334 research outputs found
Further insights into the interareal connectivity of a cortical network
Over the past years, network science has proven invaluable as a means to
better understand many of the processes taking place in the brain. Recently,
interareal connectivity data of the macaque cortex was made available with
great richness of detail. We explore new aspects of this dataset, such as a
correlation between connection weights and cortical hierarchy. We also look at
the link-community structure that emerges from the data to uncover the major
communication pathways in the network, and moreover investigate its reciprocal
connections, showing that they share similar properties
A laminar organization for selective cortico-cortical communication
The neocortex is central to mammalian cognitive ability, playing critical roles in sensory perception, motor skills and executive function. This thin, layered structure comprises distinct, functionally specialized areas that communicate with each other through the axons of pyramidal neurons. For the hundreds of such cortico-cortical pathways to underlie diverse functions, their cellular and synaptic architectures must differ so that they result in distinct computations at the target projection neurons. In what ways do these pathways differ? By originating and terminating in different laminae, and by selectively targeting specific populations of excitatory and inhibitory neurons, these “interareal” pathways can differentially control the timing and strength of synaptic inputs onto individual neurons, resulting in layer-specific computations. Due to the rapid development in transgenic techniques, the mouse has emerged as a powerful mammalian model for understanding the rules by which cortical circuits organize and function. Here we review our understanding of how cortical lamination constrains long-range communication in the mammalian brain, with an emphasis on the mouse visual cortical network. We discuss the laminar architecture underlying interareal communication, the role of neocortical layers in organizing the balance of excitatory and inhibitory actions, and highlight the structure and function of layer 1 in mouse visual cortex
Corticocortical evoked potentials reveal projectors and integrators in human brain networks.
The cerebral cortex is composed of subregions whose
functional specialization is largely determined by their
incoming and outgoing connections with each other. In the
present study, we asked which cortical regions can exert the
greatest influence over other regions and the cortical
network as a whole. Previous research on this question has
relied on coarse anatomy (mapping large fiber pathways) or
functional connectivity (mapping inter-regional statistical
dependencies in ongoing activity). Here we combined direct
electrical stimulation with recordings from the cortical
surface to provide a novel insight into directed, inter-
regional influence within the cerebral cortex of awake
humans. These networks of directed interaction were
reproducible across strength thresholds and across subjects.
Directed network properties included (1) a decrease in the
reciprocity of connections with distance; (2) major projector
nodes (sources of influence) were found in peri-Rolandic
cortex and posterior, basal and polar regions of the temporal
lobe; and (3) major receiver nodes (receivers of influence)
were found in anterolateral frontal, superior parietal, and
superior temporal regions. Connectivity maps derived from
electrical stimulation and from resting electrocorticography
(ECoG) correlations showed similar spatial distributions for
the same source node. However, higher-level network topology
analysis revealed differences between electrical stimulation
and ECoG that were partially related to the reciprocity of
connections. Together, these findings inform our
understanding of large-scale corticocortical influence as
well as the interpretation of functional connectivity
networks
Recruitment of inhibition and excitation across mouse visual cortex depends on the hierarchy of interconnecting areas
Diverse features of sensory stimuli are selectively processed in distinct brain areas. The relative recruitment of inhibitory and excitatory neurons within an area controls the gain of neurons for appropriate stimulus coding. We examined how such a balance of inhibition and excitation is differentially recruited across multiple levels of a cortical hierarchy by mapping the locations and strengths of synaptic inputs to pyramidal and parvalbumin (PV)-expressing neurons in feedforward and feedback pathways interconnecting primary (V1) and two higher visual areas. While interareal excitation was stronger in PV than in pyramidal neurons in all layer 2/3 pathways, we observed a gradual scaling down of the inhibition/excitation ratio from the most feedforward to the most feedback pathway. Our results indicate that interareal gain control depends on the hierarchical position of the source and the target, the direction of information flow through the network, and the laminar location of target neurons. DOI: http://dx.doi.org/10.7554/eLife.19332.00
Brain rhythms define distinct interaction networks with differential dependence on anatomy
Cognitive functions are subserved by rhythmic neuronal synchronization across widely distributed brain areas. In 105 area pairs, we investigated functional connectivity (FC) through coherence, power correlation, and Granger causality (GC) in the theta, beta, high-beta, and gamma rhythms. Between rhythms, spatial FC patterns were largely independent. Thus, the rhythms defined distinct interaction networks. Importantly, networks of coherence and GC were not explained by the spatial distributions of the strengths of the rhythms. Those networks, particularly the GC networks, contained clear modules, with typically one dominant rhythm per module. To understand how this distinctiveness and modularity arises on a common anatomical backbone, we correlated, across 91 area pairs, the metrics of functional interaction with those of anatomical projection strength. Anatomy was primarily related to coherence and GC, with the largest effect sizes for GC. The correlation differed markedly between rhythms, being less pronounced for the beta and strongest for the gamma rhythm
Constraints and spandrels of interareal connectomes
Interareal connectomes are whole-brain wiring diagrams of white-matter pathways. Recent studies have identified modules, hubs, module hierarchies and rich clubs as structural hallmarks of these wiring diagrams. An influential current theory postulates that connectome modules are adequately explained by evolutionary pressures for wiring economy, but that the other hallmarks are not explained by such pressures and are therefore less trivial. Here, we use constraint network models to test these postulates in current gold-standard vertebrate and invertebrate interareal-connectome reconstructions. We show that empirical wiring-cost constraints inadequately explain connectome module organization, and that simultaneous module and hub constraints induce the structural byproducts of hierarchies and rich clubs. These byproducts, known as spandrels in evolutionary biology, include the structural substrate of the default-mode network. Our results imply that currently standard connectome characterizations are based on circular analyses or double dipping, and we emphasize an integrative approach to future connectome analyses for avoiding such pitfalls.M.R. was funded by the NARSAD Young Investigator Award, the Isaac Newton Grant for Research Purposes, and the Parke Davis Exchange Fellowship. The BCNI was funded by the MRC and the Wellcome Trust
Weight Consistency Specifies Regularities of Macaque Cortical Networks
To what extent cortical pathways show significant weight differences and whether these differences are consistent across animals (thereby comprising robust connectivity profiles) is an important and unresolved neuroanatomical issue. Here we report a quantitative retrograde tracer analysis in the cynomolgus macaque monkey of the weight consistency of the afferents of cortical areas across brains via calculation of a weight index (fraction of labeled neurons, FLN). Injection in 8 cortical areas (3 occipital plus 5 in the other lobes) revealed a consistent pattern: small subcortical input (1.3% cumulative FLN), high local intrinsic connectivity (80% FLN), high-input form neighboring areas (15% cumulative FLN), and weak long-range corticocortical connectivity (3% cumulative FLN). Corticocortical FLN values of projections to areas V1, V2, and V4 showed heavy-tailed, lognormal distributions spanning 5 orders of magnitude that were consistent, demonstrating significant connectivity profiles. These results indicate that 1) connection weight heterogeneity plays an important role in determining cortical network specificity, 2) high investment in local projections highlights the importance of local processing, and 3) transmission of information across multiple hierarchy levels mainly involves pathways having low FLN values
The natural axis of transmitter receptor distribution in the human cerebral cortex
Transmitter receptors constitute a key component of the molecular machinery for intercellular communication in the brain. Recent efforts have mapped the density of diverse transmitter receptors across the human cerebral cortex with an unprecedented level of detail. Here, we distill these observations into key organizational principles. We demonstrate that receptor densities form a natural axis in the human cerebral cortex, reflecting decreases in differentiation at the level of laminar organization and a sensory-to-association axis at the functional level. Along this natural axis, key organizational principles are discerned: progressive molecular diversity (increase of the diversity of receptor density); excitation/inhibition (increase of the ratio of excitatory-to-inhibitory receptor density); and mirrored, orderly changes of the density of ionotropic and metabotropic receptors. The uncovered natural axis formed by the distribution of receptors aligns with the axis that is formed by other dimensions of cortical organization, such as the myelo- and cytoarchitectonic levels. Therefore, the uncovered natural axis constitutes a unifying organizational feature linking multiple dimensions of the cerebral cortex, thus bringing order to the heterogeneity of cortical organization
Micro-, Meso- and Macro-Connectomics of the Brain
Neurosciences, Neurolog
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