A connectome — the complete wiring diagram of a brain — was long an impossible dream, and the bottleneck was never the microscope. It was reconstruction: tracing every wispy neurite through petabytes of electron-microscopy images, a segmentation problem at a scale only AI can touch. Flood-filling networks cracked it, tracing neurons an order of magnitude more accurately than before. In 2024 the payoff landed: FlyWire published the first complete wiring diagram of an adult brain — 139,255 neurons and about 50 million synapses in the fruit fly — annotated into 8,453 cell types; and H01 reconstructed a cubic millimeter of human cortex, 57,000 cells and 150 million synapses in 1.4 petabytes. But the honest frontier is sharp: even with AI, proofreading whole brains ‘will require many person-years of effort,’ and a wiring diagram is not function — you still need dynamics and neuromodulation to know what a circuit does. It’s the most extreme version of the problem the lab lives on: AI reading images at a scale no human can, on open platforms, with humans in the loop to keep it honest.
