In pursuit of a mind map,slice by slice

“You are born with your genes,and they don’t change afterward,” said H. Sebastian Seung,a professor of computational neuroscience at the Massachusetts Institute of Technology who is working on the computer side of connectomics. “The connectome is a product of your genes and your experiences. It’s where nature meets nurture.”

The task is arduous and years from fruition,and even the biggest zealots acknowledge that their work may not pay off. But connectomics has gotten some meaningful financing: In September,the US National Institutes of Health handed out $40 million in grants to researchers at Harvard,Washington University in St. Louis,the University of Minnesota and the University of California,Los Angeles,to pursue connectomics. Together,their research efforts comprise the Human Connectome Project.

Since the 1970s,researchers have only had one connectome to play with—that of a worm with a measly 300 neurons. Now they are trying a mouse brain,with its 100 million neurons. So far the notion of creating a human-scale connectome—which would illuminate all of the connections among more than 100 billion neurons and unravel the millions of miles of wires in the brain—has proved too daunting.

About one petabyte of computer memory will be needed to store the images needed to form a picture of a one-millimeter cube of mouse brain,the scientists say. By comparison,it takes Facebook about one petabyte of data storage space to hold 40 billion photos. “The world is not yet ready for the million-petabyte data set the human brain would be,” Lichtman said. “But it will be.”

Neuroscientists say that a connectome could give them insights about the brain’s function and prove useful in the exploration of mental illness. For the first time,researchers and doctors might be able to determine how someone was wired—literally—and compare that picture with “regular” brains. Surgeons armed with a connectome might also be able to make more calculated cuts in the brain.

“The connectome project is going to show where all the white matter—all the connecting fibers—are,” said Stanley Klein,a professor of optometry and vision science at the University of California,Berkeley.

Scientists around the world,have developed varying techniques for mapping the brains and nervous systems of human as well as other creatures. Harvard recruited Lichtman to push the connectome quest to its limits by tackling an entire mouse brain at the finest scale and allowed him to set up his own connectome research laboratory,staffed with four other people.

Machines built by Kenneth J. Hayworth,another one of the researchers,can sheer off slices of a mouse brain just 29.4 nanometers thin using a diamond knife blade. To provide a sense of the accomplishment,the researchers liken the cutting to shaving off the entire surface of a football field at a thickness of one-hundredth of an inch. The researchers are marching across a mouse brain in linear fashion,gathering the slices,imaging them and then putting the puzzle back together. Once assembled by a computer,the images of the brain are beautiful. It takes about three days for the researchers to carve 7,000 sections of a mouse’s cerebral cortex. “The cutting is easy,” Lichtman said. “The big time sink is imaging.”

Lichtman estimates it will be several years before they can contemplate a connectome of a mouse brain,but there are some technological advances on the horizon that could cut that time significantly. Needless to say,a human brain would be far more complex and time-consuming. ashlee vance