The representation of social facial touch in rat barrel cortex

A reduced ability to discriminate social from non-social touch at the circuit level may underlie social avoidance in autism

Social touch is critical for communication to impart emotions and intentions. However, certain autistic individuals experience aversion to social touch. Here, we used Neuropixels probes to record neural responses to social vs. non-social interactions in somatosensory cortex, tail of striatum, and basolateral amygdala. We find that wild type mice show aversion to repeated presentations of an inanimate object but not of another mouse. Cortical neurons are modulated especially by touch context (social vs. object), while striatal neurons change their preference depending on whether mice could choose or not to interact. In contrast, Fmr1 knockout (KO) mice, a model of autism, find social and non-social interactions equally aversive, especially at close proximity, and their cortical/striatal neurons are less able to discriminate social valence. A linear model shows that the encoding of certain avoidance/aversive behaviors in cortical neuron activity differed between genotypes. Thus, a reduced capacity to represent social stimuli at the circuit level may underlie social avoidance in autism.

Mapping the landscape of social behavior

Social interaction is integral to animal behavior. However, lacking tools to describe it in quantitative and rigorous ways has limited our understanding of its structure, underlying principles, and the neuropsychiatric disorders, like autism, that perturb it. Here, we present a technique for high-resolution 3D tracking of postural dynamics and social touch in freely interacting animals, solving the challenging subject occlusion and part-assignment problems using 3D geometric reasoning, graph neural networks, and semi-supervised learning. We collected over 110 million 3D pose samples in interacting rats and mice, including seven monogenic autism rat lines. Using a multi-scale embedding approach, we identified a rich landscape of stereotyped actions, interactions, synchrony, and body contacts. This high-resolution phenotyping revealed a spectrum of changes in autism models and in response to amphetamine not resolved by conventional measurements. Our framework and large library of interactions will facilitate studies of social behaviors and their neurobiological underpinnings.

Neural control of sex differences in affiliative and prosocial behaviors

Social interactions are vital for various taxa and species. Prosocial and affiliative dynamics within a group and between individuals are not only pleasurable and rewarding, but also appear to actively contribute to well-being, cognitive performance, and disease prevention. Moreover, disturbances in acting or being prosocial can represent a major burden for an individual and their affective partners. These disruptions are evident across a spectrum of neuropsychiatric conditions, including depression and autism spectrum disorders. Importantly, interactive patterns of prosocial and affiliative behavior can vary with sex. The fact that genders are differentially affected by neuropsychiatric disorders associated with social impairment underscores the high importance of this research in uncovering the underlying neural correlates and mechanisms. This review focuses on elucidating sex-related differences in prosocial and affiliative behaviors and their potential association with sexually different neural correlates. Specifically, we aim to shed light on the complex interplay between sex, behavior, and neurobiology in affiliative and prosocial interaction patterns.

The modified elevated gap interaction test: a novel paradigm to assess social preference

Social deficits play a role in numerous psychiatric, neurological and neurodevelopmental disorders. Relating complex behaviour, such as social interaction, to brain activity remains one of the biggest goals and challenges in neuroscience. Availability of standardized tests that assess social preference is however, limited. Here, we present a novel behavioural paradigm that we developed to measure social behaviour, the modified elevated gap interaction test (MEGIT). In this test, animals are placed on one of two elevated platforms separated by a gap, in which they can engage in whisker interaction with either a conspecific or an object. This allows quantification of social preference in real interaction rather than just proximity and forms an ideal setup for social behaviour-related neuronal recordings. We provide a detailed description of the paradigm and its highly reliable, deep-learning based analysis, and show results obtained from wild-type animals as well as mouse models for disorders characterized by either hyposocial (autism spectrum disorder; ASD) or hypersocial (Williams Beuren syndrome; WBS) behaviour. Wild-type animals show a clear social preference. This preference is significantly smaller in an ASD mouse model, whereas it is larger in WBS mice. The results indicate that MEGIT is a sensitive and reliable test for detecting social phenotypes.

Similarity and characterization of structural and functional neural connections within species under isoflurane anesthesia in the common marmoset

The common marmoset is an essential model for understanding social cognition and neurodegenerative diseases. This study explored the structural and functional brain connectivity in a marmoset under isoflurane anesthesia, aiming to statistically overcome the effects of high inter-individual variability and noise-related confounds such as physiological noise, ensuring robust and reliable data. Similarities and differences in individual subject data, including assessments of functional and structural brain connectivities derived from resting-state functional MRI and diffusion tensor imaging were meticulously captured. The findings highlighted the high consistency of structural neural connections within the species, indicating a stable neural architecture, while functional connectivity under anesthesia displayed considerable variability. Through independent component and dual regression analyses, several distinct brain connectivities were identified, elucidating their characteristics under anesthesia. Insights into the structural and functional features of the marmoset brain from this study affirm its value as a neuroscience research model, promising advancements in the field through fundamental and translational studies.

Mapping the landscape of social behavior

Social interaction is integral to animal behavior. However, we lack tools to describe it with quantitative rigor, limiting our understanding of its principles and neuropsychiatric disorders, like autism, that perturb it. Here, we present a technique for high-resolution 3D tracking of postural dynamics and social touch in freely interacting animals, solving the challenging subject occlusion and part assignment problems using 3D geometric reasoning, graph neural networks, and semi-supervised learning. We collected over 140 million 3D postures in interacting rodents, featuring new monogenic autism rat lines lacking reports of social behavioral phenotypes. Using a novel multi-scale embedding approach, we identified a rich landscape of stereotyped actions, interactions, synchrony, and body contact. This enhanced phenotyping revealed a spectrum of changes in autism models and in response to amphetamine that were inaccessible to conventional measurements. Our framework and large library of interactions will greatly facilitate studies of social behaviors and their neurobiological underpinnings.

A failure to discriminate social from non-social touch at the circuit level may underlie social avoidance in autism

Social touch is critical for communication and to impart emotions and intentions. However, certain autistic individuals experience aversion to social touch, especially when it is unwanted. We used a novel social touch assay and Neuropixels probes to compare neural responses to social vs. non-social interactions in three relevant brain regions: vibrissal somatosensory cortex, tail of striatum, and basolateral amygdala. We find that wild type (WT) mice showed aversion to repeated presentations of an inanimate object but not of another mouse. Cortical neurons cared most about touch context (social vs. object) and showed a preference for social interactions, while striatal neurons changed their preference depending on whether mice could choose or not to interact. Amygdalar and striatal neurons were preferentially modulated by forced object touch, which was the most aversive. In contrast, the Fmr1 knockout (KO) model of autism found social and non-social interactions equally aversive and displayed more aversive facial expressions to social touch when it invaded their personal space. Importantly, when Fmr1 KO mice could choose to interact, neurons in all three regions did not discriminate social valence. Thus, a failure to differentially encode social from non-social stimuli at the circuit level may underlie social avoidance in autism.

Out of touch? How trauma shapes the experience of social touch - Neural and endocrine pathways

Trauma can shape the way an individual experiences the world and interacts with other people. Touch is a key component of social interactions, but surprisingly little is known about how trauma exposure influences the processing of social touch. In this review, we examine possible neurobiological pathways through which trauma can influence touch processing and lead to touch aversion and avoidance in trauma-exposed individuals. Emerging evidence indicates that trauma may affect sensory touch thresholds by modulating activity in the primary sensory cortex and posterior insula. Disturbances in multisensory integration and oxytocin reactivity combined with diminished reward-related and anxiolytic responses may induce a bias towards negative appraisal of touch contexts. Furthermore, hippocampus deactivation during social touch may reflect a dissociative state. These changes depend not only on the type and severity of the trauma but also on the features of the touch. We hypothesise that disrupted touch processing may impair social interactions and confer elevated risk for future stress-related disorders.

Sensory and behavioral modulation of thalamic head-direction cells

Head-direction (HD) neurons are thought to exclusively encode directional heading. In awake mice, we found that sensory stimuli evoked robust short-latency responses in thalamic HD cells, but not in non-HD neurons. The activity of HD cells, but not that of non-HD neurons, was tightly correlated to brain-state fluctuations and dynamically modulated during social interactions. These data point to a new role for the thalamic compass in relaying sensory and behavioral-state information.

Meso-Py: Dual Brain Cortical Calcium Imaging in Mice during Head-Fixed Social Stimulus Presentation

We present a cost-effective, compact foot-print, and open-source Raspberry Pi-based widefield imaging system. The compact nature allows the system to be used for close-proximity dual-brain cortical mesoscale functional-imaging to simultaneously observe activity in two head-fixed animals in a staged social touch-like interaction. We provide all schematics, code, and protocols for a rail system where head-fixed mice are brought together to a distance where the macrovibrissae of each mouse make contact. Cortical neuronal functional signals (GCaMP6s; genetically encoded Ca2+ sensor) were recorded from both mice simultaneously before, during, and after the social contact period. When the mice were together, we observed bouts of mutual whisking and cross-mouse correlated cortical activity across the cortex. Correlations were not observed in trial-shuffled mouse pairs, suggesting that correlated activity was specific to individual interactions. Whisking-related cortical signals were observed during the period where mice were together (closest contact). The effects of social stimulus presentation extend outside of regions associated with mutual touch and have global synchronizing effects on cortical activity.

Neural Circuit Transitions Supporting Developmentally Specific Social Behavior

Environmentally appropriate social behavior is critical for survival across the lifespan. To support this flexible behavior, the brain must rapidly perform numerous computations taking into account sensation, memory, motor-control, and many other systems. Further complicating this process, individuals must perform distinct social behaviors adapted to the unique demands of each developmental stage; indeed, the social behaviors of the newborn would not be appropriate in adulthood and vice versa. However, our understanding of the neural circuit transitions supporting these behavioral transitions has been limited. Recent advances in neural circuit dissection tools, as well as adaptation of these tools for use at early time points, has helped uncover several novel mechanisms supporting developmentally appropriate social behavior. This review, and associated Minisymposium, bring together social neuroscience research across numerous model organisms and ages. Together, this work highlights developmentally regulated neural mechanisms and functional transitions in the roles of the sensory cortex, prefrontal cortex, amygdala, habenula, and the thalamus to support social interaction from infancy to adulthood. These studies underscore the need for synthesis across varied model organisms and across ages to advance our understanding of flexible social behavior.

A Novel Head-Fixed Assay for Social Touch in Mice Uncovers Aversive Responses in Two Autism Models

Social touch, an important aspect of social interaction and communication, is essential to kinship across animal species. How animals experience and respond to social touch has not been thoroughly investigated, in part because of the lack of appropriate assays. Previous studies that examined social touch in freely moving rodents lacked the necessary temporal and spatial control over individual touch interactions. We designed a novel head-fixed assay for social touch in mice, in which the experimenter has complete control to elicit highly stereotyped bouts of social touch between two animals. The user determines the number, duration, context, and type of social touch interactions, while monitoring an array of complex behavioral responses with high resolution cameras. We focused on social touch to the face because of its high translational relevance to humans. We validated this assay in two different models of autism spectrum disorder (ASD), the Fmr1 knock-out (KO) model of Fragile X syndrome (FXS) and maternal immune activation (MIA) mice. We observed higher rates of avoidance running, hyperarousal, and aversive facial expressions (AFEs) to social touch than to object touch, in both ASD models compared with controls. Fmr1 KO mice showed more AFEs to mice of the same sex but whether they were stranger or familiar mice mattered less. Because this new social touch assay for head-fixed mice can be used to record neural activity during repeated bouts of social touch it could be used to uncover underlying circuit differences.SIGNIFICANCE STATEMENT Social touch is important for communication in animals and humans. However, it has not been extensively studied and current assays to measure animals' responses to social touch have limitations. We present a novel head-fixed assay to quantify how mice respond to social facial touch with another mouse. We validated this assay in autism mouse models since autistic individuals exhibit differences in social interaction and touch sensitivity. We find that mouse models of autism exhibit more avoidance, hyperarousal, and aversive facial expressions (AFEs) to social touch compared with controls. Thus, this novel assay can be used to investigate behavioral responses to social touch and the underlying brain mechanisms in rodent models of neurodevelopmental conditions, and to evaluate therapeutic responses in preclinical studies.

The Neurobiology of Love and Pair Bonding from Human and Animal Perspectives

Love is a powerful emotional experience that is rooted in ancient neurobiological processes shared with other species that pair bond. Considerable insights have been gained into the neural mechanisms driving the evolutionary antecedents of love by studies in animal models of pair bonding, particularly in monogamous species such as prairie voles (Microtus ochrogaster). Here, we provide an overview of the roles of oxytocin, dopamine, and vasopressin in regulating neural circuits responsible for generating bonds in animals and humans alike. We begin with the evolutionary origins of bonding in mother-infant relationships and then examine the neurobiological underpinnings of each stage of bonding. Oxytocin and dopamine interact to link the neural representation of partner stimuli with the social reward of courtship and mating to create a nurturing bond between individuals. Vasopressin facilitates mate-guarding behaviors, potentially related to the human experience of jealousy. We further discuss the psychological and physiological stress following partner separation and their adaptive function, as well as evidence of the positive health outcomes associated with being pair-bonded based on both animal and human studies.

Prosocial behavior, social reward and affective state discrimination in adult male and female mice

Prosocial behavior, defined as voluntary behavior intended to benefit another, has long been regarded as a primarily human characteristic. In recent years, it was reported that laboratory animals also favor prosocial choices in various experimental paradigms, thus demonstrating that prosocial behaviors are evolutionarily conserved. Here, we investigated prosocial choices in adult male and female C57BL/6 laboratory mice in a task where a subject mouse was equally rewarded for entering any of the two compartments of the experimental cage, but only entering of the compartment designated as "prosocial" rewarded an interaction partner. In parallel we have also assessed two traits that are regarded as closely related to prosociality: sensitivity to social reward and the ability to recognize the affective state of another individual. We found that female, but not male, mice increased frequency of prosocial choices from pretest to test. However, both sexes showed similar rewarding effects of social contact in the conditioned place preference test, and similarly, there was no effect of sex on affective state discrimination measured as the preference for interaction with a hungry or relieved mouse over a neutral animal. These observations bring interesting parallels to differences between sexes observed in humans, and are in line with reported higher propensity for prosocial behavior in human females, but differ with regard to sensitivity to social stimuli in males.

High-precision spatial analysis of mouse courtship vocalization behavior reveals sex and strain differences

Mice display a wide repertoire of vocalizations that varies with sex, strain, and context. Especially during social interaction, including sexually motivated dyadic interaction, mice emit sequences of ultrasonic vocalizations (USVs) of high complexity. As animals of both sexes vocalize, a reliable attribution of USVs to their emitter is essential. The state-of-the-art in sound localization for USVs in 2D allows spatial localization at a resolution of multiple centimeters. However, animals interact at closer ranges, e.g. snout-to-snout. Hence, improved algorithms are required to reliably assign USVs. We present a novel algorithm, SLIM (Sound Localization via Intersecting Manifolds), that achieves a 2-3-fold improvement in accuracy (13.1-14.3 mm) using only 4 microphones and extends to many microphones and localization in 3D. This accuracy allows reliable assignment of 84.3% of all USVs in our dataset. We apply SLIM to courtship interactions between adult C57Bl/6J wildtype mice and those carrying a heterozygous Foxp2 variant (R552H). The improved spatial accuracy reveals that vocalization behavior is dependent on the spatial relation between the interacting mice. Female mice vocalized more in close snout-to-snout interaction while male mice vocalized more when the male snout was in close proximity to the female's ano-genital region. Further, we find that the acoustic properties of the ultrasonic vocalizations (duration, Wiener Entropy, and sound level) are dependent on the spatial relation between the interacting mice as well as on the genotype. In conclusion, the improved attribution of vocalizations to their emitters provides a foundation for better understanding social vocal behaviors.

A novel head-fixed assay for social touch in mice uncovers aversive responses in two autism models

Social touch, an important aspect of social interaction and communication, is essential to kinship across animal species. How animals experience and respond to social touch has not been thoroughly investigated, in part due to the lack of appropriate assays. Previous studies that examined social touch in freely moving rodents lacked the necessary temporal and spatial control over individual touch interactions. We designed a novel head-fixed assay for social touch in mice, in which the experimenter has complete control to elicit highly stereotyped bouts of social touch between two animals. The user determines the number, duration, context, and type of social touch interactions, while monitoring with high frame rate cameras an array of complex behavioral responses. We focused on social touch to the face because of their high translational relevance to humans. We validated this assay in two different models of autism spectrum disorder (ASD), the Fmr1 knockout model of Fragile X Syndrome and maternal immune activation mice. We observed increased avoidance, hyperarousal, and more aversive facial expressions to social touch, but not to object touch, in both ASD models compared to controls. Because this new social touch assay for head-fixed mice can be used to record neural activity during repeated bouts of social touch it should be of interest to neuroscientists interested in uncovering the underlying circuits.

Neonatal bilateral whisker trimming in male mice age-dependently alters brain neurotransmitter levels and causes adolescent onsets of social behavior abnormalities

Tactile perception via whiskers is important in rodent behavior. Whisker trimming during the neonatal period affects mouse behaviors related to both whisker-based tactile cognition and social performance. However, the molecular basis of these phenomena is not completely understood. To solve this issue, we investigated developmental changes in transmitters and metabolites in various brain regions of male mice subjected to bilateral whisker trimming during the neonatal period (10 days after birth [BWT10 mice]). We discovered significantly lower levels of 3-methoxy-4-hydroxyphenyl glycol (MHPG), the major noradrenaline metabolite, in various brain regions of male BWT10 mice at both early/late adolescent stages (at P4W and P8W). However, reduced levels of dopamine (DA) and their metabolites were more significantly identified at P8W in the nuclear origins of monoamine (midbrain and medulla oblongata) and the limbic system (frontal cortex, amygdala, and hippocampus) than at P4W. Furthermore, the onset of social behavior deficits (P6W) was observed later to the impairment of whisker-based tactile cognitive behaviors (P4W). Taken together, these findings suggest that whisker-mediated tactile cognition may contribute toprogressive abnormalities in social behaviors in BWT10 mice accompanied by impaired development of dopaminergic systems.

Physiological state gates sensorimotor cortical processing and goal-directed behavior

Goal-directed behavior is often studied under food- and water-restricted states. A study by Matteucci et al.¹ in this issue of Neuron reveals that task performance and sensorimotor cortical encoding are impaired under both low and high motivational states but improve with physiological adaption.

Neural and behavioral plasticity across the female reproductive cycle

Sex is fundamental for the evolution and survival of most species. However, sex can also pose danger, because it increases the risk of predation and disease transmission, among others. Thus, in many species, cyclic fluctuations in the concentration of sex hormones coordinate sexual receptivity and attractiveness with female reproductive capacity, promoting copulation when fertilization is possible and preventing it otherwise. In recent decades, numerous studies have reported a wide variety of sex hormone-dependent plastic rearrangements across the entire brain, including areas relevant for female sexual behavior. By contrast, how sex hormone-induced plasticity alters the computations performed by such circuits, such that collectively they produce the appropriate periodic switches in female behavior, is mostly unknown. In this review, we highlight the myriad sex hormone-induced neuronal changes known so far, the full repertoire of behavioral changes across the reproductive cycle, and the few examples where the relationship between sex hormone-dependent plasticity, neural activity, and behavior has been established. We also discuss current challenges to causally link the actions of sex hormones to the modification of specific cellular pathways and behavior, focusing on rodents as a model system while drawing a comparison between rodents and humans wherever possible.

Hearing, touching, and multisensory integration during mate choice

Mate choice is a potent generator of diversity and a fundamental pillar for sexual selection and evolution. Mate choice is a multistage affair, where complex sensory information and elaborate actions are used to identify, scrutinize, and evaluate potential mating partners. While widely accepted that communication during mate assessment relies on multimodal cues, most studies investigating the mechanisms controlling this fundamental behavior have restricted their focus to the dominant sensory modality used by the species under examination, such as vision in humans and smell in rodents. However, despite their undeniable importance for the initial recognition, attraction, and approach towards a potential mate, other modalities gain relevance as the interaction progresses, amongst which are touch and audition. In this review, we will: (1) focus on recent findings of how touch and audition can contribute to the evaluation and choice of mating partners, and (2) outline our current knowledge regarding the neuronal circuits processing touch and audition (amongst others) in the context of mate choice and ask (3) how these neural circuits are connected to areas that have been studied in the light of multisensory integration.

Regional Differences in BDNF Expression and Behavior as a Function of Sex and Enrichment Type: Oxytocin Matters

The early environment is critical to brain development, but the relative contribution of physical versus social stimulation is unclear. Here, we investigated in male and female rats the response to early physical and social environmental enrichment in relation to oxytocin (OT) and brain-derived neurotrophic factor (BDNF) expression. The findings show that males and females respond differently to prolonged sensorimotor stimulation from postnatal days 21-110 in terms of functional, structural, and molecular changes in the hippocampus versus medial prefrontal cortex (mPFC). Physical enrichment promoted motor and cognitive functions and hippocampal BDNF mRNA and protein expression in both sexes. Combined physical and social enrichment, however, promoted functional and structural gain in females. These changes were accompanied by elevated plasma oxytocin (OT) levels and BDNF mRNA expression in the mPFC, while the hippocampus was not affected. Administration of an OT antagonist in females blocked the beneficial effects of enrichment and led to reduced cortical BDNF signaling. These findings suggest that an OT-based mechanism selectively stimulates a region-specific BDNF response which is dependent on the type of experience.

Olfactory modulation of barrel cortex activity during active whisking and passive whisker stimulation

Rodents depend on olfaction and touch to meet many of their fundamental needs. However, the impact of simultaneous olfactory and tactile inputs on sensory representations in the cortex remains elusive. To study these interactions, we recorded large populations of barrel cortex neurons using 2-photon calcium imaging in head-fixed mice during olfactory and tactile stimulation. Here we show that odors bidirectionally alter activity in a small but significant population of barrel cortex neurons through at least two mechanisms, first by enhancing whisking, and second by a central mechanism that persists after whisking is abolished by facial nerve sectioning. Odor responses have little impact on tactile information, and they are sufficient for decoding odor identity, while behavioral parameters like whisking, sniffing, and facial movements are not odor identity-specific. Thus, barrel cortex activity encodes specific olfactory information that is not linked with odor-induced changes in behavior.

Rodents use both touch and smell to get around. This work describes how olfactory information is combined with touch perception in the cortex to guide behavior.

The marmoset as a model for investigating the neural basis of social cognition in health and disease

Social-cognitive processes facilitate the use of environmental cues to understand others, and to be understood by others. Animal models provide vital insights into the neural underpinning of social behaviours. To understand social cognition at even deeper behavioural, cognitive, neural, and molecular levels, we need to develop more representative study models, which allow testing of novel hypotheses using human-relevant cognitive tasks. Due to their cooperative breeding system and relatively small size, common marmosets (Callithrix jacchus) offer a promising translational model for such endeavours. In addition to having social behavioural patterns and group dynamics analogous to those of humans, marmosets have cortical brain areas relevant for the mechanistic analysis of human social cognition, albeit in simplified form. Thus, they are likely suitable animal models for deciphering the physiological processes, connectivity and molecular mechanisms supporting advanced cognitive functions. Here, we review findings emerging from marmoset social and behavioural studies, which have already provided significant insights into executive, motivational, social, and emotional dysfunction associated with neurological and psychiatric disorders.

Social Relationship as a Factor for the Development of Stress Incubation in Adult Mice

While stress reactions can emerge long after the triggering event, it remains elusive how they emerge after a protracted, seemingly stress-free period during which stress incubates. Here, we study the behavioral development in mice isolated after observing an aggressive encounter inflicted upon their pair-housed partners. We developed a spatially resolved fine-scale behavioral analysis and applied it to standard behavioral tests. It reveals that the seemingly sudden behavioral changes developed gradually. These behavioral changes were not observed if the aggressive encounter happened to a stranger mouse, suggesting that social bonding is a prerequisite for stress incubation in this paradigm. This finding was corroborated by hemisphere-specific morphological changes in cortex regions centering at the anterior cingulate cortex, a cognitive and emotional center. Our non-invasive analytical methods to capture informative behavioral details may have applications beyond laboratory animals.

NMDA receptor-mediated synaptic transmission in prefrontal neurons underlies social memory retrieval in female mice

Social memory is the ability to discriminate familiar conspecific from the unknown ones. Prefrontal neurons are essentially required for social memory, but the mechanism associated with this regulation remains unknown. It is also unclear to what extent the neuronal representations of social memory formation and retrieval events overlap in the prefrontal cortex (PFC) and which event drives social memory strength. Here we asked these questions by using a repeated social training paradigm for social recognition in FosTRAP mice. We found that after 4 days' repeated social training, female mice developed stable social memory. Specifically, repeated social training activated more cells that were labeled with tdTomato during memory retrieval compared with the first day of memory encoding. Besides, combining TRAP with c-Fos immunostaining, we found about 30% of the FosTRAPed cells were reactivated during retrieval. Moreover, the number of retrieval-induced but not first-day encoding-induced tdTomato neurons correlates with the social recognition ratio in the prelimbic but not other subregions. The activated cells during the retrieval session also showed increased NMDA receptor-mediated synaptic transmission compared with that in non-labeled pyramidal neurons. Blocking NMDA receptors by MK-801 impaired social memory but not sociability. Therefore, our results reveal that repetitive training elevates mPFC involvement in social memory retrieval via enhancing NMDA receptor-mediated synaptic transmission, thus rendering stable social memory.

Adaptive Whisking in Mice

Rodents generate rhythmic whisking movements to explore their environment. Whisking trajectories, for one, appear as a fixed pattern of whisk cycles at 5–10 Hz driven by a brain stem central pattern generator. In contrast, whisking behavior is thought to be versatile and adaptable to behavioral goals. To begin to systematically investigate such behavioral adaptation, we established a whisking task, in which mice altered the trajectories of whisking in a goal-oriented fashion to gain rewards. Mice were trained to set the whisker to a defined starting position and generate a protraction movement across a virtual target (no touch-related tactile feedback). By ramping up target distance based on reward history, we observed that mice are able to generate highly specific whisking patterns suited to keep reward probability constant. On a sensorimotor level, the behavioral adaptation was realized by adjusting whisker kinematics: more distant locations were targeted using higher velocities (i.e., pointing to longer force generation), rather than by generating higher acceleration (i.e., pointing to stronger forces). We tested the suitability of the paradigm of tracking subtle alteration in whisking motor commands using small lesions in the rhythmic whisking subfield (RW) of the whisking-related primary motor cortex. Small contralateral RW lesions generated the deterioration of whisking kinematics with a latency of 12 days post-lesion, a change that was readily discriminated from changes in the behavioral adaptation by the paradigm.

Mixed selectivity coding of sensory and motor social signals in the thalamus of a weakly electric fish

High-level neural activity often exhibits mixed selectivity to multivariate signals. How such representations arise and modulate natural behavior is poorly understood. We addressed this question in weakly electric fish, whose social behavior is relatively low dimensional and can be easily reproduced in the laboratory. We report that the preglomerular complex, a thalamic region exclusively connecting midbrain with pallium, implements a mixed selectivity strategy to encode interactions related to courtship and rivalry. We discuss how this code enables the pallial recurrent networks to control social behavior, including dominance in male-male competition and female mate selection. Notably, response latency analysis and computational modeling suggest that corollary discharge from premotor regions is implicated in flagging outgoing communications and thereby disambiguating self- versus non-self-generated signals. These findings provide new insights into the neural substrates of social behavior, multi-dimensional neural representation, and its role in perception and decision making.

Body language signals for rodent social communication

Integration of social cues to initiate adaptive emotional and behavioral responses is a fundamental aspect of animal and human behavior. In humans, social communication includes prominent nonverbal components, such as social touch, gestures and facial expressions. Comparative studies investigating the neural basis of social communication in rodents has historically been centered on olfactory signals and vocalizations, with relatively less focus on non-verbal social cues. Here, we outline two exciting research directions: First, we will review recent observations pointing to a role of social facial expressions in rodents. Second, we will review observations that point to a role of 'non-canonical' rodent body language: body posture signals beyond stereotyped displays in aggressive and sexual behavior. In both sections, we will outline how social neuroscience can build on recent advances in machine learning, robotics and micro-engineering to push these research directions forward towards a holistic systems neurobiology of rodent body language.

Top-down acetylcholine signaling via olfactory bulb vasopressin cells contributes to social discrimination in rats

Social discrimination in rats requires activation of the intrinsic bulbar vasopressin system, but it is unclear how this system comes into operation, as olfactory nerve stimulation primarily inhibits bulbar vasopressin cells (VPCs). Here we show that stimulation with a conspecific can activate bulbar VPCs, indicating that VPC activation depends on more than olfactory cues during social interaction. A series of in vitro electrophysiology, pharmacology and immunohistochemistry experiments implies that acetylcholine, probably originating from centrifugal projections, can enable olfactory nerve-evoked action potentials in VPCs. Finally, cholinergic activation of the vasopressin system contributes to vasopressin-dependent social discrimination, since recognition of a known rat was blocked by bulbar infusion of the muscarinic acetylcholine receptor antagonist atropine and rescued by additional bulbar application of vasopressin. Thus, our results implicate that top-down cholinergic modulation of bulbar VPC activity is involved in social discrimination in rats.

Prolonged anesthesia alters brain synaptic architecture

Prolonged medically induced coma (pMIC) is carried out routinely in intensive care medicine. pMIC leads to cognitive impairment, yet the underlying neuromorphological correlates are still unknown, as no direct studies of MIC exceeding ∼6 h on neural circuits exist. Here, we establish pMIC (up to 24 h) in adolescent and mature mice, and combine longitudinal two-photon imaging of cortical synapses with repeated behavioral object recognition assessments. We find that pMIC affects object recognition, and that it is associated with enhanced synaptic turnover, generated by enhanced synapse formation during pMIC, while the postanesthetic period is dominated by synaptic loss. Our results demonstrate major side effects of prolonged anesthesia on neural circuit structure.

Encoding of social exploration by neural ensembles in the insular cortex

The insular cortex (IC) participates in diverse complex brain functions, including social function, yet their cellular bases remain to be fully understood. Using microendoscopic calcium imaging of the agranular insular cortex (AI) in mice interacting with freely moving and restrained social targets, we identified 2 subsets of AI neurons—a larger fraction of “Social-ON” cells and a smaller fraction of “Social-OFF” cells—that change their activity in opposite directions during social exploration. Social-ON cells included those that represented social investigation independent of location and consisted of multiple subsets, each of which was preferentially active during exploration under a particular behavioral state or with a particular target of physical contact. These results uncover a previously unknown function of AI neurons that may act to monitor the ongoing status of social exploration while an animal interacts with unfamiliar conspecifics.

The insular cortex participates in diverse complex brain functions, including social function, but their cellular basis remains unclear. This study uses microendoscopic calcium imaging in mice interacting with conspecifics to identify “social cells” in the agranular insular cortex; multiple subsets of neurons encode distinct aspects of ongoing social behavior.

Neuronal Circuits in Barrel Cortex for Whisker Sensory Perception

The array of whiskers on the snout provides rodents with tactile sensory information relating to the size, shape and texture of objects in their immediate environment. Rodents can use their whiskers to detect stimuli, distinguish textures, locate objects and navigate. Important aspects of whisker sensation are thought to result from neuronal computations in the whisker somatosensory cortex (wS1). Each whisker is individually represented in the somatotopic map of wS1 by an anatomical unit named a 'barrel' (hence also called barrel cortex). This allows precise investigation of sensory processing in the context of a well-defined map. Here, we first review the signaling pathways from the whiskers to wS1, and then discuss current understanding of the various types of excitatory and inhibitory neurons present within wS1. Different classes of cells can be defined according to anatomical, electrophysiological and molecular features. The synaptic connectivity of neurons within local wS1 microcircuits, as well as their long-range interactions and the impact of neuromodulators, are beginning to be understood. Recent technological progress has allowed cell-type-specific connectivity to be related to cell-type-specific activity during whisker-related behaviors. An important goal for future research is to obtain a causal and mechanistic understanding of how selected aspects of tactile sensory information are processed by specific types of neurons in the synaptically connected neuronal networks of wS1 and signaled to downstream brain areas, thus contributing to sensory-guided decision-making.

Layer 4 organization and respiration locking in the rodent nose somatosensory cortex

Rodents and other mammals acquire sensory information by precisely orchestrated head, whisker, and respiratory movements. We have, however, only limited information about integration of these signals. In the somatosensory domain, the integration of somatosensory information with other modalities is particularly pertinent for body parts such as eyes, ears, and nose, which serve another modality. Here we analyzed the nose/nostril representation in the rodent somatosensory cortex. We identified the representation of the nose/nostril in the rat somatosensory cortex by receptive field mapping and subsequent histological reconstruction. In tangential somatosensory cortical sections, the rat nostril cortex was evident as a prominent stripe-like recess of layer 4 revealed by cytochrome-c oxidase reactivity or by antibodies against the vesicular glutamate-transporter-2 (identifying thalamic afferents). We compared flattened somatosensory cortices of various rodents including rats, mice, gerbils, chinchillas, and chipmunks. We found that such a nose/nostril module was evident as a region with thinned or absent layer 4 at the expected somatotopic position of the nostril. Extracellular spike activity was strongly modulated by respiration in the rat somatosensory cortex, and field potential recordings revealed a stronger locking of nostril recording sites to respiration than for whisker/barrel cortex recoding sites. We conclude that the rodent nose/nostril representation has a conserved architecture and specifically interfaces with respiration signals.NEW & NOTEWORTHY We characterized the rodent nose somatosensory cortex. The nostril representation appeared as a kind of "hole" (i.e., as a stripe-like recess of layer 4) in tangential cortical sections. Neural activity in nose somatosensory cortex was locked to respiration, and simultaneous field recordings indicate that this locking was specific to this region. Our results reveal previously unknown cytoarchitectonic and physiological properties of the rodent nose somatosensory cortex, potentially enabling it to integrate multiple sensory modalities.

Social touch promotes interfemale communication via activation of parvocellular oxytocin neurons

Oxytocin (OT) is a great facilitator of social life but, although its effects on socially relevant brain regions have been extensively studied, OT neuron activity during actual social interactions remains unexplored. Most OT neurons are magnocellular neurons, which simultaneously project to the pituitary and forebrain regions involved in social behaviors. In the present study, we show that a much smaller population of OT neurons, parvocellular neurons that do not project to the pituitary but synapse onto magnocellular neurons, is preferentially activated by somatosensory stimuli. This activation is transmitted to the larger population of magnocellular neurons, which consequently show coordinated increases in their activity during social interactions between virgin female rats. Selectively activating these parvocellular neurons promotes social motivation, whereas inhibiting them reduces social interactions. Thus, parvocellular OT neurons receive particular inputs to control social behavior by coordinating the responses of the much larger population of magnocellular OT neurons.

Effects of Sexual Experience and Puberty on Mouse Genital Cortex revealed by Chronic Imaging

The topographic map in layer 4 of somatosensory cortex is usually specified early postnatally and stable thereafter. Genital cortex, however, undergoes a sex-hormone- and sexual-touch-dependent pubertal expansion. Here, we image pubertal development of genital cortex in Scnn1a-Tg3-Cre mice, where transgene expression has been shown to be restricted to layer 4 neurons with primary sensory cortex identity. Interestingly, during puberty, the number of Scnn1a+ neurons roughly doubled within genital cortex. The increase of Scnn1a+ neurons was gradual and rapidly advanced by initial sexual experience. Neurons that gained Scnn1a expression comprised stellate and pyramidal neurons in layer 4. Unlike during neonatal development, pyramids did not retract their apical dendrites during puberty. Calcium imaging revealed stronger genital-touch responses in Scnn1a+ neurons in males versus females and a developmental increase in responsiveness in females. The first sexual interaction is a unique physical experience that often creates long-lasting memories. We suggest such experience uniquely alters somatosensory body maps.

Highly structured, partner-sex- and subject-sex-dependent cortical responses during social facial touch

Touch is a fundamental aspect of social, parental and sexual behavior. In contrast to our detailed knowledge about cortical processing of non-social touch, we still know little about how social touch impacts cortical circuits. We investigated neural activity across five frontal, motor and sensory cortical areas in rats engaging in naturalistic social facial touch. Information about social touch and the sex of the interaction partner (a biologically significant feature) is a major determinant of cortical activity. 25.3% of units were modulated during social touch and 8.3% of units displayed ‘sex-touch’ responses (responded differently, depending on the sex of the interaction partner). Single-unit responses were part of a structured, partner-sex- and, in some cases, subject-sex-dependent population response. Spiking neural network simulations indicate that a change in inhibitory drive might underlie these population dynamics. Our observations suggest that socio-sexual characteristics of touch (subject and partner sex) widely modulate cortical activity and need to be investigated with cellular resolution.

Touch is an important sensory modality during social encounters. Here the authors report that during naturalistic social encounters in rats, the cortical activity in widespread areas at the level of single neurons is modulated by sociosexual characteristics such as the subject and partner sex.

Behavioral and Cortical Correlates of Self-Suppression, Anticipation, and Ambivalence in Rat Tickling

The relationship between tickling, sensation, and laughter is complex. Tickling or its mere anticipation makes us laugh, but not when we self-tickle. We previously showed rat somatosensory cortex drives tickling-evoked vocalizations and now investigated self-tickle suppression and tickle anticipation. We recorded somatosensory cortex activity while tickling and touching rats and while rats touched themselves. Allo-touch and tickling evoked somatotopic cortical excitation and vocalizations. Self-touch induced wide-ranging inhibition and vocalization suppression. Self-touch also suppressed vocalizations and cortical responses evoked by allo-touch or cortical microstimulation. We suggest a global-inhibition model of self-tickle suppression, which operates without the classically assumed self versus other distinction. Consistent with this inhibition hypothesis, blocking cortical inhibition with gabazine abolished self-tickle suppression. We studied anticipation in a nose-poke-for-tickling paradigm. Although rats nose poked for tickling, they also showed escaping, freezing, and alarm calls. Such ambivalence ("Nervenkitzel") resembles tickle behaviors in children. We conclude that self-touch-induced GABAergic cortical inhibition prevents self-tickle, whereas anticipatory layer 5 activity drives anticipatory laughter. VIDEO ABSTRACT.

An insular view of the social decision-making network

Social animals must detect, evaluate and respond to the emotional states of other individuals in their group. A constellation of gestures, vocalizations, and chemosignals enable animals to convey affect and arousal to others in nuanced, multisensory ways. Observers integrate social information with environmental and internal factors to select behavioral responses to others via a process call social decision-making. The Social Decision Making Network (SDMN) is a system of brain structures and neurochemicals that are conserved across species (mammals, reptiles, amphibians, birds) that are the proximal mediators of most social behaviors. However, how sensory information reaches the SDMN to shape behavioral responses during a social encounter is not well known. Here we review the empirical data that demonstrate the necessity of sensory systems in detecting social stimuli, as well as the anatomical connectivity of sensory systems with each node of the SDMN. We conclude that the insular cortex is positioned to link integrated social sensory cues to this network to produce flexible and appropriate behavioral responses to socioemotional cues.

Twitches, Blinks, and Fidgets: Important Generators of Ongoing Neural Activity

Animals and humans continuously engage in small, spontaneous motor actions, such as blinking, whisking, and postural adjustments ("fidgeting"). These movements are accompanied by changes in neural activity in sensory and motor regions of the brain. The frequency of these motions varies in time, is affected by sensory stimuli, arousal levels, and pathology. These fidgeting behaviors can be entrained by sensory stimuli. Fidgeting behaviors will cause distributed, bilateral functional activation in the 0.01 to 0.1 Hz frequency range that will show up in functional magnetic resonance imaging and wide-field calcium neuroimaging studies, and will contribute to the observed functional connectivity among brain regions. However, despite the large potential of these behaviors to drive brain-wide activity, these fidget-like behaviors are rarely monitored. We argue that studies of spontaneous and evoked brain dynamics in awake animals and humans should closely monitor these fidgeting behaviors. Differences in these fidgeting behaviors due to arousal or pathology will "contaminate" ongoing neural activity, and lead to apparent differences in functional connectivity. Monitoring and accounting for the brain-wide activations by these behaviors is essential during experiments to differentiate fidget-driven activity from internally driven neural dynamics.

Sensorimotor processing in the rodent barrel cortex

Tactile sensory information from facial whiskers provides nocturnal tunnel-dwelling rodents, including mice and rats, with important spatial and textural information about their immediate surroundings. Whiskers are moved back and forth to scan the environment (whisking), and touch signals from each whisker evoke sparse patterns of neuronal activity in whisker-related primary somatosensory cortex (wS1; barrel cortex). Whisking is accompanied by desynchronized brain states and cell-type-specific changes in spontaneous and evoked neuronal activity. Tactile information, including object texture and location, appears to be computed in wS1 through integration of motor and sensory signals. wS1 also directly controls whisker movements and contributes to learned, whisker-dependent, goal-directed behaviours. The cell-type-specific neuronal circuitry in wS1 that contributes to whisker sensory perception is beginning to be defined.

Physiological and Anatomical Outputs of Rat Genital Cortex

Rat somatosensory genital cortex contains a large sexually monomorphic representation of the penis in males and the clitoris in females. Genital cortex microstimulation-evoked movements of legs, trunk and genitals, which showed sex-specific differences related to mating behaviors and included thrusting in males and lordosis-like movements in females. Erections/tumescence of penis or clitoris could not be evoked, however. Anterograde tracer injections into penis/clitoris cortex revealed eleven corticocortical and 10 subcortical projection targets, which were qualitatively similar in both sexes. Corticocortical genital-cortex-projections innervated about 3% of the cortical surface and most were analog to other somatosensory projections targeting motor cortex, secondary somatosensory cortex, parietal cortex and perirhinal cortex. Corticocortical projections that differed from other parts of somatosensory cortex targeted male scrotum cortex, female vulva cortex, the somatosensory–ear–auditory-cortex-region and the caudal parietal area. Aligning cytoarchitectonic borders with motor topography, sensory genital responses and corticocortical projections identified a candidate region for genital motor cortex. Most subcortical genital-cortex-projections were analog to other thalamic, tectal or pontine projections of somatosensory cortex. Genital-cortex-specific subcortical projections targeted amygdala and nucleus submedius and accumbens. Microstimulation-effects and projections support a sexual function of genital cortex and suggest that genital cortex is a major hub of sexual sensorimotor processing in rodents.

Estrus-Cycle Regulation of Cortical Inhibition

Female mammals experience cyclical changes in sexual receptivity known as the estrus cycle. Little is known about how estrus affects the cortex, although alterations in sensation, cognition and the cyclical occurrence of epilepsy suggest brain-wide processing changes. We performed in vivo juxtacellular and whole-cell recordings in somatosensory cortex of female rats and found that the estrus cycle potently altered cortical inhibition. Fast-spiking interneurons were strongly activated with social facial touch and varied their ongoing activity with the estrus cycle and estradiol in ovariectomized females, while regular-spiking excitatory neurons did not change. In situ hybridization for estrogen receptor β (Esr2) showed co-localization with parvalbumin-positive (PV⁺) interneurons in deep cortical layers, mirroring the laminar distribution of our physiological findings. The fraction of neurons positive for estrogen receptor β (Esr2) and PV co-localization (Esr2⁺PV⁺) in cortical layer V was increased in proestrus. In vivo and in vitro experiments confirmed that estrogen acts locally to increase fast-spiking interneuron excitability through an estrogen-receptor-β-dependent mechanism.

Socio-sexual processing in cortical circuits

How does social and sexual information processing map onto cortical circuits? Addressing this question has been difficult, because of a lack of circuit-oriented social neuroscience and an absence of measurements from interacting brains. Recent work showed social information is already differentially processed in the primary sensory cortices. Converging evidence suggests that prefrontal areas contribute to social interaction processing and determining social hierarchies. In social interactions, we identify gender in split seconds, but after centuries of anatomy we are still unable to distinguish male and female cortices. Novel data reinforce the idea of a bisexual layout of cortical anatomy. Physiological analysis, however, provided evidence for sex differences in cortical processing. Unlike other cortical circuits, sexual processing circuits undergo major rewiring and expansion during puberty and show lasting damage from childhood abuse.

Impact of X/Y genes and sex hormones on mouse neuroanatomy

Biological sex influences brain anatomy across many species. Sex differences in brain anatomy have classically been attributed to differences in sex chromosome complement (XX versus XY) and/or in levels of gonadal sex steroids released from ovaries and testes. Using the four core genotype (4CG) mouse model in which gonadal sex and sex chromosome complement are decoupled, we previously found that sex hormones and chromosomes influence the volume of distinct brain regions. However, recent studies suggest there may be more complex interactions between hormones and chromosomes, and that circulating steroids can compensate for and/or mask underlying chromosomal effects. Moreover, the impact of pre vs post-pubertal sex hormone exposure on this sex hormone/sex chromosome interplay is not well understood. Thus, we used whole brain high-resolution ex-vivo MRI of intact and pre-pubertally gonadectomized 4CG mice to investigate two questions: 1) Do circulating steroids mask sex differences in brain anatomy driven by sex chromosome complement? And 2) What is the contribution of pre- versus post-pubertal hormones to sex-hormone-dependent differences in brain anatomy? We found evidence of both cooperative and compensatory interactions between sex chromosomes and sex hormones in several brain regions, but the interaction effects were of low magnitude. Additionally, most brain regions affected by sex hormones were sensitive to both pre- and post-pubertal hormones. This data provides further insight into the biological origins of sex differences in brain anatomy.

Laminar-specific encoding of texture elements in rat barrel cortex

KEY POINTS

For rats texture discrimination is signalled by the large face whiskers by stick-slip events. Neural encoding of repetitive stick-slip events will be influenced by intrinsic properties of adaptation. We show that texture coding in the barrel cortex is laminar specific and follows a power function. Our results also show layer 2 codes for novel feature elements via robust firing rates and temporal fidelity. We conclude that texture coding relies on a subtle neural ensemble to provide important object information.

ABSTRACT

Texture discrimination by rats is exquisitely guided by fine-grain mechanical stick-slip motions of the face whiskers as they encounter, stick to and slip past successive texture-defining surface features such as bumps and grooves. Neural encoding of successive stick-slip texture events will be shaped by adaptation, common to all sensory systems, whereby receptor and neural responses to a stimulus are affected by responses to preceding stimuli, allowing resetting to signal novel information. Additionally, when a whisker is actively moved to contact and brush over surfaces, that motion itself generates neural responses that could cause adaptation of responses to subsequent stick-slip events. Nothing is known about encoding in the rat whisker system of stick-slip events defining textures of different grain or the influence of adaptation from whisker protraction or successive texture-defining stick-slip events. Here we recorded responses from halothane-anaesthetized rats in response to texture-defining stimuli applied to passive whiskers. We demonstrate that: across the columnar network of the whisker-recipient barrel cortex, adaptation in response to repetitive stick-slip events is strongest in uppermost layers and equally lower thereafter; neither whisker protraction speed nor stick-slip frequency impede encoding of stick-slip events at rates up to 34.08 Hz; and layer 2 normalizes responses to whisker protraction to resist effects on texture signalling. Thus, within laminar-specific response patterns, barrel cortex reliably encodes texture-defining elements even to high frequencies.

High-precision spatial localization of mouse vocalizations during social interaction

Mice display a wide repertoire of vocalizations that varies with age, sex, and context. Especially during courtship, mice emit ultrasonic vocalizations (USVs) of high complexity, whose detailed structure is poorly understood. As animals of both sexes vocalize, the study of social vocalizations requires attributing single USVs to individuals. The state-of-the-art in sound localization for USVs allows spatial localization at centimeter resolution, however, animals interact at closer ranges, involving tactile, snout-snout exploration. Hence, improved algorithms are required to reliably assign USVs. We develop multiple solutions to USV localization, and derive an analytical solution for arbitrary vertical microphone positions. The algorithms are compared on wideband acoustic noise and single mouse vocalizations, and applied to social interactions with optically tracked mouse positions. A novel, (frequency) envelope weighted generalised cross-correlation outperforms classical cross-correlation techniques. It achieves a median error of ~1.4 mm for noise and ~4–8.5 mm for vocalizations. Using this algorithms in combination with a level criterion, we can improve the assignment for interacting mice. We report significant differences in mean USV properties between CBA mice of different sexes during social interaction. Hence, the improved USV attribution to individuals lays the basis for a deeper understanding of social vocalizations, in particular sequences of USVs.

Pixying Behavior: A Versatile Real-Time and Post Hoc Automated Optical Tracking Method for Freely Moving and Head Fixed Animals

Here, we describe an automated optical method for tracking animal behavior in both head-fixed and freely moving animals, in real time and offline. It takes advantage of an off-the-shelf camera system, the Pixy camera, designed as a fast vision sensor for robotics that uses a color-based filtering algorithm at 50 Hz to track objects. Using customized software, we demonstrate the versatility of our approach by first tracking the rostro-caudal motion of individual adjacent row (D1, D2) or arc whiskers (β, γ), or a single whisker and points on the whisker pad, in head-fixed mice performing a tactile task. Next, we acquired high-speed video and Pixy data simultaneously and applied the pixy-based real-time tracking to high-speed video data. With this approach, we expand the temporal resolution of the Pixy camera and track motion (post hoc) at the limit of high-speed video frame rates. Finally, we show that this system is flexible: it can be used to track individual whisker or limb position without any sophisticated object tracking algorithm, it can be used in many lighting conditions including infrared (IR); it can be used to track head rotation and location of multiple animals simultaneously. Our system makes behavioral monitoring possible in virtually any biological setting.