The Development of Socially Directed Attention: An Functional Magnetic Resonance Imaging Study in Infant Monkeys

Socially guided visual attention, such as gaze following and joint attention, represents the building block of higher-level social cognition in primates, although their neurodevelopmental processes are still poorly understood. Atypical development of these social skills has served as early marker of autism spectrum disorder and Williams syndrome. In this study, we trace the developmental trajectories of four neural networks underlying visual and attentional social engagement in the translational rhesus monkey model. Resting-state fMRI (rs-fMRI) data and gaze following skills were collected in infant rhesus macaques from birth through 6 months of age. Developmental trajectories from subjects with both resting-state fMRI and eye-tracking data were used to explore brain-behavior relationships. Our findings indicate robust increases in functional connectivity (FC) between primary visual areas (primary visual cortex [V1] - extrastriate area 3 [V3] and V3 - middle temporal area, ventral motion areas middle temporal area - AST, as well as between TE and amygdala (AMY) as infants mature. Significant FC decreases were found in more rostral areas of the pathways, such as areas temporal area occipital part - TE in the ventral object pathway, V3 - lateral intraparietal (LIP) of the dorsal visual attention pathway and V3 - temporo-parietal area of the ventral attention pathway. No changes in FC were found between cortical areas LIP-FEF and temporo-parietal area - Area 12 of the dorsal and ventral attention pathways or between AST-AMY and AMY-insula. Developmental trajectory of gaze following revealed a period of dynamic changes with gradual increases from 1 to 2 months, followed by slight decreases from 3 to 6 months. Exploratory association findings across the 6-month period showed that infants with higher gaze following had lower FC between primary visual areas V1-V3, but higher FC in the dorsal attention areas V3-LIP, both in the right hemisphere. Together, the first 6 months of life in rhesus macaques represent a critical period for the emergence of gaze following skills associated with maturational changes in FC of socially guided attention pathways.

Episodic memory development: Bridging animal and human research

Human episodic memory is not functionally evident until about 2 years of age and continues to develop into the school years. Behavioral studies have elucidated this developmental timeline and its constituent processes. In tandem, lesion and neurophysiological studies in non-human primates and rodents have identified key neural substrates and circuit mechanisms that may underlie episodic memory development. Despite this progress, collaborative efforts between psychologists and neuroscientists remain limited, hindering progress. Here, we seek to bridge human and non-human episodic memory development research by offering a comparative review of studies using humans, non-human primates, and rodents. We highlight critical theoretical and methodological issues that limit cross-fertilization and propose a common research framework, adaptable to different species, that may facilitate cross-species research endeavors.

Functional maturation in visual pathways predicts attention to the eyes in infant rhesus macaques: Effects of social status

Differences in looking at the eyes of others are one of the earliest behavioral markers for social difficulties in neurodevelopmental disabilities, including autism. However, it is unknown how early visuo-social experiences relate to the maturation of infant brain networks that process visual social stimuli. We investigated functional connectivity (FC) within the ventral visual object pathway as a contributing neural system. Densely sampled, longitudinal eye-tracking and resting state fMRI (rs-fMRI) data were collected from infant rhesus macaques, an important model of human social development, from birth through 6 months of age. Mean trajectories were fit for both datasets and individual trajectories from subjects with both eye-tracking and rs-fMRI data were used to test for brain-behavior relationships. Exploratory findings showed infants with greater increases in FC between left V1 to V3 visual areas have an earlier increase in eye-looking before 2 months. This relationship was moderated by social status such that infants with low social status had a stronger association between left V1 to V3 connectivity and eye-looking than high status infants. Results indicated that maturation of the visual object pathway may provide an important neural substrate supporting adaptive transitions in social visual attention during infancy.

The role of puberty on physical and brain development: A longitudinal study in male Rhesus Macaques

This study examined the role of male pubertal maturation on physical growth and development of neurocircuits that regulate stress, emotional and cognitive control using a translational nonhuman primate model. We collected longitudinal data from male macaques between pre- and peri-puberty, including measures of physical growth, pubertal maturation (testicular volume, blood testosterone -T- concentrations) and brain structural and resting-state functional MRI scans to examine developmental changes in amygdala (AMY), hippocampus (HIPPO), prefrontal cortex (PFC), as well as functional connectivity (FC) between those regions. Physical growth and pubertal measures increased from pre- to peri-puberty. The indexes of pubertal maturation -testicular size and T- were correlated at peri-puberty, but not at pre-puberty (23 months). Our findings also showed ICV, AMY, HIPPO and total PFC volumetric growth, but with region-specific changes in PFC. Surprisingly, FC in these neural circuits only showed developmental changes from pre- to peri-puberty for HIPPO-orbitofrontal FC. Finally, testicular size was a better predictor of brain structural maturation than T levels -suggesting gonadal hormones-independent mechanisms-, whereas T was a strong predictor of functional connectivity development. We expect that these neural circuits will show more drastic pubertal-dependent maturation, including stronger associations with pubertal measures later, during and after male puberty.

Rhesus monkeys with damage to amygdala or orbitofrontal cortex perform well on novelty-based memory tasks

The amygdala and orbitofrontal cortex (OFC) are interconnected regions that serve as key nodes in brain circuits supporting social and affective behaviors. An important question that has come into focus is whether these regions also play a fundamental role in responding to novelty. One possibility is that these regions are important for discriminating novel from familiar stimuli. An alternative possibility is that these regions contribute to affective responses to stimuli in novelty-based tasks. For example, the amygdala and OFC could contribute to assessing novel stimuli as being threatening or previously selected stimuli as having reward value. The present study tested rhesus macaque monkeys with damage to the amygdala or OFC, along with sham-operated control monkeys, across six variants of novelty-based memory tasks. The results showed that monkeys with damage to the amygdala or OFC performed better overall than control monkeys across the tasks. The results indicated that neither region was essential for discriminating novel from familiar stimuli. Instead, the findings suggested that the improved performance observed in novelty-based tasks following damage to these regions was more likely attributable to influences on affect. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Differential responses toward conditioned and unconditioned stimuli, but decreased hypothalamic-pituitary-adrenal axis responsiveness in neonatal hippocampal lesioned monkeys

The hippocampus is important for long-term memory storage, but also plays a role in regulating the hypothalamic-pituitary-adrenal (HPA) axis and emotional behaviors. We previously reported that early hippocampal damage in monkeys result in increased anxious expression and blunted HPA responses to an acute stressor. Here, we further probe their responses toward aversive stimuli (conditioned and unconditioned) and evaluate HPA axis dysfunction. Responses toward social, innate, and learned aversive stimuli, fear potentiated acoustic startle, and pituitary-adrenal function were investigated in 13 adult rhesus monkeys with neonatal hippocampal lesions (Neo-Hibo=6) and controls (Neo-C=7). Neo-Hibo monkeys' responses depend on the type of unconditioned stimulus, with increased anxiety behaviors toward social and learned, but decreased reactivity toward innate stimuli. Neo-C and Neo-Hibo monkeys exhibited similar performance learning conditioned cues and safety signals. Neo-Hibo monkeys were less sensitive to HPA axis stimulation, potentially suggesting adrenal fatigue. Current findings suggest that the hippocampus plays a large role in regulating not only anxiety behaviors, but also the HPA-axis, a neural system crucial to regulate how we respond to the world around us. These data have important clinical significance considering that many developmental neuropsychiatric disorders exhibit altered hippocampal structure and function, emotional and HPA axis dysregulation.

Early life exposure to unpredictable parental sensory signals shapes cognitive development across three species

Exposure to early life adversity has long term consequences on cognitive function. Most research has focused on understanding components of early life adversities that contribute to later risk, including poverty, trauma, maltreatment, and neglect. Whereas these factors, in the aggregate, explain a significant proportion of emotional and cognitive problems, there are serious gaps in our ability to identify potential mechanisms by which early life adversities might promote vulnerability or resilience. Here we discuss early life exposure to unpredictable signals from the caretaker as an understudied type of adversity that is amenable to prevention and intervention. We employ a translational approach to discover underlying neurobiological mechanisms by which early life exposure to unpredictable signals sculpts the developing brain. First, we review evidence that exposure to unpredictable signals from the parent during sensitive periods impacts development of neural circuits. Second, we describe a method for characterizing early life patterns of sensory signals across species. Third, we present published and original data illustrating that patterns of maternal care predict memory function in humans, non-human primates, and rodents. Finally, implications are discussed for identifying individuals at risk so that early preventive-intervention can be provided.

Heritability of social behavioral phenotypes and preliminary associations with autism spectrum disorder risk genes in rhesus macaques: A whole exome sequencing study

Nonhuman primates and especially rhesus macaques (Macaca mulatta) have been indispensable animal models for studies of various aspects of neurobiology, developmental psychology, and other aspects of neuroscience. While remarkable progress has been made in our understanding of influences on atypical human social behavior, such as that observed in autism spectrum disorders (ASD), many significant questions remain. Improved understanding of the relationships among variation in specific genes and variation in expressed social behavior in a nonhuman primate would benefit efforts to investigate risk factors, developmental mechanisms, and potential therapies for behavioral disorders including ASD. To study genetic influences on key aspects of social behavior and interactions-individual competence and/or motivation for specific aspects of social behavior-we quantified individual variation in social interactions among juvenile rhesus macaques using both a standard macaque ethogram and a macaque-relevant modification of the human Social Responsiveness Scale. Our analyses demonstrate that various aspects of juvenile social behavior exhibit significant genetic heritability, with estimated quantitative genetic effects similar to that described for ASD in human children. We also performed exome sequencing and analyzed variants in 143 genes previously suggested to influence risk for human ASD. We find preliminary evidence for genetic association between specific variants and both individual behaviors and multi-behavioral factor scores. To our knowledge, this is the first demonstration that spontaneous social behaviors performed by free-ranging juvenile rhesus macaques display significant genetic heritability and then to use exome sequencing data to examine potential macaque genetic associations in genes associated with human ASD.

Cerebral Aβ deposition in an Aβ-precursor protein-transgenic rhesus monkey

With the ultimate goal of developing a more representative animal model of Alzheimer's disease (AD), two female amyloid-β-(Aβ) precursor protein-transgenic (APPtg) rhesus monkeys were generated by lentiviral transduction of the APP gene into rhesus oocytes, followed by in vitro fertilization and embryo transfer. The APP-transgene included the AD-associated Swedish K670N/M671L and Indiana V717F mutations (APPSWE/IND) regulated by the human polyubiquitin-C promoter. Overexpression of APP was confirmed in lymphocytes and brain tissue. Upon sacrifice at 10 years of age, one of the monkeys had developed Aβ plaques and cerebral Aβ-amyloid angiopathy in the occipital, parietal, and caudal temporal neocortices. The induction of Aβ deposition more than a decade prior to its usual emergence in the rhesus monkey supports the feasibility of creating a transgenic nonhuman primate model for mechanistic analyses and preclinical testing of treatments for Alzheimer's disease and cerebrovascular amyloidosis.

Validation of the Social Responsiveness Scale (SRS) to screen for atypical social behaviors in juvenile macaques

Primates form strong social bonds and depend on social relationships and networks that provide shared resources and protection critical for survival. Social deficits such as those present in autism spectrum disorder (ASD) and other psychiatric disorders hinder the individual’s functioning in communities. Given that early diagnosis and intervention can improve outcomes and trajectories of ASD, there is a great need for tools to identify early markers for screening/diagnosis, and for translational animal models to uncover biological mechanisms and develop treatments. One of the most widely used screening tools for ASD in children is the Social Responsiveness Scale (SRS), a quantitative measure used to identify individuals with atypical social behaviors. The SRS has been adapted for use in adult rhesus monkeys (Macaca mulatta)–a species very close to humans in terms of social behavior, brain anatomy/connectivity and development–but has not yet been validated or adapted for a necessary downward extension to younger ages matching those for ASD diagnosis in children. The goal of the present study was to adapt and validate the adult macaque SRS (mSRS) in juvenile macaques with age equivalent to mid-childhood in humans. Expert primate coders modified the mSRS to adapt it to rate atypical social behaviors in juvenile macaques living in complex social groups at the Yerkes National Primate Research Center. Construct and face validity of this juvenile mSRS (jmSRS) was determined based on well-established and operationalized measures of social and non-social behaviors in this species using traditional behavioral observations. We found that the jmSRS identifies variability in social responsiveness of juvenile rhesus monkeys and shows strong construct/predictive validity, as well as sensitivity to detect atypical social behaviors in young male and female macaques across social status. Thus, the jmSRS provides a promising tool for translational research on macaque models of children social disorders.

Structural development of cortical lobes during the first 6 months of life in infant macaques

This study mapped the developmental trajectories of cortical regions in comparison to overall brain growth in typically developing, socially-housed infant macaques. Volumetric changes of cortical brain regions were examined longitudinally between 2-24 weeks of age (equivalent to the first 2 years in humans) in 21 male rhesus macaques. Growth of the prefrontal, frontal, parietal, occipital, and temporal cortices (visual and auditory) was examined using MRI and age-specific infant macaque brain atlases developed by our group. Results indicate that cortical volumetric development follows a cubic growth curve, but maturational timelines and growth rates are region-specific. Total intracranial volume (ICV) increased significantly during the first 5 months of life, leveling off thereafter. Prefrontal and temporal visual cortices showed fast volume increases during the first 16 weeks, followed by a plateau, and significant growth again between 20-24 weeks. Volume of the frontal and temporal auditory cortices increased substantially between 2-24 weeks. The parietal cortex showed a significant volume increase during the first 4 months, whereas the volume of the occipital lobe increased between 2-12 weeks and plateaued thereafter. These developmental trajectories show similarities to cortical growth in human infants, providing foundational information necessary to build nonhuman primate (NHP) models of human neurodevelopmental disorders.

Emotional responses in monkeys differ depending on the stimulus type, sex, and neonatal amygdala lesion status

The amygdala plays an essential role in evaluating social information, threat detection, and learning fear associations. Yet, most of that knowledge comes from studies in adult humans and animals with a fully developed amygdala. Given the considerable protracted postnatal development of the amygdala, it is important to understand how early damage to this structure may impact the long-term development of behavior. The current study examined behavioral responses toward social, innate, or learned aversive stimuli among neonatal amygdala lesion (Neo-Aibo; males = 3, females = 3) or sham-operated control (Neo-C; males = 3, females = 4) rhesus macaques. Compared with controls, Neo-Aibo animals exhibited less emotional reactivity toward aversive objects, including faster retrieval of food reward, fewer fearful responses, and more manipulation of objects. This lower reactivity was only seen in response to social and innate aversive stimuli, whereas Neo-Aibo animals had similar responses to controls for learned aversive stimuli. The current study also detected sex differences in behavioral response to aversive stimuli, such that, as compared with males, females took longer to retrieve the food reward across all aversive stimuli types, but only expressed more hostility and more coo vocalizations during learned aversive trials. Early amygdala damage impacted the expression of some, but not all, sex differences. For example, neonatal amygdala damage eliminated the sex difference in object manipulation. These findings add important information that broaden our understanding of the role of the amygdala in the expression of sexually dimorphic behaviors, as well as its role in learning fear associations and threat detection. (PsycINFO Database Record (c) 2020 APA, all rights reserved).

Damage to Orbitofrontal Areas 12 and 13, but Not Area 14, Results in Blunted Attention and Arousal to Socioemotional Stimuli in Rhesus Macaques

An earlier study in monkeys indicated that lesions to the mid-portion of the ventral orbitofrontal cortex (OFC), including Walker’s areas 11 and 13 (OFC11/13), altered the spontaneous scanning of still pictures of primate faces (neutral and emotional) and the modulation of arousal. Yet, these conclusions were limited by several shortcomings, including the lesion approach, use of static rather than dynamic stimuli, and manual data analyses. To confirm and extend these earlier findings, we compared attention and arousal to social and nonsocial scenes in three groups of rhesus macaques with restricted lesions to one of three OFC areas (OFC12, OFC13, or OFC14) and a sham-operated control group using eye-tracking to capture scanning patterns, focal attention and pupil size. Animals with damage to the lateral OFC areas (OFC12 and OFC13) showed decreased attention specifically to the eyes of negative (threatening) social stimuli and increased arousal (increased pupil diameter) to positive social scenes. In contrast, animals with damage to the ventromedial OFC area (OFC14) displayed no differences in attention or arousal in the presence of social stimuli compared to controls. These findings support the notion that areas of the lateral OFC are critical for directing attention and modulating arousal to emotional social cues. Together with the existence of face-selective neurons in these lateral OFC areas, the data suggest that the lateral OFC may set the stage for multidimensional information processing related to faces and emotion and may be involved in social judgments.

Early Developmental Trajectories of Functional Connectivity Along the Visual Pathways in Rhesus Monkeys

Early social interactions shape the development of social behavior, although the critical periods or the underlying neurodevelopmental processes are not completely understood. Here, we studied the developmental changes in neural pathways underlying visual social engagement in the translational rhesus monkey model. Changes in functional connectivity (FC) along the ventral object and motion pathways and the dorsal attention/visuo-spatial pathways were studied longitudinally using resting-state functional MRI in infant rhesus monkeys, from birth through early weaning (3 months), given the socioemotional changes experienced during this period. Our results revealed that (1) maturation along the visual pathways proceeds in a caudo-rostral progression with primary visual areas (V1-V3) showing strong FC as early as 2 weeks of age, whereas higher-order visual and attentional areas (e.g., MT-AST, LIP-FEF) show weak FC; (2) functional changes were pathway-specific (e.g., robust FC increases detected in the most anterior aspect of the object pathway (TE-AMY), but FC remained weak in the other pathways (e.g., AST-AMY)); (3) FC matures similarly in both right and left hemispheres. Our findings suggest that visual pathways in infant macaques undergo selective remodeling during the first 3 months of life, likely regulated by early social interactions and supporting the transition to independence from the mother.

Early developmental changes in visual social engagement in infant rhesus monkeys

Impairments in social interaction in Autism Spectrum Disorder (ASD) differ greatly across individuals and vary throughout an individual’s lifetime. Yet, an important marker of ASD in infancy is deviations in social-visual engagement, such as the reliably detectable early deviations in attention to the eyes or to biological movement (Klin et al., 2015). Given the critical nature of these early developmental periods, understanding its neurobehavioral underpinnings by means of a nonhuman primate model will be instrumental to understanding the pathophysiology of ASD. Like humans, rhesus macaques 1) develop in rich and complex social behaviors, 2) progressively develop social skills throughout infancy, and 3) have high similarities with humans in brain anatomy and cognitive functions (Machado and Bachevalier, 2003). In this study, male infant rhesus macaques living with their mothers in complex social groups were eye-tracked longitudinally from birth to 6 months while viewing full-faced videos of unfamiliar rhesus monkeys differing in age and sex. The results indicated a critical period for the refinement of social skills around 4–8 weeks of age in rhesus macaques. Specifically, infant monkeys’ fixation to the eyes shows an inflection in developmental trajectory, increasing from birth to 8 weeks, decreasing slowly to a trough between 14–18 weeks, before increasing again. These results parallel the developmental trajectory of social visual engagement published in human infants (Jones & Klin, 2013) and suggest the presence of a switch in the critical networks supporting these early developing social skills that is highly conserved between rhesus macaque and human infant development.

Diffusion tensor imaging reveals microstructural alterations in corpus callosum and associated transcallosal fiber tracts in adult macaques with neonatal hippocampal lesions

To investigate the effects of neonatal hippocampal lesions on the microstructural integrity of the corpus callosum (CC) in adulthood, macaque monkeys (n = 5) with neonatal bilateral neurotoxic hippocampal lesion (Neo-Hibo) and sham-operated controls (Neo-C, n = 5) were scanned using magnetic resonance diffusion tensor imaging (DTI) technique at 8-10 years old. CC was segmented into seven regionsgrouped into anterior CC (rostrum, genu, rostral body and anterior midbody) and posterior CC (posterior midbody, isthmus and splenium) for data analysis. Associated transcallosal fiber tracts were delineated using probabilistic tractography and evaluated with tract-based spatial statistics (TBSS). Neo-Hibo lesions resulted in significant increased diffusivity indices (mean, axial and radial diffusivity) in CC posterior segments. Also, significant decreased fractional anisotropy (FA) and increased diffusivity indices were seen in the associated transcallosal fiber tracts proximal to motor, posterior parietal and retrosplenial cortices. In Neo-Hibo animals, increased mean diffusivity (MD) in posterior midbody negatively correlated with reduction of CC surface areaand the magnitude of their memory impairments was significantly correlated with FA in transcallosal fiber tracts across splenium. Although no microstructural changes were observed in CC anterior segments, changes in FA values and diffusivity indices were observed in the white matter fibers of the ventromedial prefrontal cortex. Thus, Neo-H lesions resulted in enduring degradation in transcallosal fibers proximal to parietal and retrosplenial cortices, and hemispheric connections through posterior CC. The findings may provide complementary information for understanding the neural substrate of behavioral and cognitive deficits observed in patients with early insult to the hippocampus.

Topology of the Structural Social Brain Network in Typical Adults

Although a large body of research has identified discrete neuroanatomical regions involved in social cognition and behavior (the "social brain"), the existing findings are based largely on studies of specific brain structures defined within the context of particular tasks or for specific types of social behavior. The objective of the current work was to view these regions as nodes of a larger collective network and to quantitatively characterize both the topology of that network and the relative criticality of its many nodes. Large-scale data mining was performed to generate seed regions of the social brain. High-quality diffusion MRI data of typical adults were used to map anatomical networks of the social brain. Network topology and nodal centrality were analyzed using graph theory. The structural social brain network demonstrates a high degree of global functional integration with strong local segregation. Bilateral dorsomedial prefrontal cortices and amygdala play the most central roles in the network. Strong probabilistic evidence supports modular divisions of the social brain into subnetworks bearing good resemblance to functionally classified clusters. The present network-driven approach quantifies the structural topology of the social brain as a whole. This work can serve as a critical benchmark against which to compare (1) developmental change in social brain topology over time (from infancy through adolescence and beyond) and (2) atypical network topologies that may be a sign or symptom of disorder (as in conditions such as autism, Williams syndrome, schizophrenia, and others).

Early amygdala damage alters the way rhesus macaques process species-specific audio-visual vocalizations

Perceiving, integrating, and interpreting multimodal signals are essential for social success, but the neural substrates mediating these functions are not fully understood. This study examined the role of the amygdala in processing bimodal species-specific vocalizations using eye tracking in rhesus macaques. Looking behavior of 6 adult rhesus monkeys with neonatal amygdala lesions (Neo-Aibo; 3M, 3F) was compared with that of 6 sham-operated controls (Neo-C; 3M, 3F). Two side-by-side videos of unknown male conspecifics emitting different vocalizations were presented with the audio signal matching one video. The percentage of time spent looking at each video was used to assess crossmodal integration ability and the percentages of time spent looking at a priori regions of interest (ROIs; eyes, mouth, and rest of each video) were used to characterize scanning patterns. Both groups looked more to one video, indicating that early amygdalar damage did not impair crossmodal integration of complex social signals. However, scanning patterns differed across groups as a function of sex and stimulus parameter. Whereas Neo-C males exhibited differential viewing to the eye and mouth regions as a function of the relative identity of the stimulus animals and Neo-C females made similar distinctions as a function of the relative valence of the vocalizations in females, Neo-Aibo males and females scanned these regions similarly across all trial types. The results suggest that neonatal amygdala damage alters the ability to perceive the social relevance of stimulus features, and are consistent with a role of the amygdala in the recognition of the social salience of complex cues. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Impaired Cognitive Flexibility After Neonatal Perirhinal Lesions in Rhesus Macaques

Previous research indicated that monkeys with neonatal perirhinal lesions (Neo-PRh) were impaired on working memory (WM) tasks that generated proactive interference, but performed normally on WM tasks devoid of interference (Weiss et al., 2016). This finding suggested that the early lesions disrupted cognitive processes important for resolving proactive interference, such as behavioral inhibition and cognitive flexibility. To distinguish between these possibilities, the same Neo-PRh monkeys and their controls were tested using the Intradimensional/Extradimensional attentional set-shifting task (Roberts et al., 1988; Dias et al., 1997). Neo-PRh monkeys completed the Simple and Compound Discrimination stages, the Intradimensional Shift stage, and all Reversal stages comparably to controls, but made significantly more errors on the Extradimensional Shift stage of the task. These data indicate that impaired cognitive flexibility was the likely source of increased perseverative errors made by Neo-PRh monkeys when performing WM tasks, rather than impaired behavioral inhibition, and imply that the perirhinal cortex and its interactions with the PFC may play a unique and critical role in the development of attentional set shifting abilities.

Neonatal hippocampal lesions facilitate biconditional contextual discrimination learning in monkeys

This study examined whether selective neonatal hippocampal lesions in monkeys (Macaca mulatta), which left the surrounding cortical areas (parahippocampal cortex) intact, affect contextual learning and memory compared with controls. Monkeys were tested with an automated touch-screen apparatus so that stimuli and contextual cues could be manipulated independently of one another. The data suggest that animals with neonatal hippocampal lesions have sparing of function with regard to contextual learning and memory when (a) contextual information is irrelevant or (b) relevant for good discrimination performance, and (c) when transferring a contextual rule to new discriminations. These findings are at odds with studies examining contextual learning and memory in monkeys with selective adult-onset hippocampal lesions, and those with nonselective neonatal hippocampal lesions, which have demonstrated impairment in contextual learning and memory. Therefore, the sparing of function seen in this study may be attributable to the early nature of the damage and the plastic nature of the infant brain, as well as the intact medial temporal lobe cortical areas as a result of the lesion methodology. Specifically, by removing the hippocampus early in life, before it has begun to function, the parahippocampal (TH/TF) and perirhinal cortices and its interactions with the lateral prefrontal cortex may be able to support context processing throughout life. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Intact perceptual ability, but impaired familiarity judgment, after neonatal perirhinal lesions in rhesus macaques

The perirhinal cortex is known to support high-level perceptual abilities as well as familiarity judgments that may affect recognition memory. We tested whether poor perceptual abilities or a loss of familiarity judgment contributed to the recognition memory impairments reported earlier in monkeys with PRh lesions received in infancy (Neo-PRh) (Weiss and Bachevalier, 2016; Zeamer et al., 2015). Perceptual abilities were assessed using a version of the Visual Paired Comparison task with black&white (B&W) stimuli, and familiarity judgments were assessed using the Constant Negative task requiring repeated familiarization exposures. Adult monkeys with Neo-PRh lesions were able to recognize B&W stimuli after short delays, suggesting that their perceptual abilities were within the range of control animals. However, the same Neo-PRh monkeys were slower to acquire the Constant Negative task, requiring more exposures to objects before judging them as familiar compared to control animals. Taken together, the data help to account for the differential patterns of functional compensation on previously reported recognition tasks following neonatal versus adult-onset PRh lesions, and provide further support to the view that the PRh is involved in familiarity processes.

Neonatal perirhinal cortex lesions impair monkeys' ability to modulate their emotional responses

The medial temporal lobe (MTL) is a collection of brain regions best known for their role in perception, memory, and emotional behavior. Within the MTL, the perirhinal cortex (PRh) plays a critical role in perceptual representation and recognition memory, although its contribution to emotional regulation is still debated. Here, rhesus monkeys with neonatal perirhinal lesions (Neo-PRh) and controls (Neo-C) were tested on the Human Intruder (HI) task at 2 months, 4.5 months, and 5 years of age to assess the role of the PRh in the development of emotional behaviors. The HI task presents a tiered social threat to which typically developing animals modulate their emotional responses according to the level of threat. Unlike animals with neonatal amygdala or hippocampal lesions, Neo-PRh animals were not broadly hyper- or hyporesponsive to the threat presented by the HI task as compared with controls. Instead, Neo-PRh animals displayed an impaired ability to modulate their freezing and anxiety-like behavioral responses according to the varying levels of threat. Impaired transmission of perceptual representation generated by the PRh to the amygdala and hippocampus may explain the animals' inability to appropriately assess and react to complex social stimuli. Neo-PRh animals also displayed fewer hostile behaviors in infancy and more coo vocalizations in adulthood. Neither stress-reactive nor basal cortisol levels were affected by the Neo-PRh lesions. Overall, these results suggest that the PRh is indirectly involved in the expression of emotional behavior and that effects of Neo-PRh lesions are dissociable from neonatal lesions to other temporal lobe structures. (PsycINFO Database Record

Metabolism and Distribution of Clozapine-N-oxide: Implications for Nonhuman Primate Chemogenetics

The use of Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) in neuroscience has rapidly expanded in rodent studies but has lagged behind in nonhuman primate (NHP) experiments, slowing the development of this method for therapeutic use in humans. One reason for the slow adoption of DREADD technology in primates is that the pharmacokinetic properties and bioavailability of clozapine-n-oxide (CNO), the most commonly used ligand for human muscarinic (hM) DREADDs, are not fully described in primates. We report an extensive pharmacokinetic study using subcutaneous (SC) administration of CNO in five adult rhesus monkeys. CNO reached maximal plasma and cerebrospinal fluid (CSF) concentrations within 2 h after injection, with an observed dose-dependent increase in levels following a 3 and 10 mg/kg SC dose. Since CSF concentrations were below values predicted from unbound plasma concentrations, we investigated whether CNO was restricted from the CNS through active transport at the blood-brain barrier. In vitro assessment demonstrated that CNO is a substrate for P-glycoprotein (Pgp; efflux ratio, 20), thus providing a likely mechanism limiting CNO levels in the CNS. Furthermore, CNO is metabolized to the psychoactive compounds clozapine and n-desmethylclozapine in monkeys. The concentrations of clozapine detected in the CSF are sufficient to activate several types of receptor (including the hM-DREADDs). Our results suggest that CNO metabolism and distribution may interfere with reproducibility and interpretation of DREADD-related experiments in NHPs and calls for a re-evaluation of the use of CNO in DREADD-related experiments in NHPs along with the need to test alternative compounds.

An MRI study of the corpus callosum in monkeys: Developmental trajectories and effects of neonatal hippocampal and amygdala lesions

This study provides the first characterization of early developmental trajectories of corpus callosum (CC) segments in rhesus macaques using noninvasive MRI techniques and assesses long-term effects of neonatal amygdala or hippocampal lesions on CC morphometry. In Experiment 1, 10 monkeys (5 males) were scanned at 1 week-2 years of age; eight additional infants (4 males) were scanned once at 1-4 weeks of age. The first 8 months showed marked growth across all segments, with sustained, albeit slower, growth through 24 months. Males and females had comparable patterns of CC maturation overall, but exhibited slight differences in the anterior and posterior segments, with greater increases in the isthmus for males and greater increases in the rostrum for females. The developmental changes are likely a consequence of varying degrees of axonal myelination, redirection, and pruning. In Experiment 2, animals with neonatal lesions of the amygdala (n = 6; 3 males) or hippocampus (n = 6; 4 males) were scanned at 1.5 years post-surgery and compared to scans of six control animals from Experiment 1. Whereas amygdala damage yielded larger rostral and posterior body segments, hippocampal damage yielded larger rostrum and isthmus. These differences demonstrate that early perturbations to one medial temporal lobe structure may produce extensive and long-lasting repercussions in other brain areas. The current findings emphasize the complexity of neural circuitry putatively subserving neurodevelopmental disorders such as autism spectrum disorder and Williams syndrome, which are each characterized by malformations and dysfunction of complex neural networks that include regions of the medial temporal lobe.

Developmental Whole Brain White Matter Alterations in Transgenic Huntington's Disease Monkey

Transgenic Huntington’s disease monkey (HD monkey) model provides great opportunity for studying disease progression that could lead to new insight for developing biomarker, early intervention and novel therapeutics. Whole brain white matter integrity of HD-monkeys was examined longitudinally from 6 to 48 months using diffusion tensor imaging (DTI) and tract-based spatial statistics (TBSS). Progressive developmental white matter alterations in HD monkeys were widespread and were observed not only in fiber bundles connecting cortical areas to the striatum (e.g. striatal bundle and external capsule), but also in long association fiber pathways, commissural fibers, and subcortical fiber bundle. In all fiber tracts, the data indicate an arrest in white matter development around 23 months followed by slight decline until adulthood in HD monkeys. The microstructural changes parallel the progressive motor, memory and cognitive decline previously reported as HD monkeys aged. The findings revealed the widespread progressive temporal-spatial microstructural changes in HD monkey brains from infancy to adulthood, suggesting differentiated degenerations across different brain areas during brain development.

Increased anxiety-like behaviors, but blunted cortisol stress response after neonatal hippocampal lesions in monkeys

The hippocampus is most notably known for its role in cognition and spatial memory; however it also plays an essential role in emotional behaviors and neuroendocrine responses. The current study investigated the long-term effects of neonatal hippocampal lesions (Neo-Hibo) on emotional and hypothalamic-pituitary-adrenal (HPA) axis functioning. During infancy, unlike controls, Neo-Hibo monkeys exhibited enhanced expression of emotional behaviors (e.g. freezing, anxiety-like, and self-directed behaviors) when exposed to a human intruder (HI task). Upon reaching adulthood, they exhibited reduced freezing and hostility, but increased anxiety-like and self-directed behaviors during the HI task. Neo-Hibo monkeys behaved as if they systematically over-rated the risk inherent in the HI task, which supports Gray and McNaughton's septo-hippocampal theory of anxiety. Also, in adulthood, the increased levels of anxiety-like behaviors in Neo-Hibo monkeys were associated with a blunted cortisol response to the HI task. Examination of basal HPA axis function revealed that Neo-Hibo monkeys exhibited the typical diurnal cortisol decline throughout the day, but had lower cortisol concentrations in the morning as compared to controls. Taken together these data suggest that an intact hippocampus during development plays a larger role beyond that of inhibitory/negative feedback regulation of the HPA axis stress-activation, and may be critical for HPA axis basal functioning as well as for the stress response. The behavioral and neuroendocrine changes demonstrated in the current study are reminiscent of those seen in human or nonhuman primates with adult-onset hippocampal damage, demonstrating little functional compensation following early hippocampal damage.

Development of relational memory processes in monkeys

The present study tested whether relational memory processes, as measured by the transverse patterning problem, are late-developing in nonhuman primates as they are in humans. Eighteen macaques ranging from 3 to 36 months of age, were trained to solve a set of visual discriminations that formed the transverse patterning problem. Subjects were trained at 3, 4-6, 12, 15-24 or 36 months of age to solve three discriminations as follows: 1) A+ vs. B-; 2) B+ vs. C-; 3) C+ vs. A. When trained concurrently, subjects must adopt a relational strategy to perform accurately on all three problems. All 36 month old monkeys reached the criterion of 90% correct, but only one 24-month-old and one 15-month-old did, initially. Three-month-old infants performed at chance on all problems. Six and 12-month-olds performed at 75-80% correct but used a 'linear' or elemental solution (e.g. A>B>C), which only yields correct performance on two problems. Retraining the younger subjects at 12, 24 or 36 months yielded a quantitative improvement on speed of learning, and a qualitative improvement in 24-36 month old monkeys for learning strategy. The results suggest that nonspatial relational memory develops late in macaques (as in humans), maturing between 15 and 24 months of age.

Hippocampal and diencephalic pathology in developmental amnesia

Developmental amnesia (DA) is a selective episodic memory disorder associated with hypoxia-induced bilateral hippocampal atrophy of early onset. Despite the systemic impact of hypoxia-ischaemia, the resulting brain damage was previously reported to be largely limited to the hippocampus. However, the thalamus and the mammillary bodies are parts of the hippocampal-diencephalic network and are therefore also at risk of injury following hypoxic-ischaemic events. Here, we report a neuroimaging investigation of diencephalic damage in a group of 18 patients with DA (age range 11-35 years), and an equal number of controls. Importantly, we uncovered a marked degree of atrophy in the mammillary bodies in two thirds of our patients. In addition, as a group, patients had mildly reduced thalamic volumes. The size of the anterior-mid thalamic (AMT) segment was correlated with patients' visual memory performance. Thus, in addition to the hippocampus, the diencephalic structures also appear to play a role in the patients' memory deficit.

Decreased functional connectivity in dorsolateral prefrontal cortical networks in adult macaques with neonatal hippocampal lesions: Relations to visual working memory deficits

Neonatal hippocampal lesions in monkeys impairs normal performance on both relational and working memory tasks, suggesting that the early lesions have impacted the normal development of prefrontal-hippocampal functional interactions necessary for normal performance on these tasks. Given that working memory processes engage distributed neuronal networks associated with the prefrontal cortex, it is critical to explore the integrity of distributed neural networks of dorsolateral prefrontal cortex (dlPFC) following neonatal hippocampal lesions in monkeys. We used resting-state functional MRI to assess functional connectivity of dlPFC networks in monkeys with neonatal neurotoxic hippocampal lesion (Neo-Hibo, n=4) and sham-operated control animals (Neo-C, n=4). Significant differences in the patterns of dlPFC functional networks were found between Groups Neo-Hibo and Neo-C. The within-group maps and the between-group comparisons yielded a highly coherent picture showing altered interactions of core regions of the working memory network (medial prefrontal cortex and posterior parietal cortex) as well as the dorsal (fundus of superior temporal area and superior temporal cortex) and ventral (V4 and infero-temporal cortex) visual processing areas in animals with Neo-Hibo lesions. Correlations between functional connectivity changes and working memory impairment in the same animals were found only between the dlPFC and visual cortical areas (V4 and infero-temporal cortex). Thus, the impact of the neonatal hippocampal lesions extends to multiple cortical areas interconnected with the dlPFC.

Neonatal Perirhinal Lesions in Rhesus Macaques Alter Performance on Working Memory Tasks with High Proactive Interference

The lateral prefrontal cortex is known for its contribution to working memory (WM) processes in both humans and animals. Yet, recent studies indicate that the prefrontal cortex is part of a broader network of interconnected brain areas involved in WM. Within the medial temporal lobe (MTL) structures, the perirhinal cortex, which has extensive direct interactions with the lateral and orbital prefrontal cortex, is required to form active/flexible representations of familiar objects. However, its participation in WM processes has not be fully explored. The goal of this study was to assess the effects of neonatal perirhinal lesions on maintenance and monitoring WM processes. As adults, animals with neonatal perirhinal lesions and their matched controls were tested in three object-based (non-spatial) WM tasks that tapped different WM processing domains, e.g., maintenance only (Session-unique Delayed-nonmatching-to Sample, SU-DNMS), and maintenance and monitoring (Object-Self-Order, OBJ-SO; Serial Order Memory Task, SOMT). Neonatal perirhinal lesions transiently impaired the acquisition of SU-DNMS at a short (5 s) delay, but not when re-tested with a longer delay (30 s). The same neonatal lesions severely impacted acquisition of OBJ-SO task, and the impairment was characterized by a sharp increase in perseverative errors. By contrast, neonatal perirhinal lesion spared the ability to monitor the temporal order of items in WM as measured by the SOMT. Contrary to the SU-DNMS and OBJ-SO, which re-use the same stimuli across trials and thus produce proactive interference, the SOMT uses novel objects on each trial and is devoid of interference. Therefore, the impairment of monkeys with neonatal perirhinal lesions on SU-DNMS and OBJ-SO tasks is likely to be caused by an inability to solve working memory tasks with high proactive interference. The sparing of performance on the SOMT demonstrates that neonatal perirhinal lesions do not alter working memory processes per se but rather impact processes modulating impulse control and/or behavioral flexibility.

Object and spatial memory after neonatal perirhinal lesions in monkeys

The contribution of the perirhinal cortex (PRh) to recognition memory is well characterized in adults, yet the same lesions have limited effect on recognition of spatial locations. Here, we assessed whether the same outcomes will follow when perirhinal lesions are performed in infancy. Monkeys with neonatal perirhinal (Neo-PRh) lesions and control animals were tested in three operant recognition tasks as they reached adulthood: Delayed Nonmatching-to-Sample (DNMS) and Object Memory Span (OMS), measuring object recognition, and Spatial Memory Span (SMS), measuring recognition of spatial locations. Although Neo-PRh lesions did not impact acquisition of the DNMS rule, they did impair performance when the delays were extended from 30s to 600s. In contrast, the same neonatal lesions had no impact on either the object or spatial memory span tasks, suggesting that the lesions impacted the maintenance of information across longer delays and not memory capacity. Finally, the magnitude of recognition memory impairment after the Neo-PRh lesions was similar to that previously observed after adult-onset perirhinal lesions, indicating minimal, or no, functional compensation after the early PRh lesions. Overall, the results indicate that the PRh is a cortical structure that is important for the normal development of mechanisms supporting object recognition memory. Its contribution may be relevant to the memory impairment observed with human cases of temporal lobe epilepsy without hippocampal sclerosis, but not to the memory impairment found in developmental amnesia cases.

Progressive cognitive deficit, motor impairment and striatal pathology in a transgenic Huntington disease monkey model from infancy to adulthood

One of the roadblocks to developing effective therapeutics for Huntington disease (HD) is the lack of animal models that develop progressive clinical traits comparable to those seen in patients. Here we report a longitudinal study that encompasses cognitive and motor assessment, and neuroimaging of a group of transgenic HD and control monkeys from infancy to adulthood. Along with progressive cognitive and motor impairment, neuroimaging revealed a progressive reduction in striatal volume. Magnetic resonance spectroscopy at 48 months of age revealed a decrease of N-acetylaspartate (NAA), further suggesting neuronal damage/loss in the striatum. Postmortem neuropathological analyses revealed significant neuronal loss in the striatum. Our results indicate that HD monkeys share similar disease patterns with HD patients, making them potentially suitable as a preclinical HD animal model.

Neonatal amygdala lesions advance pubertal timing in female rhesus macaques

Social context influences the timing of puberty in both humans and nonhuman primates, such as delayed first ovulation in low-ranking rhesus macaques, but the brain region(s) mediating the effects of social context on pubertal timing are unknown. The amygdala is important for responding to social information and thus, is a potential brain region mediating the effects of social context on pubertal timing. In this study, female rhesus macaques living in large, species-typical, social groups received bilateral neurotoxic amygdala lesions at one month of age and pubertal timing was examined beginning at 14 months of age. Pubertal timing was affected in neonatal amygdala-lesioned females (Neo-A), such that they experienced significantly earlier menarche and first ovulation than did control females (Neo-C). Duration between menarche and first ovulation did not differ between Neo-A and Neo-C females, indicating earlier first ovulation in Neo-A females was likely a consequence of earlier menarche. Social rank of Neo-A females was related to age at menarche, but not first ovulation, and social rank was not related to either event in Neo-C females. It is more likely that amygdalectomy affects pubertal timing through its modulation of GABA-ergic mechanisms rather than as a result of the removal of a social-contextual inhibition on pubertal timing.

Mother recognition and preference after neonatal amygdala lesions in rhesus macaques (Macaca mulatta) raised in a semi-naturalistic environment

Attachment to the caregiver, typically the biological mother, is crucial to young mammals' socio-emotional development. Although studies in nonprimate species suggest that the amygdala regulates social preference and attachment development, its role in primate filial attachment development has been little investigated and has produced mixed results. This study assessed the effects of neonatal amygdala- (Neo-A, N = 16) and sham- (Neo-C, N = 12) lesions on mother recognition and discrimination in macaques raised in species-typical social groups. Neonatal amygdalectomy did not affect social discriminative abilities and mother preference at 3 and 6 months of age, strongly suggesting that the amygdala is not involved in the cognitive processes underlying the development of filial attachment at least when the amygdala damage occurred after the third to fourth weeks of age. Nevertheless, as compared to sham-operated controls, amygdalectomized infants initiated physical contact with their mothers less frequently. The findings are discussed in relation to the known contribution of the amygdala to filial attachment in both rodents and humans.

Effects of Separate or Combined Neonatal Damage to the Orbital Frontal Cortex and the Inferior Convexity on Object Recognition in Monkeys

Unlike adult damage, neonatal damage to the inferior prefrontal convexity (IC) in monkeys spares learning and performance on the delayed nonmatching-to-sample (DNMS) task ( Málková et al. 2000). We investigated whether this sparing was due to compensation by undamaged orbital frontal cortex (O), an area also critical for DNMS, by comparing combined IC and O damage (Neo-ICO) with damage to O alone (Neo-O). Group Neo-ICO was impaired on DNMS learning at 3 months and 2 years of age. In contrast, Group Neo-O was impaired at 3 months, but recovered this function by 2 years, compared with Neo-IC and controls (N). We propose that the intact IC assumed the function of learning the DNMS rule for Group Neo-O. The persistent impairment after Neo-ICO lesions suggests that whereas O may likely support the rule acquisition in the absence of IC, no compensatory mechanisms are available after the combined damage. For the memory of lists of items, all groups were impaired at 3 months. At 2 years, the performance of Groups N and Neo-IC dramatically improved, whereas that of groups with O damage (Neo-O and Neo-ICO) remained impaired, indicating a critical role for O in recognition memory that cannot be substituted by another area.

Alterations of hippocampal projections in adult macaques with neonatal hippocampal lesions: a Diffusion Tensor Imaging study

Neuropsychological and brain imaging studies have demonstrated persistent deficits in memory functions and structural changes after neonatal neurotoxic hippocampal lesion in monkeys. However, the relevant microstructural changes in the white matter of affected brain regions following this early insult remain unknown. This study assessed white matter integrity in the main hippocampal projections of adult macaque monkeys with neonatal hippocampal lesions, using diffusion tensor imaging (DTI). Data analysis was performed using tract-based spatial statistics (TBSS) and compared with volume of interest statistics. Alterations of fractional anisotropy (FA) and diffusivity indices were observed in fornix, temporal stem, ventromedial prefrontal cortex and optical radiations. To further validate the lesion effects on the prefrontal cortex, probabilistic diffusion tractography was used to examine the integrity of the fiber connections between hippocampus and ventromedial prefrontal cortex, and alterations were found in these connections. In addition, increased radial diffusivity in the left ventromedial prefrontal cortex correlated negatively with the severity of deficits in working memory in the same monkeys. The findings revealed microstructural changes due to neonatal hippocampal lesion, and confirmed that neonatal neurotoxic hippocampal lesions resulted in significant and enduring functional alterations in the hippocampal projection system.

The influence of context on recognition memory in monkeys: effects of hippocampal, parahippocampal and perirhinal lesions

This study further investigated the specific contributions of the medial temporal lobe structures to contextual recognition memory. Monkeys (Macaca mulatta) with either neurotoxic lesions of the hippocampus, aspiration lesions of the perirhinal cortex and parahippocampal areas TH/TF, or sham operations were tested on five conditions of a visual-paired comparison (VPC) task in which 3-dimensional objects were presented over multicolored backgrounds. In two conditions (Conditions 1 and 2: Context-changes), the sample object was presented on a new background during the retention tests, whereas in the three others (Conditions 3-5: No-context-changes) the sample object was presented over its familiar background. Novelty preference scores of control animals were weaker, but still significantly different from chance, in the Context-changes conditions than on the No-context-changes conditions. Animals in the three experimental groups showed strong preference for novelty on the No-context-change conditions, but weaker novelty preference on the Context-change conditions than controls. Thus, animals in all three lesion types had greater difficulty recognizing an object when its background was different from that used during encoding. The data are consistent with the view that the hippocampal formation, areas TH/TF, and perirhinal cortex contribute interactively to contextual memory processes.

Neonatal amygdala lesions alter mother-infant interactions in rhesus monkeys living in a species-typical social environment

The current study examined the effects of neonatal amygdala lesions on mother-infant interactions in rhesus monkeys reared in large species-typical social groups. Focal observations of mother-infant interactions were collected in their social group for the first 12 months postpartum on infants that had received amygdala lesions (Neo-A) at 24-25 days of age and control infants. Early amygdala lesions resulted in subtle behavioral alterations. Neo-A females exhibited earlier emergence of independence from the mother than did control females, spending more time away from their mother, whereas Neo-A males did not. Also, a set of behaviors, including coo vocalizations, time in contact, and time away from the mother, accurately discriminated Neo-A females from control females, but not Neo-A and control males. Data suggest that neonatal amygdalectomy either reduced fear, therefore increasing exploration in females, or reduced the positive reward value of maternal contact. Unlike females, neonatal amygdala lesions had little measurable effects on male mother-infant interactions. The source of this sex difference is unknown.

Face recognition in primates: a cross-species study

Recognition for human faces, monkey faces, and objects was assessed in both adult humans (Homo sapiens) and monkeys (Macaca mulatta) with a visual paired-comparison task. The results demonstrated that while both species showed strong novelty preference for objects, human participants showed novelty preference for human faces but not for monkey faces, and vice versa for the monkeys. This `species-specific effect' in face recognition is discussed in relation with data on both the `other-race effect' observed in humans and the effects of experience or training on face recognition processes in primates.

Neonatal lesions of orbital frontal areas 11/13 in monkeys alter goal-directed behavior but spare fear conditioning and safety signal learning

Recent studies in monkeys have demonstrated that damage to the lateral subfields of orbital frontal cortex (OFC areas 11/13) yields profound changes in flexible modulation of goal-directed behaviors and deficits in fear regulation. Yet, little consideration has been placed on its role in emotional and social development throughout life. The current study investigated the effects of neonatal lesions of the OFC on the flexible modulation of goal-directed behaviors and fear responses in monkeys. Infant monkeys received neonatal lesions of OFC areas 11/13 or sham-lesions during the first post-natal week. Modulation of goal-directed behaviors was measured with a devaluation task at 3–4 and 6–7 years. Modulation of fear reactivity by safety signals was assessed with the AX+/BX− fear-potentiated-startle paradigm at 6–7 years. Similar to adult-onset OFC lesions, selective neonatal lesions of OFC areas 11/13 yielded a failure to modulate behavioral responses guided by changes in reward value, but spared the ability to modulate fear responses in the presence of safety signals. These results suggest that these areas play a critical role in the development of behavioral adaptation during goal-directed behaviors, but not or less so, in the development of the ability to process emotionally salient stimuli and to modulate emotional reactivity using environmental contexts, which could be supported by other OFC subfields, such as the most ventromedial subfields (i.e., areas 14/25). Given similar impaired decision-making abilities and spared modulation of fear after both neonatal lesions of either OFC areas 11 and 13 or amygdala (Kazama et al., 2012; Kazama and Bachevalier, 2013), the present results suggest that interactions between these two neural structures play a critical role in the development of behavioral adaptation; an ability essential for the self-regulation of emotion and behavior that assures the maintenance of successful social relationships.

A two years longitudinal study of a transgenic Huntington disease monkey

Background

A two-year longitudinal study composed of morphometric MRI measures and cognitive behavioral evaluation was performed on a transgenic Huntington’s disease (HD) monkey. rHD1, a transgenic HD monkey expressing exon 1 of the human gene encoding huntingtin (HTT) with 29 CAG repeats regulated by a human polyubiquitin C promoter was used together with four age-matched wild-type control monkeys. This is the first study on a primate model of human HD based on longitudinal clinical measurements.

Results

Changes in striatal and hippocampal volumes in rHD1 were observed with progressive impairment in motor functions and cognitive decline, including deficits in learning stimulus-reward associations, recognition memory and spatial memory. The results demonstrate a progressive cognitive decline and morphometric changes in the striatum and hippocampus in a transgenic HD monkey.

Conclusions

This is the first study on a primate model of human HD based on longitudinal clinical measurements. While this study is based a single HD monkey, an ongoing longitudinal study with additional HD monkeys will be important for the confirmation of our findings. A nonhuman primate model of HD could complement other animal models of HD to better understand the pathogenesis of HD and future development of diagnostics and therapeutics through longitudinal assessment.

Crossmodal integration of conspecific vocalizations in rhesus macaques

Crossmodal integration of audio/visual information is vital for recognition, interpretation and appropriate reaction to social signals. Here we examined how rhesus macaques process bimodal species-specific vocalizations by eye tracking, using an unconstrained preferential looking paradigm. Six adult rhesus monkeys (3M, 3F) were presented two side-by-side videos of unknown male conspecifics emitting different vocalizations, accompanied by the audio signal corresponding to one of the videos. The percentage of time animals looked to each video was used to assess crossmodal integration ability and the percentages of time spent looking at each of the six a priori ROIs (eyes, mouth, and rest of each video) were used to characterize scanning patterns. Animals looked more to the congruent video, confirming reports that rhesus monkeys spontaneously integrate conspecific vocalizations. Scanning patterns showed that monkeys preferentially attended to the eyes and mouth of the stimuli, with subtle differences between males and females such that females showed a tendency to differentiate the eye and mouth regions more than males. These results were similar to studies in humans indicating that when asked to assess emotion-related aspects of visual speech, people preferentially attend to the eyes. Thus, the tendency for female monkeys to show a greater differentiation between the eye and mouth regions than males may indicate that female monkeys were slightly more sensitive to the socio-emotional content of complex signals than male monkeys. The current results emphasize the importance of considering both the sex of the observer and individual variability in passive viewing behavior in nonhuman primate research.

Long-term effects of neonatal hippocampal lesions on novelty preference in monkeys

In a recent longitudinal study to assess the development of incidental recognition memory processes in monkeys, we showed that the effects of neonatal hippocampal lesions did alter incidental recognition memory only when the animals reached the juvenile period (Zeamer et al., ). The current follow-up study tested whether this incidental memory loss was long-lasting, i.e., present in adulthood, or only transitory, due to functional compensation with further brain maturation. The same animals with neonatal hippocampal lesions and their sham-operated controls were re-tested in the visual paired-comparison task when they reached adulthood (48 months). The results demonstrated that, at least for easily discriminable color pictures of objects, the involvement of the hippocampus was only transitory, given that when re-tested as adults, animals with neonatal hippocampal lesions performed as well as sham-operated controls at all delays. Yet, significant recognition memory impairment was re-instated when the discriminability of the stimuli was made more difficult (black/white pictures of similar objects). The data demonstrate profound functional remodeling within the hippocampus and its interactions with different medial temporal lobe structures from the juvenile period to adulthood, which is substantiated by a parallel morphological maturation of hippocampal intrinsic circuits (Lavenex et al., ; Jabès et al., ).

Pervasive alterations of emotional and neuroendocrine responses to an acute stressor after neonatal amygdala lesions in rhesus monkeys

The current study examined the long-term effects of neonatal amygdala lesions on emotional and hypothalamic-pituitary-adrenal (HPA) axis reactivity to an acute stressor in rhesus monkeys. Rhesus monkeys received either bilateral MRI-guided ibotenic acid amygdala (Neo-Aibo; n=6) or sham (Neo-C; n=7) lesions between 7 and 14 days of age. Emotional reactivity was assessed using the Human Intruder paradigm at 2 months, 4.5 months, and 6-8 years of age, whereas stress neuroendocrine response was only assessed in adulthood (6-8 years). The modulation of defensive and emotional behaviors based on the gaze direction of the intruder emerged between 2 and 4 months of age in surrogate-peer reared sham-operated infant monkeys, as already shown for mother-reared infants. Although neonatal amygdala lesions did not impair the ability to exhibit defensive and emotional behaviors, it altered the modulation of these responses based on the intruder's gaze direction. The changes in emotional reactivity after neonatal amygdala lesions emerged in infancy and persisted throughout adulthood when they were associated with a reduction of basal cortisol levels and a blunted cortisol response to the stressor. These changes are reminiscent of those found after adult-onset amygdala lesions, demonstrating little functional compensation following early amygdala damage.

Neonatal amygdala lesions alter basal cortisol levels in infant rhesus monkeys

The amygdala is mostly thought to exert an excitatory influence on the hypothalamic-pituitary-adrenal (HPA) axis, although its role regulating HPA basal tone is less clear, particularly during primate development. The current study examined the effects of neonatal amygdala lesions on basal HPA function and the postnatal testosterone (T) surge of rhesus monkeys reared with their mothers in large outdoor social groups. An early morning basal blood sample was collected at 2.5 months of age, whereas at 5 months samples were collected not only at sunrise, but also at mid-day and sunset to examine the diurnal rhythm of cortisol. At 2.5 months of age sham-operated males exhibited higher cortisol than females, but this sex difference was abolished by neonatal amygdalectomy, with lesioned males also showing lower basal cortisol than controls. Although neonatal amygdalectomy did not alter the postnatal T surge, there was a positive relationship between T and basal cortisol levels. At 5 months of age, neither the sex difference in cortisol, nor its correlation with T levels were apparent any longer. Instead, the diurnal cortisol rhythm of both males and females with amygdalectomy showed a blunted decline from mid-day to sunset compared to controls. These results indicate that neonatal amygdala damage alters basal HPA function in infant rhesus monkeys, affecting males only at early ages (at 2.5 months), while leaving the postnatal T surge intact, and resulting in a flattened diurnal rhythm in both genders at the later ages. Thus, the primate amygdala has a critical influence on the HPA axis in the first few months of life.

Sex-dependent role of the amygdala in the development of emotional and neuroendocrine reactivity to threatening stimuli in infant and juvenile rhesus monkeys

Amygdala dysfunction and abnormal fear and stress reactivity are common features of several developmental neuropsychiatric disorders. Yet, little is known about the exact role the amygdala plays in the development of threat detection and emotional modulation. The current study examined the effects of neonatal amygdala lesions on defensive, emotional, and neuroendocrine reactivity of infant rhesus monkeys reared with their mothers in large species-typical social groups. Monkeys received either bilateral MRI-guided ibotenic acid amygdala (Neo-A; n = 16) or sham (Neo-C; n = 12) lesions at 24.8 ± 1.2 days of age, or served as behavioral control (Neo-BC; n = 3). Defensive and emotional responses were assessed using the Human Intruder paradigm as infants and as juveniles (2.5 and 12 months of age, respectively), whereas neuroendocrine reactivity was only examined during the juvenile period. As infants, Neo-A animals expressed similar levels of freezing and hostile behaviors as compared to controls, whereas during the juvenile period Neo-A animals expressed significantly less freezing compared to controls. Interestingly, the sex of the infant modulated the behavioral effects of neonatal amygdalectomy, leading to different patterns of behavior depending on the sex and lesion status of the infant. Unlike controls, Neo-A infants did not modulate their behavioral responses based on the salience of the threat. The impact of neonatal amygdalectomy increased with age, such that Neo-A juveniles exhibited fewer emotional behaviors and increased cortisol response to the stressor as compared to controls. These data indicate that the amygdala plays a critical role in the development of both emotional and neuroendocrine reactivity as well as the expression of sexually dimorphic emotional expression.