In a groundbreaking achievement for neuroscience, researchers have unveiled the most comprehensive and detailed 3D map of a mammalian brain region ever created, revealing not only the dense wiring of over 200,000 cells but also the real-time activity of tens of thousands of neurons. The findings, part of the Machine Intelligence from Cortical Networks (MICrONS) project, were published in a remarkable package of eight papers in Nature and Nature Methods.
This unprecedented feat, accomplished by an international team of more than 150 scientists, presents the first large-scale brain map to overlay neuronal activity directly onto a structural wiring diagram. Focusing on a cubic millimetre of the mouse visual cortex—a region that processes visual input—the researchers charted 82,000 neurons, over 500 million synapses, and more than four kilometres of neuronal wiring.
The scale and detail of the map are staggering. Previous efforts in human brain mapping covered a similar-sized tissue volume but captured only 16,000 neurons and 150 million synapses. The MICrONS project pushes the boundaries of what’s possible in the field of connectomics, which aims to map the brain’s neural connections in hopes of better understanding cognition, memory, and behavior.
“This is something we’ve never seen before—neuronal activity linked directly to physical connections on such a massive scale,” said Mariela Petkova, a Harvard neuroscientist not involved with the project, in comments to Nature.
To achieve this breakthrough, the team recorded activity from nearly 76,000 neurons while a mouse watched video clips—including scenes from The Matrix—for two hours. They then preserved and sliced the brain tissue into thousands of ultra-thin layers, each thinner than a strand of human hair, before reconstructing them into a three-dimensional map. Using artificial intelligence and machine learning, they labeled neurons, their intricate branches, and their synapses—an endeavor never before attempted at this volume or resolution.
“The data are really stunningly beautiful,” said Forrest Collman, co-author and neuroscientist at the Allen Institute for Brain Science. “Looking at it really gives you an awe about the complexity of the brain that’s very much akin to looking up at the stars at night.”
Beyond visual impact, the map is already yielding scientific insights. One key finding reinforces and refines a classic neuroscience theory: “neurons that fire together, wire together.” The research showed that neurons responding to similar visual stimuli formed more connections—even across long distances—than those tuned to different features. This large-scale evidence strengthens a foundational concept that has previously only been tested on small neuron populations.
“This is a very impressive endeavor and success,” said Moritz Helmstaedter of the Max Planck Institute for Brain Research. “The combination of function and structure at this scale is unprecedented.”
The MICrONS dataset has been made publicly available, and researchers across the globe are already exploring its implications—from how the brain encodes visual memories to how it organizes and processes information.
According to co-author Nuno Maçarico da Costa from the Allen Institute, the mapped section represents just 0.2% of the mouse brain, but the team is now working to apply their methods on a whole-brain scale.
