A conserved pattern of differential expansion of cortical areas in simian primates

High-Expanding Regions in Primate Cortical Brain Evolution Support Supramodal Cognitive Flexibility

Primate cortical evolution has been characterized by massive and disproportionate expansion of a set of specific regions in the neocortex. The associated increase in neocortical neurons comes with a high metabolic cost, thus the functions served by these regions must have conferred significant evolutionary advantage. In the present series of analyses, we show that evolutionary high-expanding cortex - as estimated from patterns of surface growth from several primate species - shares functional connections with different brain networks in a context-dependent manner. Specifically, we demonstrate that high-expanding cortex is characterized by high internetwork functional connectivity; is recruited flexibly over many different cognitive tasks; and changes its functional coupling pattern between rest and a multimodal task-state. The capacity of high-expanding cortex to connect flexibly with various specialized brain networks depending on particular cognitive requirements suggests that its selective growth and sustainment in evolution may have been linked to an involvement in supramodal cognition. In accordance with an evolutionary-developmental view, we find that this observed ability of high-expanding regions - to flexibly modulate functional connections as a function of cognitive state - emerges gradually through childhood, with a prolonged developmental trajectory plateauing in young adulthood.

Divergence of rodent and primate medial frontal cortex functional connectivity

With the medial frontal cortex (MFC) centrally implicated in several major neuropsychiatric disorders, it is critical to understand the extent to which MFC organization is comparable between humans and animals commonly used in preclinical research (namely rodents and nonhuman primates). Although the cytoarchitectonic structure of the rodent MFC has mostly been conserved in humans, it is a long-standing question whether the structural analogies translate to functional analogies. Here, we probed this question using ultra high field fMRI data to compare rat, marmoset, and human MFC functional connectivity. First, we applied hierarchical clustering to intrinsically define the functional boundaries of the MFC in all three species, independent of cytoarchitectonic definitions. Then, we mapped the functional connectivity "fingerprints" of these regions with a number of different brain areas. Because rats do not share cytoarchitectonically defined regions of the lateral frontal cortex (LFC) with primates, the fingerprinting method also afforded the unique ability to compare the rat MFC and marmoset LFC, which have often been suggested to be functional analogs. The results demonstrated remarkably similar intrinsic functional organization of the MFC across the species, but clear differences between rodent and primate MFC whole-brain connectivity. Rat MFC patterns of connectivity showed greatest similarity with premotor regions in the marmoset, rather than dorsolateral prefrontal regions, which are often suggested to be functionally comparable. These results corroborate the viability of the marmoset as a preclinical model of human MFC dysfunction, and suggest divergence of functional connectivity between rats and primates in both the MFC and LFC.

Severe nausea and vomiting in pregnancy: psychiatric and cognitive problems and brain structure in children

BACKGROUND

Two studies have suggested that severe prolonged nausea and vomiting during pregnancy is associated with emotional and behavioral problems in offspring, with smaller sample size and short-term follow-up. Moreover, little information is available on the role of the brain structure in the associations.

METHODS

In a US-based cohort, the association was investigated between severe prolonged nausea and vomiting in pregnancy (extending after the second trimester and termed SNVP), psychiatric and cognitive problems, and brain morphology, from the Adolescent Brain Cognitive Development (ABCD) study, from 10,710 children aged 9-11 years. We validated the emotional including psychiatric findings using the Danish National Cohort Study with 2,092,897 participants.

RESULTS

SNVP was significantly associated with emotional and psychiatric problems (t = 8.89, Cohen's d = 0.172, p = 6.9 × 10-19) and reduced global cognitive performance (t = - 4.34, d = - 0.085, p = 1.4 × 10-5) in children. SNVP was associated with low cortical area and volume, especially in the cingulate cortex, precuneus, and superior medial prefrontal cortex. These lower cortical areas and volumes significantly mediated the relation between SNVP and the psychiatric and cognitive problems in children. In the Danish National Cohort, severe nausea and vomiting in pregnancy were significantly associated with increased risks of behavioral and emotional disorders in children (hazard ratio, 1.24; 95% confidence interval, 1.16-1.33).

CONCLUSIONS

SNVP is strongly associated with psychiatric and cognitive problems in children, with mediation by brain structure. These associations highlight the clinical importance and potential benefits of the treatment of SNVP, which could reduce the risk of psychiatric disorder in the next generation.

Longitudinal connections and the organization of the temporal cortex in macaques, great apes, and humans

The temporal association cortex is considered a primate specialization and is involved in complex behaviors, with some, such as language, particularly characteristic of humans. The emergence of these behaviors has been linked to major differences in temporal lobe white matter in humans compared with monkeys. It is unknown, however, how the organization of the temporal lobe differs across several anthropoid primates. Therefore, we systematically compared the organization of the major temporal lobe white matter tracts in the human, gorilla, and chimpanzee great apes and in the macaque monkey. We show that humans and great apes, in particular the chimpanzee, exhibit an expanded and more complex occipital-temporal white matter system; additionally, in humans, the invasion of dorsal tracts into the temporal lobe provides a further specialization. We demonstrate the reorganization of different tracts along the primate evolutionary tree, including distinctive connectivity of human temporal gray matter.

Towards HCP-Style macaque connectomes: 24-Channel 3T multi-array coil, MRI sequences and preprocessing

Macaque monkeys are an important animal model where invasive investigations can lead to a better understanding of the cortical organization of primates including humans. However, the tools and methods for noninvasive image acquisition (e.g. MRI RF coils and pulse sequence protocols) and image data preprocessing have lagged behind those developed for humans. To resolve the structural and functional characteristics of the smaller macaque brain, high spatial, temporal, and angular resolutions combined with high signal-to-noise ratio are required to ensure good image quality. To address these challenges, we developed a macaque 24-channel receive coil for 3-T MRI with parallel imaging capabilities. This coil enables adaptation of the Human Connectome Project (HCP) image acquisition protocols to the in-vivo macaque brain. In addition, we adapted HCP preprocessing methods to the macaque brain, including spatial minimal preprocessing of structural, functional MRI (fMRI), and diffusion MRI (dMRI). The coil provides the necessary high signal-to-noise ratio and high efficiency in data acquisition, allowing four- and five-fold accelerations for dMRI and fMRI. Automated FreeSurfer segmentation of cortex, reconstruction of cortical surface, removal of artefacts and nuisance signals in fMRI, and distortion correction of dMRI all performed well, and the overall quality of basic neurobiological measures was comparable with those for the HCP. Analyses of functional connectivity in fMRI revealed high sensitivity as compared with those from publicly shared datasets. Tractography-based connectivity estimates correlated with tracer connectivity similarly to that achieved using ex-vivo dMRI. The resulting HCP-style in vivo macaque MRI data show considerable promise for analyzing cortical architecture and functional and structural connectivity using advanced methods that have previously only been available in studies of the human brain.

A resource for the detailed 3D mapping of white matter pathways in the marmoset brain

While the fundamental importance of the white matter in supporting neuronal communication is well known, existing publications of primate brains do not feature a detailed description of its complex anatomy. The main barrier to achieving this is that existing primate neuroimaging data have insufficient spatial resolution to resolve white matter pathways fully. Here we present a resource that allows detailed descriptions of white matter structures and trajectories of fiber pathways in the marmoset brain. The resource includes: (1) the highest-resolution diffusion-weighted MRI data available to date, which reveal white matter features not previously described; (2) a comprehensive three-dimensional white matter atlas depicting fiber pathways that were either omitted or misidentified in previous atlases; and (3) comprehensive fiber pathway maps of cortical connections combining diffusion-weighted MRI tractography and neuronal tracing data. The resource, which can be downloaded from marmosetbrainmapping.org, will facilitate studies of brain connectivity and the development of tractography algorithms in the primate brain.

Spinal cord neural interfacing in common marmosets (Callithrix jacchus)

Objective:

Spinal cord injury (SCI) remains an ailment with no comprehensive cure, and affected patients suffer from a greatly diminished quality of life. This large population could significantly benefit from prosthetic technologies to replace missing limbs, reanimate nonfunctional limbs, and enable new modes of technologies to restore muscle control and function. While cortically driven brain machine interfaces (BMIs) have achieved great success in interfacing with an external device to restore lost functions, interfacing with the spinal cord can provide an additional site to record motor control signals, which can have its own advantages, albeit challenges from using a smaller non-human primate (NHP) model. The goal of this study is to develop such a spinal cord neural interface to record motor signals from the high cervical levels of the spinal cord in a common marmoset (Callithrix jacchus) model.

Approach and Main Results:

Detailed methods are discussed for this smaller NHP model that includes behavioral training, surgical methods for electrode placement, connector placement and wire handling, electrode specifications and modifications for accessing high cervical level interneurons and motorneurons. The study also discusses the methods and challenges involved in behavioral multi-channel extracellular recording from the marmoset spinal cord, including the major recording failure mechanisms encountered during the study.

Significance:

Marmosets provide a good step between rodent and larger NHP models due to their small size, ease of handling, cognitive abilities, and similarities to other primate motor systems. The study shows the feasibility of recording spinal cord signals and using marmosets as a smaller NHP model in behavioral neuroscience studies. Interfacing with the spinal cord in chronically implanted animals can provide useful information about how motor control signals within the spinal cord are transformed to cause limb movements.

Cerebral cortical folding, parcellation, and connectivity in humans, nonhuman primates, and mice

Advances in neuroimaging and neuroanatomy have yielded major insights concerning fundamental principles of cortical organization and evolution, thus speaking to how well different species serve as models for human brain function in health and disease. Here, we focus on cortical folding, parcellation, and connectivity in mice, marmosets, macaques, and humans. Cortical folding patterns vary dramatically across species, and individual variability in cortical folding increases with cortical surface area. Such issues are best analyzed using surface-based approaches that respect the topology of the cortical sheet. Many aspects of cortical organization can be revealed using 1 type of information (modality) at a time, such as maps of cortical myelin content. However, accurate delineation of the entire mosaic of cortical areas requires a multimodal approach using information about function, architecture, connectivity, and topographic organization. Comparisons across the 4 aforementioned species reveal dramatic differences in the total number and arrangement of cortical areas, particularly between rodents and primates. Hemispheric variability and bilateral asymmetry are most pronounced in humans, which we evaluated using a high-quality multimodal parcellation of hundreds of individuals. Asymmetries include modest differences in areal size but not in areal identity. Analyses of cortical connectivity using anatomical tracers reveal highly distributed connectivity and a wide range of connection weights in monkeys and mice; indirect measures using functional MRI suggest a similar pattern in humans. Altogether, a multifaceted but integrated approach to exploring cortical organization in primate and nonprimate species provides complementary advantages and perspectives.

Thalamic Inputs to Posterior Parietal Cortical Areas Involved in Skilled Forelimb Movement and Tool Use in the Capuchin Monkey

The posterior parietal cortex (PPC) is a central hub for the primate forebrain networks that control skilled manual behavior, including tool use. Here, we quantified and compared the sources of thalamic input to electrophysiologically-identified hand/forearm-related regions of several PPC areas, namely areas 5v, AIP, PFG, and PF, of the capuchin monkey (Sapajus sp). We found that these areas receive most of their thalamic connections from the Anterior Pulvinar (PuA), Lateral Posterior (LP) and Medial Pulvinar (PuM) nuclei. Each PPC area receives a specific combination of projections from these nuclei, and fewer additional projections from other nuclei. Moreover, retrograde labeling of the cells innervating different PPC areas revealed substantial intermingling of these cells within the thalamus. Differences in thalamic input may contribute to the different functional properties displayed by the PPC areas. Furthermore, the observed innervation of functionally-related PPC domains from partly intermingled thalamic cell populations accords with the notion that higher-order thalamic inputs may dynamically regulate functional connectivity between cortical areas.

Spatial encoding in primate hippocampus during free navigation

The hippocampus comprises two neural signals-place cells and θ oscillations-that contribute to facets of spatial navigation. Although their complementary relationship has been well established in rodents, their respective contributions in the primate brain during free navigation remains unclear. Here, we recorded neural activity in the hippocampus of freely moving marmosets as they naturally explored a spatial environment to more explicitly investigate this issue. We report place cells in marmoset hippocampus during free navigation that exhibit remarkable parallels to analogous neurons in other mammalian species. Although θ oscillations were prevalent in the marmoset hippocampus, the patterns of activity were notably different than in other taxa. This local field potential oscillation occurred in short bouts (approximately .4 s)-rather than continuously-and was neither significantly modulated by locomotion nor consistently coupled to place-cell activity. These findings suggest that the relationship between place-cell activity and θ oscillations in primate hippocampus during free navigation differs substantially from rodents and paint an intriguing comparative picture regarding the neural basis of spatial navigation across mammals.

Exploring the cerebral substrate of voice perception in primate brains

One can consider human language to be the Swiss army knife of the vast domain of animal communication. There is now growing evidence suggesting that this technology may have emerged from already operational material instead of being a sudden innovation. Sharing ideas and thoughts with conspecifics via language constitutes an amazing ability, but what value would it hold if our conspecifics were not first detected and recognized? Conspecific voice (CV) perception is fundamental to communication and widely shared across the animal kingdom. Two questions that arise then are: is this apparently shared ability reflected in common cerebral substrate? And, how has this substrate evolved? The paper addresses these questions by examining studies on the cerebral basis of CV perception in humans' closest relatives, non-human primates. Neuroimaging studies, in particular, suggest the existence of a ‘voice patch system’, a network of interconnected cortical areas that can provide a common template for the cerebral processing of CV in primates.

This article is part of the theme issue ‘What can animal communication teach us about human language?’

Large-scale comparative neuroimaging: Where are we and what do we need?

Neuroimaging has a lot to offer comparative neuroscience. Although invasive "gold standard" techniques have a better spatial resolution, neuroimaging allows fast, whole-brain, repeatable, and multi-modal measurements of structure and function in living animals and post-mortem tissue. In the past years, comparative neuroimaging has increased in popularity. However, we argue that its most significant potential lies in its ability to collect large-scale datasets of many species to investigate principles of variability in brain organisation across whole orders of species-an ambition that is presently unfulfilled but achievable. We briefly review the current state of the field and explore what the current obstacles to such an approach are. We propose some calls to action.

Critique of Pure Marmoset

The common marmoset, a New World (platyrrhine) monkey, is currently being fast-tracked as a non-human primate model species, especially for genetic modification but also as a general-purpose model for research on the brain and behavior bearing on the human condition. Compared to the currently dominant primate model, the catarrhine macaque monkey, marmosets are notable for certain evolutionary specializations, including their propensity for twin births, their very small size (a result of phyletic dwarfism), and features related to their small size (rapid development and relatively short lifespan), which result in these animals yielding experimental results more rapidly and at lower cost. Macaques, however, have their own advantages. Importantly, macaques are more closely related to humans (which are also catarrhine primates) than are marmosets, sharing approximately 20 million more years of common descent, and are demonstrably more similar to humans in a variety of genomic, molecular, and neurobiological characteristics. Furthermore, the very specializations of marmosets that make them attractive as experimental subjects, such as their rapid development and short lifespan, are ways in which marmosets differ from humans and in which macaques more closely resemble humans. These facts warrant careful consideration of the trade-offs between convenience and cost, on the one hand, and biological realism, on the other, in choosing between non-human primate models of human biology. Notwithstanding the advantages marmosets offer as models, prudence requires continued commitment to research on macaques and other primate species.

Sulcal organization in the medial frontal cortex provides insights into primate brain evolution

Although the relative expansion of the frontal cortex in primate evolution is generally accepted, the nature of the human uniqueness, if any, and between-species anatomo-functional comparisons of the frontal areas remain controversial. To provide a novel interpretation of the evolution of primate brains, sulcal morphological variability of the medial frontal cortex was assessed in Old World monkeys (macaque/baboon) and Hominoidea (chimpanzee/human). We show that both Hominoidea possess a paracingulate sulcus, which was previously thought to be unique to the human brain and linked to higher cognitive functions, such as mentalizing. Also, we show systematic sulcal morphological organization of the medial frontal cortex that can be traced from Old World monkeys to Hominoidea species, demonstrating an evolutionarily conserved organizational principle. These data provide a new framework to compare sulcal morphology, cytoarchitectonic areal distribution, connectivity, and function across the primate order, leading to clear predictions about how other primate brains might be anatomo-functionally organized.

Sex Differences in Cognitive Flexibility and Resting Brain Networks in Middle-Aged Marmosets

Sex differences in human cognitive performance are well characterized. However, the neural correlates of these differences remain elusive. This issue may be clarified using nonhuman primates, for which sociocultural influences are minimized. We used the marmoset (Callithrix jacchus) to investigate sex differences in two aspects of executive function: reversal learning and intradimensional/extradimensional (ID/ED) set shifting. Stress reactivity and motor function were also assessed. In agreement with human literature, females needed more trials than males to acquire the reversals. No sex differences in ED set shifting or motivational measures were observed. The findings suggest enhanced habit formation in females, perhaps due to striatal estrogenic effects. Both sexes showed increased urinary cortisol during social separation stressor, but females showed an earlier increase in cortisol and a greater increase in agitated locomotion, possibly indicating enhanced stress reactivity. Independent of sex, basal cortisol predicted cognitive performance. No sex differences were found in motor performance. Associations between brain networks and reversal learning performance were investigated using resting state fMRI. Resting state functional connectivity (rsFC) analyses revealed sex differences in cognitive networks, with differences in overall neural network metrics and specific regions, including the prefrontal cortex, caudate, putamen, and nucleus accumbens. Correlations between cognitive flexibility and neural connectivity indicate that sex differences in cognitive flexibility are related to sex-dependent patterns of resting brain networks. Overall, our findings reveal sex differences in reversal learning, brain networks, and their relationship in the marmoset, positioning this species as an excellent model to investigate the biological basis of cognitive sex differences.

Distinct Patterns of Hippocampal and Neocortical Evolution in Primates

Because of the central role of the hippocampus in representing spatial and temporal details of experience, comparative studies of its volume and structure are relevant to understanding the evolution of representational memory across species. The hippocampal formation, however, is organized into separate anatomical subregions with distinct functions, and little is known about the evolutionary diversification of these subregions. We investigate relative volumetric changes in hippocampal subregions across a large sample of primate species. We then compare the evolution of the hippocampal formation to the neocortex. Results across hippocampal subregions indicate that, compared to strepsirrhines, the anthropoid lineage displays a decrease in relative CA3, fascia dentata, subiculum, and rhinal cortex volume in tandem with an increase in relative neocortical volume. These findings indicate that hippocampal function in anthropoids might be substantially augmented by the executive decision-making functions of the neocortex. Humans are found to have a unique cerebral organization combining increased relative CA3, subiculum, and rhinal cortex with increased relative neocortical volumes, suggesting that these regions may play a role in behaviors that are uniquely specialized in humans.

Topographic Organization of the 'Third-Tier' Dorsomedial Visual Cortex in the Macaque

The boundaries of the visual areas located anterior to V2 in the dorsomedial region of the macaque cortex remain contentious. This region is usually conceptualized as including two functional subdivisions: the dorsal component of area V3 (V3d) laterally and another area named the parietooccipital area (PO) or V6 medially. However, the nature of the putative border between V3d and PO/V6 has remained undefined. We recorded the receptive fields of multiunit clusters in male macaques and reconstructed the locations of recording sites using histological sections and computer-generated maps. Immediately adjacent to dorsomedial V2, we observed a representation of the lower contralateral quadrant that represented the vertical meridian at its rostral border. This region formed a simple eccentricity gradient from ∼<5° in the annectant gyrus to >60° in the parietooccipital medial sulcus. There was no topographic reversal where one would expect to find the border between V3d and PO/V6. Rather, near the midline, this lower quadrant map continued directly into a representation of the peripheral upper visual field without an intervening lower quadrant representation. Therefore, cortex previously assigned to the medial part of V3d and to PO/V6 forms a single map that includes parts of both quadrants. Together with previous observations that V3d and PO/V6 are densely myelinated relative to adjacent cortex and share similar input from V1, these results suggest that they are parts of a single area (for which we suggest the designation V6), which is distinct from the one forming the ventral component of the third-tier complex.SIGNIFICANCE STATEMENT The primate visual cortex has a large number of areas. Knowing the extent of each visual area and how they can be distinguished from each other is essential for the interpretation of experiments aimed at understanding visual processing. Currently, there are conflicting models of the organization of the dorsomedial visual cortex rostral to area V2 (one of the earliest stages of cortical processing of vision). By conducting large-scale electrophysiological recordings, we found that what were originally thought to be distinct areas in this region (dorsal V3 and the parietooccipital area PO/V6), together form a single map of the visual field. This will help to guide future functional studies and the interpretation of the outcomes of lesions involving the dorsal visual cortex.

Macroscale cortical organization and a default-like apex transmodal network in the marmoset monkey

Networks of widely distributed regions populate human association cortex. One network, often called the default network, is positioned at the apex of a gradient of sequential networks that radiate outward from primary cortex. Here, extensive anatomical data made available through the Marmoset Brain Architecture Project are explored to show a homologue exists in marmoset. Results reveal that a gradient of networks extend outward from primary cortex to progressively higher-order transmodal association cortex in both frontal and temporal cortex. The apex transmodal network comprises frontopolar and rostral temporal association cortex, parahippocampal areas TH / TF, the ventral posterior midline, and lateral parietal association cortex. The positioning of this network in the gradient and its composition of areas make it a candidate homologue to the human default network. That the marmoset, a physiologically- and genetically-accessible primate, might possess a default-network-like candidate creates opportunities for study of higher cognitive and social functions.

Cortical Afferents of Area 10 in Cebus Monkeys: Implications for the Evolution of the Frontal Pole

Area 10, located in the frontal pole, is a unique specialization of the primate cortex. We studied the cortical connections of area 10 in the New World Cebus monkey, using injections of retrograde tracers in different parts of this area. We found that injections throughout area 10 labeled neurons in a consistent set of areas in the dorsolateral, ventrolateral, orbital, and medial parts of the frontal cortex, superior temporal association cortex, and posterior cingulate/retrosplenial region. However, sites on the midline surface of area 10 received more substantial projections from the temporal lobe, including clear auditory connections, whereas those in more lateral parts received >90% of their afferents from other frontal areas. This difference in anatomical connectivity reflects functional connectivity findings in the human brain. The pattern of connections in Cebus is very similar to that observed in the Old World macaque monkey, despite >40 million years of evolutionary separation, but lacks some of the connections reported in the more closely related but smaller marmoset monkey. These findings suggest that the clearer segregation observed in the human frontal pole reflects regional differences already present in early simian primates, and that overall brain mass influences the pattern of cortico-cortical connectivity.

Prehensile kinematics of the marmoset monkey: Implications for the evolution of visually-guided behaviors

Throughout the primate lineage, there is a wide diversity of prehensile capacity that is thought to stem from individual species foraging patterns. While many studies have explored primates with precise hand grips, such as higher apes, few have considered primates that lack opposition movements. The New World marmoset monkey occupies an intriguing niche, displaying adept control of their hand movements yet their absence of opposable digits results in relatively imprecise grasping actions when compared with those observed in Old World monkeys, apes, and humans. The marmoset monkey offers a unique composition of ancestral primate corticospinal organization combined with skilled hand use to explore the evolution and development of visually-guided actions. In this study, four adult marmosets were trained to perform a series of visually-guided tasks, designed to assess their control over locating and retrieving objects of differing dimensions. Two of these animals received a neonatal lesion of the inferior pulvinar (unilateral), a thalamic nucleus previously demonstrated to be involved in visuomotor development. The kinematics of their reaching and grasping patterns were recorded for offline analysis. Predictive modeling revealed that maximum grip aperture, time to reach peak velocity and hand use were reliable predictors of distinguishing between cohorts. A consistent feature observed across all tasks was that they do not precisely scale their grip according to the dimensions of the target object which may be attributed to their lack of independent digit control. Therefore, the marmoset monkey represents a previously understudied position in the evolution of primate reach and grasp behavior.

Astrocytic Response to Acutely- and Chronically-Implanted Microelectrode Arrays in the Marmoset (Callithrix jacchus) Brain

Microelectrode implants are an important tool in neuroscience research and in developing brain–machine interfaces. Data from rodents have consistently shown that astrocytes are recruited to the area surrounding implants, forming a glial scar that increases electrode impedance and reduces chronic utility. However, studies in non-human primates are scarce, with none to date in marmosets. We used glial fibrillary acidic protein (GFAP) immunostaining to characterize the acute and chronic response of the marmoset brain to microelectrodes. By using densitometry, we showed that marmoset astrocytes surround brain implants and that a glial scar is formed over time, with significant increase in the chronic condition relative to the acute condition animal.

Not all cortical expansions are the same: the coevolution of the neocortex and the dorsal thalamus in mammals

A central question in comparative neurobiology concerns how evolution has produced brains with expanded neocortices, composed of more areas with unique connectivity and functional properties. Some mammalian lineages, such as primates, exhibit exceptionally large cortices relative to the amount of sensory inputs from the dorsal thalamus, and this expansion is associated with a larger number of distinct cortical areas, composing a larger proportion of the cortical sheet. We propose a link between the organization of the neocortex and its expansion relative to the size of the dorsal thalamus, based on a combination of work in comparative neuroanatomy and experimental research.

Adverse Effects of Aromatase Inhibition on the Brain and Behavior in a Nonhuman Primate

Breast cancer patients using aromatase inhibitors (AIs) as an adjuvant therapy often report side effects, including hot flashes, mood changes, and cognitive impairment. Despite long-term use in humans, little is known about the effects of continuous AI administration on the brain and cognition. We used a primate model of human cognitive aging, the common marmoset, to examine the effects of a 4-week daily administration of the AI letrozole (20 μg, p.o.) on cognition, anxiety, thermoregulation, brain estrogen content, and hippocampal pyramidal cell physiology. Letrozole treatment was administered to both male and female marmosets and reduced peripheral levels of estradiol (E2), but unexpectedly increased E2 levels in the hippocampus. Spatial working memory and intrinsic excitability of hippocampal neurons were negatively affected by the treatment possibly due to increased hippocampal E2. While no changes in hypothalamic E2 were observed, thermoregulation was disrupted by letrozole in females only, indicating some impact on hypothalamic activity. These findings suggest adverse effects of AIs on the primate brain and call for new therapies that effectively prevent breast cancer recurrence while minimizing side effects that further compromise quality of life.SIGNIFICANCE STATEMENT Aromatase inhibitors (AIs) are used as an adjuvant therapy for estrogen-receptor-positive breast cancer and are associated with side effects, including hot flashes, depression/anxiety, and memory deficits severe enough for many women to discontinue this life-saving treatment. AIs are also used by men, yet sex differences in the reported side effects have not been systematically studied. We show that AI-treated male and female marmosets exhibit behavioral changes consistent with these CNS symptoms, as well as elevated hippocampal estradiol and compromised hippocampal physiology. These findings illustrate the need for (1) a greater understanding of the precise mechanisms by which AIs impact brain function and (2) the development of new treatment approaches for breast cancer patients that minimize adverse effects on the brain.

Gradients of connectivity distance in the cerebral cortex of the macaque monkey

Cortical connectivity conforms to a series of organizing principles that are common across species. Spatial proximity, similar cortical type, and similar connectional profile all constitute factors for determining the connectivity between cortical regions. We previously demonstrated another principle of connectivity that is closely related to the spatial layout of the cerebral cortex. Using functional connectivity from resting-state fMRI in the human cortex, we found that the further a region is located from primary cortex, the more distant are its functional connections with the other areas of the cortex. However, it remains unknown whether this relationship between cortical layout and connectivity extends to other primate species. Here, we investigated this relationship using both resting-state functional connectivity as well as gold-standard tract-tracing connectivity in the macaque monkey cortex. For both measures of connectivity, we found a gradient of connectivity distance extending between primary and frontoparietal regions. In the human cortex, the further a region is located from primary areas, the stronger its connections to distant portions of the cortex, with connectivity distance highest in frontal and parietal regions. The similarity between the human and macaque findings provides evidence for a phylogenetically conserved relationship between the spatial layout of cortical areas and connectivity.

Recognition Memory in Marmoset and Macaque Monkeys: A Comparison of Active Vision

The core functional organization of the primate brain is remarkably conserved across the order, but behavioral differences evident between species likely reflect derived modifications in the underlying neural processes. Here, we performed the first study to directly compare visual recognition memory in two primate species-rhesus macaques and marmoset monkeys-on the same visual preferential looking task as a first step toward identifying similarities and differences in this cognitive process across the primate phylogeny. Preferences in looking behavior on the task were broadly similar between the species, with greater looking times for novel images compared with repeated images as well as a similarly strong preference for faces compared with other categories. Unexpectedly, we found large behavioral differences among the two species in looking behavior independent of image familiarity. Marmosets exhibited longer looking times, with greater variability compared with macaques, regardless of image content or familiarity. Perhaps most strikingly, marmosets shifted their gaze across the images more quickly, suggesting a different behavioral strategy when viewing images. Although such differences limit the comparison of recognition memory across these closely related species, they point to interesting differences in the mechanisms underlying active vision that have significant implications for future neurobiological investigations with these two nonhuman primate species. Elucidating whether these patterns are reflective of species or broader phylogenetic differences (e.g., between New World and Old World monkeys) necessitates a broader sample of primate taxa from across the Order.

Comparing Adult Hippocampal Neurogenesis Across Species: Translating Time to Predict the Tempo in Humans

Comparison of neurodevelopmental sequences between species whose initial period of brain organization may vary from 100 days to 1,000 days, and whose progress is intrinsically non-linear presents large challenges in normalization. Comparing adult timelines when lifespans stretch from 1 year to 75 years, when underlying cellular mechanisms under scrutiny do not scale similarly, presents challenges to simple detection and comparison. The question of adult hippocampal neurogenesis has generated numerous controversies regarding its simple presence or absence in humans versus rodents, whether it is best described as the tail of a distribution centered on early neural development, or is several distinct processes. In addition, adult neurogenesis may have substantially changed in evolutionary time in different taxonomic groups. Here, we extend and adapt a model of the cross-species transformation of early neurodevelopmental events which presently reaches up to the equivalent of the third human postnatal year for 18 mammalian species (www.translatingtime.net) to address questions relevant to hippocampal neurogenesis, which permit extending the database to adolescence or perhaps to the whole lifespan. We acquired quantitative data delimiting the envelope of hippocampal neurogenesis from cell cycle markers (i.e., Ki67 and DCX) and RNA sequencing data for two primates (macaque and humans) and two rodents (rat and mouse). To improve species coverage in primates, we gathered the same data from marmosets (Callithrix jacchus), but additionally gathered data on a number of developmental milestones to find equivalent developmental time points between marmosets and other species. When all species are so modeled, and represented in a common time frame, the envelopes of hippocampal neurogenesis are essentially superimposable. Early developmental events involving the olfactory and limbic system start and conclude possibly slightly early in primates than rodents, and we find a comparable early conclusion of primate hippocampal neurogenesis (as assessed by the relative number of Ki67 cells) suggesting a plateau to low levels at approximately 2 years of age in humans. Marmosets show equivalent patterns within neurodevelopment, but unlike macaque and humans may have wholesale delay in the initiation of neurodevelopment processes previously observed in some precocial mammals such as the guinea pig and multiple large ungulates.

One-year change in cognitive flexibility and fine motor function in middle-aged male and female marmosets (Callithrix jacchus)

The common marmoset (Callithrix jacchus) is uniquely suited for longitudinal studies of cognitive aging, due to a relatively short lifespan, sophisticated cognitive abilities, and patterns of brain aging that resemble those of humans. We examined cognitive function and fine motor skills in male and female marmosets (mean age ∼5 at study entry) followed longitudinally for 2 years. Each year, monkeys were tested on a reversal learning task with three pairs of stimuli (n = 18, 9 females) and a fine motor task requiring them to grasp small rewards from two staircases (Hill and Valley test, n = 12, 6 females). There was little evidence for a decline in cognitive flexibility between the two time points, in part because of practice effects. However, independent of year of testing, females took longer than males to reach criterion in the reversals, indicating impaired cognitive flexibility. Motivation was unlikely to contribute to this effect, as males refused a greater percentage of trials than females in the reversals. With regards to motor function, females were significantly faster than males in the Hill and Valley task. From Year 1 to Year 2, a slight slowing of motor function was observed in both sexes, but accuracy decreased significantly in males only. This study (1) demonstrates that marmosets exhibit sex differences in cognitive flexibility and fine motor function that resemble those described in humans; (2) that changes in fine motor function can already be detected at middle-age; and (3) that males may experience greater age-related changes in fine motor skills than females. Additional data points will determine whether these sex and age differences persist over time.

[Usage of common marmoset to drug discovery research]

Rodent laboratory animals, such as mice and rats have been greatly contributing to biomedical research. Although its usefulness would not change in the future, nonhuman primates (NHPs) also offer excellent models for preclinical research to assess safety and efficacy of developing novel therapeutic approaches because of their similarities of genetics, metabolism and physiological characteristics to humans. Recent years, the gene modification technology in nonhuman primates has been developed. In fact, pre-clinical studies using nonhuman primates are increasing in the world, especially in the neuroscience research field. Among the NHPs, the common marmoset (Callithrix jacchus) is one of a suitable NHP laboratory animal for producing genetically modified models because they are fecund animal. This article outlines the common marmoset and that of the disease models.

Development and Evolution of Cerebral and Cerebellar Cortex

Cerebral cortex and cerebellar cortex both vary enormously across species in their size and complexity of convolutions. We discuss the development and evolution of cortical structures in terms of anatomy and functional organization. We propose that the distinctive shapes of cerebral and cerebellar cortex can be explained by relatively few developmental processes, notably including mechanical tension along axons and dendrites. Regarding functional organization, we show how maps of myelin content in cerebral cortex are evolutionarily conserved across primates but differ in the proportion of cortex devoted to sensory, cognitive, and other functions. We summarize recent progress and challenges in (i) parcellating cerebral cortex into a mosaic of distinct areas, (ii) distinguishing cortical areas that correspond across species from those that are present in one species but not another, and (iii) using this information along with surface-based interspecies registration to gain deeper insights into cortical evolution. We also comment on the methodological challenges imposed by the differences in anatomical and functional organization of cerebellar cortex relative to cerebral cortex.

Connectivity and the search for specializations in the language-capable brain

The search for the anatomical basis of language has traditionally been a search for specializations. More recently such research has focused both on aspects of brain organization that are unique to humans and aspects shared with other primates. This work has mostly concentrated on the architecture of connections between brain areas. However, as specializations can take many guises, comparison of anatomical organization across species is often complicated. We demonstrate how viewing different types of specializations within a common framework allows one to better appreciate both shared and unique aspects of brain organization. We illustrate this point by discussing recent insights into the anatomy of the dorsal language pathway to the frontal cortex and areas for laryngeal control in the motor cortex.

Quantitative assessment of prefrontal cortex in humans relative to nonhuman primates

A longstanding controversy in neuroscience pertains to differences in human prefrontal cortex (PFC) compared with other primate species; specifically, is human PFC disproportionately large? Distinctively human behavioral capacities related to higher cognition and affect presumably arose from evolutionary modifications since humans and great apes diverged from a common ancestor about 6–8 Mya. Accurate determination of regional differences in the amount of cortical gray and subcortical white matter content in humans, great apes, and Old World monkeys can further our understanding of the link between structure and function of the human brain. Using tissue volume analyses, we show a disproportionately large amount of gray and white matter corresponding to PFC in humans compared with nonhuman primates.

Humans have the largest cerebral cortex among primates. The question of whether association cortex, particularly prefrontal cortex (PFC), is disproportionately larger in humans compared with nonhuman primates is controversial: Some studies report that human PFC is relatively larger, whereas others report a more uniform PFC scaling. We address this controversy using MRI-derived cortical surfaces of many individual humans, chimpanzees, and macaques. We present two parcellation-based PFC delineations based on cytoarchitecture and function and show that a previously used morphological surrogate (cortex anterior to the genu of the corpus callosum) substantially underestimates PFC extent, especially in humans. We find that the proportion of cortical gray matter occupied by PFC in humans is up to 1.9-fold greater than in macaques and 1.2-fold greater than in chimpanzees. The disparity is even more prominent for the proportion of subcortical white matter underlying the PFC, which is 2.4-fold greater in humans than in macaques and 1.7-fold greater than in chimpanzees.

Frontoparietal Functional Connectivity in the Common Marmoset

In contrast to the well established macaque monkey, little is known about functional connectivity patterns of common marmoset monkey (Callithrix jacchus) that is poised to become the leading transgenic primate model. Here, we used resting-state ultra-high-field fMRI data collected from anesthetized marmosets and macaques along with awake human subjects, to examine and compare the brain's functional organization, with emphasis on the saccade system. Exploratory independent component analysis revealed eight resting-state networks in marmosets that greatly overlapped with corresponding macaque and human networks including a distributed frontoparietal network. Seed-region analyses of the superior colliculus (SC) showed homolog areas in macaques and marmosets. The marmoset SC displayed the strongest frontal functional connectivity with area 8aD at the border to area 6DR. Functional connectivity of this frontal region revealed a similar functional connectivity pattern as the frontal eye fields in macaques and humans. Furthermore, areas 8aD, 8aV, PG,TPO, TE2, and TE3 were identified as major hubs based on region-wise evaluation of betweeness centrality, suggesting that these cortical regions make up the functional core of the marmoset brain. The results support an evolutionarily preserved frontoparietal system and provide a starting point for invasive neurophysiological studies in the marmoset saccade and visual systems.

Development of an optogenetic toolkit for neural circuit dissection in squirrel monkeys

Optogenetic tools have opened a rich experimental landscape for understanding neural function and disease. Here, we present the first validation of eight optogenetic constructs driven by recombinant adeno-associated virus (AAV) vectors and a WGA-Cre based dual injection strategy for projection targeting in a widely-used New World primate model, the common squirrel monkey Saimiri sciureus. We observed opsin expression around the local injection site and in axonal projections to downstream regions, as well as transduction to thalamic neurons, resembling expression patterns observed in macaques. Optical stimulation drove strong, reliable excitatory responses in local neural populations for two depolarizing opsins in anesthetized monkeys. Finally, we observed continued, healthy opsin expression for at least one year. These data suggest that optogenetic tools can be readily applied in squirrel monkeys, an important first step in enabling precise, targeted manipulation of neural circuits in these highly trainable, cognitively sophisticated animals. In conjunction with similar approaches in macaques and marmosets, optogenetic manipulation of neural circuits in squirrel monkeys will provide functional, comparative insights into neural circuits which subserve dextrous motor control as well as other adaptive behaviors across the primate lineage. Additionally, development of these tools in squirrel monkeys, a well-established model system for several human neurological diseases, can aid in identifying novel treatment strategies.

Of Men and Mice: Modeling the Fragile X Syndrome

The Fragile X Syndrome (FXS) is one of the most common forms of inherited intellectual disability in all human societies. Caused by the transcriptional silencing of a single gene, the fragile x mental retardation gene FMR1, FXS is characterized by a variety of symptoms, which range from mental disabilities to autism and epilepsy. More than 20 years ago, a first animal model was described, the Fmr1 knock-out mouse. Several other models have been developed since then, including conditional knock-out mice, knock-out rats, a zebrafish and a drosophila model. Using these model systems, various targets for potential pharmaceutical treatments have been identified and many treatments have been shown to be efficient in preclinical studies. However, all attempts to turn these findings into a therapy for patients have failed thus far. In this review, I will discuss underlying difficulties and address potential alternatives for our future research.

Glutamine/glutamate (Glx) concentration in prefrontal cortex predicts reversal learning performance in the marmoset

This study used Magnetic Resonance Spectroscopy (MRS) to identify potential neurometabolitic markers of cognitive performance in male (n = 7) and female (n = 8) middle-aged (∼5 years old) common marmosets (Callithrix jacchus). Anesthetized marmosets were scanned with a 4.7 T/40 cm horizontal magnet equipped with 450 mT/m magnetic field gradients and a 20 G/cm magnetic field gradient insert, within 3 months of completing the CANTAB serial Reversal Learning task. Neurometabolite concentrations of N-Acetyl Asparate, Myo-Inositol, Choline, Phosphocreatine + creatine, Glutamate and Glutamine were acquired from a 3 mm³ voxel positioned in the Prefrontal Cortex (PFC). Males acquired the reversals (but not simple discriminations) faster than the females. Higher PFC Glx (glutamate + glutamine) concentration was associated with faster acquisition of the reversals. Interestingly, the correlation between cognitive performance and Glx was significant in males, but not in females. These results suggest that MRS is a useful tool to identify biochemical markers of cognitive performance in the healthy nonhuman primate brain and that biological sex modulates the relationship between neurochemical composition and cognition.

Specialized Representations of Value in the Orbital and Ventrolateral Prefrontal Cortex: Desirability versus Availability of Outcomes

Advantageous foraging choices benefit from an estimation of two aspects of a resource's value: its current desirability and availability. Both orbitofrontal and ventrolateral prefrontal areas contribute to updating these valuations, but their precise roles remain unclear. To explore their specializations, we trained macaque monkeys on two tasks: one required updating representations of a predicted outcome's desirability, as adjusted by selective satiation, and the other required updating representations of an outcome's availability, as indexed by its probability. We evaluated performance on both tasks in three groups of monkeys: unoperated controls and those with selective, fiber-sparing lesions of either the OFC or VLPFC. Representations that depend on the VLPFC but not the OFC play a necessary role in choices based on outcome availability; in contrast, representations that depend on the OFC but not the VLPFC play a necessary role in choices based on outcome desirability.

Towards a comprehensive atlas of cortical connections in a primate brain: Mapping tracer injection studies of the common marmoset into a reference digital template

The marmoset is an emerging animal model for large‐scale attempts to understand primate brain connectivity, but achieving this aim requires the development and validation of procedures for normalization and integration of results from many neuroanatomical experiments. Here we describe a computational pipeline for coregistration of retrograde tracing data on connections of cortical areas into a 3D marmoset brain template, generated from Nissl‐stained sections. The procedure results in a series of spatial transformations that are applied to the coordinates of labeled neurons in the different cases, bringing them into common stereotaxic space. We applied this procedure to 17 injections, placed in the frontal lobe of nine marmosets as part of earlier studies. Visualizations of cortical patterns of connections revealed by these injections are supplied as Supplementary Materials. Comparison between the results of the automated and human‐based processing of these cases reveals that the centers of injection sites can be reconstructed, on average, to within 0.6 mm of coordinates estimated by an experienced neuroanatomist. Moreover, cell counts obtained in different areas by the automated approach are highly correlated (r = 0.83) with those obtained by an expert, who examined in detail histological sections for each individual. The present procedure enables comparison and visualization of large datasets, which in turn opens the way for integration and analysis of results from many animals. Its versatility, including applicability to archival materials, may reduce the number of additional experiments required to produce the first detailed cortical connectome of a primate brain. J. Comp. Neurol. 524:2161–2181, 2016. © 2016 The Authors The Journal of Comparative Neurology Published by Wiley Periodicals, Inc.

Generation of transgenic marmosets expressing genetically encoded calcium indicators

Chronic monitoring of neuronal activity in the living brain with optical imaging techniques became feasible owing to the continued development of genetically encoded calcium indicators (GECIs). Here we report for the first time the successful generation of transgenic marmosets (Callithrix jacchus), an important nonhuman primate model in neurophysiological research, which were engineered to express the green fluorescent protein (GFP)-based family of GECIs, GCaMP, under control of either the CMV or the hSyn promoter. High titer lentiviral vectors were produced, and injected into embryos collected from donor females. The infected embryos were then transferred to recipient females. Eight transgenic animals were born and shown to have stable and functional GCaMP expression in several different tissues. Germline transmission of the transgene was confirmed in embryos generated from two of the founder transgenic marmosets that reached sexual maturity. These embryos were implanted into six recipient females, three of which became pregnant and are in advanced stages of gestation. We believe these transgenic marmosets will be invaluable non-human primate models in neuroscience, allowing chronic in vivo monitoring of neural activity with functional confocal and multi-photon optical microscopy imaging of intracellular calcium dynamics.

Controversies about the visual areas located at the anterior border of area V2 in primates

Anatomical and electrophysiological studies have provided us with detailed information regarding the extent and topography of the primary (V1) and secondary (V2) visual areas in primates. The consensus about the V1 and V2 maps, however, is in sharp contrast with controversies regarding the organization of the cortical areas lying immediately rostral to V2. In this review, we address the contentious issue of the extent of the third visual area (V3). Specifically, we will argue for the existence of both ventral (V3v) and dorsal (V3d) segments of V3, which are located, respectively, adjacent to the anterior border of ventral and dorsal V2. V3v and V3d would together constitute a single functional area with a complete representation of both upper and lower visual hemifields. Another contentious issue is the organization of the parietal-occipital (PO) area, which also borders the rostral edge of the medial portion of dorsal V2. Different from V1, V2, and V3, which exhibit a topography based on the defined lines of isoeccentricity and isopolar representation, area PO only has a systematic representation of polar angles, with an emphasis on the peripheral visual field (isoeccentricity lines are not well defined). Based on the connectivity patterns of area PO with distinct cytochrome oxidase modules in V2, we propose a subdivision of the dorsal stream of visual information processing into lateral and medial domains. In this model, area PO constitutes the first processing instance of the dorsal-medial stream, coding for the full-field flow of visual cues during navigation. Finally, we compare our findings with those in other species of Old and New World monkeys and argue that larger animals, such as macaque and capuchin monkeys, have similar organizations of the areas rostral to V2, which is different from that in smaller New World monkeys.

Neural plasticity following lesions of the primate occipital lobe: The marmoset as an animal model for studies of blindsight

For nearly a century it has been observed that some residual visually guided behavior can persist after damage to the primary visual cortex (V1) in primates. The age at which damage to V1 occurs leads to different outcomes, with V1 lesions in infancy allowing better preservation of visual faculties in comparison with those incurred in adulthood. While adult V1 lesions may still allow retention of some limited visual abilities, these are subconscious-a characteristic that has led to this form of residual vision being referred to as blindsight. The neural basis of blindsight has been of great interest to the neuroscience community, with particular focus on understanding the contributions of the different subcortical pathways and cortical areas that may underlie this phenomenon. More recently, research has started to address which forms of neural plasticity occur following V1 lesions at different ages, including work using marmoset monkeys. The relatively rapid postnatal development of this species, allied to the lissencephalic brains and well-characterized visual cortex provide significant technical advantages, which allow controlled experiments exploring visual function in the absence of V1. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 314-327, 2017.