Chimpanzee Brain Research Articles

Comparative analysis of the chimpanzee and human brain superficial structural connectivities

Diffusion MRI tractography (dMRI) has fundamentally transformed our ability to investigate white matter pathways in the human brain. While long-range connections have extensively been studied, superficial white matter bundles (SWMBs) have remained a relatively underexplored aspect of brain connectivity. This study undertakes a comprehensive examination of SWMB connectivity in both the human and chimpanzee brains, employing a novel combination of empirical and geometric methodologies to classify SWMB morphology in an objective manner. Leveraging two anatomical atlases, the Ginkgo Chauvel chimpanzee atlas and the Ginkgo Chauvel human atlas, comprising respectively 844 and 1375 superficial bundles, this research focuses on sparse representations of the morphology of SWMBs to explore the little-understood superficial connectivity of the chimpanzee brain and facilitate a deeper understanding of the variability in shape of these bundles. While similar, already well-known in human U-shape fibers were observed in both species, other shapes with more complex geometry such as 6 and J shapes were encountered. The localisation of the different bundle morphologies, putatively reflecting the brain gyrification process, was different between humans and chimpanzees using an isomap-based shape analysis approach. Ultimately, the analysis aims to uncover both commonalities and disparities in SWMBs between chimpanzees and humans, shedding light on the evolution and organization of these crucial neural structures.

A comparative anatomical network analysis of the human and chimpanzee brains.

Spatial interactions among anatomical elements help to identify topological factors behind morphological variation and can be investigated through network analysis. Here, a whole-brain network model of the chimpanzee (Pan troglodytes, Blumenbach 1776) is presented, based on macroanatomical divisions, and compared with a previous equivalent model of the human brain. The goal was to contrast which regions are essential in the geometric balance of the brains of the two species, to compare underlying phenotypic patterns of spatial variation, and to understand how these patterns might have influenced the evolution of human brain morphology. The human and chimpanzee brains share morphologically complex inferior-medial regions and a topological organization that matches the spatial constraints exerted by the surrounding braincase. These shared topological features are interesting because they can be traced back to the Chimpanzee-Human Last Common Ancestor, 7-10 million years ago. Nevertheless, some key differences are found in the human and chimpanzee brains. In humans, the temporal lobe, particularly its deep and medial limbic aspect (the parahippocampal gyrus), is a crucial node for topological complexity. Meanwhile, in chimpanzees, the cerebellum is, in this sense, more embedded in an intricate spatial position. This information helps to interpret brain macroanatomical change in fossil hominids.

Detailed mapping of the complex fiber structure and white matter pathways of the chimpanzee brain

Long-standing questions about human brain evolution may only be resolved through comparisons with close living evolutionary relatives, such as chimpanzees. This applies in particular to structural white matter (WM) connectivity, which continuously expanded throughout evolution. However, due to legal restrictions on chimpanzee research, neuroscience research currently relies largely on data with limited detail or on comparisons with evolutionarily distant monkeys. Here, we present a detailed magnetic resonance imaging resource to study structural WM connectivity in the chimpanzee. This open-access resource contains (1) WM reconstructions of a postmortem chimpanzee brain, using the highest-quality diffusion magnetic resonance imaging data yet acquired from great apes; (2) an optimized and validated method for high-quality fiber orientation reconstructions; and (3) major fiber tract segmentations for cross-species morphological comparisons. This dataset enabled us to identify phylogenetically relevant details of the chimpanzee connectome, and we anticipate that it will substantially contribute to understanding human brain evolution.

Astrocytes Drive Divergent Metabolic Gene Expression in Humans and Chimpanzees.

The human brain utilizes ∼20% of all of the body's metabolic resources, while chimpanzee brains use <10%. Although previous work shows significant differences in metabolic gene expression between the brains of primates, we have yet to fully resolve the contribution of distinct brain cell types. To investigate cell type-specific interspecies differences in brain gene expression, we conducted RNA-seq on neural progenitor cells, neurons, and astrocytes generated from induced pluripotent stem cells from humans and chimpanzees. Interspecies differential expression analyses revealed that twice as many genes exhibit differential expression in astrocytes (12.2% of all genes expressed) than neurons (5.8%). Pathway enrichment analyses determined that astrocytes, rather than neurons, diverged in expression of glucose and lactate transmembrane transport, as well as pyruvate processing and oxidative phosphorylation. These findings suggest that astrocytes may have contributed significantly to the evolution of greater brain glucose metabolism with proximity to humans.

In vivo mapping of the deep and superficial white matter connectivity in the chimpanzee brain

Mapping the chimpanzee brain connectome and comparing it to that of humans is key to our understanding of similarities and differences in primate evolution that occurred after the split from their common ancestor around 6 million years ago. In contrast to studies on macaque species’ brains, fewer studies have specifically addressed the structural connectivity of the chimpanzee brain and its comparison with the human brain. Most comparative studies in the literature focus on the anatomy of the cortex and deep nuclei to evaluate how their morphology and asymmetry differ from that of the human brain, and some studies have emerged concerning the study of brain connectivity among humans, monkeys, and apes. In this work, we established a new white matter atlas of the deep and superficial white matter structural connectivity in chimpanzees. In vivo anatomical and diffusion-weighted magnetic resonance imaging (MRI) data were collected on a 3-Tesla MRI system from 39 chimpanzees. These datasets were subsequently processed using a novel fiber clustering pipeline adapted to the chimpanzee brain, enabling us to create two novel deep and superficial white matter connectivity atlases representative of the chimpanzee brain. These atlases provide the scientific community with an important and novel set of reference data for understanding the commonalities and differences in structural connectivity between the human and chimpanzee brains. We believe this study to be innovative both in its novel approach and in mapping the superficial white matter bundles in the chimpanzee brain, which will contribute to a better understanding of hominin brain evolution.

High angular resolution susceptibility imaging and estimation of fiber orientation distribution functions in primate brain

Uncovering brain-tissue microstructure including axonal characteristics is a major neuroimaging research focus. Within this scope, anisotropic properties of magnetic susceptibility in white matter have been successfully employed to estimate primary axonal trajectories using mono-tensorial models. However, anisotropic susceptibility has not yet been considered for modeling more complex fiber structures within a voxel, such as intersecting bundles, or an estimation of orientation distribution functions (ODFs). This information is routinely obtained by high angular resolution diffusion imaging (HARDI) techniques. In applications to fixed tissue, however, diffusion-weighted imaging suffers from an inherently low signal-to-noise ratio and limited spatial resolution, leading to high demands on the performance of the gradient system in order to mitigate these limitations. In the current work, high angular resolution susceptibility imaging (HARSI) is proposed as a novel, phase-based methodology to estimate ODFs. A multiple gradient-echo dataset was acquired in an entire fixed chimpanzee brain at 61 orientations by reorienting the specimen in the magnetic field. The constant solid angle method was adapted for estimating phase-based ODFs. HARDI data were also acquired for comparison. HARSI yielded information on whole-brain fiber architecture, including identification of peaks of multiple bundles that resembled features of the HARDI results. Distinct differences between both methods suggest that susceptibility properties may offer complementary microstructural information. These proof-of-concept results indicate a potential to study the axonal organization in post-mortem primate and human brain at high resolution.

The association of astrogliosis and microglial activation with aging and Alzheimer's disease pathology in the chimpanzee brain.

Aging and neurodegenerative disorders, such as Alzheimer's disease (AD), trigger an immune response known as glial activation in the brain. Recent evidence indicates species differences in inflammatory responses to AD pathology, highlighting the need for additional comparative studies to further understand human-specific neuropathologies. In the present study, we report on the occurrence of astrogliosis, microglial activation, and their relationship with age and AD-like pathology in a cohort of male and female chimpanzees (Pan troglodytes). Chimpanzees with severe astrogliosis exhibited widespread upregulation of hypertrophic astrocytes immunoreactive for glial fibrillary acidic protein (GFAP) throughout all layers of the dorsolateral prefrontal cortex and a loss of the interlaminar palisade. In addition, extreme astrogliosis was associated with increased astrocyte density in the absence of significant microglial activation and AD lesions. A shift from decreased resting to increased phagocytotic microglia occurred with aging, although proliferation was absent and no changes in astrogliosis was observed. Vascular amyloid correlated with decreased astrocyte and microglia densities, while tau lesions were associated with morphological changes in microglia and greater total glia density and glia: neuron ratio. These results further our understanding of inflammatory processes within the chimpanzee brain and provide comparative data to improve our understanding of human aging and neuropathological processes.

-correction of magnetization transfer saturation maps optimized for 7T postmortem MRI of the brain.

Magnetization transfer saturation ( ) is a useful marker to probe tissue macromolecular content and myelination in the brain. The increased -inhomogeneity at T and significantly larger saturation pulse flip angles which are often used for postmortem studies exceed the limits where previous correction methods are applicable. Here, we develop a calibration-based correction model and procedure, and validate and evaluate it in postmortem 7T data of whole chimpanzee brains. The dependence of was investigated by varying the off-resonance saturation pulse flip angle. For the range of saturation pulse flip angles applied in typical experiments on postmortem tissue, the dependence was close to linear. A linear model with a single calibration constant is proposed to correct bias in by mapping it to the reference value of the saturation pulse flip angle. was estimated voxel-wise in five postmortem chimpanzee brains. "Individual-based global parameters" were obtained by calculating the mean within individual specimen brains and "group-based global parameters" by calculating the means of the individual-based global parameters across the five brains. The linear calibration model described the data well, though was not entirely independent of the underlying tissue and . Individual-based correction parameters and a group-based global correction parameter ( ) led to visible, quantifiable reductions of -biases in high-resolution maps. The presented model and calibration approach effectively corrects for inhomogeneities in postmortem 7T data.

Time required for commitment to T cell proliferation depends on TCR affinity and cytokine response.

T cell activation and effector functions are determined by the affinity of the interaction between T cell receptor (TCR) and its antigenic peptide MHC (pMHC) ligand. A better understanding of the quantitative aspects of TCR-pMHC affinity-dependent T cell activation is critical for the development of new immunotherapeutic strategies. However, the role of TCR-pMHC affinity in regulating the kinetics of CD8+ T cell commitment to proliferation and differentiation is unknown. Here, we show that the stronger the TCR-pMHC affinity, the shorter the time of T cell-APC co-culture required to commit CD8+ T cells to proliferation. The time threshold for T cell cytokine production is much lower than that for cell proliferation. There is a strong correlation between affinity-dependent differences in AKT phosphorylation and T cell proliferation. The cytokine IL-15 increases the poor proliferation of T cells stimulated with low affinity pMHC, suggesting that pro-inflammatory cytokines can override the affinity-dependent features of T cell proliferation.

Equally potent: Nlrp3 mutation in macrophages or neutrophils is sufficient to drive autoinflammation.

Gain-of-function mutation in NLRP3 is associated with a spectrum of autoinflammatory disorders including familial cold autoinflammatory syndrome, Muckle-Wells syndrome, and neonatal onset multisystem inflammatory disease, collectively known as cryopyrin-associated periodic syndrome (CAPS). However, the cell types mediating the pathogenesis of CAPS are not completely understood. Two studies in EMBO Reports now demonstrate that gain-of-function Nlrp3 mutation in either macrophages or neutrophils alone is sufficient to trigger systemic autoinflammation and lethality in mice.

Gray Matter Variation in the Posterior Superior Temporal Gyrus Is Associated with Polymorphisms in the KIAA0319 Gene in Chimpanzees (Pan troglodytes).

Determining the impact that the KIAA0319 gene has on primate brain morphology can provide insight into the evolution of human cognition and language systems. Here, we tested whether polymorphisms in KIAA0319 in chimpanzees account for gray matter volumetric variation in brain regions implicated in language and communication (particularly within the posterior superior temporal gyrus and inferior frontal gyrus). First, we identified the nature and frequencies of single nucleotide variants (SNVs) in KIAA0319 in a sample of unrelated chimpanzees (Pan troglodytes spp.). Next, we genotyped a subset of SNVs (those important for gene regulation or likely to alter protein structure/function) in a sample of chimpanzees for which in vivo T1-structural magnetic resonance imaging scans had been obtained. We then used source-based morphometry (SBM) to test for whole-brain gray matter covariation differences between chimpanzees with different KIAA0319 alleles. Finally, using histologic sections of 15 postmortem chimpanzee brains, we analyzed microstructural variation related to KIAA0319 polymorphisms in the posterior superior temporal cortex. We found that the SNVs were associated with variation in gray matter within several brain regions, including the posterior superior temporal gyrus (a region associated with language comprehension and production in humans). The microstructure analysis further revealed hemispheric differences in neuropil fraction, indicating that KIAA0319 expression may be involved in regulation of processes related to the formation and maintenance of synapses, dendrites, or axons within regions associated with communication.

Individual variability in the size and organization of the human arcuate nucleus of the medulla.

The arcuate nucleus (Arc) of the medulla is found in almost all human brains and in a small percentage of chimpanzee brains. It is absent in the brains of other mammalian species including mice, rats, cats, and macaque monkeys. The Arc is classically considered a precerebellar relay nucleus, receiving input from the cerebral cortex and projecting to the cerebellum via the inferior cerebellar peduncle. However, several studies have found aplasia of the Arc in babies who died of SIDS (Sudden Infant Death Syndrome), and it was suggested that the Arc is the locus of chemosensory neurons critical for brainstem control of respiration. Aplasia of the Arc, however, has also been reported in adults, suggesting that it is not critical for survival. We have examined the Arc in closely spaced Nissl-stained sections in thirteen adult human cases to acquire a better understanding of the degree of variability of its size and location in adults. We have also examined immunostained sections to look for neurochemical compartments in this nucleus. Caudally, neurons of the Arc are ventrolateral to the pyramidal tracts (py); rostrally, they are ventro-medial to the py and extend up along the midline. In some cases, the Arc is discontinuous, with a gap between sections with the ventrolaterally located and the ventromedially located neurons. In all cases, there is some degree of left-right asymmetry in Arc position, size, and shape at all rostro-caudal levels. Somata of neurons in the Arc express calretinin (CR), neuronal nitric oxide synthase (nNOS), and nonphosphorylated neurofilament protein (NPNFP). Calbindin (CB) is expressed in puncta whereas there is no expression of parvalbumin (PV) in somata or puncta. There is also immunostaining for GAD and GABA receptors suggesting inhibitory input to Arc neurons. These properties were consistent among cases. Our data show differences in location of caudal and rostral Arc neurons and considerable variability among cases in the size and shape of the Arc. The variability in size suggests that "hypoplasia" of the Arc is difficult to define. The discontinuity of the Arc in many cases suggests that establishing aplasia of the Arc requires examination of many closely spaced sections through the brainstem.

Distribution of brain oxytocin and vasopressin V1a receptors in chimpanzees (Pan troglodytes): comparison with humans and other primate species.

Despite our close genetic relationship with chimpanzees, there are notable differences between chimpanzee and human social behavior. Oxytocin and vasopressin are neuropeptides involved in regulating social behavior across vertebrate taxa, including pair bonding, social communication, and aggression, yet little is known about the neuroanatomy of these systems in primates, particularly in great apes. Here, we used receptor autoradiography to localize oxytocin and vasopressin V1a receptors, OXTR and AVPR1a respectively, in seven chimpanzee brains. OXTR binding was detected in the lateral septum, hypothalamus, medial amygdala, and substantia nigra. AVPR1a binding was observed in the cortex, lateral septum, hypothalamus, mammillary body, entire amygdala, hilus of the dentate gyrus, and substantia nigra. Chimpanzee OXTR/AVPR1a receptor distribution is compared to previous studies in several other primate species. One notable difference is the lack of OXTR in reward regions such as the ventral pallidum and nucleus accumbens in chimpanzees, whereas OXTR is found in these regions in humans. Our results suggest that in chimpanzees, like in most other anthropoid primates studied to date, OXTR has a more restricted distribution than AVPR1a, while in humans the reverse pattern has been reported. Altogether, our study provides a neuroanatomical basis for understanding the function of the oxytocin and vasopressin systems in chimpanzees.

The distribution, number, and certain neurochemical identities of infracortical white matter neurons in a chimpanzee (Pan troglodytes) brain.

We examined the number, distribution, and immunoreactivity of the infracortical white matter neuronal population, also termed white matter interstitial cells (WMICs), throughout the telencephalic white matter of an adult female chimpanzee. Staining for neuronal nuclear marker (NeuN) revealed WMICs throughout the infracortical white matter, these cells being most numerous and dense close to the inner border of cortical layer VI, decreasing significantly in density with depth in the white matter. Stereological analysis of NeuN-immunopositive cells revealed an estimate of approximately 137.2million WMICs within the infracortical white matter of the chimpanzee brain studied. Immunostaining revealed subpopulations of WMICs containing neuronal nitric oxide synthase (nNOS, approximately 14.4million in number), calretinin (CR, approximately 16.7million), very few WMICs containing parvalbumin (PV), and no calbindin-immunopositive neurons. The nNOS, CR, and PV immunopositive WMICs, possibly all inhibitory neurons, represent approximately 22.6% of the total WMIC population. As the white matter is affected in many cognitive conditions, such as schizophrenia, autism, epilepsy, and also in neurodegenerative diseases, understanding these neurons across species is important for the translation of findings of neural dysfunction in animal models to humans. Furthermore, studies of WMICs in species such as apes provide a crucial phylogenetic context for understanding the evolution of these cell types in the human brain.

The Paracingulate Sulcus Is a Unique Feature of the Medial Frontal Cortex Shared by Great Apes and Humans

Primate brains display a wide range of variation in size and cerebral gyrification, leading to the appearance of novel sulci in particular groups of species. We investigated sulcal organization in the medial frontal cortex of great apes, with a particular focus on the paracingulate sulcus (PCGS). Until recently, the presence of the PCGS was thought to be a structural feature unique to the human brain. However, upon closer examination, the PCGS has been observed as a variable feature that also may appear in chimpanzee brains. To understand the evolutionary origins of the sulcal anatomy in the medial frontal cortex of apes, we examined high-resolution MRI scans for the presence or absence of the PCGS and, when present, measured its length in a sample of ape brains (chimpanzees, bonobos, gorillas, orangutans, gibbons, and siamangs). We found that the PCGS is variable in its appearance among these species, being present in 23 to 50% of great ape individuals depending on the species, but not present in gibbons or siamangs. We did not find population level hemispheric lateralization patterns or sex differences in PCGS presence across species, and we did not detect a relationship between cerebral volume and PCGS occurrence or length. Our data suggest that the PCGS is a common sulcal variant present in great apes and humans due to a shared evolutionary ancestry.

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

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

Age- and cognition-related differences in the gray matter volume of the chimpanzee brain (Pan troglodytes): A voxel-based morphometry and conjunction analysis.

Several primate species have been shown to exhibit age-related changes in cognition, brain, and behavior. However, severe neurodegenerative illnesses, such as Alzheimer's disease (AD), were once thought to be uniquely human. Recently, some chimpanzees naturally were documented to develop both neurofibrillary tangles and amyloid plaques, the main characteristics of AD pathology. In addition, like humans and other primates, chimpanzees show similar declines in cognition and motor function with age. Here, we used voxel-based morphometry to examine the relationships among gray matter volume, age, and cognition using magnetic resonance imaging scans previously acquired from chimpanzees (N = 216). We first determined the relationship between age and gray matter volume, identifying the regions that declined with age. With a subset of our sample (N = 103), we also determined differences in gray matter volume between older chimpanzees with higher cognition scores than expected for their age, and older chimpanzees with lower than expected scores. Finally, we ran a conjunction analysis to determine any overlap in brain regions between these two analyses. We found that as chimpanzees age, they lose gray matter in regions associated with cognition. In addition, cognitively healthy older chimpanzees (those performing better for their age) have greater gray matter volume in many brain regions compared with chimpanzees who underperform for their age. Finally, the conjunction analysis revealed that regions of age-related decline overlap with the regions that differ between cognitively healthy chimpanzees and those who underperform. This study provides further evidence that chimpanzees are an important model for research on the neurobiology of aging. Future studies should investigate the effects of cognitive stimulation on both cognitive performance and brain structure in aging nonhuman primates.

Chimpanzee histology and functional brain imaging show that the paracingulate sulcus is not human-specific

The paracingulate sulcus -PCGS- has been considered for a long time to be specific to the human brain. Its presence/absence has been discussed in relation to interindividual variability of personality traits and cognitive abilities. Recently, a putative PCGS has been observed in chimpanzee brains. To demonstrate that this newly discovered sulcus is the homologue of the PCGS in the human brain, we analyzed cytoarchitectonic and resting-state functional magnetic resonance imaging data in chimpanzee brains which did or did not display a PCGS. The results show that the organization of the mid-cingulate cortex of the chimpanzee brain is comparable to that of the human brain, both cytoarchitectonically and in terms of functional connectivity with the lateral frontal cortex. These results demonstrate that the PCGS is not human-specific but is a shared feature of the primate brain since at least the last common ancestor to humans and great apes ~6 mya.

In-vivo diffusion MRI protocol optimization for the chimpanzee brain and examination of aging effects on the primate optic nerve at 3T

BackgroundDiffusion MRI (dMRI) data acquisition protocols are well-established on modern high-field clinical scanners for human studies. However, these protocols are not suitable for the chimpanzee (or other large-brained mammals) because of its substantial difference in head geometry and brain volume compared with humans. Therefore, an optimal dMRI data acquisition protocol dedicated to chimpanzee neuroimaging is needed. MethodsA multi-shot (4 segments) double spin-echo echo-planar imaging (MS-EPI) sequence and a single-shot double spin-echo EPI (SS-EPI) sequence were optimized separately for in vivo dMRI data acquisition of chimpanzees using a clinical 3T scanner. Correction for severe susceptibility-induced image distortion and signal drop-off of the chimpanzee brain was performed and evaluated using FSL software. DTI indices in different brain regions and probabilistic tractography were compared. A separate DTI data set from n=34 chimpanzees (13 to 56 years old) was collected using the optimal protocol. Age-related changes in diffusivity indices of optic nerve fibers were evaluated. ResultsThe SS-EPI sequence acquired dMRI data of the chimpanzee brain with approximately doubled the SNR as the MS-EPI sequence given the same scan time. The quality of white matter fiber tracking from the SS-EPI data was much higher than that from MS-EPI data. However, quantitative analysis of DTI indices showed no difference in most ROIs between the SS-EPI and MS-EPI sequences. The progressive evolution of diffusivity indices of optic nerves indicated mild changes in fiber bundles of chimpanzees aged 40 years and above. ConclusionThe single-shot EPI-based acquisition protocol provided better image quality of dMRI for chimpanzee brains and is recommended for in vivo dMRI study or clinical diagnosis of chimpanzees (or other large animals) using a clinical scanner. Also, the tendency of FA decrease or diffusivity increase in the optic nerve of aged chimpanzees was seen but did not show significant age-related changes, suggesting aging may have less impact on optic nerve fiber integrity of chimpanzees, in contrast to previous results for both macaque monkeys and humans.

Chimpanzee brain morphometry utilizing standardized MRI preprocessing and macroanatomical annotations.

Chimpanzees are among the closest living relatives to humans and, as such, provide a crucial comparative model for investigating primate brain evolution. In recent years, human brain mapping has strongly benefited from enhanced computational models and image processing pipelines that could also improve data analyses in animals by using species-specific templates. In this study, we use structural MRI data from the National Chimpanzee Brain Resource (NCBR) to develop the chimpanzee brain reference template Juna.Chimp for spatial registration and the macro-anatomical brain parcellation Davi130 for standardized whole-brain analysis. Additionally, we introduce a ready-to-use image processing pipeline built upon the CAT12 toolbox in SPM12, implementing a standard human image preprocessing framework in chimpanzees. Applying this approach to data from 194 subjects, we find strong evidence for human-like age-related gray matter atrophy in multiple regions of the chimpanzee brain, as well as, a general rightward asymmetry in brain regions.