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Connectomics is an emerging field of science defined by technologies that provide accurate tracings of fine wiring in the brain.
Previous efforts to map brain wiring focused on larger anatomical features, such as connective wiring between different parts of the brain, or on the paths of single neurons. Scientists now use imaging techniques and machine-learning that support high-throughput data generation and mining.
The connectome (map) annotates the structure of a neural network, listing all synaptic connections between the neurons inside a brain or brain region.
These maps assist scientists who examine how neural networks perform their precise functions.
One complete wiring diagram exists, for the model organism C. elegans.
The worm's brain contains 302 neurons, but the mapping effort took more than a decade. The human brain has an estimated 100 billion neurons and 100 trillion synapses.
Winfried Denk, a neuroscientist at the Max Planck Institute for Medical Research, estimates that standard methods would take roughly three billion person years to generate the wiring diagram of a single cortical column, a narrow functional unit of neurons in the cortex.
“The brain is essentially a computer that wires itself up during development and can rewire itself” (Sebastian Seung, MIT computational neuroscientist).
As a consequence, there are practical benefits from accurate brain maps, such as investigations into various birth conditions that result from alternative nerve pathways, or even outright cases of faulty wiring, such as autism and schizophrenia.
Ultimately, the usefulness of any connectome is the contribution to scientific understanding of the framework for processing and transferring data in the brain.
Over the horizon, research is underway to generate maps that incorporate the biochemical and physiological properties of various cells into the wiring diagrams, which will provide deep content access in real time.
Bloomington, IN, USA. An international team of researchers has created the first complete high-resolution map of how millions of neural fibers in the human cerebral cortex — the outer layer of the brain responsible for higher level thinking — connect and communicate. They have identified a single network core, or hub, that integrates both brain hemispheres.
The map shows a core of brain regions with highly interconnected structures (the brain "connectome"). The groundbreaking work also describes a novel application of a non-invasive technique that can be used by other scientists to continue mapping the trillions of neural connections in the brain at even greater resolution, which is becoming a new field of science ("connectomics").
Mapping the Structural Core of Human Cerebral Cortex. Patric Hagmann, Leila Cammoun, Xavier Gigandet, Reto Meuli, Christopher J. Honey, Van J. Wedeen, Olaf Sporns. PLoS Biology 6(7) e159 doi: 10.1371 / journal.pbio.0060159 [ Download PDF ]
A map of the human cerebral cortex identifies a single network core that could be key to the workings of both hemispheres of the brain. Image courtesy of Indiana University.
Until now, scientists have mostly used functional magnetic resonance imaging (fMRI) technology to measure brain activity — locating which parts of the brain become active during perception or cognition — but there has been little understanding of the role of the underlying anatomy in generating this activity.
What is known of neural fiber connections and pathways has largely been learned from animal studies, and so far, no complete map of brain connections in the human brain exists.
In this new study, a team of neuroimaging researchers led by Patric Hagmann used state-of-the-art diffusion MRI technology, which is a non-invasive scanning technique that estimates fiber connection trajectories based on gradient maps of the diffusion of water molecules through brain tissue. A highly sensitive variant of the method, called diffusion spectrum imaging (DSI), can depict the orientation of multiple fibers that cross a single location.
The study applies this technique to the entire human cortex, resulting in maps of millions of neural fibers running throughout this highly furrowed part of the brain.
Olaf Sporns, co-author of the study and neuroscientist at IU, carried out a computational analysis trying to identify regions of the brain that played a more central role in the connectivity, serving as hubs in the cortical network. Surprisingly, these analyses revealed a single highly and densely connected structural core in the brain of all participants.
"We found that the core, the most central part of the brain, is in the medial posterior portion of the cortex, and it straddles both hemispheres," Sporns said. "This wasn't known before. Researchers have been interested in this part of the brain for other reasons. For example, when you're at rest, this area uses up a lot of metabolic energy, but until now it hasn't been clear why."
"This is one of the first steps necessary for building large-scale computational models of the human brain to help us understand processes that are difficult to observe, such as disease states and recovery processes to injuries," said Sporns.
The researchers asked whether the structural connections of the brain in fact shape its dynamic activity, Sporns said. The study examined the brains of five human participants who were imaged using both fMRI and DSI techniques to compare how closely the brain activity observed in the fMRI mapped to the underlying fiber networks.
"It turns out they're quite closely related," Sporns said. "We can measure a significant correlation between brain anatomy and brain dynamics. This means that if we know how the brain is connected we can predict what the brain will do."
Sporns said he and Hagmann plan to look at more brains soon, to map brain connectivity as brains develop and age, and as they change in the course of disease and dysfunction.
Mapping the Structural Core of Human Cerebral Cortex. Patric Hagmann, Leila Cammoun, Xavier Gigandet, Reto Meuli, Christopher J. Honey, Van J. Wedeen, Olaf Sporns. PLoS Biology 6(7) e159 doi: 10.1371 / journal.pbio.0060159
[ Download PDF ]
Abstract
Structurally segregated and functionally specialized regions of the human cerebral cortex are interconnected by a dense network of cortico-cortical axonal pathways. By using diffusion spectrum imaging, we noninvasively mapped these pathways within and across cortical hemispheres in individual human participants. An analysis of the resulting large-scale structural brain networks reveals a structural core within posterior medial and parietal cerebral cortex, as well as several distinct temporal and frontal modules. Brain regions within the structural core share high degree, strength, and betweenness centrality, and they constitute connector hubs that link all major structural modules. The structural core contains brain regions that form the posterior components of the human default network. Looking both within and outside of core regions, we observed a substantial correspondence between structural connectivity and resting-state functional connectivity measured in the same participants. The spatial and topological centrality of the core within cortex suggests an important role in functional integration.
Author Summary
In the human brain, neural activation patterns are shaped by the underlying structural connections that form a dense network of fiber pathways linking all regions of the cerebral cortex. Using diffusion imaging techniques, which allow the noninvasive mapping of fiber pathways, we constructed connection maps covering the entire cortical surface. Computational analyses of the resulting complex brain network reveal regions of cortex that are highly connected and highly central, forming a structural core of the human brain. Key components of the core are portions of posterior medial cortex that are known to be highly activated at rest, when the brain is not engaged in a cognitively demanding task. Because we were interested in how brain structure relates to brain function, we also recorded brain activation patterns from the same participant group. We found that structural connection patterns and functional interactions between regions of cortex were significantly correlated. Based on our findings, we suggest that the structural core of the brain may have a central role in integrating information across functionally segregated brain regions.
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The connectome (map) annotates the structure of a neural network, listing all synaptic connections between the neurons inside a brain or brain region. 
In this new study, a team of neuroimaging researchers led by Patric Hagmann used state-of-the-art diffusion MRI technology, which is a non-invasive scanning technique that estimates fiber connection trajectories based on gradient maps of the diffusion of water molecules through brain tissue. A highly sensitive variant of the method, called diffusion spectrum imaging (DSI), can depict the orientation of multiple fibers that cross a single location.
"We found that the core, the most central part of the brain, is in the medial posterior portion of the cortex, and it straddles both hemispheres," Sporns said. "This wasn't known before. Researchers have been interested in this part of the brain for other reasons. For example, when you're at rest, this area uses up a lot of metabolic energy, but until now it hasn't been clear why."
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