Scientists on Wednesday unveiled a detailed, three-dimensional map of the 75 million neurons of the mouse brain, the most complex diagram of a mammalian brain ever created. Researchers say it is an important step toward understanding the way the human brain works.
The Seattle-based Allen Institute for Brain Science revealed its Mouse Brain Connectivity Atlas, in which scientists used high-resolution scans to render detailed images of the neural connections of every portion of the mouse brain.
The so-called “connectome” shows the circuits formed by connections between individual brain cells, or neurons, which work together to process and code information that ultimately produces emotions and bodily functions. The atlas, which anyone can access online for free, wraps up four years of research from the Allen Institute involving the scans of 1,700 mouse brains, detailed in a paper published Wednesday in the journal Nature.
"Understanding how the brain is wired is among the most crucial steps to understanding how the brain encodes information," Hongkui Zeng, senior director of research science at the Allen Institute, said in a news release.
The researches said the atlas contains a total of 1.8 petabytes of data — the equivalent of 23.9 years of continuous HD video.
The news comes exactly one year after President Obama unveiled the “BRAIN” Initiative, a $100 million government investment in research on the human brain and brain disorders including Alzheimer’s, schizophrenia and autism.
Despite the importance of neuron connections in understanding how the brain works, “our knowledge of it remains remarkably incomplete,” the authors wrote in the Nature paper.
The only other species for which scientists have completely mapped the neural connections is the C. elegans — a worm with just 302 neurons. The mouse, by comparison, has 75 million neurons, and humans have an estimated 100 billion.
But much of the available data has been scattered or incomplete, the authors say, showing just a few images or extreme detail of one small region of the brain. Otherwise, details on brain connections have been binary — meaning that the presence of brain connections are known in “yes” or “no” terms, without any quantifying details.
That simple “yes” or “no” doesn’t tell us much about the strength or nature of connections in the brain, Zeng told Al Jazeera. And as a result, scientists lack a comprehensive understanding in terms of whole-brain connectivity.
So they created an “atlas,” by imaging one mouse brain at a time, injecting a synthetic virus that would light up a particular area of the brain with a fluorescent green protein.
The scientists then did high-resolution scans of the entire mouse brain to see the neural connections in each illuminated site.
“We found amazing diversity in the connectional strengths,” Zeng said.
The connections were found to vary in five orders of magnitude — that means there were 100,000-fold differences in strength between neural connections in different regions of the mouse brain. Some regions of the mouse brain only have a few connection fibers, and others have hundreds of thousands of them.
What also interested Zeng and her group was that there was a small number of very strong brain connections, and a large number of weaker connections. “So it may be that the brain is processing information through a few small pathways, but then there is a large number of weaker connections” that moderates the processes of those few, strong connections, she said.
But at this point, Zeng added, it’s just a hypothesis.
Since the database will be completely open and free to the public, scientists will be able to mine the data about specific connections to guide more research. For example, if this is what connections in a normal mouse brain look like, what do they look like in animal models of human diseases such as, say, Alzheimer’s? How and where do those brain connections change?
“We believe that [the atlas is] only scratching the surface,” Zeng said.
She said the project raises more questions than it offers in terms of distinct answers.
“Why does the brain need all these different types of connections with all these different strengths? Our result is always raising more questions than providing answers,” Zeng said. “That’s always the case.”