It is made up of grey matter, the cell bodies of neurons; and white matter, bundles of nerve fibers that connect the cells.
An average human brain is thought to have about ten billion neurons, and these cells and their connections are so densely packed that traditional MRI scans can't show individual neurons and nerve fibers.
To trace the brain's wiring, study co-author Patric Hagmann of the University of Lausanne in Switzerland and colleagues used a version of MRI called diffusion spectrum imaging that tracks the diffusion of water.
"Nerve fibers are like thin wires that are wrapped in a layer of fat called myelin," Sporns explained. "Water tends to diffuse not through these fibers but alongside them. That's because water and fat don't easily mix."
By following the water, Hagmann's neuroimaging team figured out the most likely orientation of axons and created a map of various pathways that connect hundreds of thousands of neurons.
Sporns and colleagues then used computational analysis to look for hubs in the neural network where many bundles of fibers intersect.
What they were surprised to find is that all of their study participants had the same single structural core in roughly the same region of their brains.
Functional MRI scans—which show brain activity based on bloodflow—then revealed the link between at-rest activity and the core.
The team's results appeared yesterday in the journal PLoS Biology.
Marcus Raichle is a professor of neurology at Washington University in St. Louis who was not involved in the research.
He lauded the work, noting that "the idea that there might be a 'superhub' that talks across systems would seem like an obvious one, but that it's true is very neat."
The new paper, he said, builds on previous research that ties what had long been considered "noise" in scans of brain activity to actual function.
"If you put somebody in an MR scanner and you don't ask them to do anything, the brain is still active—just as active as when you are doing something," he noted.
And just as specific regions light up when a person is focused on a task, the brain's core stands out when the only main activity is general communication.
"You have these different systems that are talking to themselves. What this beautiful paper says is that you not only have communication within systems but also among systems," Raichle said.
This makes intuitive sense, he added. For example, when someone reaches for a coffee cup, the visual cortex must be communicating closely with regions of the brain that handle motor control.
Personal Brain Map?
According to Sporns, his team's "wiring diagram" of the cortex is just a first step forward for connectomics.
His colleagues and other researchers are working furiously to create a more complete neural map, and he predicts that more detailed versions could appear in the next year or two.
"The connectome is a desirable thing to have, because we have so much information coming from functional brain imaging, but without a structural basis it's kind of floating in the air. We need to ground it."
And like genome scientists looking for a cheap way to map an individual's DNA, connectomics researchers might one day be looking for ways to create personalized brain maps.
"We need to pay much more attention to individual brains, actually. In the past, most published studies reported their results by referring to a standardized brain or group averages," Sporns said.
"My hunch here is that, if it is in fact true that wiring is related to function, that wiring might help to predict and explain the individual variation we see in cognitive performance even in healthy brains."
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