Mechanisms and functional implications of social buffering in infants: Lessons from animal models

Reconsidering the nature of threat in infancy: Integrating animal and human studies on neurobiological effects of infant stress

Early life stress has been associated with elevated risk for later psychopathology. One mechanism that may contribute to such long-term risk is alterations in amygdala development, a brain region critical to stress responsivity. Yet effects of stress on the amygdala during human infancy, a period of particularly rapid brain development, remain largely unstudied. In order to model how early stressors may affect infant amygdala development, several discrepancies across the existing literatures on early life stress among rodents and early threat versus deprivation among older human children and adults need to be reconciled. We briefly review the key findings of each of these literatures. We then consider them in light of emerging findings from studies of human infants regarding relations among maternal caregiving, infant cortisol response, and infant amygdala volume. Finally, we advance a developmental salience model of how early threat may impact the rapidly developing infant brain, a model with the potential to integrate across these divergent literatures. Future work to assess the value of this model is also proposed.

Social support of virtual characters reduces pain perception

BACKGROUND

Psychosocial factors, such as social support, can reduce pain. Virtual reality (VR) is a powerful tool to decrease pain, but social factors in VR-based pain analgesia have rarely been studied. Specifically, it is unclear whether social support by virtual characters can reduce pain and whether the perceived control behind virtual characters (agency) and varying degrees of social cues impact pain perception.

METHODS

Healthy participants (N = 97) received heat pain stimulation while undergoing four within-subject conditions in immersive VR: (1) virtual character with a low number of social cues (virtual figure) provided verbal support, (2) virtual character with a high number of social cues (virtual human) provided verbal support, (3) no social support (hearing neutral words), (4) no social support. Perceived agency of the virtual characters served as between-subjects factor. Participants in the avatar group were led to believe that another participant controlled the virtual characters. Participants in the agent group were told they interacted with a computer. However, in both conditions, virtual characters were computer-controlled. Pain ratings, psychophysiological measurements and presence ratings were recorded.

RESULTS

Virtual social support decreased pain intensity and pain unpleasantness ratings but had no impact on electrodermal activity nor heart rate. A virtual character with a high number of social cues led to lower pain unpleasantness and higher feelings of presence. Agency had no significant impact.

CONCLUSIONS

Virtual characters providing social support can reduce pain independent of perceived agency. A more human visual appearance can have beneficial effects on social pain modulation by virtual characters.

SIGNIFICANCE

Social influences are important factors in pain modulation. The current study demonstrated analgesic effects through verbal support provided by virtual characters and investigated modulating factors. A more human appearance of a virtual character resulted in a higher reduction of pain unpleasantness. Importantly, agency of the virtual characters had no impact. Given the increasing use of digital health interventions, the findings suggest a positive impact of virtual characters for digital pain treatments.

Characterizing the mechanisms of social connection

Understanding how individuals form and maintain strong social networks has emerged as a significant public health priority as a result of the increased focus on the epidemic of loneliness and the myriad protective benefits conferred by social connection. In this review, we highlight the psychological and neural mechanisms that enable us to connect with others, which in turn help buffer against the consequences of stress and isolation. Central to this process is the experience of rewards derived from positive social interactions, which encourage the sharing of perspectives and preferences that unite individuals. Sharing affective states with others helps us to align our understanding of the world with another's, thereby continuing to reinforce bonds and strengthen relationships. These psychological processes depend on neural systems supporting reward and social cognitive function. Lastly, we also consider limitations associated with pursuing healthy social connections and outline potential avenues of future research.

The stress-buffering role of friendships in young people with childhood threat experiences: a preliminary report

Background: High-quality friendships have a positive impact on the mental health of young people with childhood adversity (CA). Social stress buffering, the phenomenon of a social partner attenuating acute stress responses, is a potential yet unexplored mechanism that may underlie this relationship.Objective: This study examined whether perceived friendship quality was related to better mental health and lower neural stress response in young people with CA.Method: A total of N = 102 young people (aged 16-26) with low to moderate CA were included in the study. We first investigated associations between friendship quality, mental health, and CA. In a representative subset (n = 62), we assessed neural stress responses using the Montreal Imaging Stress Task. In our sample, CA was best described along two dimensions resembling threat or deprivation like experiences. Hence, we investigated both cumulative and dimensional effects of CA.Results: We found no support for social thinning after CA, meaning that the severity of CA (cumulative or dimensional) did not differentially impact friendship quality. High-quality friendships, on the other hand, were strongly associated with better mental health. Furthermore, acute stress increased state anxiety and enhanced neural activity in five frontolimbic brain regions, including the left hippocampus. We found weak support that threat experiences interacted with friendship quality to predict left hippocampal reactivity to stress. However, this effect did not survive multiple comparison correction.Conclusion: The absence of social thinning in our sample may suggest that the risk of developing impoverished social networks is low for rather well-functioning young people with low to moderate CA. Regardless, our findings align with prior research, consistently showing a strong association between high-quality friendships and better mental health in young people with CA. Future research is needed to examine whether friendships aid neural stress responses in young people with childhood threat experiences.

Infant fearful distress, parent intrusiveness, and adversity in families experiencing homelessness

Infants experiencing homelessness face risk for developing mental health problems in the future. Parents residing in shelters experience adverse events at elevated rates compared to non-homeless individuals, which can impact their infants during a time of rapid development depending on how the parent copes. For some, trauma linked to these events may manifest in an intrusive parenting style that interferes with the child's developing capacity for self-regulation. We utilized a sample of 60 parent-infant dyads recruited while residing in emergency homeless shelters to evaluate associations among parents' history of adversity, intrusive parenting behaviors, and infants' fearful distress. Observed parent intrusiveness during a free play interaction was significantly associated with infant fearfulness observed in a separate behavioral task during which infants viewed a series of masks. Furthermore, there was a moderating effect for parent history of adversity such that the association between parent intrusiveness and infant fearful distress was stronger among parents with more past experiences of adversity.

Dynamic developmental changes in neurotransmitters supporting infant attachment learning

Infant survival relies on rapid identification, remembering and behavioral responsiveness to caregivers' sensory cues. While neural circuits supporting infant attachment learning have largely remained elusive in children, use of invasive techniques has uncovered some of its features in rodents. During a 10-day sensitive period from birth, newborn rodents associate maternal odors with maternal pleasant or noxious thermo-tactile stimulation, which gives rise to a preference and approach behavior towards these odors, and blockade of avoidance learning. Here we review the neural circuitry supporting this neonatal odor learning, unique compared to adults, focusing specifically on the early roles of neurotransmitters such as glutamate, GABA (Gamma-AminoButyric Acid), serotonin, dopamine and norepinephrine, in the olfactory bulb, the anterior piriform cortex and amygdala. The review highlights the importance of deepening our knowledge of age-specific infant brain neurotransmitters and behavioral functioning that can be translated to improve the well-being of children during typical development and aid in treatment during atypical development in childhood clinical practice, and the care during rearing of domestic animals.

Age-related change in task-evoked amygdala-prefrontal circuitry: A multiverse approach with an accelerated longitudinal cohort aged 4-22 years

The amygdala and its connections with medial prefrontal cortex (mPFC) play central roles in the development of emotional processes. While several studies have suggested that this circuitry exhibits functional changes across the first two decades of life, findings have been mixed - perhaps resulting from differences in analytic choices across studies. Here we used multiverse analyses to examine the robustness of task-based amygdala-mPFC function findings to analytic choices within the context of an accelerated longitudinal design (4-22 years-old; N = 98; 183 scans; 1-3 scans/participant). Participants recruited from the greater Los Angeles area completed an event-related emotional face (fear, neutral) task. Parallel analyses varying in preprocessing and modeling choices found that age-related change estimates for amygdala reactivity were more robust than task-evoked amygdala-mPFC functional connectivity to varied analytical choices. Specification curves indicated evidence for age-related decreases in amygdala reactivity to faces, though within-participant changes in amygdala reactivity could not be differentiated from between-participant differences. In contrast, amygdala-mPFC functional connectivity results varied across methods much more, and evidence for age-related change in amygdala-mPFC connectivity was not consistent. Generalized psychophysiological interaction (gPPI) measurements of connectivity were especially sensitive to whether a deconvolution step was applied. Our findings demonstrate the importance of assessing the robustness of findings to analysis choices, although the age-related changes in our current work cannot be overinterpreted given low test-retest reliability. Together, these findings highlight both the challenges in estimating developmental change in longitudinal cohorts and the value of multiverse approaches in developmental neuroimaging for assessing robustness of results.

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.

Systematic Input-Output Mapping Reveals Structural Plasticity of VTA Dopamine Neurons-Zona Incerta Loop Underlying the Social Buffering Effects in Learned Helplessness

A common phenomenon called social buffering (SB), communication within conspecific animals is a benefit for a stressed individual to better recover from aversive events, is crucial to all mammals. Although the dopamine reward system has been implicated in SB, it is not clear which neuronal populations are relevant and how they contribute. Here, we adopted a learned helplessness (LH) animal model of depression and found that LH subjects housed with a conspecific partner show better performance in the shuttle box test, showing that SB improves the stress-coping abilities to deal with stress. Bidirectional manipulation of ventral tegmental area (VTA) dopamine neurons by chemogenetic tools can mimic or block the SB effect in LH mice. To screen for SB-induced structure plasticity of VTA dopamine neurons, we employed viral genetic tools for mapping input and output architecture and found LH- and SB-triggered circuit-level changes in neuronal ensembles. Zona incerta (ZI), an overlapping brain region, was significantly changed in both anterograde and retrograde tracing during LH and SB. These results reveal a neural loop with structural plasticity between VTA dopamine neurons and ZI underlies the SB effects in LH and lays a foundation for studying how VTA dopamine neurons regulate SB-related neural circuits.

Infant Attachment and Social Modification of Stress Neurobiology

Decades of research have informed our understanding of how stress impacts the brain to perturb behavior. However, stress during development has received specific attention as this occurs during a sensitive period for scaffolding lifelong socio-emotional behavior. In this review, we focus the developmental neurobiology of stress-related pathology during infancy and focus on one of the many important variables that can switch outcomes from adaptive to maladaptive outcome: caregiver presence during infants' exposure to chronic stress. While this review relies heavily on rodent neuroscience research, we frequently connect this work with the human behavioral and brain literature to facilitate translation. Bowlby's Attachment Theory is used as a guiding framework in order to understand how early care quality impacts caregiver regulation of the infant to produce lasting outcomes on mental health.

Elevated infant cortisol is necessary but not sufficient for transmission of environmental risk to infant social development: Cross-species evidence of mother-infant physiological social transmission

Environmental adversity increases child susceptibility to disrupted developmental outcomes, but the mechanisms by which adversity can shape development remain unclear. A translational cross-species approach was used to examine stress-mediated pathways by which poverty-related adversity can influence infant social development. Findings from a longitudinal sample of low-income mother-infant dyads indicated that infant cortisol (CORT) on its own did not mediate relations between early-life scarcity-adversity exposure and later infant behavior in a mother-child interaction task. However, maternal CORT through infant CORT served as a mediating pathway, even when controlling for parenting behavior. Findings using a rodent "scarcity-adversity" model indicated that pharmacologically blocking pup corticosterone (CORT, rodent equivalent to cortisol) in the presence of a stressed mother causally prevented social transmission of scarcity-adversity effects on pup social behavior. Furthermore, pharmacologically increasing pup CORT without the mother present was not sufficient to disrupt pup social behavior. Integration of our cross-species results suggests that elevated infant CORT may be necessary, but without elevated caregiver CORT, may not be sufficient in mediating the effects of environmental adversity on development. These findings underscore the importance of considering infant stress physiology in relation to the broader social context, including caregiver stress physiology, in research and interventional efforts.

Why Psychotropic Drugs Don't Cure Mental Illness-But Should They?

While pharmacological treatments for psychiatric disorders have offered great promise and have provided clinically meaningful symptom relief these treatments have had less effect on altering the course of these disorders. Research has provided many new insights about the effects of different psychotropic agents on the functions of various brain systems as investigators have embraced the "translational research model." However, this theoretical approach of deconstructing complex behaviors into smaller measurable behavioral units and targeting brain systems that are hypothesized to underlie these discrete behaviors has offered little of practical clinical relevance to significantly improve the treatment of psychiatric disorders in this century. Radical new treatments have not emerged, and available treatments continue to provide symptom relief without resolution of the underlying conditions. Recent publications on the subject have attempted to identify the barriers to progress and have pointed out some of the limitations of the translational approach. It is our position that, given the present limitations of our therapeutic arsenal, both researchers and clinicians would be well-advised to pay closer attention to human specific factors such as the role of language, the creation of personal narratives, and how factors such as these interface with underlying biological diatheses in mental illness. These interactions between pathophysiology and intrapersonal processes may be critical to both the in vivo expression of the underlying biological mechanisms of psychiatric disease states, and to the development of enhancements in therapeutic efficacy. Lastly, we discuss the implications of more coherently integrating neuroscientific research and clinical practice for more effectively addressing the challenges of understanding and treating mental illness.

Is there evidence for sensitive periods in emotional development?

During sensitive periods an individual's development is especially receptive to information from the environment in ways that it is not at earlier and later developmental stages. Here, we describe challenges in applying the concept of sensitive periods to the domain of socio-emotional development, review what applications of this approach have accomplished, and point to promising future directions. We also argue that since emotional development consists of higher-order cognitive processes, it likely involves multiple and overlapping sensitive periods tied to different mechanisms (e.g., facial recognition, reward processing, fear conditioning). Moreover, we note a distinction between the construct of a sensitive period versus the identification of an effect of early experience-two ideas that are often confused in the literature. Progress in the study of emotion will require understanding the mechanisms involved in developmental change and models that predict children's behavior based on their prior experience.

Neural signatures of child cognitive emotion regulation are bolstered by parental social regulation in two cultures

Caregiver impact on the efficacy of cognitive emotion regulation (ER; i.e. reappraisal) during childhood is poorly understood, particularly across cultures. We tested the hypothesis that in children from Japan and the USA, a neurocognitive signature of effective reappraisal, the late positive potential (LPP), will be bolstered by cognitive scaffolding by parents, and explored whether the two cultures differed in whether mere physical proximity of parents provides similar benefit. Five-to-seven-year-olds (N = 116; n_Japan = 58; n_USA = 58) completed a directed reappraisal task (EEG-recorded) in one of three contexts: (i) parent-scaffolding, (ii) parent-present and (iii) parent-absent. Across cultures, those in the parent-scaffolding group and parent-present group showed effective reappraisal via the LPP relative to those in the parent-absent group. Results suggest that scaffolding is an effective method through which parents in these two cultures buttress child ER, and even parental passive proximity appears to have a meaningful effect on child ER across cultures.

Gut Feelings Begin in Childhood: the Gut Metagenome Correlates with Early Environment, Caregiving, and Behavior

Psychosocial environments impact normative behavioral development in children, increasing the risk of problem behaviors and psychiatric disorders across the life span. Converging evidence demonstrates that early normative development is affected by the gut microbiome, which itself can be altered by early psychosocial environments. However, much of our understanding of the gut microbiome's role in early development stems from nonhuman animal models and predominately focuses on the first years of life, during peri- and postnatal microbial colonization. As a first step to identify if these findings translate to humans and the extent to which these relationships are maintained after initial microbial colonization, we conducted a metagenomic investigation among a cross-sectional sample of early school-aged children with a range of adverse experiences and caregiver stressors and relationships. Our results indicate that the taxonomic and functional composition of the gut microbiome correlates with behavior during a critical period of child development. Furthermore, our analysis reveals that both socioeconomic risk exposure and child behaviors associate with the relative abundances of specific taxa (e.g., Bacteroides and Bifidobacterium species) as well as functional modules encoded in their genomes (e.g., monoamine metabolism) that have been linked to cognition and health. While we cannot infer causality within this study, these findings suggest that caregivers may moderate the gut microbiome's link to environment and behaviors beyond the first few years of life.IMPORTANCE Childhood is a formative period of behavioral and biological development that can be modified, for better or worse, by the psychosocial environment that is in part determined by caregivers. Not only do our own genes and the external environment influence such developmental trajectories, but the community of microbes living in, on, and around our bodies-the microbiome-plays an important role as well. By surveying the gut microbiomes of a cross-sectional cohort of early school-aged children with a range of psychosocial environments and subclinical mental health symptoms, we demonstrated that caregiving behaviors modified the child gut microbiome's association to socioeconomic risk and behavioral dysregulation.

During infant maltreatment, stress targets hippocampus, but stress with mother present targets amygdala and social behavior

Infant maltreatment increases vulnerability to physical and mental disorders, yet specific mechanisms embedded within this complex infant experience that induce this vulnerability remain elusive. To define critical features of maltreatment-induced vulnerability, rat pups were reared from postnatal day 8 (PN8) with a maltreating mother, which produced amygdala and hippocampal deficits and decreased social behavior at PN13. Next, we deconstructed the maltreatment experience to reveal sufficient and necessary conditions to induce this phenotype. Social behavior and amygdala deficits (volume, neurogenesis, c-Fos, local field potential) required combined chronic high corticosterone and maternal presence (not maternal behavior). Hippocampal deficits were induced by chronic high corticosterone regardless of social context. Causation was shown by blocking corticosterone during maltreatment and suppressing amygdala activity during social behavior testing. These results highlight (1) that early life maltreatment initiates multiple pathways to pathology, each with distinct causal mechanisms and outcomes, and (2) the importance of social presence on brain development.

Perception of Ultrasonic Vocalizations by Socially Housed and Isolated Mice

It is currently unclear whether mice use their ultrasonic vocalizations (USVs) for communication purposes. It is also unknown whether mice require previous experience with USVs to understand conspecifics. There is some evidence that experience changes the perception of juvenile USVs; however, it is unclear whether similar plasticity also occurs for adult USVs. To examine whether social exposure or deprivation throughout development leads to changes in USV perception, eleven female CBA/CaJ mice were trained to discriminate between 18 USVs of three different categories using operant conditioning procedures. Mice were group housed with four females or housed individually from weaning for the duration of the experiment. Socially housed and isolated mice differed in initial training times on pure tones, suggesting isolated mice had a more difficult time learning the task. Both groups completed USV discrimination conditions quicker at the end of the testing phases relative to the beginning. The overall discrimination of USVs did not differ between the two housing conditions, but a multidimensional scaling analysis revealed that socially experienced and isolated mice perceive some USVs differently, illustrated by differences in locations of USVs on the scaling maps from the two groups. Finally, a negative correlation was found between spectrotemporal similarity and percent discrimination, and analyses support the idea that mice may show categorical perception of at least two of the three USV categories. Thus, experience with USVs changes USV perception.

Infant Trauma Alters Social Buffering of Threat Learning: Emerging Role of Prefrontal Cortex in Preadolescence

Within the infant-caregiver attachment system, the primary caregiver holds potent reward value to the infant, exhibited by infants' strong preference for approach responses and proximity-seeking towards the mother. A less well-understood feature of the attachment figure is the caregiver's ability to reduce fear via social buffering, commonly associated with the notion of a "safe haven" in the developmental literature. Evidence suggests this infant system overlaps with the neural network supporting social buffering (attenuation) of fear in the adults of many species, a network known to involve the prefrontal cortex (PFC). Here, using odor-shock conditioning in young developing rats, we assessed when the infant system transitions to the adult-like PFC-dependent social buffering of threat system. Rat pups were odor-shock conditioned (0.55 mA-0.6 mA) at either postnatal day (PN18; dependent on mother) or 28 (newly independent, weaned at PN23). Within each age group, the mother was present or absent during conditioning, with PFC assessment following acquisition using 14C 2-DG autoradiography and cue testing the following day. Since the human literature suggests poor attachment attenuates the mother's ability to socially buffer the infants, half of the pups at each age were reared with an abusive mother from PN8-12. The results showed that for typical control rearing, the mother attenuated fear in both PN18 and PN28 pups, although the PFC [infralimbic (IL) and ventral prelimbic (vPL) cortices] was only engaged at PN28. Abuse rearing completely disrupted social buffering of pups by the mother at PN18. The results from PN28 pups showed that while the mother modulated learning in both control and abuse-reared pups, the behavioral and PFC effects were attenuated after maltreatment. Our data suggest that pups transition to the adult-like PFC social support circuit after independence from the mother (PN28), and this circuit remains functional after early-life trauma, although its effectiveness appears reduced. This is in sharp contrast to the effects of early life trauma during infancy, where social buffering of the infant is more robustly impacted. We suggest that the infant social buffering circuit is disengaged by early-life trauma, while the adolescent PFC-dependent social buffering circuit may use a safety signal with unreliable safety value.

Developing a neurobehavioral animal model of poverty: Drawing cross-species connections between environments of scarcity-adversity, parenting quality, and infant outcome

Children reared in impoverished environments are at risk for enduring psychological and physical health problems. Mechanisms by which poverty affects development, however, remain unclear. To explore one potential mechanism of poverty's impact on social-emotional and cognitive development, an experimental examination of a rodent model of scarcity-adversity was conducted and compared to results from a longitudinal study of human infants and families followed from birth (N = 1,292) who faced high levels of poverty-related scarcity-adversity. Cross-species results supported the hypothesis that altered caregiving is one pathway by which poverty adversely impacts development. Rodent mothers assigned to the scarcity-adversity condition exhibited decreased sensitive parenting and increased negative parenting relative to mothers assigned to the control condition. Furthermore, scarcity-adversity reared pups exhibited decreased developmental competence as indicated by disrupted nipple attachment, distress vocalization when in physical contact with an anesthetized mother, and reduced preference for maternal odor with corresponding changes in brain activation. Human results indicated that scarcity-adversity was inversely correlated with sensitive parenting and positively correlated with negative parenting, and that parenting fully mediated the association of poverty-related risk with infant indicators of developmental competence. Findings are discussed from the perspective of the usefulness of bidirectional-translational research to inform interventions for at-risk families.

Socioeconomic risk moderates the association between caregiver cortisol levels and infant cortisol reactivity to emotion induction at 24 months

Relations between maternal baseline cortisol and infant cortisol reactivity to an emotion induction procedure at child ages 7, 15, and 24 months were analyzed using data from the Family Life Project (N = 1,292). The emotion induction consisted of a series of standardized and validated tasks, including an arm restraint, toy removal, and mask presentation, intended to elicit responses of fear and frustration. Results revealed that at 7 and 15 months, maternal baseline cortisol was negatively related to child cortisol reactivity, such that children of mothers with lower cortisol exhibited steeper cortisol increases in response to the emotion induction. At 24 months, the association between mother and infant cortisol was moderated by socioeconomic risk, such that maternal baseline cortisol was associated with child cortisol reactivity only in dyads characterized by low socioeconomic risk. Furthermore, at 24 months, children of mothers with low baseline cortisol and low socioeconomic risk exhibited decreasing cortisol responses, whereas children of mothers with low baseline cortisol but high risk exhibited flat cortisol responses. Children in dyads characterized by high baseline maternal cortisol also exhibited flat cortisol responses regardless of socioeconomic risk. The role of caregiver physiology in the regulation of the child's stress response in the context of adversity is discussed.

Comparative studies of social buffering: A consideration of approaches, terminology, and pitfalls

KIYOKAWA, Y. and HENNESSY, M.B. Comparative studies of social buffering: A consideration of approaches, terminology, and pitfalls…NEUROSCI BIOBEHAV REV XXX-XXX, .- Over the past decades, there has been an increasing number of investigations of the impact of social variables on neural, endocrine, and immune outcomes. Among these are studies of "social buffering"-or the phenomenon by which affiliative social partners mitigate the response to stressors. Yet, as social buffering studies have become more commonplace, the variety of approaches taken, definitions employed, and divergent results obtained in different species can lead to confusion and miscommunication. The aim of the present paper, therefore, is to address terminology and approaches and to highlight potential pitfalls to the study of social buffering across nonhuman species. We review and categorize variables currently being employed in social buffering studies and provide an overview of responses measured, mediating sensory modalities and underlying mechanisms. It is our hope that the paper will be useful to those contemplating examination of social buffering in the context of their own research.

The neurobiology of safety and threat learning in infancy

What an animal needs to learn to survive is altered dramatically as they change from dependence on the parent for protection to independence and reliance on self-defense. This transition occurs in most altricial animals, but our understanding of the behavioral neurobiology has mostly relied on the infant rat. The transformation from dependence to independence occurs over three weeks in pups and is accompanied by complex changes in responses to both natural and learned threats and the supporting neural circuitry. Overall, in early life, the threat system is quiescent and learning is biased towards acquiring attachment related behaviors to support attachment to the caregiver and proximity seeking. Caregiver-associated cues learned in infancy have the ability to provide a sense of safety throughout lifetime. This attachment/safety system is activated by learning involving presumably pleasurable stimuli (food, warmth) but also painful stimuli (tailpinch, moderate shock). At about the midway point to independence, pups begin to have access to the adult-like amygdala-dependent threat system and amygdala-dependent responses to natural dangers such as predator odors. However, pups have the ability to switch between the infant and adult-like system, which is controlled by maternal presence and modification of stress hormones. Specifically, if the pup is alone, it will learn fear but if with the mother it will learn attachment (10-15days of age). As pups begin to approach weaning, pups lose access to the attachment system and rely only on the amygdala-dependent threat system. However, pups learning system is complex and exhibits flexibility that enables the mother to override the control of the attachment circuit, since newborn pups may acquire threat responses from the mother expressing fear in their presence. Together, these data suggest that the development of pups' threat learning system is not only dependent upon maturation of the amygdala, but it is also exquisitely controlled by the environment. Most notably the mother can switch pup learning between attachment to threat learning in a moment's notice. This enables the mother to navigate pup's learning about the world and what is threatening and what is safe.

Development of Odor Hedonics: Experience-Dependent Ontogeny of Circuits Supporting Maternal and Predator Odor Responses in Rats

A major component of perception is hedonic valence: perceiving stimuli as pleasant or unpleasant. Here, we used early olfactory experiences that shape odor preferences and aversions to explore developmental plasticity in circuits mediating odor hedonics. We used 2-deoxyglucose autoradiographic mapping of neural activity to identify circuits differentially activated by biologically relevant preferred and avoided odors across rat development. We then further probed this system by increasing or decreasing hedonic value. Using both region of interest and functional connectivity analyses, we identified regions within primary olfactory, amygdala/hippocampal, and prefrontal cortical networks that were activated differentially by maternal and male odors. Although some activated regions remained stable across development (postnatal days 7-23), there was a developmental emergence of others that resulted in an age-dependent elaboration of hedonic-response-specific circuitry despite stable behavioral responses (approach/avoidance) to the odors across age. Hedonic responses to these biologically important odors were modified through diet suppression of the maternal odor and co-rearing with a male. This allowed assessment of hedonic circuits in isolation of the specific odor quality and/or intensity. Early experience significantly modified odor-evoked circuitry in an age-dependent manner. For example, co-rearing with a male, which induced pup attraction to male odor, reduced activity in amygdala regions normally activated by the unfamiliar avoided male odor, making this region more consistent with maternal odor. Understanding the development of odor hedonics, particularly within the context of altered early life experience, provides insight into the development of sensory processes, food preferences, and the formation of social affiliations, among other behaviors.

SIGNIFICANCE STATEMENT

Odor hedonic valence controls approach-avoidance behaviors, but also modulates ongoing behaviors ranging from food preferences and social affiliation with the caregiver to avoidance of predator odors. Experiences can shape hedonic valence. This study explored brain circuitry involved in odor hedonic encoding throughout development using maternal and predator odors and assessed the effects of early life experience on odor hedonic encoding by increasing/decreasing the hedonic value of these odors. Understanding the role of changing brain circuitry during development and its impact on behavioral function is critical for understanding sensory processing across development. These data converge with exciting literature on the brain's hedonic network and highlight the significant role of early life experience in shaping the neural networks of highly biologically relevant stimuli.

The developing amygdala: a student of the world and a teacher of the cortex

Amygdala and prefrontal cortex (PFC) function subserving emotional behavior has largely been examined from the perspective of their adult roles, with a tremendous focus on the regulatory influence of the PFC over amygdala activity. Here we consider the circuit's function in its developmental context, when maximal learning about emotion and incentives from the environment is necessary. We argue that during development the amygdala exhibits an overwhelming influence over the developmental destiny of circuitry function, and the amygdala's learning and experiential history are conveyed to the cortex to modulate subsequent PFC development. We present recent findings on the different developmental trajectories of the amygdala and PFC, their functional connectivity, and the timing of environmental influences as evidence supporting our position.

Freezing suppression by oxytocin in central amygdala allows alternate defensive behaviours and mother-pup interactions

When animals and their offspring are threatened, parents switch from self-defense to offspring protection. How self-defense is suppressed remains elusive. We postulated that suppression of the self-defense response, freezing, is gated via oxytocin acting in the centro-lateral amygdala (CeL). We found that rat dams conditioned to fear an odor, froze when tested alone, whereas if pups were present, they remained in close contact with them or targeted the threat. Furthermore, blocking oxytocin signaling in the CeL prevented the suppression of maternal freezing. Finally, pups exposed to the odor in the presence of the conditioned dam later froze when re-exposed alone. However, if oxytocin signaling in the dam had been blocked, pups failed to learn. This study provides a functional role for the well-described action of oxytocin in the central amygdala, and demonstrates that self-defense suppression allows for active pup protection and mother-pup interactions crucial for pup threat learning.

DOI:http://dx.doi.org/10.7554/eLife.24080.001

Animals have many mechanisms to avoid or defend themselves against deadly encounters with predators. However, adult animals frequently put themselves at risk while protecting their more vulnerable offspring from attacks. For example, a killdeerbird with young will fake a broken wing and lead a predator away from its nest. This helps ensure that the parent’s genes live on and contribute to the survival of their species. To do this, the parent must override his or her own defense mechanisms and protect the young instead of themselves.

Little is known about the exact mechanisms that allow animals to suppress their own defense mechanisms while protecting their young. Freezing is one tactic that animals will use when they are unable to escape a predator. Previously, studies have shown that the hormone oxytocin, which is produced in the brain, suppresses freezing behavior. Oxytocin plays an important role in birth and breastfeeding, but it is also known to strengthen the bond between individuals, in particular between mother and child. Until now, it was not known whether this hormone also blocks self-defense behaviors in animals protecting their offspring.

Now, Rickenbacher et al. show that oxytocin does indeed block freezing behavior, enabling mother rats to protect their offspring in the face of a threatening smell. In the experiments, mother rats were taught to fear the scent of peppermint. Without their young, these rats would freeze whenever they smelled peppermint. Yet, when mother rats with their pups were exposed to the scent, they did not freeze. Instead, they tried to defend their young. Blocking oxytocin in a part of the mothers’ brains called the amygdala, however, caused them to freeze in response to the scent of peppermint, even in the presence of their pups.

The experiments show that oxytocin helps mother rats suppress their self-defense mechanisms and is necessary for the mothers to protect their young. Rickenbacher et al. also showed that pups of oxytocin-treated mothers did not learn to freeze in response to the threat. But pups of untreated mothers who defended them, learned to freeze when they were exposed to the scent of peppermint. A next step will be to record neurons that produce oxytocin to better understand how the presence of the pups stimulate its production in their mothers. In addition, it is still unclear how pups learn from their mothers to freeze in response to a threat. One possibility is that the mother produces a molecule that signals danger. Identifying this molecule would be the next step.

DOI:http://dx.doi.org/10.7554/eLife.24080.002

From attachment to independence: Stress hormone control of ecologically relevant emergence of infants' responses to threat

Young infant rat pups learn to approach cues associated with pain rather than learning amygdala-dependent fear. This approach response is considered caregiver-seeking and ecologically relevant within the context of attachment. With maturation, increases in the stress hormone corticosterone permit amygdala-dependent fear, which is crucial for survival during independent living. During the developmental transition from attachment to fear learning, maternal presence suppresses corticosterone elevation to block amygdala-dependent fear learning and re-engage the attachment circuitry. Early life trauma disrupts this developmental sequence by triggering a precocious increase of corticosterone, which permits amygdala-dependent threat responses. In this review, we explore the importance of the stress hormone corticosterone in infants' transition from complete dependence on the caregiver to independence, with consideration for environmental influences on threat response ontogeny and mechanistic importance of social buffering of the stress response.

Sensitive Periods of Emotion Regulation: Influences of Parental Care on Frontoamygdala Circuitry and Plasticity

Early caregiving experiences play a central role in shaping emotional development, stress physiology, and refinement of limbic circuitry. Converging evidence across species delineates a sensitive period of heightened neuroplasticity when frontoamygdala circuitry is especially amenable to caregiver inputs early in life. During this period, parental buffering regulates emotional behaviors and stress physiology as emotion regulation circuitry continues to mature. By contrast, disorganized or poor quality caregiving has profound and lasting consequences on the maturation of frontoamygdala circuitry essential for emotion regulation, even following termination of this early life stressor (e.g., adoption from orphanage). This article highlights how interactions between caregiving experiences and the biological state of the developing brain have broad implications for long-term health.

Early life adversity during the infant sensitive period for attachment: Programming of behavioral neurobiology of threat processing and social behavior

Animals, including humans, require a highly coordinated and flexible system of social behavior and threat evaluation. However, trauma can disrupt this system, with the amygdala implicated as a mediator of these impairments in behavior. Recent evidence has further highlighted the context of infant trauma as a critical variable in determining its immediate and enduring consequences, with trauma experienced from an attachment figure, such as occurs in cases of caregiver-child maltreatment, as particularly detrimental. This review focuses on the unique role of caregiver presence during early-life trauma in programming deficits in social behavior and threat processing. Using data primarily from rodent models, we describe the interaction between trauma and attachment during a sensitive period in early life, which highlights the role of the caregiver's presence in engagement of attachment brain circuitry and suppressing threat processing by the amygdala. These data suggest that trauma experienced directly from an abusive caregiver and trauma experienced in the presence of caregiver cues produce similar neurobehavioral deficits, which are unique from those resulting from trauma alone. We go on to integrate this information into social experience throughout the lifespan, including consequences for complex scenarios, such as dominance hierarchy formation and maintenance.

Neurobehavioral assessment of maternal odor in developing rat pups: implications for social buffering

Social support can attenuate the behavioral and stress hormone response to threat, a phenomenon called social buffering. The mother's social buffering of the infant is one of the more robust examples; yet we understand little about the neurobiology. Using a rodent model, we explore the neurobiology of social buffering by assessing neural processing of the maternal odor, a major cue controlling social buffering in rat pups. We used pups before (postnatal day (PN) 7) and after (PN14, PN23) the functional emergence of social buffering. Pups were injected with 14C 2-deoxyglucose (2-DG) and presented with the maternal odor, a control preferred odor incapable of social buffering (acetophenone), or no odor. Brains were removed, processed for autoradiography and brain areas identified as important in adult social buffering were assessed, including the amygdala basolateral complex (Basolateral Amygdala [BLA]), medial prefrontal cortex (mPFC), and anterior cingulate cortex (ACC). Results suggest dramatic changes in the processing of maternal odor. PN7 pups show mPFC and ACC activation, although PN14 pups showed no activation of the mPFC, ACC, or BLA. All brain areas assessed were recruited by PN23. Additional analysis suggests substantial changes in functional connectivity across development. Together, these results imply complex nonlinear transitions in the neurobiology of social buffering in early life that may provide insight into the changing role of the mother in supporting social buffering.

Attachment Figure's Regulation of Infant Brain and Behavior

Altricial infants (i.e., requiring parental care for survival), such as humans and rats, form an attachment to their caregiver and receive the nurturing and protections needed for survival. Learning has a strong role in attachment, as is illustrated by strong attachment formed to non-biological caregivers of either sex. Here we summarize and integrate results from animal and human infant attachment research that highlights the important role of social buffering (social presence) of the stress response by the attachment figure and its effect on infant processing of threat and fear through modulation of the amygdala. Indeed, this work suggests the caregiver switches off amygdala function in rodents, although recent human research suggests a similar process in humans and nonhuman primates. This cross-species analysis helps provide insight and unique understanding of attachment and its role in the neurobiology of infant behavior within attachment.

Maternal buffering of fear-potentiated startle in children and adolescents with trauma exposure

Parental availability influences fear expression and learning across species, but the effect of maternal buffering on fear learning in humans is unknown. Here we investigated the effect of maternal availability during fear conditioning in a group of children (ages 8-10) and adolescents (ages 11-13) from a low-income population with a range of trauma exposure. Acoustic startle response data were collected to measure fear-potentiated startle (FPS) in 104 participants. A total of 62 participants were tested with the mother available and 42 when the mother was not in the testing room. We observed that maternal availability during fear conditioning interacted with age to affect FPS discrimination between CS+ and CS-. In line with previous findings suggesting an absence of maternal buffering in adolescents, fear discrimination was affected by maternal availability only in children. Second, we observed that the effect of maternal buffering on FPS discrimination in children was not influenced by maternally reported warmth. In conclusion, we demonstrated that maternal availability improved discrimination in children, regardless of the quality of the relationship. Adolescents discriminated irrespective of maternal status, suggesting that childhood may be a sensitive period for environmental influences on key processes such as learning of danger and safety signals.

Sexual trauma and the female brain

Sexual aggression and violence against women (VAM) are not only social problems; they are mental health problems. Women who experience sexual trauma often express disruptions in emotional and cognitive processes, some of which lead to depression and post-traumatic stress disorder (PTSD). Animal models of neurogenesis and learning suggest that social yet aggressive interactions between a pubescent female and an adult male can disrupt processes of learning related to maternal care, which in turn reduce survival of new neurons in the female hippocampus. Mental and Physical (MAP) Training is a novel clinical intervention that was translated from neurogenesis research. The intervention, which combines meditation and aerobic exercise, is currently being used to help women learn to recover from traumatic life experiences, especially those related to sexual violence and abuse.