Cellular heterogeneity in cerebral cortex: a study of the morphology of pyramidal neurones in visual areas of the marmoset monkey

Marmoset core visual object recognition behavior is comparable to that of macaques and humans

Among the smallest simian primates, the common marmoset offers promise as an experimentally tractable primate model for neuroscience with translational potential to humans. However, given its exceedingly small brain and body, the gap in perceptual and cognitive abilities between marmosets and humans requires study. Here, we performed a comparison of marmoset behavior to that of three other species in the domain of high-level vision. We first found that marmosets outperformed rats - a marmoset-sized rodent - on a simple recognition task, with marmosets robustly recognizing objects across views. On a more challenging invariant object recognition task used previously in humans, marmosets also achieved high performance. Notably, across hundreds of images, marmosets' image-by-image behavior was highly similar to that of humans - nearly as human-like as macaque behavior. Thus, core aspects of visual perception are conserved across monkeys and humans, and marmosets present salient behavioral advantages over other small model organisms for visual neuroscience.

Neuronal density and expression of calcium-binding proteins across the layers of the superior colliculus in the common marmoset (Callithrix jacchus)

The superior colliculus (SC) is a layered midbrain structure with functions that include polysensory and sensorimotor integration. Here, we describe the distribution of different immunohistochemically identified classes of neurons in the SC of adult marmoset monkeys (Callithrix jacchus). Neuronal nuclei (NeuN) staining was used to determine the overall neuronal density in the different SC layers. In addition, we studied the distribution of neurons expressing different calcium-binding proteins (calbindin [CB], parvalbumin [PV] and calretinin [CR]). Our results indicate that neuronal density in the SC decreases from superficial to deep layers. Although the neuronal density within the same layer varies little across the mediolateral axis, it tends to be lower at rostral levels, compared to caudal levels. Cells expressing different calcium-binding proteins display differential gradients of density according to depth. Both CB- and CR-expressing neurons show markedly higher densities in the stratum griseum superficiale (SGS), compared to the stratum opticum and intermediate and deep layers. However, CR-expressing neurons are twice as common as CB-expressing neurons outside the SGS. The distribution of PV-expressing cells follows a shallow density gradient from superficial to deep layers. When normalized relative to total neuronal density, the proportion of CR-expressing neurons increases between the superficial and intermediate layers, whereas that of CB-expressing neurons declines toward the deep layers. The proportion of PV-expressing neurons remains constant across layers. Our data provide layer-specific and accurate estimates of neuronal density, which may be important for the generation of biophysical models of how the primate SC transforms sensory inputs into motor signals.

The large numbers of minicolumns in the primary visual cortex of humans, chimpanzees and gorillas are related to high visual acuity

Minicolumns are thought to be a fundamental neural unit in the neocortex and their replication may have formed the basis of the rapid cortical expansion that occurred during primate evolution. We sought evidence of minicolumns in the primary visual cortex (V-1) of three great apes, three rodents and representatives from three other mammalian orders: Eulipotyphla (European hedgehog), Artiodactyla (domestic pig) and Carnivora (ferret). Minicolumns, identified by the presence of a long bundle of radial, myelinated fibers stretching from layer III to the white matter of silver-stained sections, were found in the human, chimpanzee, gorilla and guinea pig V-1. Shorter bundles confined to one or two layers were found in the other species but represent modules rather than minicolumns. The inter-bundle distance, and hence density of minicolumns, varied systematically both within a local area that might represent a hypercolumn but also across the whole visual field. The distance between all bundles had a similar range for human, chimpanzee, gorilla, ferret and guinea pig: most bundles were 20-45 μm apart. By contrast, the space between bundles was greater for the hedgehog and pig (20-140 μm). The mean density of minicolumns was greater in tangential sections of the gorilla and chimpanzee (1,243-1,287 bundles/mm2) than in human (314-422 bundles/mm2) or guinea pig (643 bundles/mm2). The minicolumnar bundles did not form a hexagonal lattice but were arranged in thin curving and branched bands separated by thicker bands of neuropil/somata. Estimates of the total number of modules/minicolumns within V-1 were strongly correlated with visual acuity.

Common Microscale and Macroscale Principles of Connectivity in the Human Brain

The brain requires efficient information transfer between neurons and large-scale brain regions. Brain connectivity follows predictable organizational principles. At the cellular level, larger supragranular pyramidal neurons have larger, more branched dendritic trees, more synapses, and perform more complex computations; at the macroscale, region-to-region connections display a diverse architecture with highly connected hub areas facilitating complex information integration and computation. Here, we explore the hypothesis that the branching structure of large-scale region-to-region connectivity follows similar organizational principles as the neuronal scale. We examine microscale connectivity of basal dendritic trees of supragranular pyramidal neurons (300+) across 10 cortical areas in five human donor brains (1 male, 4 female). Dendritic complexity was quantified as the number of branch points, tree length, spine count, spine density, and overall branching complexity. High-resolution diffusion-weighted MRI was used to construct white matter trees of corticocortical wiring. Examining complexity of the resulting white matter trees using the same measures as for dendritic trees shows heteromodal association areas to have larger, more complex white matter trees than primary areas (p < 0.0001) and macroscale complexity to run in parallel with microscale measures, in terms of number of inputs (r = 0.677, p = 0.032), branch points (r = 0.797, p = 0.006), tree length (r = 0.664, p = 0.036), and branching complexity (r = 0.724, p = 0.018). Our findings support the integrative theory that brain connectivity follows similar principles of connectivity at neuronal and macroscale levels and provide a framework to study connectivity changes in brain conditions at multiple levels of organization.SIGNIFICANCE STATEMENT Within the human brain, cortical areas are involved in a wide range of processes, requiring different levels of information integration and local computation. At the cellular level, these regional differences reflect a predictable organizational principle with larger, more complexly branched supragranular pyramidal neurons in higher order regions. We hypothesized that the 3D branching structure of macroscale corticocortical connections follows the same organizational principles as the cellular scale. Comparing branching complexity of dendritic trees of supragranular pyramidal neurons and of MRI-based regional white matter trees of macroscale connectivity, we show that macroscale branching complexity is larger in higher order areas and that microscale and macroscale complexity go hand in hand. Our findings contribute to a multiscale integrative theory of brain connectivity.

Variation in Pyramidal Cell Morphology Across the Human Anterior Temporal Lobe

Pyramidal neurons are the most abundant and characteristic neuronal type in the cerebral cortex and their dendritic spines are the main postsynaptic elements of cortical excitatory synapses. Previous studies have shown that pyramidal cell structure differs across layers, cortical areas, and species. However, within the human cortex, the pyramidal dendritic morphology has been quantified in detail in relatively few cortical areas. In the present work, we performed intracellular injections of Lucifer Yellow at several distances from the temporal pole. We found regional differences in pyramidal cell morphology, which showed large inter-individual variability in most of the morphological variables measured. However, some values remained similar in all cases. The smallest and least complex cells in the most posterior temporal region showed the greatest dendritic spine density. Neurons in the temporal pole showed the greatest sizes with the highest number of spines. Layer V cells were larger, more complex, and had a greater number of dendritic spines than those in layer III. The present results suggest that, while some aspects of pyramidal structure are conserved, there are specific variations across cortical regions, and species.

Developmental Changes in Pyramidal Cell Morphology in Multiple Visual Cortical Areas Using Cluster Analysis

Neuronal morphology is characterized by salient features such as complex axonal and dendritic arbors. In the mammalian brain, variations in dendritic morphology among cell classes, brain regions, and animal species are thought to underlie known differences in neuronal function. In this work, we obtained a large dataset from http://neuromorpho.org/ comprising layer III pyramidal cells in different cortical areas of the ventral visual pathway (V1, V2, V4, TEO, and TE) of the macaque monkey at different developmental stages. We performed an in depth quantitative analysis of pyramidal cell morphology throughout development in an effort to determine which aspects mature early in development and which features require a protracted period of maturation. We were also interested in establishing if developmental changes in morphological features occur simultaneously or hierarchically in multiple visual cortical areas. We addressed these questions by performing principal component analysis (PCA) and hierarchical clustering analysis on relevant morphological features. Our analysis indicates that the maturation of pyramidal cell morphology is largely based on early development of topological features in most visual cortical areas. Moreover, the maturation of pyramidal cell morphology in V1, V2, V4, TEO, and TE is characterized by unique developmental trajectories.

Brain connectomes come of age

Databases of consistent, directed- and weighted inter-areal connectivity for mouse, macaque and marmoset monkeys have recently become available and begun to be used to build structural and dynamical models. A structural hierarchy can be defined based by laminar patterns of cortical connections. A large-scale dynamical model of the macaque cortex endowed with a laminar structure accounts for empirically observed frequency-modulated interplay between bottom-up and top-down processes. Signal propagation in the model with spiking neurons displays a threshold of stimulus amplitude for the activity to gain access to the prefrontal cortex, reminiscent of the ignition phenomenon associated with conscious perception. These two examples illustrate how connectomics inform structurally based dynamic models of multi-regional brain systems. Theory raises novel questions for future anatomical and physiological empirical research, in a back-and-forth collaboration between experimentalists and theorists. Directed- and weighted inter-areal cortical connectivity matrices of macaque, marmoset and mouse exhibit similarities as well as marked differences. The new connectomic data provide quantitative information for structural and dynamical modeling of multi-regional cortical circuit providing insight to the global cortical function. Quantification of cortical hierarchy guides investigations of interplay between bottom-up and top-down information processes.

Marmosets have a greater diversity of c-Fos response after hyperstimulation in distinct cortical regions as compared to rats

Previous evidence indicated a potential mechanism that might support the fact that primates exhibit greater neural integration capacity as a result of the activation of different structures of the central nervous system, as compared to rodents. The current study aimed to provide further evidence to confirm previous findings by analyzing the patterns of c-Fos expression in more neocortical structures of rats and marmosets using a more robust quantitative technique and evaluating a larger number of brain areas. Nineteen Wistar rats and 21 marmosets (Callithrix jacchus) were distributed among control groups (animals without injections) and animals injected with pentylenetetrazol (PTZ) and euthanized at different time points after stimulus. Immunohistochemical detection of c-Fos was quantified using unbiased and efficient stereological cell counting in eight neocortical regions. Marmosets had a c-Fos expression that was notably more widely expressed (5× more cells) and longer lasting (up to 3 hr) than rats. c-Fos expression in rats presented similar patterns of expression according to the function of the brain cortical structures (associative, sensorial, and motor functions), which was not observed for marmosets (in which no clear pattern could be drawn, and a more diverse profile emerged). Our results provide evidence that the marmoset brain has a greater neuronal activation after intense stimulation by means of PTZ and a more complex pattern of brain activation. We speculate that these functional differences may contribute for the understanding of the different neuronal processing capacities of the neocortex in these mammals' orders.

BigBrain 3D atlas of cortical layers: Cortical and laminar thickness gradients diverge in sensory and motor cortices

Histological atlases of the cerebral cortex, such as those made famous by Brodmann and von Economo, are invaluable for understanding human brain microstructure and its relationship with functional organization in the brain. However, these existing atlases are limited to small numbers of manually annotated samples from a single cerebral hemisphere, measured from 2D histological sections. We present the first whole-brain quantitative 3D laminar atlas of the human cerebral cortex. It was derived from a 3D histological atlas of the human brain at 20-micrometer isotropic resolution (BigBrain), using a convolutional neural network to segment, automatically, the cortical layers in both hemispheres. Our approach overcomes many of the historical challenges with measurement of histological thickness in 2D, and the resultant laminar atlas provides an unprecedented level of precision and detail. We utilized this BigBrain cortical atlas to test whether previously reported thickness gradients, as measured by MRI in sensory and motor processing cortices, were present in a histological atlas of cortical thickness and which cortical layers were contributing to these gradients. Cortical thickness increased across sensory processing hierarchies, primarily driven by layers III, V, and VI. In contrast, motor-frontal cortices showed the opposite pattern, with decreases in total and pyramidal layer thickness from motor to frontal association cortices. These findings illustrate how this laminar atlas will provide a link between single-neuron morphology, mesoscale cortical layering, macroscopic cortical thickness, and, ultimately, functional neuroanatomy.

Using deep learning to segment the layers of the cerebral cortex, this study presents the first whole brain quantitative atlas of cortical and laminar structure. This laminar atlas provides a novel framework for bridging between the scales of neuroscience.

Developmental duration as an organizer of the evolving mammalian brain: scaling, adaptations, and exceptions

Neurodevelopmental duration plays a central role in the evolution of the retina and neocortex in mammals. In the diurnal primate eye and retina, it is necessary to scale the number of cones versus the number of rods with different exponents to defend their respective functions of spatial acuity and sensitivity in eyes of different sizes. The order of photoreceptor precursor specification, cones specified first, rods second, couples their respective cell numbers at maturity to the kinetics of embryonic stem cell proliferation. Different durations of retinogenesis change the ratio of rods to cones produced so as to defend both functions over a range of eye diameters. In the evolution of nocturnality, the same coupling of photoreceptor specification to neurogenesis is altered to fewer cones and many more rods in nocturnal eyes, by delaying the onset of retinogenesis. Similarly, the neocortex also shows coupling of the specification of laminar position with duration of neurogenesis. Overall, duration of neurogenesis directly predicts neocortex volume in most mammalian clades. In larger brains with longer neocortical neurogenesis, its organization changes progressively, differentiating the frontal pole from the occipital pole in volume of connectivity and number of neurons per unit column. This permits greater, hierarchically organized information abstraction with increasing neocortex volume. Exceptions do exist, however, in species of three separate taxa, marsupials, naked mole rats, and bats, which break the correlation of neurodevelopmental duration and brain size. Naked mole rats and bats both have small brains and unusual longevity, coupled with neurodevelopmental periods characteristic of much bigger-brained animals, raising the possibility that developmental duration and lifespan have some genetic or mechanistic control in common. The role of duration of development in mediating between the mechanistic levels of construction of retinal and cortical organization, and the different life histories associated with larger brains, such as duration of parental care, learning and overall longevity are discussed.

Postnatal Dendritic Growth and Spinogenesis of Layer-V Pyramidal Cells Differ between Visual, Inferotemporal, and Prefrontal Cortex of the Macaque Monkey

Pyramidal cells in the primate cerebral cortex, particularly those in layer III, exhibit regional variation in both the time course and magnitude of postnatal growth and pruning of dendrites and spines. Less is known about the development of pyramidal cell dendrites and spines in other cortical layers. Here we studied dendritic morphology of layer-V pyramidal cells in primary visual cortex (V1, sensory), cytoarchitectonic area TE in the inferotemporal cortex (sensory association), and granular prefrontal cortex (Walker's area 12, executive) of macaque monkeys at the ages of 2 days, 3 weeks, 3.5 months, and 4.5 years. We found that changes in the basal dendritic field area of pyramidal cells were different across the three areas. In V1, field size became smaller over time (largest at 2 days, half that size at 4.5 years), in TE it did not change, and in area 12 it became larger over time (smallest at 2 days, 1.5 times greater at 4.5 years). In V1 and TE, the total number of branch points in the basal dendritic trees was similar between 2 days and 4.5 years, while in area 12 the number was greater in the adult monkeys than in the younger ones. Spine density peaked at 3 weeks and declined in all areas by adulthood, with V1 exhibiting a faster decline than area TE or area 12. Estimates of the total number of spines in the dendritic trees revealed that following the onset of visual experience, pyramidal cells in V1 lose more spines than they grow, whereas those in TE and area 12 grow more spines than they lose during the same period. These data provide further evidence that the process of synaptic refinement in cortical pyramidal cells differs not only according to time, but also location within the cortex. Furthermore, given the previous finding that layer-III pyramidal cells in all these areas exhibit the highest density and total number of spines at 3.5 months, the current results indicate that pyramidal cells in layers III and V develop spines at different rates.

Functional hierarchy underlies preferential connectivity disturbances in schizophrenia

Schizophrenia may involve an elevated excitation/inhibition (E/I) ratio in cortical microcircuits. It remains unknown how this regulatory disturbance maps onto neuroimaging findings. To address this issue, we implemented E/I perturbations within a neural model of large-scale functional connectivity, which predicted hyperconnectivity following E/I elevation. To test predictions, we examined resting-state functional MRI in 161 schizophrenia patients and 164 healthy subjects. As predicted, patients exhibited elevated functional connectivity that correlated with symptom levels, and was most prominent in association cortices, such as the fronto-parietal control network. This pattern was absent in patients with bipolar disorder (n = 73). To account for the pattern observed in schizophrenia, we integrated neurobiologically plausible, hierarchical differences in association vs. sensory recurrent neuronal dynamics into our model. This in silico architecture revealed preferential vulnerability of association networks to E/I imbalance, which we verified empirically. Reported effects implicate widespread microcircuit E/I imbalance as a parsimonious mechanism for emergent inhomogeneous dysconnectivity in schizophrenia.

In Vivo Two-Photon Imaging of Dendritic Spines in Marmoset Neocortex

Two-photon microscopy in combination with a technique involving the artificial expression of fluorescent protein has enabled the direct observation of dendritic spines in living brains. However, the application of this method to primate brains has been hindered by the lack of appropriate labeling techniques for visualizing dendritic spines. Here, we developed an adeno-associated virus vector-based fluorescent protein expression system for visualizing dendritic spines in vivo in the marmoset neocortex. For the clear visualization of each spine, the expression of reporter fluorescent protein should be both sparse and strong. To fulfill these requirements, we amplified fluorescent signals using the tetracycline transactivator (tTA)–tetracycline-responsive element system and by titrating down the amount of Thy1S promoter-driven tTA for sparse expression. By this method, we were able to visualize dendritic spines in the marmoset cortex by two-photon microscopy in vivo and analyze the turnover of spines in the prefrontal cortex. Our results demonstrated that short spines in the marmoset cortex tend to change more frequently than long spines. The comparison of in vivo samples with fixed samples showed that we did not detect all existing spines by our method. Although we found glial cell proliferation, the damage of tissues caused by window construction was relatively small, judging from the comparison of spine length between samples with or without window construction. Our new labeling technique for two-photon imaging to visualize in vivo dendritic spines of the marmoset neocortex can be applicable to examining circuit reorganization and synaptic plasticity in primates.

Pyramidal cell development: postnatal spinogenesis, dendritic growth, axon growth, and electrophysiology

Here we review recent findings related to postnatal spinogenesis, dendritic and axon growth, pruning and electrophysiology of neocortical pyramidal cells in the developing primate brain. Pyramidal cells in sensory, association and executive cortex grow dendrites, spines and axons at different rates, and vary in the degree of pruning. Of particular note is the fact that pyramidal cells in primary visual area (V1) prune more spines than they grow during postnatal development, whereas those in inferotemporal (TEO and TE) and granular prefrontal cortex (gPFC; Brodmann's area 12) grow more than they prune. Moreover, pyramidal cells in TEO, TE and the gPFC continue to grow larger dendritic territories from birth into adulthood, replete with spines, whereas those in V1 become smaller during this time. The developmental profile of intrinsic axons also varies between cortical areas: those in V1, for example, undergo an early proliferation followed by pruning and local consolidation into adulthood, whereas those in area TE tend to establish their territory and consolidate it into adulthood with little pruning. We correlate the anatomical findings with the electrophysiological properties of cells in the different cortical areas, including membrane time constant, depolarizing sag, duration of individual action potentials, and spike-frequency adaptation. All of the electrophysiological variables ramped up before 7 months of age in V1, but continued to ramp up over a protracted period of time in area TE. These data suggest that the anatomical and electrophysiological profiles of pyramidal cells vary among cortical areas at birth, and continue to diverge into adulthood. Moreover, the data reveal that the “use it or lose it” notion of synaptic reinforcement may speak to only part of the story, “use it but you still might lose it” may be just as prevalent in the cerebral cortex.

A simpler primate brain: the visual system of the marmoset monkey

Humans are diurnal primates with high visual acuity at the center of gaze. Although primates share many similarities in the organization of their visual centers with other mammals, and even other species of vertebrates, their visual pathways also show unique features, particularly with respect to the organization of the cerebral cortex. Therefore, in order to understand some aspects of human visual function, we need to study non-human primate brains. Which species is the most appropriate model? Macaque monkeys, the most widely used non-human primates, are not an optimal choice in many practical respects. For example, much of the macaque cerebral cortex is buried within sulci, and is therefore inaccessible to many imaging techniques, and the postnatal development and lifespan of macaques are prohibitively long for many studies of brain maturation, plasticity, and aging. In these and several other respects the marmoset, a small New World monkey, represents a more appropriate choice. Here we review the visual pathways of the marmoset, highlighting recent work that brings these advantages into focus, and identify where additional work needs to be done to link marmoset brain organization to that of macaques and humans. We will argue that the marmoset monkey provides a good subject for studies of a complex visual system, which will likely allow an important bridge linking experiments in animal models to humans.

Pyramidal cells in V1 of African rodents are bigger, more branched and more spiny than those in primates

Pyramidal cells are characterized by markedly different sized dendritic trees, branching patterns, and spine density across the cortical mantle. Moreover, pyramidal cells have been shown to differ in structure among homologous cortical areas in different species; however, most of these studies have been conducted in primates. Whilst pyramidal cells have been quantified in a few cortical areas in some other species there are, as yet, no uniform comparative data on pyramidal cell structure in a homologous cortical area among species in different Orders. Here we studied layer III pyramidal cells in V1 of three species of rodents, the greater cane rat, highveld gerbil, and four-striped mouse, by the same methodology used to sample data from layer III pyramidal cells in primates. The data reveal markedly different trends between rodents and primates: there is an appreciable increase in the size, branching complexity, and number of spines in the dendritic trees of pyramidal cells with increasing size of V1 in the brain in rodents, whereas there is relatively little difference in primates. Moreover, pyramidal cells in rodents are larger, more branched and more spinous than those in primates. For example, the dendritic trees of pyramidal cells in V1 of the adult cane rat are nearly three times larger, and have more than 10 times the number of spines in their basal dendritic trees, than those in V1 of the adult macaque (7900 and 600, respectively), which has a V1 40 times the size that of the cane rat. It remains to be determined to what extent these differences may result from development or reflect evolutionary and/or processing specializations.

Dendritic morphology of pyramidal neurons in the chimpanzee neocortex: regional specializations and comparison to humans

The primate cerebral cortex is characterized by regional variation in the structure of pyramidal neurons, with more complex dendritic arbors and greater spine density observed in prefrontal compared with sensory and motor cortices. Although there are several investigations in humans and other primates, virtually nothing is known about regional variation in the morphology of pyramidal neurons in the cerebral cortex of great apes, humans' closest living relatives. The current study uses the rapid Golgi stain to quantify the dendritic structure of layer III pyramidal neurons in 4 areas of the chimpanzee cerebral cortex: Primary somatosensory (area 3b), primary motor (area 4), prestriate visual (area 18), and prefrontal (area 10) cortex. Consistent with previous studies in humans and macaque monkeys, pyramidal neurons in the prefrontal cortex of chimpanzees exhibit greater dendritic complexity than those in other cortical regions, suggesting that prefrontal cortical evolution in primates is characterized by increased potential for integrative connectivity. Compared with chimpanzees, the pyramidal neurons of humans had significantly longer and more branched dendritic arbors in all cortical regions.

The evolutions of large brain size in mammals: the 'over-700-gram club quartet'

The current paper details our developing understanding of the evolution of large brains in mammals. In order to do this, we first define brains that we consider to be large--those that have passed the apparent 700-gram ceiling on brain mass evolution in the class Mammalia. The over-700-gram club includes certain species within the genus Homo, order Cetacea, order Proboscidea, and suborder Pinnipedia. Our analysis suggests that selection for body size appears to be the most important factor in the evolution of large brain size, but there also appear to be internal morphophysiological constraints on large brain size evolution that need to be overcome in order for brains to break the 700-gram barrier. These two aspects appear to be common themes in the evolution of large brains. This significantly diminishes the explanatory value of selection for greater cognitive capacities as a principal factor in the evolution of enlarged brain sizes above the 700-gram threshold.

Principal component and cluster analysis of layer V pyramidal cells in visual and non-visual cortical areas projecting to the primary visual cortex of the mouse

The long-distance corticocortical connections between visual and nonvisual sensory areas that arise from pyramidal neurons located within layer V can be considered as a subpopulation of feedback connections. The purpose of the present study is to determine if layer V pyramidal neurons from visual and nonvisual sensory cortical areas that project onto the visual cortex (V1) constitute a homogeneous population of cells. Additionally, we ask whether dendritic arborization relates to the target, the sensory modality, the hierarchical level, or laterality of the source cortical area. Complete 3D reconstructions of dendritic arbors of retrogradely labeled layer V pyramidal neurons were performed for neurons of the primary auditory (A1) and somatosensory (S1) cortices and from the lateral (V2L) and medial (V2M) parts of the secondary visual cortices of both hemispheres. The morphological parameters extracted from these reconstructions were subjected to principal component analysis (PCA) and cluster analysis. The PCA showed that neurons are distributed within a continuous range of morphologies and do not form discrete groups. Nevertheless, the cluster analysis defines neuronal groups that share similar features. Each cortical area includes neurons belonging to several clusters. We suggest that layer V feedback connections within a single cortical area comprise several cell types.

Cognitive reserve, cortical plasticity and resistance to Alzheimer's disease

There are aspects of the ageing brain and cognition that remain poorly understood despite intensive efforts to understand how they are related. Cognitive reserve is the concept that has been developed to explain how it is that some elderly people with extensive neuropathology associated with dementia show little in the way of cognitive decline. Cognitive reserve is intimately related to cortical plasticity but this also, as it relates to ageing, remains poorly understood at the present time. Despite the shortcomings in understanding, we do have some knowledge on which to base efforts to minimise the likelihood of an elderly person developing dementia. For some risks the evidence is far from secure, but resistance to Alzheimer's disease (AD) appears from epidemiological studies to be contributed to by avoiding hypertension in middle life, obesity, depression, smoking and diabetes and head injury and by undertaking extended years of education, physical exercise, and social and intellectual pursuits in middle and late life. Nutritional factors may also promote healthy brain ageing. Resistance to AD is also contributed to by genetic factors, particularly apolipoprotein E2, but some combinations of other genetic polymorphisms as well. Although multiple factors and possible interventions may influence cognitive reserve and susceptibility to dementia, much more work is required on the mechanisms of action in order to determine which, if any, may improve the clinical and epidemiological picture. Understanding of how such factors operate may lead to new initiatives to keep the elderly population in the 21st century able to lead active and fulfilling lives.

Differential expression patterns of striate cortex-enriched genes among Old World, New World, and prosimian primates

A group of 5 genes, OCC1, testican-1, testican-2, 5-HT1B, and 5-HT2A, are selectively expressed in layer 4 (4C of Brodmann) of striate cortex (visual area V1) of both Old World macaques and New World marmoset monkeys. The expression of these genes is activity dependent, as expression is reduced after blocking retinal activity. Surprisingly, the pronounced expression pattern has not been found in rodents or carnivores. Thus, these genes may be highly expressed in V1 of some but perhaps not all primates. Here, we compared the gene expression in members of 3 major branches of primate evolution: prosimians, New World monkeys, and Old World monkeys. Although the expression pattern of 5-HT1B was well conserved, those of the other genes varied from the least distinct in prosimian galagos to successively more in New World owl monkeys, marmosets, squirrel monkeys, and Old World macaque monkeys. In owl monkeys, the expression of 5-HT2A was significantly reduced by monocular tetrodotoxin injection, while those of OCC1 and 5-HT1B were not. Thus, we propose that early primates had low levels of expression and higher levels emerged with anthropoid primates and became further enhanced in the Old World catarrhine monkeys that are more closely related to humans.

Pyramidal cells in prefrontal cortex of primates: marked differences in neuronal structure among species

The most ubiquitous neuron in the cerebral cortex, the pyramidal cell, is characterized by markedly different dendritic structure among different cortical areas. The complex pyramidal cell phenotype in granular prefrontal cortex (gPFC) of higher primates endows specific biophysical properties and patterns of connectivity, which differ from those in other cortical regions. However, within the gPFC, data have been sampled from only a select few cortical areas. The gPFC of species such as human and macaque monkey includes more than 10 cortical areas. It remains unknown as to what degree pyramidal cell structure may vary among these cortical areas. Here we undertook a survey of pyramidal cells in the dorsolateral, medial, and orbital gPFC of cercopithecid primates. We found marked heterogeneity in pyramidal cell structure within and between these regions. Moreover, trends for gradients in neuronal complexity varied among species. As the structure of neurons determines their computational abilities, memory storage capacity and connectivity, we propose that these specializations in the pyramidal cell phenotype are an important determinant of species-specific executive cortical functions in primates.

Immunohistochemical parcellation of the ferret (Mustela putorius) visual cortex reveals substantial homology with the cat (Felis catus)

Electrophysiological mapping of the adult ferret visual cortex has until now determined the existence of 12 retinotopically distinct areas; however, in the cat, another member of the Carnivora, 20 distinct visual areas have been identified by using retinotopic mapping and immunolabeling. In the present study, the immunohistochemical approach to demarcate the areal boundaries of the adult ferret visual cortex was applied in order to overcome the difficulties in accessing the sulcal surfaces of a small, gyrencephalic brain. Nonphosphorylated neurofilament (NNF) expression profiles were compared with another classical immunostain of cortical nuclei, Cat-301 chondroitin sulfate proteoglycan (CSPG). Together, these two markers reliably demarcated the borders of the 12 previously defined areas and revealed further arealization beyond those borders to a total of 19 areas: 21a and 21b; the anterolateral, posterolateral, dorsal, and ventral lateral suprasylvian areas (ALLS, PLLS, DLS, and VLS, respectively); and the splenial and cingulate visual areas (SVA and CVA). NNF expression profile and location of the newly defined areas correlate with previously defined areas in the cat. Moreover, NNF and Cat-301 together revealed discrete expression domains in the posteroparietal (PP) cortex, demarcating four subdivisions in the caudal lateral and medial domains (PPcL and PPcM) and rostral lateral and medial domains (PPrL and PPrM), where only two retinotopic maps have been previously identified (PPc and PPr). Taken together, these studies suggest that NNF and Cat-301 can illustrate the homology between cortical areas in different species and draw out the principles that have driven evolution of the visual cortex.

Morphological variability of NADPH diaphorase neurons across areas V1, V2, and V3 of the common agouti

Previous studies have shown a noticeable phenotypic diversity for pyramidal cells among cortical areas in the cerebral cortex. Both the extent and systematic nature of this variation suggests a correlation with particular aspects of cortical processing. Nevertheless, regional variations in the morphology of inhibitory cells have not been evaluated with the same detail. In the present study we performed a 3D morphometric analysis of 120 NADPH diaphorase (NADPH-d) type I neurons in the visual cortex of a South American Hystricomorph rodent, the diurnal agouti (Dasyprocta sp.). We found significant differences in morphology of NADPH-d type I neurons among visual cortical areas: cells became progressively larger and more branched from V1 to V2 and V3. Presumably, the specialized morphology of these cells is correlated with different sampling geometry and function. The data suggest that area-specific specializations of cortical inhibitory circuitry are also present in rodents.

Spinogenesis and pruning from early visual onset to adulthood: an intracellular injection study of layer III pyramidal cells in the ventral visual cortical pathway of the macaque monkey

Neocortical pyramidal cells are characterized by markedly different structure among cortical areas in the mature brain. In the ventral visual pathway of adult primates, pyramidal cells become increasingly more branched and more spinous with anterior progression through the primary (V1), second (V2), and fourth (V4) visual areas and cytoarchitectonic areas TEO and TE. It is not known how these regional specializations in neuron structure develop. Here, we report that the basal dendritic trees of layer III pyramidal cells in V1, V2, V4, TEO, and TE were characterized by unique growth profiles. Different numbers of spines were grown in the dendritic trees of cells among these cortical areas and then subsequently pruned. In V1, V2, and V4, more spines were pruned than grew resulting in a net decrease in the number of spines in the dendritic trees following the onset of visual experience. In TEO and TE, neurons grew more spines than they pruned from visual onset to adulthood. These data suggest that visual experience may influence neuronal maturation in different ways in different cortical areas.

Auditory cortex asymmetry, altered minicolumn spacing and absence of ageing effects in schizophrenia

The superior temporal gyrus, which contains the auditory cortex, including the planum temporale, is the most consistently altered neocortical structure in schizophrenia (Shenton ME, Dickey CC, Frumin M, McCarley RW. A review of MRI findings in schizophrenia. Schizophr Res 2001; 49: 1-52). Auditory hallucinations are associated with abnormalities in this region and activation in Heschl's gyrus. Our review of 34 MRI and 5 post-mortem studies of planum temporale reveals that half of those measuring region size reported a change in schizophrenia, usually consistent with a reduction in the left hemisphere and a relative increase in the right hemisphere. Furthermore, female subjects are under-represented in the literature and insight from sex differences may be lost. Here we present evidence from post-mortem brain (N = 21 patients, compared with 17 previously reported controls) that normal age-associated changes in planum temporale are not found in schizophrenia. These age-associated differences are reported in an adult population (age range 29-90 years) and were not found in the primary auditory cortex of Heschl's gyrus, indicating that they are selective to the more plastic regions of association cortex involved in cognition. Areas and volumes of Heschl's gyrus and planum temporale and the separation of the minicolumns that are held to be the structural units of the cerebral cortex were assessed in patients. Minicolumn distribution in planum temporale and Heschl's gyrus was assessed on Nissl-stained sections by semi-automated microscope image analysis. The cortical surface area of planum temporale in the left hemisphere (usually asymmetrically larger) was positively correlated with its constituent minicolumn spacing in patients and controls. Surface area asymmetry of planum temporale was reduced in patients with schizophrenia by a reduction in the left hemisphere (F = 7.7, df 1,32, P < 0.01). The relationship between cortical asymmetry and the connecting, interhemispheric callosal white matter was also investigated; minicolumn asymmetry of both Heschl's gyrus and planum temporale was correlated with axon number in the wrong subregions of the corpus callosum in patients. The spacing of minicolumns was altered in a sex-dependent manner due to the absence of age-related minicolumn thinning in schizophrenia. This is interpreted as a failure of adult neuroplasticity that maintains neuropil space. The arrested capacity to absorb anomalous events and cognitive demands may confer vulnerability to schizophrenic symptoms when adult neuroplastic demands are not met.

A cost-benefit analysis of neuronal morphology

Over hundreds of millions of years, evolution has optimized brain design to maximize its functionality while minimizing costs associated with building and maintenance. This observation suggests that one can use optimization theory to rationalize various features of brain design. Here, we attempt to explain the dimensions and branching structure of dendritic arbors by minimizing dendritic cost for given potential synaptic connectivity. Assuming only that dendritic cost increases with total dendritic length and path length from synapses to soma, we find that branching, planar, and compact dendritic arbors, such as those belonging to Purkinje cells in the cerebellum, are optimal. The theory predicts that adjacent Purkinje dendritic arbors should spatially segregate. In addition, we propose two explicit cost function expressions, falsifiable by measuring dendritic caliber near bifurcations.

A novel computational approach for automatic dendrite spines detection in two-photon laser scan microscopy

BACKGROUND

Recent research has shown that there is a strong correlation between the functional properties of a neuron and its morphologic structure. Current morphologic analyses typically involve a significant component of computer-assisted manual labor, which is very time-consuming and is susceptible to operator bias. The existing semi-automatic approaches largely reduce user efforts. However, some manual interventions, such as setting a global threshold for segmentation, are still needed during image processing.

METHODS

We present an automated approach, which can greatly help neurobiologists obtain quantitative morphological information about a neuron and its spines. The automation includes an adaptive thresholding method, which can yield better segment results than the prevalent global thresholding method. It also introduces an efficient backbone extraction method, a SNR based, detached spine component detection method, and an attached spine component detection method based on the estimation of local dendrite morphology.

RESULTS

The morphology information obtained both manually and automatically are compared in detail. Using the Kolmogov-Smirnov test, we find a 99.13% probability that the dendrite length distributions are the same for the automatic and manual processing methods. The spine detection results are also compared with other existing semi-automatic approaches. The comparison results show that our approach has 33% fewer false positives and 77% fewer false negatives on average.

CONCLUSIONS

Because the proposed detection algorithm requires less user input and performs better than existing algorithms, our approach can quickly and accurately process neuron images without user intervention.

Ipsilateral corticocortical projections to the primary and middle temporal visual areas of the primate cerebral cortex: area-specific variations in the morphology of connectionally identified pyramidal cells

We quantified the morphology of over 350 pyramidal neurons with identified ipsilateral corticocortical projections to the primary (V1) and middle temporal (MT) visual areas of the marmoset monkey, following intracellular injection of Lucifer Yellow into retrogradely labelled cells. Paralleling the results of studies in which randomly sampled pyramidal cells were injected, we found that the size of the basal dendritic tree of connectionally identified cells differed between cortical areas, as did the branching complexity and spine density. We found no systematic relationship between dendritic tree structure and axon target or length. Instead, the size of the basal dendritic tree increased roughly in relation to increasing distance from the occipital pole, irrespective of the length of the connection or the cortical layer in which the neurons were located. For example, cells in the second visual area had some of the smallest and least complex dendritic trees irrespective of whether they projected to V1 or MT, while those in the dorsolateral area (DL) were among the largest and most complex. We also observed that systematic differences in spine number were more marked among V1-projecting cells than MT-projecting cells. These data demonstrate that the previously documented systematic differences in pyramidal cell morphology between areas cannot simply be attributed to variable proportions of neurons projecting to different targets, in the various areas. Moreover, they suggest that mechanisms intrinsic to the area in which neurons are located are strong determinants of basal dendritic field structure.

Minicolumnar structure in Heschl’s gyrus and planum temporale: Asymmetries in relation to sex and callosal fiber number

AIM

To investigate the cytoarchitectural basis of asymmetries in human auditory cortex. Minicolumn spacing and number, and regional cortical volume and surface area were measured in the primary auditory region (Heschl's gyrus, HG) and posterior auditory association region (planum temporale, PT) in 17 neurologically normal adults (10 female, seven male). PT surface area, minicolumn spacing and minicolumn number were greater in the left hemisphere. HG surface area was larger in the left hemisphere. Asymmetries of minicolumn number in primary and association auditory regions correlated with axonal fiber numbers in the subregions of the corpus callosum through which they project. PT minicolumn number was more asymmetrical in men than women but total number was similar in the two sexes. We conclude that asymmetry of the surface area of the PT is a function of minicolumn spacing. Fewer callosal projections between the plana are found when the minicolumn spacing is more asymmetrical.

Loose-patch–juxtacellular recording in vivo—A method for functional characterization and labeling of neurons in macaque V1

We describe a method that uses a modified version of juxtacellular labeling [Pinault D. A novel single-cell staining procedure performed in vivo under electrophysiological control: morpho-functional features of juxtacellularly labeled thalamic cells and other central neurons with biocytin or neurobiotin. J Neurosci Meth 1996;65:113-36], which allows us to functionally characterize and subsequently label single neurons in vivo in macaque V1. The method is generally applicable in acute in vivo preparations. Extracellular recording is made with a patch electrode when the electrode is attached to the cell membrane. Initially a 'blind' search method is used as a guide to obtaining a cell attached configuration that we refer to as a loose-patch (LP). The neuron's receptive field properties are functionally characterized, the neuron is labeled and then characterization is confirmed, all in the LP configuration. There are a number of advantages of the method that we describe over other methods. First, we have found that we can obtain stable extracellular recordings for periods of hours that enable us to make a relatively comprehensive visual functional characterization of a neuron's receptive field properties. Second, because the electrode is closely apposed to the cell we obtain excellent isolation of the extracellular spike. Third, the method provides labeling that gives complete dendritic and axonal filling that survives over a number of days, which is an important feature in acute primate experiments. Fourth, the in vivo method of labeling and reconstructing neurons gives complete three-dimensional structure of the neuron including its intra-cortical axonal arbor. These features overcome known limits of the established methods of studying neuronal morphology including the Golgi stain (limited when adult tissue is used) and in vitro whole cell methods (incomplete axonal filling due to limited slice thickness). They also overcome the known limits of the established method of combined function-morphology studies i.e. intracellular recording in vivo. The modified juxtacellular method provides a reliable alternative to the difficult method of characterization by extracellular recording and subsequent intracellular labeling [Anderson JC, Martin KAC, Whitteridge D. Form, function and intracortical projections of neurons in the striate cortex of the monkey Macacus nemestrinus. Cerebral Cortex 1993;3:412-20]. We show the method can be used to record at a range of depths through V1 cortex allowing for sampling of neurons in the different layers and functional subpopulations. Links can then be made with existing knowledge about the anatomical organization of V1, the various morphological classes of neurons found therein, their functional connectivity and visual response properties.

Specialization of pyramidal cell structure in the visual areas V1, V2 and V3 of the South American rodent, Dasyprocta primnolopha

Marked phenotypic variation has been reported in pyramidal cells in the primate cerebral cortex. These extent and systematic nature of these specializations suggest that they are important for specialized aspects of cortical processing. However, it remains unknown as to whether regional variations in the pyramidal cell phenotype are unique to primates or if they are widespread amongst mammalian species. In the present study we determined the receptive fields of neurons in striate and extrastriate visual cortex, and quantified pyramidal cell structure in these cortical regions, in the diurnal, large-brained, South American rodent Dasyprocta primnolopha. We found evidence for a first, second and third visual area (V1, V2 and V3, respectively) forming a lateral progression from the occipital pole to the temporal pole. Pyramidal cell structure became increasingly more complex through these areas, suggesting that regional specialization in pyramidal cell phenotype is not restricted to primates. However, cells in V1, V2 and V3 of the agouti were considerably more spinous than their counterparts in primates, suggesting different evolutionary and developmental influences may act on cortical microcircuitry in rodents and primates.

Specializations of the granular prefrontal cortex of primates: implications for cognitive processing

The biological underpinnings of human intelligence remain enigmatic. There remains the greatest confusion and controversy regarding mechanisms that enable humans to conceptualize, plan, and prioritize, and why they are set apart from other animals in their cognitive abilities. Here we demonstrate that the basic neuronal building block of the cerebral cortex, the pyramidal cell, is characterized by marked differences in structure among primate species. Moreover, comparison of the complexity of neuron structure with the size of the cortical area/region in which the cells are located revealed that trends in the granular prefrontal cortex (gPFC) were dramatically different to those in visual cortex. More specifically, pyramidal cells in the gPFC of humans had a disproportionately high number of spines. As neuron structure determines both its biophysical properties and connectivity, differences in the complexity in dendritic structure observed here endow neurons with different computational abilities. Furthermore, cortical circuits composed of neurons with distinguishable morphologies will likely be characterized by different functional capabilities. We propose that 1. circuitry in V1, V2, and gPFC within any given species differs in its functional capabilities and 2. there are dramatic differences in the functional capabilities of gPFC circuitry in different species, which are central to the different cognitive styles of primates. In particular, the highly branched, spinous neurons in the human gPFC may be a key component of human intelligence.

Specialization in pyramidal cell structure in the sensory-motor cortex of the Chacma baboon (Papio ursinus) with comparative notes on macaque and vervet monkeys

The systematic study of pyramidal cell structure has revealed new insights into specialization of the phenotype in the primate cerebral cortex. Regional specialization in the neuronal phenotype may influence patterns of connectivity and the computational abilities of the circuits they compose. The comparative study of pyramidal cells in homologous cortical areas is beginning to yield data on the evolution and development of such specialized circuitry in the primate cerebral cortex. Recently, we have focused our efforts on sensory-motor cortex. Based on our intracellular injection methodology, we have demonstrated a progressive increase in the size of, the branching structure in, and the spine density of the basal dendritic trees of pyramidal cells through somatosensory areas 3b, 1, 2, 5, and 7 in the macaque and vervet monkeys. In addition, we have shown that pyramidal cells in premotor area 6 are larger, more branched, and more spinous than those in the primary motor cortex (MI or area 4) in the macaque monkey, vervet monkey, and baboon. Here we expand the basis for comparison by studying the basal dendritic trees of layer III pyramidal cells in these same sensory-motor areas in the chacma baboon. The baboon was selected because it has a larger cerebral cortex than either the macaque or vervet monkeys; motor cortex has expanded disproportionately in these three species; and motor cortex in the baboon reportedly has differentiated to include a new cortical area not present in either the macaque or vervet monkeys. We found, as in monkeys, a progressive increase in the morphological complexity of pyramidal cells through areas 3b, 5, and 7, as well as from area 4 to area 6, suggesting that areal specialization in microcircuitry was likely to be present in a common ancestor of primates. In addition, we found subtle differences in the extent of the interareal differences in pyramidal cell structure between homologous cortical areas in the three species.

Subtle changes in the ageing human brain

Subtle changes in the human brain constitute a third element in addition to plaques and tangles as markers of vulnerability to Alzheimer's disease (AD). Neurofibrillary tangle (NFT) distribution shows the closest relationship to the severity of dementia. Two features of the distribution (regional selectivity and columnar clustering) provide clues about the structural changes due to normal ageing that may precede tangle formation. It is hypothesized that the columnar organization of the cortex, determines the pattern of pathological spread in AD and, consequently, the pattern of function loss. Minicolum thinning occurs in normal ageing and echoes the selective regional distribution of NFT formation in AD. NFT vulnerability appears to emerge from hierarchical variation in neural plasticity associated with the hierarchical variation in size and spacing of mini and macro-columns in the cortex. Regional differences may involve regional variation in gene expression. Dietary Omega 3 fatty acid intake has been shown to have neuroprotective effects on the cytoarchitectural features that contribute to this cortical hierarchy.

Pyramidal neurons of granular prefrontal cortex of the galago: complexity in evolution of the psychic cell in primates

Typically, cognitive abilities of humans have been attributed to their greatly expanded cortical mantle, granular prefrontal cortex (gPFC) in particular. Recently we have demonstrated systematic differences in microstructure of gPFC in different species. Specifically, pyramidal cells in adult human gPFC are considerably more spinous than those in the gPFC of the macaque monkey, which are more spinous than those in the gPFC of marmoset and owl monkeys. As most cortical dendritic spines receive at least one excitatory input, pyramidal cells in these different species putatively receive different numbers of inputs. These differences in the gPFC pyramidal cell phenotype may be of fundamental importance in determining the functional characteristics of prefrontal circuitry and hence the cognitive styles of the different species. However, it remains unknown as to why the gPFC pyramidal cell phenotype differs between species. Differences could be attributed to, among other things, brain size, relative size of gPFC, or the lineage to which the species belong. Here we investigated pyramidal cells in the dorsolateral gPFC of the prosimian galago to extend the basis for comparison. We found these cells to be less spinous than those in human, macaque, and marmoset.

Pyramidal cell specialization in the occipitotemporal cortex of the Chacma baboon (Papio ursinus)

Pyramidal cell structure varies systematically in occipitotemporal visual areas in monkeys. The dendritic trees of pyramidal cells, on average, become larger, more branched and more spinous with progression from the primary visual area (V1) to the second visual area (V2), the fourth (V4, or dorsolateral DL visual area) and inferotemporal (IT) cortex. Presently available data reveal that the extent of this increase in complexity parallels the expansion of occipitotemporal cortex. Here we extend the basis for comparison by studying pyramidal cell structure in occipitotemporal cortical areas in the chacma baboon. We found a systematic increase in the size of and branching complexity in the basal dendritic trees, as well as a progressive increase in the spine density along the basal dendrites of layer III pyramidal cells through V1, V2 and V4. These data suggest that the trend for more complex pyramidal cells with anterior progression through occipitotemporal visual areas is not a feature restricted to monkeys and prosimians, but is a widespread feature of occipitotemporal cortex in primates.

Dendritic branch typing and spine expression patterns in cortical nonpyramidal cells

To understand the dendritic differentiation in various types of cortical nonpyramidal cells, we analyzed quantitatively their dendritic branching and spine expression. The dendritic internode and interspine interval obeyed exponential distributions with type-specific decay constants. The initial branching pattern, internode interval and spine density at the light microscopic level divided nonpyramidal cells into three dendritic types, correlated with axonal, neurochemical and firing types. The initial branching pattern determined the overall vertical spread of dendrites. Basket cell subtypes with different firing and chemical expression patterns were distinct in the vertical and horizontal spatial spread, providing diverse input territories. Internode densities of dendritic spines, as well as those of axonal synaptic boutons, did not correlate with the tortuosities and intervals, suggesting a tendency to distribute synapses homogeneously over the arbor. Dendritic spines identified at the electron microscopic level were different in length and shape among subtypes. Although the density was lower than that of pyramidal cells, spines themselves were also composed of several morphological types such as mushroom and multihead ones, which were expressed differentially among subtypes. Correlation of dendritic branching characteristics with differences in spine structure suggests distinct ways to receive specific inputs among the subtypes.

Regional specialization in pyramidal cell structure in the visual cortex of the galago: an intracellular injection study of striate and extrastriate areas with comparative notes on new world and old world monkeys

Recent studies have revealed marked differences in the basal dendritic structure of layer III pyramidal cells in the cerebral cortex of adult simian primates. In particular, there is a consistent trend for pyramidal cells of increasing complexity with anterior progression through occipitotemporal cortical visual areas. These differences in pyramidal cell structure, and their systematic nature, are believed to be important for specialized aspects of visual processing within, and between, cortical areas. However, it remains unknown whether this regional specialization in the pyramidal cell phenotype is unique to simians, is unique to primates in general or is widespread amongst mammalian species. In the present study we investigated pyramidal cell structure in the prosimian galago (Otolemur garnetti). We found, as in simians, that the basal dendritic arbors of pyramidal cells differed between cortical areas. More specifically, pyramidal cells became progressively more spinous through the primary (V1), second (V2), dorsolateral (DL) and inferotemporal (IT) visual areas. Moreover, pyramidal neurons in V1 of the galago are remarkably similar to those in other primate species, in spite of large differences in the sizes of this area. In contrast, pyramidal cells in inferotemporal cortex are quite variable among primate species. These data suggest that regional specialization in pyramidal cell phenotype was a likely feature of cortex in a common ancestor of simian and prosimian primates, but the degree of specialization varies between species.

Areal specialization of pyramidal cell structure in the visual cortex of the tree shrew: a new twist revealed in the evolution of cortical circuitry

Cortical pyramidal cells, while having a characteristic morphology, show marked phenotypic variation in primates. Differences have been reported in their size, branching structure and spine density between cortical areas. In particular, there is a systematic increase in the complexity of the structure of pyramidal cells with anterior progression through occipito-temporal cortical visual areas. These differences reflect area-specific specializations in cortical circuitry, which are believed to be important for visual processing. However, it remains unknown as to whether these regional specializations in pyramidal cell structure are restricted to primates. Here we investigated pyramidal cell structure in the visual cortex of the tree shrew, including the primary (V1), second (V2) and temporal dorsal (TD) areas. As in primates, there was a trend for more complex branching structure with anterior progression through visual areas in the tree shrew. However, contrary to the trend reported in primates, cells in the tree shrew tended to become smaller with anterior progression through V1, V2 and TD. In addition, pyramidal cells in V1 of the tree shrew are more than twice as spinous as those in primates. These data suggest that variables that shape the structure of adult cortical pyramidal cells differ among species.

Specialization in pyramidal cell structure in the sensory-motor cortex of the vervet monkey (Cercopethicus pygerythrus)

Recent studies have revealed systematic differences in the pyramidal cell structure between functionally related cortical areas of primates. Trends for a parallel in pyramidal cell structure and functional complexity have been reported in visual, somatosensory, motor, cingulate and prefrontal cortex in the macaque monkey cortex. These specializations in structure have been interpreted as being fundamental in determining cellular and systems function, endowing circuits in these different cortical areas with different computational power. In the present study we extend our initial finding of systematic specialization of pyramidal cell structure in sensory-motor cortex in the macaque monkey [Cereb Cortex 12 (2002) 1071] to the vervet monkey. More specifically, we investigated pyramidal cell structure in somatosensory and motor areas 1/2, 5, 7, 4 and 6. Neurones in fixed, flat-mounted, cortical slices were injected intracellularly with Lucifer Yellow and processed for a light-stable 3,3'-diaminobenzidine reaction product. The size of, number of branches in, and spine density of the basal dendritic arbors varied systematically such that there was a trend for increasing complexity in arbor structure with progression through 1/2, 5 and 7. In addition, cells in area 6 were larger, more branched, and more spinous than those in area 4.

Neuronal circuits of the neocortex

We explore the extent to which neocortical circuits generalize, i.e., to what extent can neocortical neurons and the circuits they form be considered as canonical? We find that, as has long been suspected by cortical neuroanatomists, the same basic laminar and tangential organization of the excitatory neurons of the neocortex is evident wherever it has been sought. Similarly, the inhibitory neurons show characteristic morphology and patterns of connections throughout the neocortex. We offer a simple model of cortical processing that is consistent with the major features of cortical circuits: The superficial layer neurons within local patches of cortex, and within areas, cooperate to explore all possible interpretations of different cortical input and cooperatively select an interpretation consistent with their various cortical and subcortical inputs.

Automated Algorithms for Multiscale Morphometry of Neuronal Dendrites

We describe the synthesis of automated neuron branching morphology and spine detection algorithms to provide multiscale three-dimensional morphological analysis of neurons. The resulting software is applied to the analysis of a high-resolution (0.098 μm × 0.098 μm × 0.081 μ m) image of an entire pyramidal neuron from layer III of the superior temporal cortex in rhesus macaque monkey. The approach provides a highly automated, complete morphological analysis of the entire neuron; each dendritic branch segment is characterized by several parameters, including branch order, length, and radius as a function of distance along the branch, as well as by the locations, lengths, shape classification (e.g., mushroom, stubby, thin), and density distribution of spines on the branch. Results for this automated analysis are compared to published results obtained by other computer-assisted manual means.

Parvalbumin-, calbindin-, and calretinin-immunoreactive neurons in the prefrontal cortex of the owl monkey (Aotus trivirgatus): a standardized quantitative comparison with sensory and motor areas

Recent studies have revealed regional variation in the density and distribution of inhibitory neurons in different cortical areas, which are thought to reflect area-specific specializations in cortical circuitry. However, there are as yet few standardized quantitative data regarding how the inhibitory circuitry in prefrontal cortex (PFC), which is thought to be involved in executive functions such as cognition, emotion and decision making, compares to that in other cortical areas. Here we used immunohistochemical techniques to determine the density and distribution of parvalbumin (PV)-, calbindin (CB)-, and calretinin (CR)-immunoreactive (ir) neurons and axon terminals in the dorsolateral and orbital PFC of the owl monkey (Aotus trivirgatus), and compared them directly with data obtained using the same techniques in 11 different visual, somatosensory and motor areas. We found marked differences in the density of PV-ir, CB-ir, and CR-ir interneurons in several cortical areas. One hundred and twenty eight of all 234 possible between-area pair-wise comparisons were significantly different. The density of specific subpopulations of these cells also varied among cortical areas, as did the density of axon terminals. Comparison of PFC with other cortical areas revealed that 40 of all 66 possible statistical comparisons of the density of PV-ir, CB-ir, and CR-ir cells were significantly different. We also found evidence for heterogeneity in the pattern of labeling of PV-ir, CB-ir, and CR-ir cells and axon terminals between the dorsolateral and orbital subdivisions of PFC. These data are likely to reflect basic differences in interneuron circuitry, which are likely to influence inhibitory function in the cortex.

Pyramidal cell heterogeneity in the visual cortex of the nocturnal New World owl monkey (Aotus trivirgatus)

Recent studies have revealed marked variation in pyramidal cell structure in the visual cortex of macaque and marmoset monkeys. In particular, there is a systematic increase in the size of, and number of spines in, the arbours of pyramidal cells with progression through occipitotemporal (OT) visual areas. In the present study we extend the basis for comparison by investigating pyramidal cell structure in OT visual areas of the nocturnal owl monkey. As in the diurnal macaque and marmoset monkeys, pyramidal cells became progressively larger and more spinous with anterior progression through OT visual areas. These data suggest that: 1. the trend for more complex pyramidal cells with anterior progression through OT visual areas is a fundamental organizational principle in primate cortex; 2. areal specialization of the pyramidal cell phenotype provides an anatomical substrate for the reconstruction of the visual scene in OT areas; 3. evolutionary specialization of different aspects of visual processing may determine the extent of interareal variation in the pyramidal cell phenotype in different species; and 4. pyramidal cell structure is not necessarily related to brain size.