To the uninitiated, the brain’s connections may look like a plate of tangled spaghetti, but new research suggests that there is symmetry hidden in the apparent chaos. The brain’s wiring is in fact laid out in a three-dimensional grid-like structure that is surprisingly simple, according to the study published in Science.
“We hadn’t expected a strong three-dimensional order to be present in the brain,” says Dr. Van Wedeen, associate professor of radiology at Harvard Medical School, who led the research. “We had assumed that in order to be as flexible as possible, the wiring of the brain was a very free business and loose, like a telephone switchboard where any point might connect to just about any other point. Instead, we find that the basic map is like Manhattan. The streets go in two axes and the buildings go up and down.”
Since the brain is so curved and so complex, these patterns are not immediately apparent. And since brain scientists tend to focus only on specific brain regions associated with specific functions, research has so far left us with surprisingly little information about the patterns that run through it as a whole.
Typically, scientists look at individual regions of the brain’s cortex, or outer layer. The cortex is divided up into various Brodmann areas, so numbered and assigned by a German anatomist in the early 1900s. Although the Brodmann areas’ boundaries are much debated, certain clearly defined regions of the brain have long been recognized to be associated with specific functions. The inner portions of the brain are similarly delineated, according to physical structure, and are likewise found to have particular processing duties.
But beyond that crude understanding, little more is known. How do these regions wire themselves up? What is the nature of their overall patterning? Scientists are only just beginning to detail how brain cells follow chemical signals to create the right connections in the appropriate places during development.
“If you picture the wires of the brain like little worms tunneling through, [you would think that] each one needs its own GPS compass and instruction set and tons of information,” says Wedeen. “Instead, the wiring is infinitely simpler. There are only three directions that it can go in, only three fixed choices at this point: left or right, front or back, and up or down.”
As in most of midtown Manhattan, the brain’s throughways run parallel or perpendicular, but not diagonally. (If the brain has a Broadway exception, it wasn’t found here.)
Since the brain has billions of cells making trillions of connections between them, and the human genome contains only tens of thousands of genes to tell the organ how to wire itself, researchers suspected that there had to be simple principles underlying much of it. For evolutionary reasons, simplicity was also seen as probably: if not, mutations in genes might too often be catastrophic.
Wedeen’s work, using a technique called diffusion magnetic resonance spectrum imaging, found the grid-like structure in four non-human primates as well as in the human brain. He says the same kind of structure is reflected in the most primitive brains. “If you look at the simplest brain in a flatworm, you see a wiring diagram astonishingly similar to ours, which looks like a ladder, sort of like what the spinal cord looks like,” he says. “What we’re really saying is that the whole [brain] is organized in a way that strongly echoes this ancient simple pattern, which has become immensely elaborated.”
Understanding the fundamentals of brain structure could help scientists better recognize how a healthy brain should look, and what goes wrong in developmental disorders like autism, neurological diseases like Alzheimer’s or mental illnesses. A commentary published in Science noted that Wedeen’s study, along with another one that examined brain structures in twins, find “unifying hierarchical and geometric rules behind the organizational details.”
Karl Friston, scientific director of the Wellcome Trust Centre for Neuroimaging in the U.K., who was not associated with the study, told the Scientist that the research was “substantial” and “convincing.”
Some researchers, however, are not convinced that the research adds much what is already known about brain organization. “I am underwhelmed,” says Partha Mitra, a neuroscience professor at Cold Spring Harbor Laboratory, who was not associated with the research. “I think at some level, it’s probably right but perhaps not greatly surprising,” he says, adding that the study didn’t provide quantification for its findings.
Wedeen believes his work will be a starting point for future research. “I anticipate that this will be a rich source of inspiration for neuroscientists trying to comprehend the business of the brain,” he says.