Simple Yet Complex Primate Brain Structure Aided Human Evolution, Suggests Study

It's not a tangled mass, but a simple, yet highly complex, criss-crossing grid of neural fibers, much like the fine threads of a sheet of fabric. This is how scientists would now describe the structure of the neural lines and connections that make up the primate brain, including humans. It is, according to the leading scientist of a recently completed study, one reason why evolutionary processes were able to more easily take hold in the primate brain and lead to the changes that helped make us and our primate cousins what we are today.

The study is detailed In the March 30 issue of Science. It reports the discovery of an amazingly simple and organized, yet complex neural structure that makes up human and primate brains. Specifically, the researchers found that the neural pathways carrying signals through the brain are oriented not in a tangled, disorganized fashion, as has been typically pictured, but in a curving, three-dimensional grid. Their analysis revealed that neural fibers of the brain white matter formed a grid pattern of parallel and criss-crossing fibers, interwoven like the threads in a sheet of fabric, defining an orderly three-dimensional curved grid structure. The same pattern was observed for humans as well as for all of the four non-human primates species – rhesus monkeys, owl monkeys, marmosets and galagos - examined in the study. Says study leader Van Wedeen, MD, Associate Professor of Radiology at the Massachusetts General Hospital Martinos Center for Biomedical Imaging: "Finding this kind of simple organization in the forebrain of higher animals was completely unsuspected".

In conducting the work, researchers used diffusion spectrum MR imaging – a technology Wedeen developed that reveals the orientation of the brain's neural fibers. With diffusion imaging, a scanner detects water and chemicals injected into and moving through individual fibers to reveal their locations and orientation relative to each other. They then applied a mathematical analysis of the crossing or adjacent pathways in the brains. Previous studies of animal brains also showed similar patterns, but it was not certain that such structures were widespread or only defined certain brain systems. Observing this in the human brain has long eluded scientists because, in part, the human cortex of the brain has multiple folds that make it difficult to determine the structure of the neural connections.
Among other things, the discovery has implications for human evolution.

"The old image of the brain as a tangle of thousands of discrete, unconnected wires didn't make sense from an evolutionary standpoint," says Wedeen. "How could natural selection guide each of those wires into more efficient, advantageous configurations?......The very simplicity of this grid structure is the reason why it can accomodate the random, gradual changes of evolution. It's easier for a simple structure to change and adapt, whether we're talking about the big changes that occur across evolution or the changes that can occur during an individual's lifetime – both the normal neuroplasticity associated with development and learning or the damage that results from injury or disease. A simple grid structure makes both evolutionary and developmental sense."

The research also revealed that the more complex human and rhesus brains, for example, showed more differentiation between pathways than simpler species.

Wedeen is an associate professor of Radiology at Harvard Medical School and on the faculty of the Harvard-MIT Health Sciences and Technology Program. Additional co-authors of the Science article are Ruopeng Wang and Guangping Dai of the Martinos Center; Douglas Rosene and Farzad Mortazavi, Boston University Medical Center; Patric Hagmann, University of Lausanne, Switzerland; Jon Kaas, Vanderbilt University; and Wen-Yih Tseng, National Taiwan University College of Medicine. The study was supported by grants from the National Science Foundation, the National Institutes of Health and the Human Connectome Project of the NIH.

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Cover Photo, Top Left: This is a diffusion spectrum MR image of human brain showing curvature of two-dimensional sheets of parallel neuronal fibers that cross each other at right angles. Credit: Van Wedeen, M.D., Martinos Center for Biomedical Imaging, Massachusetts General Hospital

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