Neurosurgeon Wilder Penfield, MD, may have given scientists a glimpse of the inner workings of the human brain when he stimulated the brain electrically in the process of epilepsy surgery to map the sensory and motor regions of the cortex. Now, almost 55 years later, scientists have reconstructed the spatial patterns of cerebral white matter tracts, creating a complete map of cortical networks and how they are connected.
According to the lead scientist, Olaf Sporns, PhD, a computational neuroscientist at Indiana University in Bloomington, it is not unlike a map of the country's vast air transportation system, held together by a number of large hubs that planes fly in and out of en route to their final destination.
In the new study, published in the July 1 online issue of PLos Biology, Dr. Sporns and an international team of investigators identified a highly connected center in the back of the brain that they believe is the hub, or core, where virtually all information must go before it reaches its destination.
This new map of the anatomy of cerebral white matter suggests that the way in which information flows and is integrated within the networks of the brain is organized around hubs. The map suggests that if problems arise within these hub regions, they might have widespread effects throughout the cortex.
“No one has had this map,” said Dr. Sporns.
After studying cortical networks in other mammalian brains, including the macaque, Dr. Sporns turned to University of Lausanne's Patric Hagmann, MD, PhD, who was applying the technique of diffusion spectrum imaging, or DSI, a sophisticated modality that keeps track of crossing fiber bundles. DSI uses the brain's water signals to identify white matter tracts throughout the cortex and is more sensitive than diffusion tensor imaging. Van J. Wedeen, MD, of the Massachusetts General Hospital Martinos Center for Biomedical Imaging, who was a co-author of the PLoS study, first described the imaging technique and the software that drives the analysis in 2005 in Magnetic Resonance in Medicine, a journal of the International Society of Magnetic Resonance in Medicine.
In the current study, Dr. Sporns and colleagues asked whether DSI could help unravel the mysteries of how parts of the normal human brain are interconnected. The team got an answer after using DSI to scan the brains of five healthy young men. They were able to map the cerebral “hot spots” or hubs, areas that are densely connected. Simultaneously, they also used fMRI to compare the networks to the actual functional activations of the brain at rest.
They discovered two of the brain's most densely packed regions — the medial parietal lobe and the posterior cingulate cortex. “Many of our analyses pointed to these regions as central to the brain, forming its core,” said Dr. Sporns.
If these scientists are right, it could help explain some common brain diseases, including Alzheimer disease (AD). Evidence from several labs suggests that the posterior cingulate and other parts of the medial posterior cortex are among the first regions to show pathology in AD. These regions are highly activated at rest, which has led some scientists to suggest that high brain metabolism in this tissue wears down this part of the brain more quickly.
“When we take the patterns of correlations and compare them to the function on MRI we can make interesting comparisons,” said Dr. Sporns. “We want to know where the pattern comes from. Is it driven by anatomy? Are these networks constrained by the structural anatomy of the fibers?” He said that their imaging studies may help get at this answer.
“It is an important paper,” said Steven E. Petersen, PhD, the James S. McDonnell Professor of Cognitive Neuroscience at Washington University in St. Louis. “They have used this scanning in a new way.”
Dr. Sporns said that the next step is validation. “This is critical,” he added. “We want to be sure that the connectivity revealed by diffusion spectrum imaging is real.” While the group tested subjects at two different times and got the same results, other scientists will need to work with DSI to confirm these results.
Dr. Sporns said that the wiring diagram could ultimately help in the design of new treatments for stroke victims and other brain injuries or diseases, which might lead to a better understanding of brain recovery and reorganization.
As for the hub, the posterior medial and parietal cerebral cortex never appear to rest, Dr. Sporns said, and may play a critical role in functional integration.
Hagmann P, Cammoun L, Sporns O, et al. Mapping the structural core of human cerebral cortex. PLoS Biol 2008;6(7):e159.
Wedeen VJ, Hagmann P, Weisskoff RM, et al. Mapping complex tissue architecture with diffusion spectrum magnetic resonance imaging. Magn Reson Med 2005; 54:1377–1386.