Monday, July 07, 2008

First Detailed Map of the Human Cortex

A new imaging technique reveals previously hidden brain structures, including the central hub.

By Emily Singer

Cortical core: Scientists used a new type of brain imaging called diffusion spectrum imaging, along with mathematical analysis, to build a map of the cortical architecture of the human brain, shown here. The blue lines represent neural fibers connecting different parts of the brain. To make the map easier to understand, only the strongest connections are shown. Letters indicate anatomic subregions of the brain. Credit: Indiana University, University of Lausanne, EPFL

The first high-resolution map of the human cortical network reveals that the brain has its own version of Grand Central Station, a central hub that is structurally connected to many other parts of the brain. Scientists generated the map using a new type of brain imaging known as diffusion imaging. The technique maps the largely inaccessible tangle of the brain's white matter--the long, thin fibers that ferry nerve signals between cells. Scientists hope that using the noninvasive method to study neural connections in people with Alzheimer's, schizophrenia, and autism will shed light on how changes in brain architecture are linked to these complex diseases.

"The fact that such a core exists gives rise to many questions we can now ask about it," says Olaf Sporns, a neuroscientist at Indiana University, in Bloomington, and senior author of the study, published this week in PLoS Biology. "What goes on there? And how is it involved in passing messages between different parts of the brain?"

Conventional imaging techniques, such as structural magnetic resonance imaging (MRI), reveal major anatomical features of the brain. But in humans, the brain's finer architecture--the neural projections that connect its different parts--has, until recently, remained hidden. "The brain we've been looking at with conventional MRI or CT scans all these years is not the real brain," says Van Wedeen, a neuroscientist at Massachusetts General Hospital, in Boston, who was also involved in the study. "We're just seeing a shadow of its surfaces."

Diffusion imaging is a new twist on MRI that uses magnetic resonance signals to track the movement of water molecules in the brain.In gray matter, water tends to diffuse multidirectionally. But in white matter, it diffuses along the length of neural wires, called axons, and scientists can use these diffusion measurements to map the fibers. The newest incarnation of diffusion imaging, called diffusion spectrum imaging, allows scientists to perform a very difficult feat: determining the direction of overlapping nerve fibers. "That's very important for noninvasive mapping of brain connectivity," says Wedeen, who developed the technique.

Wedeen and his collaborators used diffusion spectrum imaging to image the brains of five healthy volunteers, generating a wiring map of the entire cortex. To reveal the core of the network, Sporns used a mathematical technique to repeatedly prune away the connection points with the fewest links. "If you do it gradually, you end up with a set of nodes remaining that are highly interconnected," he says.