西亚试剂：Connectomes make the map

A newborn baby, well fed and sleepy, is swaddled in a blanket and lying on what looks like a tea tray with a helmet attached to one end. Once the infant falls asleep, researchers pull special tabs on the blanket to ease the baby into the helmet. It is a customized receiver coil used for magnetic resonance imaging (MRI), a common method for visualizing brains in living people. The researchers slide the baby-holding contraption along a special trolley into the MRI tube and start collecting images.

From about 1,000 such scans, and another 500 of developing fetuses, UK scientists in the Developing Human Connectome Project plan to map how regions of the brain communicate with each other during development. They then hope to work out why preterm babies are at risk for conditions such as autism spectrum disorder or attention deficit hyperactivity disorder, and perhaps to do similar scans to check whether methods to prevent such disorders are working.

The project is one of many to unravel the 'connectome', the links between the brain's hundreds of areas and millions of neurons. “The days of just looking at one part of the brain are waning,” says Arthur Toga, director of the Laboratory of Neuro Imaging at the University of Southern California (USC) in Los Angeles. He and other scientists are already starting to compare the connections in healthy brains with those of people who have connectopathies, diseases caused by aberrant connections, such as schizophrenia, or disrupted connections, like Alzheimer's disease.

The subjects studied by connectome researchers range from living people to the preserved brains of tiny animals such as worms and flies. The investigative technologies range from MRI scanners to light microscopes and electron microscopes. Irrespective of the specifics, scientists — with the aid of computers — painstakingly chart connections to build an atlas. The map-makers hope that revealing the connectome's structure will help neuroscientists to navigate as they work out how different parts of the brain function together.

Like traditional cartography, brain mapping is a matter of scale (see 'Maps across magnitudes'). Researchers such as Toga who study the brains of living people are limited to a global view. “It's basically a fly-over at 39,000 feet,” Toga says. This approach, called macroscale by some, shows how bundles of axon fibres connect large regions together. With millimetre resolution, it is like a country map that marks major highways. Scientists studying animal brains slice by slice get more detail. At this mesoscale, researchers see how smaller regions of the brain communicate along single axons at micrometre or submicrometre resolution. It is like adding in the lanes of highways and local streets. Finally, microscale images reveal individual neurons and synapses at resolutions of a few nanometres — akin to a map that shows even footpaths and stepping stones.

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