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A map of excitatory neurons from the new research. The team cataloged around 57,000 cells and 150 million connections between neurons.
Google Research & Lichtman Lab (Harvard University). Renderings by D. Berger (Harvard University)
Researchers have made a digital map showing a tiny chunk of a human brain in unprecedented detail. Based on a brain tissue sample that had been surgically removed from a person, the map represents a cubic millimeter of brain—an area about half the size of a grain of rice. But even that tiny segment is overflowing with 1.4 million gigabytes of information—containing about 57,000 cells, 230 millimeters of blood vessels and 150 million synapses, the connections between neurons. The researchers published their findings in the journal Science on Friday. They have made the data set freely available online and provided tools for analyzing and proofreading it. "In one respect, our data set is miniscule," Jeff Lichtman, a co-author of the study and biologist at Harvard University, tells Popular Science's Lauren Leffer. "But it doesn't feel small, because when you get in it, you see it's like a gigantic forest." "It's a little bit humbling," Viren Jain, a co-author of the study and neuroscientist at Google, says to Nature News' Carissa Wong. "How are we ever going to really come to terms with all this complexity?" The first such map of a brain was made in 1986, when researchers cataloged the 302 neurons of a roundworm, Jain writes in a Google blog post about the new research. In 2020, the Google team mapped 25,000 neurons in part of a fruit fly brain. They've also been involved in mapping parts of the brains of the zebra finch (a small bird) and zebrafish larvae. For the new paper, the researchers turned to a piece of brain tissue from a person with epilepsy. To access a lesion in the patient's hippocampus, a surgeon had removed the tissue from their left anterior temporal lobe, a part of the brain thought to play a role in our memory of objects, people, words and facts. Scientists then used an electron microscope to image more than 5,000 slices of the tissue, each of which was only around 30 nanometers thick. The process took close to 11 months. Artificial intelligence algorithms then reconstructed the cells and their connections in 3D. "The aim was to get a high resolution view of this most mysterious piece of biology that each of us carries around on our shoulders," Lichtman tells the Guardian's Ian Sample. "The reason we haven't done it before is that it is damn challenging. It really was enormously hard to do this." "It's probably the most computer-intensive work in all of neuroscience," Michael Hawrylycz, a computational neuroscientist at the Allen Institute for Brain Science who did not contribute to the findings, tells MIT Technology Review's Cassandra Willyard. "There is a Herculean amount of work involved." The data has already yielded some unexpected findings for the researchers. "There were just so many things in it that were incompatible with what you would read in a textbook," Lichtman says to MIT Technology Review. For instance, they found some rare sites where neurons were connected by more than 50 synapses. This is incredibly uncommon—more than 96 percent of connections between neurons have just one synapse, and more than 99 percent have three or fewer synapses, per Google's blog post. These strong connections might show "what learning looks like in the brain," Lichtman hypothesizes to the Guardian, representing behaviors so well-practiced that they require almost no thought at all, such as moving your foot from the accelerator to the brake when stopping a car. The team also found cases of axons, the tendrils on neurons that transmit messages, wrapping themselves into knots—but they don't know why. "Nobody had seen anything like this before," Jain tells Nature News. While the tissue sample didn't have any evidence of disease, the researchers can't be certain whether these surprises are in some way related to the person's epilepsy or treatments they were receiving. Studying other tissue samples could give them a clearer idea. Next, the researchers would like to map the hippocampus of a mouse. But in the meantime, the human data they have so far will allow other researchers the chance to learn more about the human brain. "Not only is this an impressive technological feat, this is a tool and a resource that is really aimed at sharing with the world and getting all of this scientific information out there," Tim Mosca, a neuroscientist at Thomas Jefferson University who did not contribute to the findings, tells Popular Science. "This group has done an amazing job designing all of the new tools and the pipelines to make this available to anyone who wants to look at it, wants to think about it, wants to use this in their research." Get the latest stories in your inbox every weekday.