Brain's neuronal nexus mapped

A structural brain map -- the most detailed to date -- provides support for a controversial theory of a "default" state of brain activity, and could bring key insights into the physiological basis of illnesses such as schizophrenia, depression, and Alzheimer's disease. Researchers have identified a set of axonal pathways in the human cerebral cortex that forms structural "core" of the cortex -- a neuronal nexus that acts as the main relay station between disparate brain regions involved in cogn

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A structural brain map -- the most detailed to date -- provides support for a controversial theory of a "default" state of brain activity, and could bring key insights into the physiological basis of illnesses such as schizophrenia, depression, and Alzheimer's disease. Researchers have identified a set of axonal pathways in the human cerebral cortex that forms structural "core" of the cortex -- a neuronal nexus that acts as the main relay station between disparate brain regions involved in cognition, they report in a linkurl:paper;http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0060159 published today (July 1) in PLoS Biology. "We don't really have a very good connection map for the brain yet," said Olaf Sporns of Indiana University, the study's main author. Most studies on whole brain function have focused on functional activation with techniques such as Positron Emission Tomography (PET) or linkurl:functional MRI (fMRI).;http://www.the-scientist.com/article/display/53137/ "I've always been interested in tying these activity patterns to underlying structures." The study's findings confirm the presence of a "default network" in the brain -- the activation state present when the brain is not engaged in any specific cognitive task, said Peter Williamson of the University of Waterloo in Ontario, who was not involved in the research. "This is still a very controversial area," he said, but "the evidence seems to be accumulating that there's something very fundamental here." The researchers used a technique called diffusion spectrum imaging, which essentially tracks the diffusion of water molecules through the brain to map the fibers that make up the white matter. The technique has been used for about a decade, but "what's novel here," said Sporns, is that he and his colleagues used "a particularly sensitive variant of the technique" that allows measuring "hyperbundles," or large swaths of fiber tracts, at high accuracy. They mapped the axon pathways in five subjects, and identified a core region in the poster medial cortex in which fibers were most densely interconnected. When they compared the structural data to brain activity observed with fMRI, they found that the region corresponded to a brain area that becomes activated when the brain is in "default" mode. Researchers first proposed the existence of a linkurl:default network;http://www.ncbi.nlm.nih.gov/pubmed/11209064?ordinalpos=5&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum about seven years ago, showing that activity there drops when performing a specific mental task, such as a calculation. But critics have questioned whether it was possible to define what a resting state in the brain was, and suggested the default network may be an artifact of imaging techniques, said Williamson. "I think [the concept] has actually withstood many of these criticisms," he said. The fiber pathways the group identified "have been known for a long time -- this goes back to basic neuroanatomy," said Williamson, but the study's "tractography approach is really quite good," he said. However, he cautioned that the paper may be overstating how well the technique reflects the anatomy of the fiber tracts. Sporns noted that he and his colleagues compared connection data using diffusion spectrum imaging to anatomical data from macaque monkeys. "It turns out the overlap is quite good," though not perfect, he said. "I believe, maybe in just a matter of months, this technique will become more widely used" for broad structural mapping, said Sporns. "I think people are becoming more interested in looking at the brain this way." Research into the default network and its physiological basis may yield insights into conditions such as Alzheimer's disease, linkurl:schizophrenia and depression;http://www.the-scientist.com/article/daily/24909/, the researchers say. "Most of the conditions we deal with in psychiatry are human conditions," said Williamson. Some studies are beginning to suggest that it's abnormalities in the default network, which seems to develop over time during infant development, that makes human brains susceptible to such woes, he explained. Sporns said his group plans to use the high-resolution tract-tracing technique to begin "mapping those differences, and perhaps finding out whether they are causative or symptomatic, and whether they can be used as predictors of progression of a condition." The team is also looking closely at individual differences in brain wiring. "That might allow us to connect these wiring patterns to particular behaviors," he said.
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