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T HE MAPPING INITIATIVE — known as the Mouse Connectome Project (MCP) — recently achieved an important and unprecedented milestone: the fi rst charting of the mam- malian cerebral cortex. Results of the cortex mapping appeared in February in the journal Cell. "The mouse cerebral cortex is comprised of approximately 50 independent regions," says Houri Hintiryan, PhD, Assistant Professor of Research at the USC Institute for Neuroimaging and Informatics and a co-lead author of the study. "For decades, numerous studies have examined the connectivity of those individual cortical areas. For the most part, our connectivity data agree with those previous fi ndings. Although connectivity matrices of the cortex have also been assembled in other species, such as the cat and the monkey, none have previously shown how select groups of cortical areas are more interconnected than others to form distinct subnetworks with unique connectivity interactions." Previous cortical connectivity matrices were constructed piecemeal from data collected using a variety of methods in a multitude of small studies. Those fac- tors — combined with the limited space for publication of connection and injection site images in scientifi c journals — hampered comparison and analysis by the scientifi c community. MCP researchers have chosen to trace all of the pathways of the mouse brain and analyze the data in a standardized way. Investigators worldwide can view the data, including a cortical map, on the MCP website, http://mouseconnectome.org. "Collecting and analyzing all of the data systematically lends itself to detailed analysis that would not be possible oth- erwise," Dr. Hintiryan says. "That type of collection and analysis enabled us to identify eight subnetworks of the cortex that had gone undetected previously." Pathfi nders MCP researchers made 150 injections into a mouse subject's cortex; each injec- tion was made up of two circuit tracers that delineated four pathways using different- colored fl uorescent dyes. The methodology uncovered 600 unique cortical pathways. The researchers created a cortical connectivity map by incorporating 240 of the pathways into a conventional mouse brain atlas. "Analysis of the 600 cortical pathways showed certain cortical areas are more strongly interconnected at the cortical level than others," says Hong-Wei Dong, MD, PhD, Associate Professor of Neurology at the USC Institute for Neuroimaging and Informatics and senior author of the study. "That is to say, a group of cortical areas displayed a high level of cortical connectivity that organized them into eight distinct subnetworks." The subnetworks are: + Lateral subnetworks — One subnet- work along the edge of the brain may RESEARCHERS AT THE UNIVERSITY OF SOUTHERN CALIFORNIA ARE PAINSTAKINGLY MAPPING THE MOUSE BRAIN, A PROJECT WITH SIGNIFICANT IMPLICATIONS FOR UNDERSTANDING HOW STRUCTURES IN THE HUMAN BRAIN COMMUNICATE AND HOW DISRUPTIONS IN THEIR INTERACTIONS MAY AFFECT THE DEVELOPMENT OF NEUROLOGICAL DISEASES SUCH AS ALZHEIMER'S. Of Mice and Men: Cracking the Mammalian Cerebral Cortex's Organizational Code By Thomas Crocker 1 2 | Greater Boston MD NEWS ■ M D N E W S . CO M