Distribution, morphology, and gamma-aminobutyric acid immunoreactivity of horizontally projecting neurons in the macaque inferior temporal cortex

Difference in the generalization of response tolerance across views between the anterior and posterior part of the inferotemporal cortex

The inferotemporal cortex consists of an anterior (cytoarchitectonic area TE) and a posterior (area TEO) part, which together constitute the final areas of the ventral visual stream, which is critical for object discrimination. Area TE receives dense projections from area TEO. We have previously identified a response tolerance in the cells in area TE in monkeys to a range of viewing angles after object discrimination at each of several views. To investigate the contribution of area TEO to the establishment of such a response tolerance in area TE, we conducted electrophysiological recordings of the responses of the single cells in area TEO after performance saturation of object discrimination at several independent views, without any association across views, and compared them with those obtained from the TE cells. The cells in area TEO showed responses to the experienced object views, but not to nearby views. Comparisons of the tunings of the TE and TEO cells to different viewing angles for the same object sets in the same animal showed that cells in area TEO had a significantly narrower tuning width, whereas the response tolerance was usually observed in the TE cells across viewing angles up to 60°. Our findings revealed a significant difference in the representation of the object views between areas TE and TEO, and suggested that such a representation in area TE may be completed through neuronal mechanisms within area TE, which is not a property of the earlier stages of the ventral visual stream.

Postnatal Development of Intrinsic Horizontal Axons in Macaque Inferior Temporal and Primary Visual Cortices

Two distinct areas along the ventral visual stream of monkeys, the primary visual (V1) and inferior temporal (TE) cortices, exhibit different projection patterns of intrinsic horizontal axons with patchy terminal fields in adult animals. The differences between the patches in these 2 areas may reflect differences in cortical representation and processing of visual information. We studied the postnatal development of patches by injecting an anterograde tracer into TE and V1 in monkeys of various ages. At 1 week of age, labeled patches with distribution patterns reminiscent of those in adults were already present in both areas. The labeling intensity of patches decayed exponentially with projection distance in monkeys of all ages in both areas, but this trend was far less evident in TE. The number and extent of patches gradually decreased with age in V1, but not in TE. In V1, axonal and bouton densities increased postnatally only in patches with short projection distances, whereas in TE this density change occurred in patches with various projection distances. Thus, patches with area-specific distribution patterns are formed early in life, and area-specific postnatal developmental processes shape the connectivity of patches into adulthood.

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.

Stimulus selectivity and response latency in putative inhibitory and excitatory neurons of the primate inferior temporal cortex

The cerebral cortex is composed of many distinct classes of neurons. Numerous studies have demonstrated corresponding differences in neuronal properties across cell types, but these comparisons have largely been limited to conditions outside of awake, behaving animals. Thus the functional role of the various cell types is not well understood. Here, we investigate differences in the functional properties of two widespread and broad classes of cells in inferior temporal cortex of macaque monkeys: inhibitory interneurons and excitatory projection cells. Cells were classified as putative inhibitory or putative excitatory neurons on the basis of their extracellular waveform characteristics (e.g., spike duration). Consistent with previous intracellular recordings in cortical slices, putative inhibitory neurons had higher spontaneous firing rates and higher stimulus-evoked firing rates than putative excitatory neurons. Additionally, putative excitatory neurons were more susceptible to spike waveform adaptation following very short interspike intervals. Finally, we compared two functional properties of each neuron's stimulus-evoked response: stimulus selectivity and response latency. First, putative excitatory neurons showed stronger stimulus selectivity compared with putative inhibitory neurons. Second, putative inhibitory neurons had shorter response latencies compared with putative excitatory neurons. Selectivity differences were maintained and latency differences were enhanced during a visual search task emulating more natural viewing conditions. Our results suggest that short-latency inhibitory responses are likely to sculpt visual processing in excitatory neurons, yielding a sparser visual representation.

Cognitive consilience: primate non-primary neuroanatomical circuits underlying cognition

Interactions between the cerebral cortex, thalamus, and basal ganglia form the basis of cognitive information processing in the mammalian brain. Understanding the principles of neuroanatomical organization in these structures is critical to understanding the functions they perform and ultimately how the human brain works. We have manually distilled and synthesized hundreds of primate neuroanatomy facts into a single interactive visualization. The resulting picture represents the fundamental neuroanatomical blueprint upon which cognitive functions must be implemented. Within this framework we hypothesize and detail 7 functional circuits corresponding to psychological perspectives on the brain: consolidated long-term declarative memory, short-term declarative memory, working memory/information processing, behavioral memory selection, behavioral memory output, cognitive control, and cortical information flow regulation. Each circuit is described in terms of distinguishable neuronal groups including the cerebral isocortex (9 pyramidal neuronal groups), parahippocampal gyrus and hippocampus, thalamus (4 neuronal groups), basal ganglia (7 neuronal groups), metencephalon, basal forebrain, and other subcortical nuclei. We focus on neuroanatomy related to primate non-primary cortical systems to elucidate the basis underlying the distinct homotypical cognitive architecture. To display the breadth of this review, we introduce a novel method of integrating and presenting data in multiple independent visualizations: an interactive website (http://www.frontiersin.org/files/cognitiveconsilience/index.html) and standalone iPhone and iPad applications. With these tools we present a unique, annotated view of neuroanatomical consilience (integration of knowledge).

Modular and laminar pathology of Brodmann's area 37 in Alzheimer's disease

Previous studies suggested a relationship between severity of symptoms and the degree of neurofibrillary tangles (NFTs) clustering in different areas of the cortex in Alzheimer's disease (AD). The posterior inferior temporal cortex or Brodmann's area (BA 37) is involved in object naming and recognition memory. But the cellular architecture and connectivity and the NFT pathology of this cortex in AD received inadequate attention. In this report, we describe the laminar distribution and topography of NFT pathology of BA 37 in brains of AD patients by using Thionin staining for Nissl substance, Thioflavin-S staining for NFTs, and phosphorylated tau (AT8) immunohistochemistry. NFTs mostly occurred in cortical layers II, III, V and VI in the area 37 of AD brain. Moreover, NFTs appeared like a patch or in cluster pattern along the cortical layers III and V and within the columns of pyramidal cell layers. The abnormal, intensely labeled AT8 immunoreactive cells were clustered mainly in layers III and V. Based on previously published clinical correlations between cognitive abnormalities in AD and the patterns of laminar distributed NFT cluster pathology in other areas of the brain, we conclude that a similar NFT pathology that severely affected BA 37, may indicate disruption of some forms of naming and object recognition-related circuits in human AD.

Context familiarity enhances target processing by inferior temporal cortex neurons

Experience-dependent changes in the response properties of ventral visual stream neurons are thought to underlie our ability to rapidly and efficiently recognize visual objects. How these neural changes are related to efficient visual processing during natural vision remains unclear. Here, we demonstrate a neurophysiological correlate of efficient visual search through highly familiar object arrays. Humans and monkeys are faster at locating the same target when it is surrounded by familiar compared with unfamiliar distractors. We show that this behavioral enhancement is driven by an increased sensitivity of target-selective neurons in inferior temporal cortex. This results from an increased "signal" for target representations and decreased "noise" from neighboring familiar distractors. These data highlight the dynamic properties of the inferior temporal cortex neurons and add to a growing body of evidence demonstrating how experience shapes neural processing in the ventral visual stream.

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.

Organization of Horizontal Axons in the Inferior Temporal Cortex and Primary Visual Cortex of the Macaque Monkey

We investigated the organization of horizontal connections at two distinct hierarchical levels in the ventral visual cortical pathway of the monkey, the inferior temporal (TE) and primary visual (V1) cortices. After injections of anterograde tracers into layers 2 and 3, clusters of terminals ('patches') of labeled horizontal collaterals in TE appeared at various distances up to 8 mm from the injection site, while in V1 clear patches were distributed only within 2 mm. The size and spacing of these patches in TE were larger and more irregular than those observed in V1. The labeling intensity of patches in V1 declined sharply with distance from the injection site. This tendency was less obvious in TE; a number of densely labeled patches existed at distant sites beyond weakly labeled patches. While injections into both areas resulted in an elongated pattern of patches, the anisotropy was greater in TE than in V1 for injections of a similar size. Dual tracer injections and larger-sized injections further revealed that the adjacent sites in TE had spatially distinct horizontal projections, compared to those in V1. These area-specific characteristics of the horizontal connections may contribute to the differences in visual information processing of TE and V1.

Cell type- and region-specific expression of protein kinase C-substrate mRNAs in the cerebellum of the macaque monkey

We performed nonradioactive in situ hybridization histochemistry in the monkey cerebellum to investigate the localization of protein kinase C-substrate (growth-associated protein-43 [GAP-43], myristoylated alanine-rich C-kinase substrate [MARCKS], and neurogranin) mRNAs. Hybridization signals for GAP-43 mRNA were observed in the molecular and granule cell layers of both infant and adult cerebellar cortices. Signals for MARCKS mRNA were observed in the molecular, Purkinje cell, and granule cell layers of both infant and adult cortices. Moreover, both GAP-43 and MARCKS mRNAs were expressed in the external granule cell layer of the infant cortex. In the adult cerebellar vermis, signals for both GAP-43 and MARCKS mRNAs were more intense in lobules I, IX, and X than in the remaining lobules. In the adult hemisphere, both mRNAs were more intense in the flocculus and the dorsal paraflocculus than in other lobules. Such lobule-specific expressions were not prominent in the infant cerebellar cortex. Signals for neurogranin, a postsynaptic substrate for protein kinase C, were weak or not detectable in any regions of either the infant or adult cerebellar cortex. The prominent signals for MARCKS mRNA were observed in the deep cerebellar nuclei, but signals for both GAP-43 and neurogranin mRNAs were weak or not detectable. The prominent signals for both GAP-43 and MARCKS mRNAs were observed in the inferior olive, but signals for neurogranin were weak or not detectable. The cell type- and region-specific expression of GAP-43 and MARCKS mRNAs in the cerebellum may be related to functional specialization regarding plasticity in each type of cell and each region of the cerebellum.

Distribution of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-type glutamate receptor subunits (GluR2/3) along the ventral visual pathway in the monkey

By using immunohistochemical methods, we examined the distribution of cells expressing subunits of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-selective glutamate receptors (GluR2/3) in the cortical areas of the occipitotemporal pathway in monkeys. GluR2/3-immunoreactive (-ir) cells were primarily pyramidal cells; this category, however, also included large stellate cells in layer IVB of the striate cortex (V1) and fusiform cells in layer VI of all the areas examined. GluR2/3 immunoreactivity differed among the areas in laminar distribution and intensity. In V1, GluR2/3-ir cells were identified mainly in layers II, III, IVB, and VI. The prestriate areas V2 and V4 and the inferior temporal areas TEO and TE contained GluR2/3-ir cells in layers II, III, and VI. In the TE, GluR2/3-ir cells were also abundant in layer V. In area 36 of the perirhinal cortex, neurons in layers II, III, V, and VI were labeled in a similar manner to the TE labeling, but with greater staining intensity and numbers, especially in layer V. Thus, GluR2/3 immunoreactivity increased rostrally along the pathway. Within V1 and V2, cells strongly stained for GluR2/3 formed clusters that colocalized with cytochrome oxidase (CO)-rich regions. These distinct laminar and regional distribution patterns of GluR2/3 expression may contribute to the specific physiological properties of neurons within various visual areas and compartments.

Expression of GAP-43 and SCG10 mRNAs in lateral geniculate nucleus of normal and monocularly deprived macaque monkeys

We performed nonradioactive in situ hybridization histochemistry (ISH) in the lateral geniculate nucleus (LGN) of the macaque monkey to investigate the distribution of mRNA for two growth-associated proteins, GAP-43 and SCG10. GAP-43 and SCG10 mRNAs were coexpressed in most neurons of both magnocellular layers (layers I and II) and parvocellular layers (layers III-VI). Double-labeling using nonradioactive ISH and immunofluorescence revealed that both GAP-43 and SCG10 mRNAs were coexpressed with the alpha-subunit of type II calcium/calmodulin-dependent protein kinase, indicating that both mRNAs are expressed also in koniocellular neurons in the LGN. We also showed that GABA-immunoreactive neurons in the LGN did not contain GAP-43 and SCG10 mRNAs, indicating that neither GAP-43 nor SCG10 mRNAs were expressed in inhibitory interneurons in the LGN. GABA-immunoreactive neurons in the perigeniculate nucleus, however, contained both GAP-43 and SCG10 mRNAs, indicating that both mRNAs were expressed in inhibitory neurons in the perigeniculate nucleus, which project to relay neurons in the LGN. Furthermore, to determine whether the expression of GAP-43 and SCG10 mRNAs is regulated by visual input, we performed nonradioactive ISH in the LGN and the primary visual area of monkeys deprived of monocular visual input by intraocular injections of tetrodotoxin. Both mRNAs were downregulated in the LGN after monocular deprivation for 5 d or longer. From these results, we conclude that both GAP-43 and SCG10 mRNAs are expressed in the excitatory relay neurons of the monkey LGN in an activity-dependent manner.

Neuronal mechanisms of selectivity for object features revealed by blocking inhibition in inferotemporal cortex

The inferotemporal cortex (area TE) of monkeys, a higher station of the visual information stream for object recognition, contains neurons selective for particular object features. Little is known about how and where this selectivity is generated. We show that blockade of inhibition mediated by gamma-aminobutyric acid (GABA) markedly altered the selectivity of TE neurons by augmenting their responses to some stimuli but not to others. The effects were observed for particular groups of stimuli related to the originally effective stimuli or those that did not originally excite the neurons but activated nearby neurons. Intrinsic neuronal interactions within area TE thus determine the final characteristic of their selectivity, and GABAergic inhibition contributes to this process.

Intrinsic excitatory connections in the prefrontal cortex and the pathophysiology of schizophrenia

Working memory, a fundamental cognitive process that is disturbed in schizophrenia, appears to depend upon the sustained activity of specific populations of neurons in the prefrontal cortex. Understanding the neural mechanism(s) that may contribute to the sustained activity of these neurons represents a critical step in predicting the types of alterations in prefrontal circuitry that may be present in schizophrenia, and in determining how such alterations may contribute to the cognitive symptoms of this disorder. This article reviews recent findings which suggest that intrinsic horizontal connections among pyramidal neurons in layer 3 of the dorsolateral prefrontal cortex may provide a critical anatomical substrate for working memory processes, and that alterations in these connections may account for the observations of disturbed working memory, adolescence-related onset of clinical features, and certain pathological changes in the prefrontal cortex of subjects with schizophrenia.

Laminar distribution of neurons in extrastriate areas projecting to visual areas V1 and V4 correlates with the hierarchical rank and indicates the operation of a distance rule

The directionality of corticocortical projections is classified as feedforward (going from a lower to higher hierarchical levels), feedback (interconnecting descending levels), and lateral (interconnecting equivalent levels). Directionality is determined by the combined criteria of the laminar patterns of the axon terminals as well as the cells of origins and has been used to construct models of the visual system, which reveals a strict hierarchical organization (Felleman and Van Essen, 1991; Hilgetag et al., 1996a). However, these models are indeterminate partly because we have no indication of the distance separating adjacent levels. Here we have attempted to determine a graded parameter describing the anatomical relationship of interconnected areas. We have investigated whether the precise percentage of labeled supragranular layer neurons (SLN%) in each afferent area after injection in either visual areas V1 or V4 determines its hierarchical position in the model. This shows that pathway directionality in the Felleman and Van Essen model is characterized by a range of SLN% values. The one exception is the projection of the frontal eye field to area V4, which resembles a feedforward projection. Individual areal differences in SLN% values are highly significant, and the number of hierarchical steps separating a target area from a source area is found to be tightly correlated to SLN%. The present results show that the hierarchical rank of each afferent area is reliably indicated by SLN%, and therefore this constitutes a graded parameter that is related to hierarchical distance.