MIND
March 30, 2012

A 3-D Street Map of the Brain

Remarkable new scans of monkeys' brains completely change our understanding of how the brain is wired.

The first high resolution maps of the brain's wiring are now out. And the wiring diagram is much simpler than thought, resembling an orderly 3D street map much more than the complex tangle of wires (like winding country roads) that researchers expected to see (see Figure 1).

Figure 1.
Detail of a DSI Scan
Detail from DSI scan shows fabric-like 3-D grid structure of connections in a monkey brain.

Credit: Van Wedeen, M.D., Martinos Center and Dept. of Radiology, Massachusetts General Hospital and Harvard University Medical School

The discovery has implications for all types of brain disorders, from stroke to PTSD. It's difficult to fix an electrical malfunction, much less find one, without being able to first trace the wiring. The folds, nooks and crannies of the surrounding tissue have previously made it impossible to see the underlying wiring of the brain's nerve fibers in much detail.

According to Van Wedeen, of Massachusetts General Hospital and Harvard Medical School, whose team conducted the study: "Before, we had just driving directions. Now, we have a map showing how all the highways and byways are interconnected."

Researchers from MGH were able to visualize the brain's wiring by using a scanning technique called diffusion spectrum imaging (DSI), which pinpoints the nerve fibers by tracking the water flowing through them.

So far, this grid structure has shown up in all areas of the brains of humans and in the four types of monkeys the researchers looked at: rhesus, owl, marmoset and galago.

The overall appearance is of bundles of flat ribbon cables running in three directions: front-back, left-right and up-down. The nerve fibers don't run diagonally or at odd angles. The layout is similar to what you would see if street maps also had an up and down direction; or in the weave of a carpet, if carpets were three dimensional instead of being flat.

This network of nerve fibers carries impulses from one part of the brain to another, allowing different parts of the brain to communicate. The bundles crisscross at many points, forming a 3D grid.

So far, this grid structure has shown up in all areas of the brains of humans and in the four types of monkeys the researchers looked at: rhesus, owl, marmoset and galago.

The simplicity of the grid's structure may help cut down on errors during embryonic development, when nerve fibers first begin to grow and form connections. With only four possible choices of direction--left, right, up and down--it becomes simpler for growing nerve fibers to find their proper destination.

While DSI is a vast improvement over previous techniques, the researchers were only able to see about 25% of the grid structure of the human brain with the scanner they were using. It was only able to reveal the large central circuitry, not that in outlying areas where folding intensifies.

But the lessons they learned allowed the design of a more powerful scanner called Connectom, which was installed at Massachusetts General Hospital last fall. Connectom scans are much faster and can reveal much more of the outlying areas. So far it's been possible to see about 75% of the total grid structure. (See Figure 2, a Connectom scan of the human brain).

Figure 2.
DSI Image of a Human Brain
Curvature in this DSI image of a whole human brain turns out to be folding of 2-D sheets of parallel neuronal fibers that cross paths at right angles. This picture came from the new Connectom scanner.

Credit: Van Wedeen, M.D., Martinos Center and Dept. of Radiology, Massachusetts General Hospital and Harvard University Medical School

An article on the study appears in March 30, 2012 issue of Science.

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