On 9 April 2025, the MICrONS consortium - the Allen Institute, Baylor College of Medicine, and Princeton University - published a suite of ten papers in the Nature family of journals describing the largest combined structural and functional map of brain tissue made to date. The dataset covers a single cubic millimeter of a mouse’s visual cortex and contains more than 200,000 cells, about half a billion synapses, and roughly four kilometers of axonal wiring.
What set MICrONS apart was that it captured both the wiring and the activity of the same piece of brain. Researchers first recorded the responses of about 75,000 neurons with calcium imaging while the mouse watched videos, then reconstructed the exact physical circuitry of that same tissue with electron microscopy. Co-registering the two means a researcher can ask not only how a neuron is wired but also how it actually behaved - a functional connectome rather than a static diagram.
MICrONS stands for Machine Intelligence from Cortical Networks, and the project was explicitly motivated by AI as much as biology. The program, backed by roughly 100 million dollars of US government funding, set out to reverse-engineer the algorithms of a real cortical circuit in the hope of informing better artificial neural networks, while also illuminating a brain region affected by conditions such as Alzheimer’s disease and autism.
As with the fly connectome, machine learning did the heavy lifting of tracing neurons through enormous image volumes, and the finished resource was released openly with interactive tools so that any researcher could explore the data.