzees could extract by using tools. We conducted behavioral observations in two conditions over an approximately 2.5-month period: when the food maze was loaded (12 enrichment days), and when it was empty (12 baseline days). Data were collected using 2-min scan sampling and untimed-event focal sampling during two daily sessions of 80 min each. We expected that the chimpanzees’ interest in the enrichment would decrease over time, but that its use would be linked to an increase in the occurrence of species-typical behaviors, a reduction in negative indicators of welfare, and changes in social behaviors. We found that participation widely varied among subjects, being higher in females and decreasing through time. Furthermore, participation was linked to an increase in tool use and a decrease in inactivity, but also to an increase in aggression-related behaviors. In contrast, participation had no effect on the occurrence of abnormal behaviors, social proximity or affiliation-related behaviors. Finally, we detected an increase in self-directed behaviors only when subjects actively interacted with the device. We conclude that, in future studies, these types of devices should be evaluated for longer periods of time and more attention should be paid to individuals’ preferences and abilities.

Beyond language: The unspoken sensory-motor representation of the tongue in non-primates, non-human and human primates

The English idiom "on the tip of my tongue" commonly acknowledges that something is known, but it cannot be immediately brought to mind. This phrase accurately describes sensorimotor functions of the tongue, which are fundamental for many tongue-related behaviors (e.g., speech), but often neglected by scientific research. Here, we review a wide range of studies conducted on non-primates, non-human and human primates with the aim of providing a comprehensive description of the cortical representation of the tongue's somatosensory inputs and motor outputs across different phylogenetic domains. First, we summarize how the properties of passive non-noxious mechanical stimuli are encoded in the putative somatosensory tongue area, which has a conserved location in the ventral portion of the somatosensory cortex across mammals. Second, we review how complex self-generated actions involving the tongue are represented in more anterior regions of the putative somato-motor tongue area. Finally, we describe multisensory response properties of the primate and non-primate tongue area by also defining how the cytoarchitecture of this area is affected by experience and deafferentation.

Infants exploring objects: A cascades perspective

Infants spend much of their time exploring objects (Herzberg et al., 2021), and object exploration is linked to learning and development in various domains (e.g., social, cognitive, motor). But how does exploration develop in the first place, and how, exactly, does exploration promote learning? One way to approach these process-oriented questions is with a developmental cascades perspective, which holds that new skills emerge from earlier-developing ones and that various interactions with people and objects accumulate over time to influence multiple domains of development (Masten & Cicchetti, 2010). In this chapter, we describe object exploration from a developmental cascades perspective. In Section 2, we describe typical and atypical trajectories of exploration behaviors, noting how these behaviors emerge from earlier-developing cognitive and motor skills. In Section 3, we discuss how object exploration opens the door for new types of learning opportunities. In Section 4, we discuss early experiences that may shape the development of object exploration. Altogether, we aim to convey that new developments in exploration skills are extensions of earlier-developing skills, and that seemingly insignificant exploratory behaviors (e.g., shaking a rattle) may result in numerous and varied consequences for the developing infant.

Predicting their past: Machine language learning can discriminate the brains of chimpanzees with different early-life social rearing experiences

Early life experiences, including separation from caregivers, can result in substantial, persistent effects on neural, behavioral, and physiological systems as is evidenced in a long-standing literature and consistent findings across species, populations, and experimental models. In humans and other animals, differential rearing conditions can affect brain structure and function. We tested for whole brain patterns of morphological difference between 108 chimpanzees reared typically with their mothers (MR; N = 54) and those reared decades ago in a nursery with peers, human caregivers, and environmental enrichment (NR; N = 54). We applied support vector machine (SVM) learning to archival MRI images of chimpanzee brains to test whether we could, with any degree of significant probability, retrospectively classify subjects as MR and NR based on variation in gray matter within the entire brain. We could accurately discriminate MR and NR chimpanzee brains with nearly 70% accuracy. The combined brain regions discriminating the two rearing groups were widespread throughout the cortex. We believe this is the first report using machine language learning as an analytic method for discriminating nonhuman primate brains based on early rearing experiences. In this sense, the approach and findings are novel, and we hope they stimulate application of the technique to studies on neural outcomes associated with early experiences. The findings underscore the potential for infant separation from caregivers to leave a long-term mark on the developing brain.

Artificial Termite-Fishing Tasks as Enrichment for Sanctuary-Housed Chimpanzees: Behavioral Effects and Impact on Welfare

Simple Summary

The welfare of captive animals is nowadays a topic of major concern. In order to express their natural behavioral repertoires, however, animals require complex environments and stimuli which are difficult to reproduce in captivity. To overcome this, environmental enrichment is considered one of the most successful tools to increase behavioral opportunities and enhance animal welfare. In this study, we explored whether providing an artificial termite-fishing task, and whether participation in this task, predicted changes in the solitary and social behavior of sanctuary-housed chimpanzees (Pan troglodytes). We compared chimpanzee behavior when the enrichment was presented to different periods without enrichment. We found that the presence of the enrichment predicted an increase in tool use and feeding behavior and a decrease in inactivity, especially for those chimpanzees with higher participation. However, we did not detect significant changes in abnormal or self-directed behaviors. Furthermore, we found no variation in affiliation- or aggression-related behaviors, but social proximity increased in chimpanzees that participated more. Our results support previous studies demonstrating that artificial termite-fishing promotes species-typical behaviors in captive chimpanzees with no major effects on social activities.

Abstract

Artificial termite-fishing tasks are a common enrichment for captive great apes, promoting species-typical behaviors. Nonetheless, whether these activities are linked to changes in other behaviors and whether these changes persist over time has seldom been investigated. We assessed whether the use of an artificial termite-fishing task was linked to changes in the solitary behavior and social dynamics in two groups of sanctuary-housed chimpanzees (Pan troglodytes). Specifically, we compared chimpanzee behavior during eight enrichment sessions distributed over a two-month period, with similar periods before and after the introduction of the enrichment. Data were collected from combined interval and continuous sampling methods and were analyzed using generalized linear mixed models. We found that participation increased across sessions and that both enrichment and participation predicted an increase in tool use and feeding and a decrease in inactivity, which were all maintained throughout the sessions. Furthermore, participation was positively associated with social proximity, revealing a gathering effect of the task. However, neither enrichment nor participation were linked to changes in abnormal, self-directed, affiliation-related or aggression-related behaviors. Overall, our results support the hypothesis that artificial termite-fishing is a suitable enrichment for captive chimpanzees, maintaining the subjects’ interest and promoting species-typical behaviors, with no negative effects on social activities.

Past and present experiences with maternal touch affect infants' attention toward emotional faces

Maternal touch is considered crucial in regulating infants' internal states when facing unknown or distressing situations. Here, we explored the effects of maternal touch on 7-month-old infants' preferences towards emotions. Infants' looking times were measured through a two-trial preferential looking paradigm, while infants observed dynamic videos of happy and angry facial expressions. During the observation, half of the infants received an affective touch (i.e., stroke), while the other half received a non-affective stimulation (i.e., fingertip squeeze) from their mother. Further, we assessed the frequency of maternal touch in the mother-infant dyad through The Parent-Infant Caregiving Touch Scale (PICTS). Our results have shown that infants' attention to angry and happy facial expressions varied as a function of both present and past experiences with maternal touch. Specifically, in the affective touch condition, as the frequency of previous maternal affective tactile care increased (PICTS), the avoidance of angry faces decreased. Conversely, in the non-affective touch condition, as the frequency of previous maternal affective tactile care increased (PICTS), the avoidance of angry faces increased as well. Thus, past experience with maternal affective touch is a crucial predictor of the regulatory effects that actual maternal touch exerts on infants' visual exploration of emotional stimuli.

Capturing touch in parent-infant interaction: A comparison of methods

Naturally occurring high levels of caregiver touch promote offspring development in many animal species. Yet, caregiver touch remains a relatively understudied topic in human development, possibly due to challenges of measuring this means of interaction. While parental reports (e.g., questionnaires, diaries) are easy to collect, they may be subject to biases and memory limitations. In contrast, observing touch in a short session of parent-child interaction in the lab may not be representative of touch interaction in daily life. In the present study, we compared parent reports (one-off questionnaires and diary) and observation-based methods in a sample of German 6- to 13-month-olds and their primary caregivers (n = 71). In an attempt to characterize touching behaviors across a broad range of contexts, we measured touch both during play and while the parent was engaged in another activity. We found that context affected both the quantity and types of touch used in interaction. Parent-reported touch was moderately associated with touch observed in parent-child interactions and more strongly with touch used during play. We conclude that brief one-off questionnaires are a good indicator of touch in parent-child interaction, yet they may be biased toward representing particular daily activities and particular types of touch.

The Role of Affectionate Caregiver Touch in Early Neurodevelopment and Parent-Infant Interactional Synchrony

Though rarely included in studies of parent–infant interactions, affectionate touch plays a unique and vital role in infant development. Previous studies in human and rodent models have established that early and consistent affectionate touch from a caregiver confers wide-ranging and holistic benefits for infant psychosocial and neurophysiological development. We begin with an introduction to the neurophysiological pathways for the positive effects of touch. Then, we provide a brief review of how affectionate touch tunes the development of infant somatosensory, autonomic (stress regulation), and immune systems. Affective touch also plays a foundational role in the establishment of social affiliative bonds and early psychosocial behavior. These touch-related bonding effects are known to be mediated primarily by the oxytocin system, but touch also activates mesocorticolimbic dopamine and endogenous opioid systems which aid the development of social cognitive processes such as social learning and reward processing. We conclude by proposing a unique role for affectionate touch as an essential pathway to establishing and maintaining parent-infant interactional synchrony at behavioral and neural levels. The limitations of the current understanding of affectionate touch in infant development point to fruitful avenues for future research.

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.

Personality, cognition and behavior in chimpanzees: a new approach based on Eysenck's model

Personality has been linked to individual variation in interest and performance in cognitive tasks. Nevertheless, this relationship is still poorly understood and has rarely been considered in animal cognition research. Here, we investigated the association between personality and interest, motivation and task performance in 13 sanctuary chimpanzees (Pan troglodytes) housed at Fundació Mona (Spain). Personality was assessed with a 12-item questionnaire based on Eysenck's Psychoticism-Extraversion-Neuroticism model completed by familiar keepers and researchers. Additionally, personality ratings were compared to behavioral observations conducted over an 11-year period. Experimental tasks consisted in several puzzle boxes that needed to be manipulated in order to obtain a food reward. Dependent variables included participation (as an indicator of interest), success and latency (as measures of performance), and losing contact with the task (as an indicator of motivation). As predicted, we obtained significant correlations between Eysenck's personality traits and observed behaviors, although some expected associations were absent. We then analyzed data using Generalized Linear Mixed Models, running a model for each dependent variable. In both sexes, lower Extraversion and lower Dominance were linked to a higher probability of success, but this effect was stronger in females. Furthermore, higher Neuropsychoticism predicted higher probability of success in females, but not in males. The probability of losing contact with the task was higher in young chimpanzees, and in those rated lower on Extraversion and higher on Dominance. Additionally, chimpanzees rated higher on Neuropsychoticism were also more likely to stop interacting with the task, but again this was more evident in females. Participation and latency were not linked to any personality trait. Our findings show that the PEN may be a good model to describe chimpanzee personality, and stress the importance of considering personality when interpreting the results of cognitive research in non-human primates.

The developmental support hypothesis: adaptive plasticity in neural development in response to cues of social support

Across mammals, cues of developmental support, such as touching, licking or attentiveness, stimulate neural development, behavioural exploration and even overall body growth. Why should such fitness-related traits be so sensitive to developmental conditions? Here, we review what we term the 'developmental support hypothesis', a potential adaptive explanation of this plasticity. Neural development can be a costly process, in terms of time, energy and exposure. However, environmental variability may sometimes compromise parental care during this costly developmental period. We propose this environmental variation has led to the evolution of adaptive plasticity of neural and behavioural development in response to cues of developmental support, where neural development is stimulated in conditions that support associated costs. When parental care is compromised, offspring grow less and adopt a more resilient and stress-responsive strategy, improving their chances of survival in difficult conditions, similar to existing ideas on the adaptive value of early-life programming of stress. The developmental support hypothesis suggests new research directions, such as testing the adaptive value of reduced neural growth and metabolism in stressful conditions, and expanding the range of potential cues animals may attend to as indicators of developmental support. Considering evolutionary and ecologically appropriate cues of social support also has implications for promoting healthy neural development in humans. This article is part of the theme issue 'Life history and learning: how childhood, caregiving and old age shape cognition and culture in humans and other animals'.

Socially-relevant Visual Stimulation Modulates Physiological Response to Affective Touch in Human Infants

The human tactile system is known to discriminate different types of touches, one of these termed 'affective touch', is mainly mediated by slow conducting tactile afferents (CT fibres), which are preferentially activated by slow and gentle strokes. Human infants experience self-generated tactile stimulation during prenatal life, and they receive a large amount of affectionate touches by their caregivers from birth. This early and extended experience with tactile stimulation may likely make infants particularly sensitive to affective touch, and increasing evidence shows that this may indeed be the case. However, infants commonly experience affective touch in the context of social interactions with familiar adults (e.g., while looking at their caregiver), and recent evidence suggests that this helps them assigning affiliative and communicative meaning to the touch they are perceiving. Here we investigated the presence of visual-tactile interactions in 4-5-month-old infants' physiological (i.e., skin conductance) and behavioural (i.e., visual looking times) responses to visual and tactile stimulation of affective/social nature when the sources of both stimulation are not familiar to the infant. To explore whether the modulation of physiological arousal elicited by the socially-relevant bimodal stimulation is specific to infants or extends into adulthood, we also tested a group of adults. Infants (N = 25) and adults (N = 25) were stimulated on their forearm through slow stroking (i.e. affective touch) or tapping (i.e. non-affective touch) during the observation of dynamic images of socially-relevant (i.e., an unfamiliar face) and non-socially-relevant (i.e., a house) stimuli. We found that the simultaneous presentation of socially-relevant visual-tactile stimuli significantly decreased infants' - but not the adults' - electrodermal response, suggesting that infants easily integrate low-level properties of affective touch with socially salient visual information, and that social experience may tune and change sensitivity to affective touch across the life-span.

Revisiting the roots of attachment: A review of the biological and psychological effects of maternal skin-to-skin contact and carrying of full-term infants

During the early period of hypothesis building and empirical testing of attachment theory, a major emphasis was placed on mother-infant physical contact. In spite of this, mother-infant contact has received scant attention amongst attachment and child development researchers in the past decades. Here, a brief theoretical framework for mother-infant contact is presented, drawing on animal studies as well as human studies of preterm infants and neonates. Salient mechanisms may include an extended sensitive period during early infancy, requiring specific somatosensory stimuli for bio-behavioral homeorhesis; oxytocinergic and epigenetic pathways; kinesthetic stimuli and face-to-face proximity allowing for increased social interaction. Studies of extended human mother-full-term infant physical contact have demonstrated positive effects in multiple domains. For infants, these include sleep organization, temperature and heart rate regulation, behavioral response, crying/colic, socio-emotional development, attachment quality, speech development opportunities and mother-child interactions. For mothers, studies demonstrate improved depressive symptomatology, physiological stress regulation, contingent responsivity, breastfeeding and mother-child interactions. Parent-infant attachment quality has gained prominence as a trauma-resilience factor as well as a predictor of adult physical health. The potential role of mother-infant contact as an attachment promoting intervention as well as future research subjects are discussed. Current evidence supports the original attachment research that early maternal touch provision may influence infant socio-emotional development and attachment quality, with positive implications for mother-child relationship functioning.

Rearing condition may alter neonatal development of captive Bolivian squirrel monkeys (Saimiri boliviensis boliviensis)

Nursery rearing has well-known consequences for primate species. Relative to some other primate species, research has indicated a reduced impact of nursery rearing on squirrel monkeys, particularly in terms of rates, severity, and persistence of abnormal behavior. We administered the Primate Neonatal Neurobehavioral Assessment to 29 dam-reared and 13 nursery-reared squirrel monkeys (Saimiri boliviensis boliviensis) at 2 and 6 weeks of age. Mixed-model ANOVAs comparing composite scores and individual assessment items across age, rearing status, and sex revealed a number of developmental differences. Dam-reared infants scored higher on all four composite measures compared to nursery-reared infants (p < .05) indicating that nursery-reared animals had slower motor development, were less active and attentive, and were more passive than their dam-reared counterparts. Consistent with infant rhesus macaques, nursery-reared squirrel monkeys showed an increased sensitivity to tactile stimulation (p < .05). Altogether, these results suggest a disruption of species-typical development when squirrel monkey infants are reared in a nursery setting, with activity, orientation, and state control areas most affected, though experimental research is needed to determine if this is a causal relationship. Contrary to previous behavioral research, there are likely developmental differences between dam-reared infant squirrel monkeys and those reared in a nursery setting.

Social touch alters newborn monkey behavior

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

Impacts of early social experience on cognitive development in infant rhesus macaques

Although much is known about the influences of early life experiences on the neurobiology and behavior of macaque models of child development, there is scant literature on cognitive development with respect to early rearing. Here, we examined the effects of rearing condition on affective reactivity and cognitive development in infant rhesus macaques. Infants were pseudo-randomly assigned to one of the two rearing conditions: nursery reared (NR, N = 32; 16 peer-reared, 16 surrogate-peer-reared) or mother-peer-reared (MPR, N = 7). During the first month of life, infants were administered the Primate Neonatal Neurobehavioral Assessment (PNNA). Beginning at 4 months old, infants were tested on cognitive tasks that assessed reward association, cognitive flexibility, and impulsivity. We found no gross cognitive differences between MPR and NR infants. However, MPR infants were more reactive than NR infants on the PNNA. Additionally, reactivity on the PNNA correlated with impulsivity, such that infants who were more reactive at 1 month of age completed fewer trials correctly on this task at 8-10 months. These findings are the first to directly compare cognitive development in MPR and NR infants, and add to the existing literature elucidating the influences of early social experience on temperament and development.

Infants discriminate the source of social touch at stroking speeds eliciting maximal firing rates in CT-fibers

•

Infants’ cardiac response to touch varies depending on its social source.

•

This effect occurs only for velocities yielding maximal firing rates in CTs.

•

Infants’ responses to touch do not just depend upon its mechanical properties.

The evaluation of interpersonal touch is heavily influenced by its source. For example, a gentle stroke from a loved one is generally more pleasant than the same tactile stimulation from a complete stranger. Our study tested the early ontogenetic roots of humans’ sensitivity to the source of interpersonal touch. We measured the heart rate of three groups of nine-month-olds while their legs were stroked with a brush. The participants were stroked at a different speed in each group (0.3 cm/s, 3 cm/s, 30 cm/s). Depending on the Identity condition (stranger vs. parent), the person who acted as if she was stroking the infant’s leg was either an unfamiliar experimenter or the participant’s caregiver. In fact, the stimulation was always delivered by a second experimenter blind to the Identity condition. Infants’ heart rate decreased more in reaction to strokes when their caregiver rather than a stranger acted as the source of the touch. This effect was found only for tactile stimulations whose velocity (3 cm/s) is known to elicit maximal mean firing rates in a class of afferents named C-tactile fibers (CTs). Thus, the infants’ reaction to touch is modulated not just by its mechanical properties but also by its social source.

Social touch during development: Long-term effects on brain and behavior

In this paper, our goal is to explore what is known about the role of social touch during development. We first address the neural substrates of social touch and the role of tactile experience in neural development. We discuss natural variation in early exposure to social touch, followed by a discussion on experimental manipulations of social touch during development and "natural experiments", such as early institutionalization. We then consider the role of other developmental and experiential variables that predict social touch in adults. Throughout, we propose and consider new theoretical models of the role of social touch during development on later behavior and neurobiology.

Bridging the gap between rodents and humans: The role of non-human primates in oxytocin research

Oxytocin (OT), a neuropeptide that acts in the brain as a neuromodulator, has been long known to shape maternal physiology and behavior in mammals, however its role in regulating social cognition and behavior in primates has come to the forefront only in the recent decade. Many of the current perspectives on the role of OT in modulating social behavior emerged first from studies in rodents, where invasive techniques with a high degree of precision have permitted the mechanistic dissection of OT-related behaviors, as well as their underlying neural circuits in exquisite detail. In parallel, behavioral and imaging studies in humans have suggested that brain OT may similarly influence human social behavior and neural activity. These studies in rodents and humans have spurred interest in the therapeutic potential of targeting the OT system to remedy deficits in social cognition and behavior that are present across numerous psychiatric disorders. Yet there remains a tremendous gap in our mechanistic understanding of the influence of brain OT on social neural circuitry between rodents and man. In fact, very little is known regarding the neural mechanisms by which exogenous or endogenous OT influences human social cognition, limiting its therapeutic potential. Here we discuss how non-human primates (NHPs) are uniquely positioned to now bridge the gaps in knowledge provided by the precise circuit-level approaches widely used in rodent models and the behavioral, imaging, and clinical studies in humans. This review provides a perspective on what has been achieved, and what can be expected from exploring the role of OT in shaping social behaviors in NHPs in the coming years.