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Scientists have created the largest human brain map to date, offering a groundbreaking look into how the brain is built and functions. The achievement comes from the Machine Intelligence from Cortical Networks, or MICrONS, project—a seven-year effort involving more than 150 researchers around the world.
The findings, published Wednesday in a series of studies across the Nature family of journals, mark a turning point in neuroscience. Researchers mapped the brain activity and structure of a tiny portion of a mouse’s visual cortex using tools that trace both cell connections and the way they communicate.
The sample measured just one cubic millimeter—smaller than a grain of sand—but contained an entire ecosystem of brain cells. The data set is now publicly available through the MICrONS Explorer, totaling 1.6 petabytes, the equivalent of 22 years of nonstop high-definition video. Scientists say it is the largest and most complete brain map ever made.
“The MICrONS advances published in this special issue of Nature are a watershed moment for neuroscience, comparable to the Human Genome Project in their transformative potential,” said David A. Markowitz, Ph.D., former IARPA program manager who coordinated this work.
U.S. SCIENTISTS CREATE FIRST-EVER COMPLETE BRAIN CIRCUIT MAP
A groundbreaking new 3D map of a single cubic millimeter of mouse brain—just a speck—has revealed a staggering 84,000 neurons connected by half a billion synapses and over 5.4 kilometers of neural wiring.… pic.twitter.com/hAilDY3aUy
— Mario Nawfal (@MarioNawfal) April 10, 2025
The process began at Baylor College of Medicine, where scientists recorded the activity of brain cells as a mouse watched videos, including YouTube clips. The tissue was then sent to the Allen Institute, where researchers sliced it into more than 25,000 layers—each thinner than a human hair—and captured high-resolution images using electron microscopes.
At Princeton University, scientists used artificial intelligence to reconstruct the images into a three-dimensional model, revealing over 200,000 cells, four kilometers of neural wiring, and more than 500 million synapses, or connection points between cells.
“Inside that tiny speck is an entire architecture like an exquisite forest,” said Dr. Clay Reid, a senior researcher at the Allen Institute. “It has all sorts of rules of connections that we knew from various parts of neuroscience, and within the reconstruction itself, we can test the old theories and hope to find new things that no one has ever seen before.”
Among the key findings was a shift in understanding how inhibitory brain cells function. These cells, long thought to simply suppress activity across the brain, were found to operate in a much more selective and coordinated way.
Instead of acting randomly, inhibitory cells carefully target specific excitatory cells—those responsible for activating signals—creating a complex network of control. Some inhibitory cells work together to calm broad regions, while others focus on precise cell types.
The research also identified new cell types and organizational patterns, prompting scientists to rethink how cells are classified and how brain circuits are structured.
Experts say the findings could lead to new insights into how the brain processes information, and may one day help inform treatments for neurological conditions.
