Morphometric variability of minicolumns in the striate cortex of Homo sapiens, Macaca mulatta, and Pan troglodytes

Neuropil Variation in the Prefrontal, Motor, and Visual Cortex of Six Felids

INTRODUCTION

Felids have evolved a specialized suite of morphological adaptations for obligate carnivory. Although the musculoskeletal anatomy of the Felidae has been studied extensively, the comparative neuroanatomy of felids is relatively unexplored. Little is known about how variation in the cerebral anatomy of felids relates to species-specific differences in sociality, hunting strategy, or activity patterns.

METHODS

We quantitatively analyzed neuropil variation in the prefrontal, primary motor, and primary visual cortices of six species of Felidae (Panthera leo, Panthera uncia, Panthera tigris, Panthera leopardus, Acinonyx jubatus, Felis sylvestris domesticus) to investigate relationships with brain size, neuronal cell parameters, and select behavioral and ecological factors. Neuropil is the dense, intricate network of axons, dendrites, and synapses in the brain, playing a critical role in information processing and communication between neurons.

RESULTS

There were significant species and regional differences in neuropil proportions, with African lion, cheetah, and tiger having more neuropil in all three cortical regions in comparison to the other species. Based on regression analyses, we find that the increased neuropil fraction in the prefrontal cortex supports social and behavioral flexibility, while in the primary motor cortex, this facilitates the neural activity needed for hunting movements. Greater neuropil fraction in the primary visual cortex may contribute to visual requirements associated with diel activity patterns.

CONCLUSION

These results provide a cross-species comparison of neuropil fraction variation in the Felidae, particularly the understudied Panthera, and provide evidence for convergence of the neuroanatomy of Panthera and cheetahs.

Homology of neocortical areas in rats and primates based on cortical type analysis: an update of the Hypothesis on the Dual Origin of the Neocortex

Sixty years ago, Friedrich Sanides traced the origin of the tangential expansion of the primate neocortex to two ancestral anlagen in the allocortex of reptiles and mammals, and proposed the Hypothesis on the Dual Origin of the Neocortex. According to Sanides, paraolfactory and parahippocampal gradients of laminar elaboration expanded in evolution by addition of successive concentric rings of gradually different cortical types inside the allocortical ring. Rodents had fewer rings and primates had more rings in the inner part of the cortex. In the present article, we perform cortical type analysis of the neocortex of adult rats, Rhesus macaques, and humans to propose hypotheses on homology of cortical areas applying the principles of the Hypothesis on the Dual Origin of the Neocortex. We show that areas in the outer rings of the neocortex have comparable laminar elaboration in rats and primates, while most 6-layer eulaminate areas in the innermost rings of primate neocortex lack homologous counterparts in rats. We also represent the topological distribution of cortical types in simplified flat maps of the cerebral cortex of monotremes, rats, and primates. Finally, we propose an elaboration of the Hypothesis on the Dual Origin of the Neocortex in the context of modern studies of pallial patterning that integrates the specification of pallial sectors in development of vertebrate embryos. The updated version of the hypothesis of Sanides provides explanation for the emergence of cortical hierarchies in mammals and will guide future research in the phylogenetic origin of neocortical areas.

Multivariate patterns of brain-behavior associations across the adult lifespan

The nature of brain-behavior covariations with increasing age is poorly understood. In the current study, we used a multivariate approach to investigate the covariation between behavioral-health variables and brain features across adulthood. We recruited healthy adults aged 20–73 years-old (29 younger, mean age = 25.6 years; 30 older, mean age = 62.5 years), and collected structural and functional MRI (s/fMRI) during a resting-state and three tasks. From the sMRI, we extracted cortical thickness and subcortical volumes; from the fMRI, we extracted activation peaks and functional network connectivity (FNC) for each task. We conducted canonical correlation analyses between behavioral-health variables and the sMRI, or the fMRI variables, across all participants. We found significant covariations for both types of neuroimaging phenotypes (ps = 0.0004) across all individuals, with cognitive capacity and age being the largest opposite contributors. We further identified different variables contributing to the models across phenotypes and age groups. Particularly, we found behavior was associated with different neuroimaging patterns between the younger and older groups. Higher cognitive capacity was supported by activation and FNC within the executive networks in the younger adults, while it was supported by the visual networks’ FNC in the older adults. This study highlights how the brain-behavior covariations vary across adulthood and provides further support that cognitive performance relies on regional recruitment that differs between older and younger individuals.

Translational Neuroscience in Autism: From Neuropathology to Transcranial Magnetic Stimulation Therapies

The presence of heterotopias, increased regional density of neurons at the gray-white matter junction, and focal cortical dysplasias all suggest an abnormality of neuronal migration in autism spectrum disorder (ASD). The abnormality is borne from a dissonance in timing between radial and tangentially migrating neuroblasts to the developing cortical plate. The uncoupling of excitatory and inhibitory cortical cells disturbs the coordinated interactions of neurons within local networks, thus providing abnormal patterns of brainwave activity in the gamma bandwidth. In ASD, gamma oscillation abnormalities and autonomic markers offer measures of therapeutic progress and help in the identification of subgroups.

The modular organization of the cerebral cortex: Evolutionary significance and possible links to neurodevelopmental conditions

The recognition of discernible anatomical regularities that appear to self-organize during development makes apparent the modular organization of the cerebral cortex. The metabolic cost engendered in sustaining interneuronal communications has emphasized the viability of short connections among neighboring neurons. This pattern of connectivity establishes a microcircuit which is repeated in parallel throughout the cerebral cortex. This canonical circuit is contained within the smallest module of information processing of the cerebral cortex; one which Vernon Mountcastle called the minicolumn. Plasticity within the brain is accounted, in part, by the presence of weak linkages that allow minicolumns to process information from a variety of sources and to quickly adapt to environmental exigencies without a need for genetic change. Recent research suggests that interlaminar correlated firing between minicolumns during the decision phase of target selection provides for the emergence of some executive functions. Bottlenecks of information processing within this modular minicolumnar organization may account for a variety of mental disorders observed in neurodevelopmental conditions.

Review: Cortical construction in autism spectrum disorder: columns, connectivity and the subplate

The cerebral cortex undergoes protracted maturation during human development and exemplifies how biology and environment are inextricably intertwined in the construction of complex neural circuits. Autism spectrum disorders are characterized by a number of pathological changes arising from this developmental process. These include: (i) alterations to columnar structure that have significant implications for the organization of cortical circuits and connectivity; (ii) alterations to synaptic spines on individual cortical units that may underlie specific types of connectional changes; and (iii) alterations within the cortical subplate, a region that plays a role in proper cortical development and in regulating interregional communication in the mature brain. Although the cerebral cortex is not the only structure affected in the disorder, it is a fundamental contributor to the behaviours that characterize autism. These alterations to cortical circuitry likely underlie the behavioural phenotype in autism and contribute to the unique pattern of deficits and strengths that characterize cognitive functioning. Recent findings within the cortical subplate may indicate that alterations to cortical construction begin prenatally, before activity-dependent connections are established, and are in need of further study. A better understanding of cortical development in autism spectrum disorders will draw bridges between the microanatomical computational circuitry and the atypical behaviours that arise when that circuitry is modified. In addition, it will allow us to better exploit the constructional plasticity within the brain to design more targeted interventions that better manage atypical cortical construction and that can be applied very early in postnatal life.

An automated pipeline for constructing personalized virtual brains from multimodal neuroimaging data

Large amounts of multimodal neuroimaging data are acquired every year worldwide. In order to extract high-dimensional information for computational neuroscience applications standardized data fusion and efficient reduction into integrative data structures are required. Such self-consistent multimodal data sets can be used for computational brain modeling to constrain models with individual measurable features of the brain, such as done with The Virtual Brain (TVB). TVB is a simulation platform that uses empirical structural and functional data to build full brain models of individual humans. For convenient model construction, we developed a processing pipeline for structural, functional and diffusion-weighted magnetic resonance imaging (MRI) and optionally electroencephalography (EEG) data. The pipeline combines several state-of-the-art neuroinformatics tools to generate subject-specific cortical and subcortical parcellations, surface-tessellations, structural and functional connectomes, lead field matrices, electrical source activity estimates and region-wise aggregated blood oxygen level dependent (BOLD) functional MRI (fMRI) time-series. The output files of the pipeline can be directly uploaded to TVB to create and simulate individualized large-scale network models that incorporate intra- and intercortical interaction on the basis of cortical surface triangulations and white matter tractograpy. We detail the pitfalls of the individual processing streams and discuss ways of validation. With the pipeline we also introduce novel ways of estimating the transmission strengths of fiber tracts in whole-brain structural connectivity (SC) networks and compare the outcomes of different tractography or parcellation approaches. We tested the functionality of the pipeline on 50 multimodal data sets. In order to quantify the robustness of the connectome extraction part of the pipeline we computed several metrics that quantify its rescan reliability and compared them to other tractography approaches. Together with the pipeline we present several principles to guide future efforts to standardize brain model construction. The code of the pipeline and the fully processed data sets are made available to the public via The Virtual Brain website (thevirtualbrain.org) and via github (https://github.com/BrainModes/TVB-empirical-data-pipeline). Furthermore, the pipeline can be directly used with High Performance Computing (HPC) resources on the Neuroscience Gateway Portal (http://www.nsgportal.org) through a convenient web-interface.

Distribution of vesicular glutamate transporter 2 (VGluT2) in the primary visual cortex of the macaque and human

The majority of thalamic terminals in V1 arise from lateral geniculate nucleus (LGN) afferents. Thalamic afferent terminals are preferentially labeled by an isoform of the vesicular glutamate transporter, VGluT2. The goal of our study was to determine the distribution of VGluT2-ir puncta in macaque and human visual cortex. First, we investigated the distribution of VGluT2-ir puncta in all layers of macaque monkey primary visual cortex (V1), and found a very close correspondence between the known distribution of LGN afferents from previous studies and the distribution of VGluT2-immunoreactive (-ir) puncta. There was also a close correspondence between cytochrome oxidase density and VGluT2-ir puncta distribution. After validating the correspondence in macaque, we made a comparative study in human V1. In many aspects, the distribution of VGluT2-ir puncta in human was qualitatively similar to that of the macaque: high densities in layer 4C, patches of VGluT2-ir puncta in the supragranular layer (2/3), lower but clear distribution in layers 1 and 6, and very few puncta in layers 5 and 4B. However, there were also important differences between macaques and humans. In layer 4A of human, there was a sparse distribution of VGluT2-ir puncta, whereas in macaque, there was a dense distribution with the characteristic honeycomb organization. The results suggest important changes in the pattern of cortical VGluT2 immunostaining that may be related to evolutionary differences in the cortical organization of LGN afferents between Old World monkeys and humans.

Social, communication, and cortical structural impairments in Epac2-deficient mice

Deficits in social and communication behaviors are common features of a number of neurodevelopmental disorders. However, the molecular and cellular substrates of these higher order brain functions are not well understood. Here we report that specific alterations in social and communication behaviors in mice occur as a result of loss of the EPAC2 gene, which encodes a protein kinase A-independent cAMP target. Epac2-deficient mice exhibited robust deficits in social interactions and ultrasonic vocalizations, but displayed normal olfaction, working and reference memory, motor abilities, anxiety, and repetitive behaviors. Epac2-deficient mice displayed abnormal columnar organization in the anterior cingulate cortex, a region implicated in social behavior in humans, but not in somatosensory cortex. In vivo two-photon imaging revealed reduced dendritic spine motility and density on cortical neurons in Epac2-deficient mice, indicating deficits at the synaptic level. Together, these findings provide novel insight into the molecular and cellular substrates of social and communication behavior.

Human Evolution and the Chimpanzee Referential Doctrine

Chimpanzees are our closest living genomic relatives, but they lack the bipedal locomotion, markedly enlarged brains, and advanced communication skills of humans. This has led many to view them as “primitive” and to presume that their behavior and anatomy are also primitive. If true, they could serve as models of our last common ancestor (LCA), i.e., a territorially aggressive knuckle walker, reliant on vertical climbing and below-branch suspension to access the high canopy as a ripe-fruit frugivore. Ardipithecus now provides abundant information that the LCA differed substantially from chimpanzees (as well as bonobos and gorillas), both anatomically and behaviorally, and exhibited many characters that are more similar to those of modern humans than to any living ape. This major extension of the hominoid fossil record contravenes strict referential modeling based on the extant chimpanzee and greatly improves our ability to reconstruct the LCA more accurately, but only when viewed within the broader context of evolutionary ecology.

Positive selection on NIN, a gene involved in neurogenesis, and primate brain evolution

A long-held dogma in comparative neurobiology has been that the number of neurons under a given area of cortical surface is constant. As such, the attention of those seeking to understand the genetic basis of brain evolution has focused on genes with functions in the lateral expansion of the developing cerebral cortex. However, new data suggest that cortical cytoarchitecture is not constant across primates, raising the possibility that changes in radial cortical development played a role in primate brain evolution. We present the first analysis of a gene with functions relevant to this dimension of brain evolution. We show that NIN, a gene necessary for maintaining asymmetric, neurogenic divisions of radial glial cells (RGCs), evolved adaptively during anthropoid evolution. We explored how this selection relates to neural phenotypes and find a significant association between selection on NIN and neonatal brain size in catarrhines. Our analyses suggest a relationship with prenatal neurogenesis and identify the human data point as an outlier, possibly explained by postnatal changes in development on the human lineage. A similar pattern is found in platyrrhines, but the highly encephalized genus Cebus departs from the general trend. We further show that the evolution of NIN may be associated with variation in neuron number not explained by increases in surface area, a result consistent with NIN's role in neurogenic divisions of RGCs. Our combined results suggest a role for NIN in the evolution of cortical development.

Columnar processing in primate pFC: evidence for executive control microcircuits

A common denominator for many cognitive disorders of human brain is the disruption of neural activity within pFC, whose structural basis is primarily interlaminar (columnar) microcircuits or "minicolumns." The importance of this brain region for executive decision-making has been well documented; however, because of technological constraints, the minicolumnar basis is not well understood. Here, via implementation of a unique conformal multielectrode recording array, the role of interlaminar pFC minicolumns in the executive control of task-related target selection is demonstrated in nonhuman primates performing a visuomotor DMS task. The results reveal target-specific, interlaminar correlated firing during the decision phase of the trial between multielectrode recording array-isolated minicolumnar pairs of neurons located in parallel in layers 2/3 and layer 5 of pFC. The functional significance of individual pFC minicolumns (separated by 40 μm) was shown by reduced correlated firing between cell pairs within single minicolumns on error trials with inappropriate target selection. To further demonstrate dependence on performance, a task-disrupting drug (cocaine) was administered in the middle of the session, which also reduced interlaminar firing in minicolumns that fired appropriately in the early (nondrug) portion of the session. The results provide a direct demonstration of task-specific, real-time columnar processing in pFC indicating the role of this type of microcircuit in executive control of decision-making in primate brain.

Laws of conservation as related to brain growth, aging, and evolution: symmetry of the minicolumn

Development, aging, and evolution offer different time scales regarding possible anatomical transformations of the brain. This article expands on the perspective that the cerebral cortex exhibits a modular architecture with invariant properties in regards to these time scales. These properties arise from morphometric relations of the ontogenetic minicolumn as expressed in Noether’s first theorem, i.e., that for each continuous symmetry there is a conserved quantity. Whenever minicolumnar symmetry is disturbed by either developmental or aging processes the principle of least action limits the scope of morphometric alterations. Alternatively, local and global divergences from these laws apply to acquired processes when the system is no longer isolated from its environment. The underlying precepts to these physical laws can be expressed in terms of mathematical equations that are conservative of quantity. Invariant properties of the brain include the rotational symmetry of minicolumns, a scaling proportion or “even expansion” between pyramidal cells and core minicolumnar size, and the translation of neuronal elements from the main axis of the minicolumn. It is our belief that a significant portion of the architectural complexity of the cerebral cortex, its response to injury, and its evolutionary transformation, can all be captured by a small set of basic physical laws dictated by the symmetry of minicolumns. The putative preservations of parameters related to the symmetry of the minicolumn suggest that the development and final organization of the cortex follows a deterministic process.

Spatial organization of neurons in the frontal pole sets humans apart from great apes

Few morphological differences have been identified so far that distinguish the human brain from the brains of our closest relatives, the apes. Comparative analyses of the spatial organization of cortical neurons, including minicolumns, can aid our understanding of the functionally relevant aspects of microcircuitry. We measured horizontal spacing distance and gray-level ratio in layer III of 4 regions of human and ape cortex in all 6 living hominoid species: frontal pole (Brodmann area [BA] 10), and primary motor (BA 4), primary somatosensory (BA 3), and primary visual cortex (BA 17). Our results identified significant differences between humans and apes in the frontal pole (BA 10). Within the human brain, there were also significant differences between the frontal pole and 2 of the 3 regions studied (BA 3 and BA 17). Differences between BA 10 and BA 4 were present but did not reach significance. These findings in combination with earlier findings on BA 44 and BA 45 suggest that human brain evolution was likely characterized by an increase in the number and width of minicolumns and the space available for interconnectivity between neurons in the frontal lobe, especially the prefrontal cortex.

Neural activity in frontal cortical cell layers: evidence for columnar sensorimotor processing

The mammalian frontal cortex (FCx) is at the top of the brain's sensorimotor hierarchy and includes cells in the supragranular Layer 2/3, which integrate convergent sensory information for transmission to infragranular Layer 5 cells to formulate motor system outputs that control behavioral responses. Functional interaction between these two layers of FCx was examined using custom-designed ceramic-based microelectrode arrays (MEAs) that allowed simultaneous recording of firing patterns of FCx neurons in Layer 2/3 and Layer 5 in nonhuman primates performing a simple go/no-go discrimination task. This unique recording arrangement showed differential encoding of task-related sensory events by cells in each layer with Layer 2/3 cells exhibiting larger firing peaks during presentation of go target and no-go target task images, whereas Layer 5 cells showed more activity during reward contingent motor responses in the task. Firing specificity to task-related events was further demonstrated by synchronized firing between pairs of cells in different layers that occupied the same vertically oriented "column" on the MEA. Pairs of cells in different layers recorded at adjacent "noncolumnar" orientations on the MEA did not show synchronized firing during the same task-related events. The results provide required evidence in support of previously suggested task-related sensorimotor processing in the FCx via functionally segregated minicolumns.

Cortical organization: a description and interpretation of anatomical findings based on systems theory

The organization of the cortex can be understood as a complex system comprised of interconnected modules called minicolumns. Comparative anatomical studies suggest that evolution has prompted a scale free world network of connectivity within the white matter while simultaneously increasing the complexity of minicolumnar composition. It is this author's opinion that this complex system is poised to collapse under the weight of environmental exigencies. Some mental disorders may be the manifestations of this collapse.

A topographic study of minicolumnar core width by lamina comparison between autistic subjects and controls: possible minicolumnar disruption due to an anatomical element in-common to multiple laminae

Radial cell minicolumns are basic cytoarchitectonic motifs of the mammalian neocortex. Recent studies reveal that autism is associated with a "minicolumnopathy" defined by decreased columnar width and both a diminished and disrupted peripheral neuropil compartment. This study further characterizes this cortical deficit by comparing minicolumnar widths across layers. Brains from seven autistic patients and an equal number of age-matched controls were celloidin embedded, serially sectioned at 200 microm and Nissl stained with gallocyanin. Photomicrograph mosaics of the cortex were analyzed with computerized imaging methods to determine minicolumnar width at nine separate neocortical areas: Brodmann Area's (BA) 3b, 4, 9, 10, 11, 17, 24, 43 and 44. Each area was assessed at supragranular, granular and infragranular levels. Autistic subjects had smaller minicolumns whose dimensions varied according to neocortical area. The greatest difference between autistic and control groups was observed in area 44. The interaction of diagnosis x cortical area x lamina (F(16,316) = 1.33; P = 0.175) was not significant. Diminished minicolumnar width across deep and superficial neocortical layers most probably reflects involvement of shared constituents among the different layers. In this article we discuss the possible role of double bouquet and pyramidal cells in the translaminar minicolumnar width narrowing observed in autistic subjects.

A comparative perspective on minicolumns and inhibitory GABAergic interneurons in the neocortex

Neocortical columns are functional and morphological units whose architecture may have been under selective evolutionary pressure in different mammalian lineages in response to encephalization and specializations of cognitive abilities. Inhibitory interneurons make a substantial contribution to the morphology and distribution of minicolumns within the cortex. In this context, we review differences in minicolumns and GABAergic interneurons among species and discuss possible implications for signaling among and within minicolumns. Furthermore, we discuss how abnormalities of both minicolumn disposition and inhibitory interneurons might be associated with neuropathological processes, such as Alzheimer's disease, autism, and schizophrenia. Specifically, we explore the possibility that phylogenetic variability in calcium-binding protein-expressing interneuron subtypes is directly related to differences in minicolumn morphology among species and might contribute to neuropathological susceptibility in humans.

Minicolumnar width: Comparison between supragranular and infragranular layers

The minicolumn derives from the radial migration of neurons along glial scaffoldings during gestation. Investigators have presumed the minicolumn to be a single-cell wide structure based on their rectilinear migratory origin. The present study measures the width of minicolumnar cores in both supra- and infra-granular layers. Postmortem tissue was obtained from 9 brain areas in 7 normative individuals. Examined tissues were celloidin embedded and Nissl stained. Digital images were denoised and then analyzed with a step-wise algorithm involving region growing and recursive line tracing. Significant differences were noted between the minicolumnar core widths of supra- and infra-granular layers. A review of the literature on corticogenesis provides some ideas as to how these laminar differences in minicolumnar core width are engendered.